Because the number of potential configurations is infinite, only a limited number of finite difference time domain (FDTD) simulations were performed. A neural network was then trained with the simulation results to find the core-shells configurations with optimal optical absorption across different wavelength ranges. This demonstrates that core-shells nanoparticles can make an important contribution to improving solar cell performance and that neural networks can be used to find optimal results in such nanophotonic systems.Core-shells metallic nanoparticles have the advantage of possessing two plasmon resonances, one in the visible and one in the infrared part of the spectrum. This special property is used in this work to enhance the efficiency of thin film solar cells by improving the optical absorption at both wavelength ranges simultaneously by using a neural network. Although many thin-film solar cell compositions can benefit from such a design, in this work, different silver core-shell configurations were explored inside a Halide Perovskite (CH The efficiency improvement of commercial single junction solar cells, with active materials, such as silicon, CdTe and copper indium gallium selenide (CIGS), has been very gradual. This slow pace of solar cell efficiency improvement has led to the exploration of other compositions, such as organic, dye-sensitized copper zinc tin sulfide (CZTS) and Perovskite , which sTo improve the plasmonic properties with respect to optical absorption in the surrounding semiconductor, such as Perovskite, the obvious parameters, such as size and shape, have been explored with little room for improvement . In core3NH3PbI3) layers with plasmonic core-shells particles by training a neural network. Perovskite is a very promising solar cell material [In this work, we present a study to improve the optical absorption in Halide Perovskite  simulations. The results were then used to train a neural network to find the optimal optical absorption in the Perovskite layer, which corresponds to a particular silver core-shells particle configuration. This work not only provides a design to plasmonically improve the efficiency of Perovskite solar cells but also provides a novel method to improve solar cell efficiency while making use of a neural network.3 with 12 perfectly matched layers (PML) on each side. The silver core-shells particles were embedded on different positions in a 200 nm thick Halide Perovskite (CH3NH3PbI3 (MAPI)) [The FDTD simulations were performed with commercial software  on core-shells particles using silver , with a (MAPI))  film, wh (MAPI)) . Two squThe neural network was created by commercial software , and three models were used for the nanoparticle array. The scaling layers were always kept at automatic, the principal components layer was not used while the unscaling and bounding layers alternated between either off or on with minimum/maximum methods. The number of layers and the number of neurons in each layer differed for each model. The activation function for each layer was always the hyperbolic tangent, while the output layer always had a linear activation function. The loss index error method was the only variable that changed in the training strategy, while the order selection and input selection remained the same for each model. Three neural network models for the array were explored  with an average error of maximum of 2.5%. Each neural network model was trained once the network parameters were set, after which order selection was run to minimize the loss on the selection instances. This process was repeated twice, and after each iteration, the network automatically updated the network parameters to give the best results. The neural network code was then further analyzed to find the configurations given by the neural network with the highest absorption for each wavelength value in the three ranges and to find the configuration with the highest average absorption.Although an infinite number of neural network models could be investigated, here we only present a feasibility study in which three models are sufficient. The simulation parameters that were changed included the particle size , number of shells , and silver shell thickness  and the position of the particle within/on the Perovskite layer . The detailed combinations of realized simulations are listed in the The illustrations of The optical absorption spectra in Perovskite with and without silver core-shells particles are shown in In a solar cell, only the optical absorption of the active layer determines the light conversion efficiency. Because of this, the optical absorption spectra of Perovskite, containing the various core-shell silver particles at the different positions in the Perovskite layer, were used to train the neural network. Three different wavelength regions were used to investigate the neural network models, 500–600 nm, which is above the absorption edge, 700–800 nm which is on the absorption edge and 1000–1200, which is in the infrared, outside the absorption edge.In The neural network models were analyzed for two situations: the highest optical absorption in Perovskite with: (1) allowing a different configuration for each wavelength, and (2) the overall best configuration for all the wavelengths in the relevant wavelength range. The optical absorption curves of these two neural network results are also shown in 2 due to the presence of iodine cannot be excluded [Although the simulations in the wavelength range 1000 to 1200 nm provided increased optical absorption with core-shell particles included at the bottom of the layer, the neural network was not able to optimize this and did not even outperform the simulation. Although a plasmon resonance occurs in this wavelength range for some core-shell nanoparticles, the neural network was not able to find a pattern due to unpredictable properties of the various core-shells nanoparticle configurations. While both the best simulations and the neural network placed the core-shells nanoparticles at the bottom, the best simulations had a diameter of 30 nm with a shell thickness of mostly 3 nm, while the neural network resulted in particles with a diameter of 60 nm with a shell thickness of 4 nm. In this wavelength range Perovskite does not absorb much light, therefore, most of the incident light reaches the bottom where a nanostructure can scatter it back into the active layer. The maximum scattering power is also shifting as a function of size because the plasmon resonance wavelength of the nanoparticles inside the Perovskite depends on their size. Because only a limited wavelength range (1000–1200 nm) was used to train the neural network, the plasmon resonance wavelength can move outside this wavelength window, complicating the optimization. Interestingly, the nanoparticles which performed best in this wavelength range as obtained from the simulations and neural network almost always had a glass shell around the silver shell to optimize the plasmon resonance. Although this is a proof of principle study, modern deposition techniques will be able to manufacture such complicated systems by, for example, approximating core-shells particles by layered deposition on top of simple particles. Although for some Perovskites the formation of AgIexcluded ,53, the Core-shells nanoparticles have various “handles” to control the plasmon resonance wavelength and strength, such as the number or thickness of the shells. Some core-shells nanoparticles have two plasmon resonances which can be conveniently tuned to the wavelength region of interest with respect to the semiconductor of a solar cell. Both the FDTD simulations and the neural network showed that core-shells nanoparticles increase the optical absorption in Perovskite thin films when they are placed on top for photon energy comparable to the band-gap and at the bottom of the Perovskite thin film for photon energies smaller than the band gap. The neural network used was able to find an optimal configuration of the core-shells nanoparticles which was not simulated. This demonstrates the potential of neural networks for nanophotonics and improving solar cell efficiency with intricate light management nanostructures. Future research using neural networks for solar cells could focus on different shapes of particles, i.e., cubes, stars etc., or different compositions, such as gold or aluminium. Because of the complex structure of Perovskites, neural networks could be used to find optimized and more stable compositions.


Here we examine to what extent readers’ eye movements can instantly reveal the difficulties of mismatching constraints when participants read sentences with the structure lian + determiner phrase + object noun + subject noun + dou + verb phrase (VP) + final commenting clause. By embedding high-likelihood or neutral events in the construction, we created incongruent and underspecified sentences and compared such sentences with congruent ones describing events of low expectedness. Relative to congruent sentences, the VP region of incongruent sentences showed no significant differences on first-pass reading time measures, but the total fixation duration was reliably longer. Moreover, readers made more regressions from the VP and the sentence-final region to previous regions in the incongruent than the congruent condition. These findings suggest that the effect of pragmatic constraints is observable during naturalistic sentence reading, reflecting the activation of the construction-based pragmatic information for the late integration of linguistic and extra-linguistic information at sentential level.An event-related potential (ERP) study demonstrated that construction-based pragmatic constraints in Chinese (e.g.,  Even a rich person cannot afford such an expensive house, a less likely event a rich person cannot afford an expensive house is constrained by the even construction, denoting the unexpectedness of what is described in the construction, and implying that any event which is more likely to happen than the embedded event must occur. If the event does not rank at the lowest end of the scale, embedding such event in the construction can result in infelicitousness  information in real time . The negtousness . Howevertousness .some kids were riding bicycles) preceded by a context mismatching the pragmatic meaning of the quantifier . This negative response indexes a process of canceling or inhibiting initially built pragmatic representation, implicitly indicating that pragmatic information is instantly used for online sentence processing  has suggested that readers can immediately detect when an upcoming word is pragmatically incongruent with the prior sentential/discourse/communicative context , as indicated by an increased N400 response on the word that indexes an increased effort of integrating the word into the pragmatic context e.g., . Some stocessing . In contpragmatic implausibility  showed mixed evidence. For example, Murray and colleagues (smacked the childthe savages/uranium ). The authors reported very early parafoveal-on-foveal effects of pragmatic plausibility , such that the pragmatically implausibility of the critical word inflated first pass reading times on its preceding regions (uranium), thus can be detected parafoveally before that word was directly fixated , with the first pass reading time being increased for the former rather than for the latter condition. Furthermore, baseline/an axeimplausible to chop the large carrots for dinner last nightJohn used a knife), and found no effect of implausibility during first pass reading of carrots. The potential effect was further delayed in the condition in which the world-knowledge permits no way out . The go-past measure of eye movement, which includes the amount of time readers spent on the target word as well as the one spent on constituents preceding the target before moving forward to new portions of the sentence, was influenced by implausibility; but the effect size of this measure was fairly small, indicating that the impact of context has no immediate effect on eye movements during reading , such an effect was found on the region following the anomalous word when the context indicated that the person who exerted the action was a cartoon or fictional character (carrots and it would gobble it down happilyEvolution dictates that cats are carnivores and cows are vegetarians. Families could feed their cat a bowl of ) but as longer total reading time on words preceded by a counterfactual world context , while a highly likely event within sentences that begins with even leads to incongruity (Even students taught by the best teacher passed the examination) with readers’ world knowledge. only, there was longer first-pass reading time spent on the critical region  when information provided by the text became incongruous, indicating that the incongruence was detected fairly quickly. In contrast, for sentences with even, the effects of anomaly emerged with a longer delay, and were manifested only on the post-critical region . Furthermore, readers were more likely to make regressions back to the text that contained a focus when sentences with even described a set of likely events.Construction-based pragmatic constraints implemented by different focus particles also moderate different stages of processing during online reading. A relevant study  investigOverall, the empirical evidence concerning the time-course of processing pragmatic constraints, established with the eye-tracking approach, is mixed and mostly restricted to alphabetic languages like English. Investigating this issue in a completely different orthography may provide further evidence regarding the time course of when readers detect pragmatic anomalies. Unlike English, Chinese orthography is logographic, character-based, and word boundaries are not marked by spaces. A single Chinese character can be a word by itself, or can be a morpheme of different multi-character words when combined with other characters. Chinese words do not have inflectional markers to specify various types of grammatical properties. Furthermore, many words in Chinese are polysemous, and their meanings have to be fully established on the basis of context by invoking world knowledge or pragmatic information . To resolian…dou…construction (similar to even in English). The lian…dou… construction normally describes an event which is unlikely to occur but occurred, or an event that is highly likely to occur but did not occur, thus constraining an event of low expectedness. This construction also introduces a pragmatic scale indicating that the focused element (here the embedded event) should lie at the lowest end of the scale and that any event with a higher likelihood than the embedded event must occur. If the event does not rank at the lowest end of the scale, embedding such an event in the construction would result in infelicitousness (Here we investigate when the pragmatic information is used (activated) during reading Chinese sentences of tousness .lian…dou… construction. The sentences were presented visually in a word-by-word fashion with each word sequentially presented at the center of the screen with a fixed stimulus-onset-asynchrony (SOA = 800 ms: 400 ms for the word duration and 400 ms for the blank screen). They manipulated the congruence between the constraint of the construction and the likelihood of the event by embedding a highly likely event  or an unspecified event in the sentence . They compared both types of sentences to the sentence with a low expected event  embedded in the construction. ERPs on the critical verb phrases (VP) showed an increased N400 response in both the incongruent and underspecified conditions, as compared to the one in congruent condition. In the incongruent condition, a slightly larger N400 was elicited as compared to the one in the underspecified condition. Moreover, on the post-critical regions which served as commenting phrase and facilitated the reading of the implied meaning of the even constraint – “it is unlikely to hear clearly such a sound,” a sustained negativity was observed in the incongruent condition compared with the congruent and the underspecified conditions, which did not differ between the two. Based on these findings, Using the event-related potential (ERP) technique with a RSVP, These observations are crucial to understanding how readers use the pragmatic constraint information  to build up sentence representations in Chinese reading see also . From a lian…dou…construction  + main verb phrase (VP) + sentence-final commenting clause.” The reader cannot determine the actual event likelihood until he or she reaches main verb phrase, which makes the VP a critical region where the sentence congruency can be detected. Sentence types were varied by manipulating the determiner phrase prior to VP, allowing the observation of any earlier effect before the critical region.To achieve the goal of examining the time course of processing the constraint of event likelihood, all sentences adopted the structure as “lian…dou) is retrieved immediately whenever the building of a representation of event likelihood is possible , and the integration of the linguistic input into the pragmatic constraint takes place without delay. Therefore, an immediate processing cost would be manifested for incongruent sentences, such that readers may experience difficulties as soon as they encounter the VP during first-pass reading; On VP, longer first pass reading time would be expected in the incongruent condition, compared with the one in the congruent condition on the VP region, or even on the pre-VP region such that the effect of event likelihood may be apparent during parafoveal processing  from Tianjin Normal University participated in the experiment. All participants were native speakers of Chinese with normal or corrected to normal vision. None of them reported any neurological, psychiatric, or reading-related disorders. Informed consent was obtained from each participant before the experiment. The study was in accordance with the Declaration of Helsinski and was approved by the Academic Committee of the Academy of Psychology and Behaviour, Tianjin Normal University.Thirty students  at a sampling rate of 1000 Hz. Only eye movements of the right eye were analyzed. Sentences were presented on a 17-inch SUMSUNG SyncMaster 959NF monitor with a 1,024 × 768 pixel resolution and a refresh rate of 120 Hz. Stimuli were presented in black on white background in lian…dou… construction were selected from the original stimuli pool developed and used in lian + determiner phrase (DP) + object noun + subject noun + dou + modal verb (MV) + main verb phrase (VP) + commenting clause (CC)” ” see . Grammatbu (not) either absent or present immediately before the main VP, creating the affirmative and the negative version of the sentences. Specifically, we replaced the affirmative modal verb with a negative counterpart and switched the adjectives in the congruent and incongruent conditions in the affirmative version to the opposite counterparts in the negative version. The purpose of using the negation form of the stimuli was to prevent readers from expecting the congruence of the sentence based on contextual information preceding the main VP.1 Consequently, six sentences were constructed for each set of stimuli.The main VP consisted of an action verb and a verb complement. The embedded event was manipulated by varying the DP, such that the DP was either a scalar adjective phrase “zheme/name/ruci [so] + adjective” to specify the likelihood of the event in the congruent and incongruent conditions or a demonstrative modifier “zheyangde/nayangde/rucide [such]” in the underspecified condition. In each set, the MV was in either affirmative or negative form, with a negation marker lian…dou construction) and the likelihood of an embedded event happening in daily life (without lian…dou construction) were rated on 7-point scales for each sentence by two independent groups of readers in two off-line tests , lower for the underspecified sentences , and the lowest for the incongruent sentences . Reversely, the event likelihood was the highest for the incongruent sentences , lower for the underspecified sentences  and the lowest for the congruent sentences .Both the global sentence comprehensibility  was defined as the critical region, where the congruency of the sentence became apparent. The adjective phrase  and the model verb  were defined as the two pre-critical regions. The MV was defined to detect any possible parafoveal effect on VP that is modulated by the congruency of the sentence. The AP was defined to examine possible regressive saccades into this region due to pragmatic inferences about the likelihood of an unspecified event against the constraints of the lian…dou… and the integration of specified event into the construction that take place on critical VP. Lexical features of AP were measured and controlled across the three conditions. The number of strokes of AP region was similar across the three conditions , and mean frequencies of this region were higher in the underspecified condition (1072/million) than those in the other two conditions . The remainder of the sentence that follows the critical region – the commenting clause  was defined as the post-critical region, making the “unexpectedness” meaning explicit.Several regions-of-interest were predefined for the analysis  and gaze duration . The measures for late processing include total fixation duration . Moreover, to investigate how participants attempted saccadic movement to deal with processing difficulty due to incongruence or under-specification, the probability of making a regression in  was reported for pre-critical regions and the probability of making a regression out  was reported for critical and post-critical regions. These two measures indicated the proportion of trials in which a participant made a regressive saccade into/out of a region.We computed different eye movement measures that represent different processing stages. The measures for early processing include lme4 package (version 1.1-7) in R , using the -7) in R . For all-7) in R , which iParticipants were instructed to read sentences in a normal way that ensured comprehension. They were informed that a simple statement would occasionally appear after a sentence, and they should verify whether the statement was consistent with the message conveyed in the critical sentence by pressing a button on the response box. Prior to the experiment, participants were required to complete a three-point horizontal calibration procedure, with an average calibration error below 0.30 degrees. Prior to the start of each trial, a fixation point was presented on the left side of the screen at reader’s eye-level. Once a stable fixation was detected by the eye tracker, the sentence was presented with the first character replacing the fixation point. Once participants finished reading a sentence, they pressed a response key on a button box to terminate the display. The experiment lasted approximately 50 minutes.There was no difference in percentages of correctly responded verification statements corresponding to different critical conditions and the overall mean accuracy was 90%, suggesting that the participants in general read and understood the sentences. Trials were removed from analysis if the track was lost or the total number of fixations were less than 5 ; additional trials were removed from analysis if an eye movement measure was beyond three standard deviations from the participants’ mean . Among all fixations of all trials, fixation durations shorter than 80 ms or longer than 1200 ms were excluded from the analyses as well. b = 0.12, SE = 0.04, t = 3.34). There was no significant difference in first-pass reading time between congruent and incongruent conditions. This result suggests that readers were requested more time to parse the sentence with underspecified adjective phrase while they encountered no increased cost for the incongruent sentences during the first-pass reading.Readers spent longer time fixating on the AP region in the underspecified condition than in the congruent condition during the first-pass reading . Furthermore, with more linguistic information accumulated for the underspecified and incongruent conditions, the readers were more likely to make regressions back to the pre-critical region .However, for the total fixation duration, readers spent longer time fixating on the AP region when reading the underspecified and incongruent sentences, as compared to reading the congruent sentences . This reduced FFD on the MV in the underspecified condition might be due to the increased FFD in the same condition on the earlier AP region. The readers may initiate the inference of missing scalar adjectives based on their knowledge or pragmatic constraints of the lian…dou… construction to deal with the uncertainty of event likelihood in the underspecified sentences. With the initial missing scalar adjectives filled, it may cost less to process the upcoming MV during the first pass reading.The measures on MV may reflect parafoveal congruency effect on the critical VP prior to the fixation. Readers spent shorter first fixations on the MV region in the underspecified sentences than in the congruent ones . These results suggest that the processing difficulty for the incongruent condition did not appear as an early parafoveal processing mechanism prior to the fixation. The incongruent condition did not affect the initial processing of MV, but the later measures, probably involving re-checking linguistic information of event likelihood at earlier regions after the incongruency, has been detected in the later regions.However, later measures showed longer TFD and more REG-IN in the incongruent relative to the congruent sentences (TFD: ps > 0.05). However, readers spent longer total fixations on and made more regressive saccades out of VP in the incongruent condition than they did in the congruent condition . This suggested that readers did not encounter any difficulties or initiate any effort to deal with the difficulties immediately after detecting the infelicitous nature of the main clause. The incongruent sentence exhibited prolonged reading time in the later measure of main VP of incongruent sentences.None of the first-pass reading time measures (including FFD and GD) showed any significant effects of incongruency or underspecification . Furthermore, there were significantly more regressive saccades out of the CC region back to previous regions in the incongruent and underspecified conditions than in the congruent condition . These data suggested that the incongruent pragmatic information did not result in the lengthening of the initial reading time but only prolonged the global reading time at the sentence-final clause. No other effects were significant for FFD and GD on CC .Similar to the findings on VP, readers spent longer TFDs on CC in the incongruent condition than in the congruent condition (lian…dou…construction (similar to even in English) in which the likelihood of the embedded event to occur was manipulated. By embedding a highly likely or an underspecified event in the sentence, we created the incongruent and the underspecified conditions, and compared each with congruent sentences in which an unexpected event was embedded.Using the same set of sentence stimuli as the previous study  and takiUnlike the previous ERP study  in whichIn addition to the eye-movement measures on critical regions (“VP”) where the nature of condition (incongruent or unspecified) was determined, we calculated such measures on regions prior to or following the VP. Depending on experimental conditions, these regions were hypothesized to attract more or less saccadic looks given the necessity to specify eventual representations (such as on “AP”), given the possible parafoveal views on the critical region that permits an early detection of pragmatic constraints (such as on “MV”), or given the possible wrap-up process for the whole discourse (such as on “CC”). On the pre-critical regions (particularly the MV region), there was no evidence indicating that the incongruent information can be processed parafoveally. Consistent with previous studies , there wlian…dou… construction does not intervene with the eye movement measures immediately as the information about the event likelihood becomes salient. Our results are comparable with some of the previous research investigating the effects of pragmatic implausibility  does not appear on the early measures on the critical word  but appears downstream of that word. Even though the plausibility effect can, in some cases, be manifested in the first-pass fixation measures on a target word (The two-stage processing is consistent with recently proposed eye movement control models. For example, the E-Z Reader 10  providedget word , the plaget word . These mlian…dou, allowing the effect of congruence-related N400 to appear on the VP.How can we reconcile the contradictory findings between plant toast and jamat the breakfast the boy would , In the current eye-tracking paradigm, sentences were presented as an entirety in one line, and the readers were allowed to preview information and initiate regressive saccades to reanalyze uncertain or incongruent linguistic input. In an ERP study when readers were allowed to read at their own pace, longer reading time was predicted by larger amplitudes of ERP on words mismatching pragmatic constraint (e.g., less plausible sentence: lian…dou…construction. When reading incongruent sentences, as compared with congruent ones, the reader spends longer total fixations, made more regressive saccades out of the critical regions where pragmatic infelicitousness is initially detected. This finding is comparable to the observation of even construction in English (even. The current study provides new evidence showing that the processing of pragmatic constraints of the Chinese lian…dou… construction may not interrupt the early stage of lexical processing during natural sentence reading, and offers a methodological perspective that promotes ecological studies of language processing.In summary, by using the eye tracking technique, the present study reveals a relatively delayed time course of processing pragmatic constraints during on-line reading of Chinese sentences with  English  which shOriginal data covered by this study can be obtained from the corresponding authors upon request.The studies involving human participants were reviewed and approved by the Academic Committee of the Academy of Psychology and Behaviour, Tianjin Normal University. The patients/participants provided their written informed consent to participate in this study.CZ, XB, GY, XJ, and XZ designed the research. CZ, LZ, and MZ performed the research and analyzed the data. CZ, XB, GY, XJ, ZH, and XZ wrote the manuscript.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


This paper introduces a novel respiratory detection system based on diaphragm wall motion tracking using an embedded ultrasound sensory system. We assess the utility and accuracy of this method in evaluating the function of the diaphragm and its contribution to respiratory workload. The developed system is able to monitor the diaphragm wall activity when the sensor is placed in the zone of apposition (ZOA). This system allows for direct measurements with only one ultrasound PZT5 piezo transducer. The system generates pulsed ultrasound waves at 2.2 MHz and amplifies reflected echoes. An added benefit of this system is that due to its design, the respiratory signal is less subject to motion artefacts. Promising results were obtained from six subjects performing six tests per subject with an average respiration detection sensitivity and specificity of 84% and 93%, respectively. Measurements were compared to a gold standard commercial spirometer. In this study, we also compared our measurements to other conventional methods such as inertial and photoplethysmography (PPG) sensors. Wearable devices providing non-invasive high resolution monitoring of living organs are essential in the hospital settings for observing physiological activity, mainly breathing and heart rate . In the Numerous non-invasive devices for respiration monitoring have been proposed. Appropriate use of current monitoring systems and correct assessment of the provided data are essential in accurate diagnosis. Some direct monitoring systems are flow meters that use pressure transducer, thermal or ultrasound flow meters ,9,10. ThAluminium nitride piezoelectric films ,12 can mIndirect methods are based on biofeedback from organs’ activities. As an example, the photoplethysmography (PPG) sensor, as the most popular and inexpensive method for heart rate monitoring, is not a direct method for respiratory monitoring, as discussed more in the sections to follow. This sensor emits infrared light into the tissue and measures the amount of light that is scattered by the blood flow. Furthermore, this sensor measures oxygen level variation due to breathing ,19, as wECG-based devices , as the Many of the above-mentioned conventional techniques are impractical in real-life conditions such as the patient being physically active or fully dressed. In a wearable health monitoring system, it is advantageous if the device monitors while operating with different body positions, with less constraints on hardware resources and wiring connectivities. In this paper, we report an ultrasound-based diaphragm motion tracking system to obtain a respiratory signal.Ultrasound technology (US) is inexpensive, safe, real-time and can be used as a direct method for cardiopulmonary monitoring. Ultrasound imaging systems with an array of piezo transducers are widely used in diagnosis and imaging . Zambon As mentioned before, the proposed system is based on the ultrasound technique. Vibration (resonant) modes of piezo-ceramic transducers depend on their shape, polarization, orientation and the direction of the electric field. The transducer used in the sensor is a 2.2 MHz PZT5 piezo disk operating in thickness mode. One main issue in piezo disk transducers operating at frequencies higher than 1 MHz is the transducer’s electrode connectivity. One side of the sensor has to be interfaced with an acoustic matching layer, with an overall thickness of λ/4 , while hTo remove air gaps between the sensor and skin, a conductive soft material such as ultrasound gel is needed between the skin and the sensor. There are many alternatives, such as water, baby oil or hand cream, which have almost an equal performance. The conductive medium enables a tight bond between the skin and the sensor, enabling wave transmissions directly to the tissues underneath. One strap band or an adhesive pad is needed to hold the sensory head in its position. Moreover, the adhesive pad will ensure the sensor’s position and lessens the motion artefact of the signals.In soft tissues, about 80% of the ultrasound wave is absorbed by the tissue, resulting in local heat production on cells . AcoustiUltrasound waves are attenuated in a medium with higher value of attenuation coefficient. In the human body, bone has the highest attenuation coefficient, which hardly allows beam transmission through itself. In addition, due to the high acoustic impedance mismatch between bone and tissues, the intensity of reflections is high. Small upper body motion does not impact the ultrasound wave propagation, and these waves can still pass through the gap between the rib cage bones. Big skin movement may result in a noticeable sensor displacement and ultrasound wave blockage if the sensor faces the bones. This misplacement leads to an error in reading ultrasound reflection. Such a big displacement rarely happens when the patient is not performing intense motion.c = 1540 m/s and f = 2.2 MHz used in this study:The acoustic axial resolution of the sensor can be calculated as below by considering the average sound velocity of tissue as Axial resolution:K. M. Langen et al.  summarizThe selected piezo ceramic disc transducer has a 15 mm diameter, which is wide enough to sense reflections within the two rib bones. The ringing effect is one of the issues to consider when designing an ultrasound system. In the previous work ,38, the In this study, the goal is to investigate the motion of the diaphragm; thus, the sensor was positioned on the zone of apposition (ZOA). The ZOA is the area of the diaphragm encompassing the cylindrical portion (the part of the muscle shaped like a dome/umbrella), which corresponds to the portion directly apposed to the inner aspect of the lower rib cage . The pieTest-T1, normal breathing: In this test, the subject was asked to disregard the apparatus and breath normally.Test-T2, normal breathing with two breath holds: In this test, the subject starts with two or three normal breaths and then a long inhalation, followed by a long exhalation. This test is done to evaluate the system on breath holds, or when the subject is suffocating.Test-T3, fast breathing: In this test, the subject breaths fast (between 25 and 40 breathing cycles per minute) to simulate breathing after an exercise.Test-T4, normal and weak breathing: This test is to evaluate the capacity of the apparatus to detect weak and slow breathing. The subject is asked to be at rest and breath gently.Test-T5, normal breathing with hand elevation: This test is intended to evaluate the skin movements when a subject abducts the arm up to the shoulder and then completely up. As mentioned before, if the sensor is placed in front of a bone, then the measurements are impossible due to the ultrasound wave blockage.Test-T6, breathing with upper body motion and rotation: In this test, the subject is asked to move his/her upper body randomly in all directions. This is to evaluate the ability of the apparatus when upper body motion is occurring.As discussed, we are monitoring the internal organ motion, mainly the diaphragm wall. Our goal is the evaluation of the proposed system on subjects performing different body motion and position conditions. Four practical tests were done in the resting state to ensure an accurate assessment versus the gold standard. Moreover, two additional tests were applied when the subject had upper body motion to examine the system under motion artefacts. All tests were done in a sitting position. The tests are detailed as follows:All participants gave their informed consent, which was approved by the appropriate ethics committee . There are no known risks associated with the experiments asked to be performed on the subjects. In some cases, it caused dry mouth, which was eliminated by providing enough time interval between each breathing pattern and drinking water. Moreover, ultrasound has been shown on many occasions to be safe when the acoustic waves are under thresholds . In a stTo validate the data obtained from the ultrasound system, a gold standard was required as a reference. Spirometer- and plethysmography-based devices are the most common clinical and commercial references. We used an SPR-BTA spirometer with GO!Link data logger software to measure oral breathing in sitting condition. A nose clip was used to prevent nasal breathing. This device measures the amount of airflow, but not the volume. Accordingly, the signal level returns to zero in breath holds. Since the proposed measurement method is a based on the volume of air the subject inhales or exhales, trapezoidal numerical integration was used to compute the approximate integral of the signal.A two-stage linear amplifier with a wide passive band-pass filter magnified reflected ultrasound beams from undesired high and low frequency components of the signal. The magnified signal was passed to an envelope detector, which helped to reduce the digital signal processing work. According to the acoustic wave attenuation ratio of 1 dB per 1 MHz per 1 cm travelled in the tissue, our ultrasound wave weakened by 2.2 dB for each cm of tissue penetration. To increase the sensing of reflections from deeper tissues, the gain of the amplifier should increase linearly with the same ratio as the ultrasound waves attenuates. For this purpose, we used an analogue front-end (AFE) designed with an integrated low noise amplifier (LNA) followed by a variable gain amplifier (VGA). The gain was controlled by an analogue voltage generated by a 10-bit digital to analogue converter (DAC). Once the switch turned to RX path, the gain started increasing from 10 to almost 30 dB within 200 To digitize the enveloped signal, we used a 10-bit resolution analogue to digital converter (ADC) with a sampling frequency of 1 Msamples/s. It converted the analogue signal once the system switched to RX mode for 200 samples  with a user-friendly GUI interface. The processing of ultrasound data could be done completely on the FPGA. However, to make a better timing comparison between all three resources, processing was done on the computer for the purposes of our experiments.The hardware setup of the proposed system architecture is shown in The average intensity of acoustic waves depends on the pulse repetition factor and the voltage level. As the voltage applied on the transducer increases, the intensity of acoustic waves increases, as well. As the repetition factor increases, the refreshment of this information increases, as well. In this study, we planned to measure the near tissue movements, meaning low energy acoustic pulses were sufficient to stimulate the transducer. In this study, we examined our system by pulses on differential voltages from j and i are the record and sample indexes, respectively (on axes Y and X). LB and UB are the lower and upper bounds of the desired window, as explained above. A series of In this test, the subject started with three normal breathing cycles. After that, two breath holds , followed by normal breathing cycles continued to the end. In this figure, the amplitude variation of each record contains the respiration information. Since these peaks were the result of an internal organ motion, rather than external motion or skin surface, the obtained information was shown to be robust to human motion. Additionally, the monitoring was based on a direct measurement of physiological human activity, and not by an indirect method. To obtain the respiratory waveform, the integral of the signal or the mean value within the desired window of each record gave a value A low-pass FIR filter was used to emit high frequency elements of the raw US signal, which was higher than 1 Hz. The same filters were used to apply to signals from the spirometer, the PPG and motion sensors. FIR filters of the same order (order 12) were used for filtering to ensure linearity in the phase of filtered data, which was an important criterion for filter selection, especially for the validation procedure. Since the respiration operation mainly changed the baseline of the PPG sensors’ data ,24,25, tFor all the listed tests, the inertial sensor was placed on the ultrasound sensor to monitor the body motion and motions due to breathing. This sensor provided three-axis accelerometer and gyroscope data at the same rate as ultrasound. We used the MAX30101 multi-sensory board, which provides a proven design to evaluate the integrated pulse-oximetry and heart rate monitoring. PPG sensor was selected as a comparative technique to the proposed ultrasound system. Some studies proposed the use of PPG sensors for respiration estimation as discussed in the Introduction. However, to our best knowledge, there is no evident practical test of respiratory monitoring when the breathing is fast or the subject is moving. In this study, we evaluated the PPG under the same conditions as our proposed system.Among the gyroscope and accelerometer sensors data, the gyroscope was found to produce a better signal. Although the magnitude of data from these two sensors was extremely small, according to our data, the respiratory waveform from the gyroscope was still more detectable than the accelerometer. Test T1 in Test T2 consisted of normal breathing with two breath holds. All subjects began with three normal breaths. Then, they were asked to hold their inhaled breaths for 10–20 s followed by a complete exhalation. The test continued with normal breathing to the end after the breath holds. This test was done to simulate an apnoea situation and the feasibility of an apnoea detection using the proposed respiratory monitoring. According to the signals of this test in Test T3 was the same as Test T1, except that the breathing was faster. This test was done not only to evaluate the performance of the proposed system, but to compare against the PPG response to fast breathing. Our analysis showed a high correlation and breath detection of the ultrasound sensor versus the gold standard. However, four breaths were not detected by the ultrasound system, which was only 11% of the total breathing cycles in this test. Inertial sensors performed better than the PPG sensor in this test. Detailed results are listed in Test T4 was similar to T1 except in comparing how the ultrasound sensor responded to weak and normal breathings versus the reference. The subject began with shallow breathing for almost 30 s and continued with normal breathing to the end. Our US signal could detect the weak breathing, but the amplitude of the signal was low and failed at three points. Note that some subjects had very weak breathing, which was the reason breath detection was missing in In this section, the proposed system is assessed versus other methods when subjects had arbitrary upper body motions. Results are shown in Finally, in Test T6, the proposed method was examined against upper body motion. An example of this test is shown in In this equation, the true positive (TP) was the number of correctly-detected full breathing operations and false negative (FN) was the number of breaths wrongly classified as negative. In this table, the total number of breaths per minute of the spirometer is listed. True negative (TN) was considered positive when both the gold standard (spirometer) and the sensor did not detect any breathing. For example, if the spirometer and the sensor did not record any breathing when no breathing was occurring in between two breathing cycle or when the subject was holding their breath, it was considered a true negative. The specificity or the true negative rate was calculated to identify the proportion of non-breaths that were correctly identified. It could be calculated using the equation below:In addition to sensitivity, the precision or positive predictive value (PPV) was calculated by Equation . In thisAccording to the above-mentioned results and visual observations, the authors did not find a relationship between the body specification  and systAs mentioned before regarding the desired window, the system looked into reflected ultrasound waves from internal organs, mainly the diaphragm. Among all the subject tests of T6 in this article, none of the tests showed any correlation with body motion. In some tests where the subject held the breath for a longer time, there were some small peaks in the signal due to the heart operation. These spikes were visible in most of the T2 tests of this article during the breath holds. Heart contractions transmitted motions to the surrounding organs, such as the pericardium sac and the diaphragm .In reference to the provided data, the proposed ultrasound system provides a stable respiratory signal using only one sensor placed in the zone of apposition. Although the system is sensitive to large sensor displacements, the respiratory signal could be monitored even in gentle body motions, at different breathing rates and intensities. In addition, it does not generate false detection in comparison to the other employed methods. Based on observations of all experiments, a weaker correlation is found between the PPG, Accl and Gyro versus the spirometer as the gold standard in the T1 to T4 test methods. As per the discussion on indirect methods for respiratory monitoring systems in the Introduction, the PPG sensor responds slower to the respiratory trend and is non-responsive when the sensor or body is not physically stable. Respiratory-induced motions on the body are negligible compared to the human motions during daily activities. This is while the respiratory signal is detectable by the proposed ultrasound system even when the body is moving, as presented in all T6 tests in To improve the displacement sensitivity, an array of transducers will enable the system to track the diaphragm motion reliably in the future. In this way, at least one transducer will trace motion changes of the diaphragm even if others are blocked by the ribs. An array of three transducers placed side by side within the intercostal space could be a solution to overcome this issue. Note that transducers placed on a flexible material will not cause any discomfort, nor inconvenience for the subjects.Our tests show that when the subjects were performing long oral breathing test with the spirometer, they felt exhausted and dizzy when the test reached a 50–60-s period. Further analysis is scheduled to apply prolonged tests based on a different reference. The system uses a bulky setup with one wired connection to the sensor. In future, integrated wireless hardware will be developed to replace the wire connectivity and bulky evaluation setup. System evaluations of the noninvasive ventilated (NIV)  subjectsIn this paper, we presented a direct respiratory monitoring system based on the pulsed ultrasound technique. The sensor consists of a PZT5 piezo transducer mounted on a flexible surface in direct-contact with the skin surface. The sensor should be placed in the zone of apposition (ZOA) on the right side of the body to observe the motion of the diaphragm. The proposed diaphragm motion tracking sensor provides fairly good detection of respiratory cycles in different breathing patterns. The system was compared against a spirometer as the gold standard. In addition, we compared our measurements with inertial and photoplethysmography (PPG) sensors. Our ultrasound sensor was less affected by upper body motions in comparison to the inertial and PPG sensors.


Signals from spaceborne polarimetric synthetic aperture radar will suffer from Faraday rotations when they propagate through the ionosphere, especially those at L-band or lower frequencies, such as signals from the Phased Array type L-band Synthetic Aperture Radar (PALSAR). For this reason, Faraday rotation compensation should be considered. On the other hand, Faraday rotation could also be retrieved from distorted echoes. Moreover, combining Faraday rotation with the radar parameters and the model of magnetic field, we could derive the total electron content (TEC) along the signal path. Benefiting from the high spatial resolution of the SAR system, TEC obtained from PALSAR could be orders of magnitude higher in spatial resolution than that from GPS. Besides, we demonstrated that the precision of TEC from PALSAR is also much higher than that from GPS. With the precise TEC available, we could fuse it with data from other ionosphere detection devices to improve their performances. In this paper, we adopted it to help modify the empirically modeled topside profile of ionosonde. The results show that the divergence between the modified profile and the referenced incoherent scattering radar profile reduced by about 30 percent when compared to the original ionosonde topside profile. Ionospheric variations have been studied for earthquake prediction, solar activity analysis, radar image modification and geomagnetic storm research ,2,3,4,5.This work relates to two parts of ionosphere detection. First, we evaluated the precision of TEC measured by the spaceborne synthetic aperture radar. Experiments showed impressive results whereby the precision was within 1 TECU when compared with the TEC obtained from ISR. Specifically, as examples, we used the L band PolSAR data from the Phased Array type L-band Synthetic Aperture Radar (PALSAR) onboard the Advanced Land Observation Satellite. The ISR station that observes the same space as the PolSAR was found. Then, TECs derived from these two facilities were compared. The results verified their consistency. The second part of this paper presents a method to improve the topside profile of the ionosonde with TEC retrieved from PolSAR data. The ionosonde station located in the scene of PolSAR was found. Since the ionosonde could directly observe the electron densities under the peak height, we were then able to derive the TEC of the topside profile by subtracting the TEC of the bottomside profile from the PolSAR TEC. This topside TEC could then be used to help model the electron density topside profile. To evaluate our method, we managed to find an ISR which was also located in the same area as the ionosonde station. A comparison of the electron density profile of the ISR and the modified topside profile of ionosonde validated our algorithm. The outline of this paper is given as follows: We first present the methods used in this paper for retrieving TEC from PolSAR data, and then introduce the algorithm for improving the topside profile of ionosonde with the retrieved TEC.Radar signals at the L band and lower frequencies will suffer strong Faraday rotation (FR) effects when they pass the ionosphere. However, these FR effects could serve as useful information for deriving the total electron density along the signal propagation path. According to ,12, the ntegrals , where TFR retrieval has been well studied in the last two decades ,19,20,21In Equation (2), the M matrix represents the measured full polarimetric scattering matrix. Therefore, when the full polarimetric data is obtained, TEC can be calculated from Equations (1) and (2).Many models are available for the topside profile of ionosonde. In this paper, we aim to improve the most commonly used model, the α-Chapman model, with a constant scale height HT :(3)Ne=NmTraditional method estimates surement . Though According to ,23,24, iIn Equation (7), Solving the nonlinear Equation (9), we can obtain https://vertex.daac.asf.alaska.edu/). Each scene of the radar data covers a rectangle area of the Earth. The TEC of each scene is estimated as the average TEC of the whole scene. For a quantitative evaluation of the reliability of TEC derived from PolSAR, we compared it to the TEC derived from ISR, because ISR is believed to be the most powerful device for monitoring the ionosphere. However, since ISRs are sparsely distributed on the earth, despite trying our best, we only managed to find three groups of corresponding data. The ISR data were collected from Poker Flat ISR station [The spaceborne L band PolSAR data for TEC retrieval were obtained from the Phased Array type L-band Synthetic Aperture Radar (PALSAR) of the Japanese Advanced Land Observation Satellite (ALOS) which orbits Sun-synchronously at about 692 km of altitude  . The topDespite doing our best, we only found one group of corresponding data, as shown in Integrating the electron density profile of ISR up to the altitude of PALSAR gave the TEC of ISR along the observing direction. The VTEC of ISR was obtained by using Equation (10), but the Equation (1) tells us that the value of PolSAR TEC is affected by the mid-value 〈Bcosθ〉 . HoweverComparing Equation (11) to the first mean value theorem for definite integrals, we getIt can be found that Take the data from ionosonde and PolSAR in The first mean value theorem for definite integrals shows that librated ,28. Therlibrated , only pi16 electrons/m2).Results of TECs retrieved from different instruments are given in From TopTEC for the ionosonde. Resorting to Equation (9), we can derive the parameter We have demonstrated that PolSAR data can be used to estimate the TEC under 700 km with a considerable precision within 1 TECU. Therefore, after the bottomside electron density profile of ionosphere has been precisely measured by an ionosonde, the corresponding PolSAR TEC can be used to calculate the TopTEC. It is clear that the bottomside profile of ionosonde matches well with that of ISR, while the two topside parts are divergent. The reason for this was discussed previously, i.e., that no topside information is considered in the original method. After modifying the topside profile with the known TEC, we can easily see that the new topside profile is more consistent with that of ISR. The divergence between the ionosonde topside profile and the ISR topside profile reduced by 30.41 percent after the presented algorithm was adopted. Here, the divergence is defined as the average absolute difference between the ISR profile and the ionosonde profile, and the percentage of reduction was calculated from Equation (13). This result proves the validity of our method.This paper first validated the ability of PolSAR as an effective device in measuring the TEC of the ionosphere, and then demonstrated the feasibility of using known TEC to help improve the topside profile of ionosonde. The results show that PolSAR is able to measure the TEC with higher precision compared to GNSS. Furthermore, the improved topside profile proved to be much more consistent with the profile of ISR than the original profile. In this paper, the α-Chapman model was used for the topside profile because of its popularity. Actually, if any other model is developed for the topside profile, the proposed TEC modification strategy could also be taken as an auxiliary process. One should note that the proposed method could only detect the TEC under the altitude of the satellite. However, it is still reliable to expand the modified ionosonde profile beyond this altitude, since TECs under 700 km cover the majority of TECs under 10,000 km.


The detection of a single photon is the most sensitive method for sensing of photon emission. A common technique for single photon detection uses microchannel plate arrays combined with photocathodes and position sensitive anodes. Here, we report on the combination of such detectors with grating diffraction spectrometers, constituting a low-noise wavelength resolving photon spectroscopy apparatus with versatile applicability. We recapitulate the operation principle of such detectors and present the details of the experimental set-up, which we use to investigate fundamental mechanisms in atomic and molecular systems after excitation with tuneable synchrotron radiation. Extensions for time and polarization resolved measurements are described and examples of recent applications in current research are given. The detection of luminescence is one of the key techniques in experimental science. Photons emitted after diverse excitation schemes carry information on the electronic and atomic structure of matter, nature and lifetime of excited states, as well as on dynamic processes like dissociation, charge and energy transfer processes. Depending on the field of science and the actual application, the photon detection schemes vary substantially. In this article, we describe dispersed single photon counting from the extreme ultraviolet to visible spectral range using microchannel plate based detection. This technique is ideally suited to be applied in fundamental research, where the interaction of an excitation source with single objects  in a dilute target is intended to be investigated.In the following paragraph, we give an overview of our experimental set-up and the working principle of our single photon detectors. In The design of a single photon detector based on a combination of microchannel plates (MCPs) and a position sensitive anode is schematically illustrated in  photons ,3,4, becDifferent anode designs allow the position resolved counting of pulses and thereby the determination of the number of incident photons on a defined detector region. In combination with a dispersing optical element, the position on the detector encodes the photon wavelength. The counting of single photons enables the investigation of weak processes with unprecedented sensitivity. To record emission spectra over a wide spectral range is time consuming. Most times, only the simultaneous detection of sufficiently large spectral intervals makes such experiments possible—utilizing position sensitive detectors.P is a function of the (wavelength dependent) efficiency of the photoelectric effect of the photocathode material d. However, the photocathode must be a thin layer because the photon is absorbed on one side of the film and the photoelectron has to escape from the other side towards the front of the MCP array, as illustrated in In the scheme described here, the creation of a photoelectron is a prerequisite for the detection of an incident photon. The sensitivity of a detector to photons within a certain spectral range is therefore governed by the photoelectron yield of the material the photon impinges on. Usually, this capability is enhanced by using a suitable photocathode material. Since most of these photocathode materials are subject to oxidation under atmospheric conditions, it is deposited as a thin film on the rear side of a window substrate with respect to the photon incidence and the complete assembly, including MCPs, is encapsulated under vacuum. The choice of photocathode and window material strongly determines the sensitivity range of the detector. The spectral response, i.e., the probability that a photon of a certain wavelength For detection of photons with 2 serve as the substrates with the lowest cut-off wavelength for transmission  with further materials available for higher wavelengths, such as fused silica (≈160 nm). In For a of channel length to diameter is an important characteristic determining the achievable amplification. The rear side of the plate is set to a high positive bias  with respect to the front side. Impinging particles, namely photons with sufficient energy or electrons from a photocathode, release secondary electrons from the channel surface. These electrons are accelerated towards the MCP rear side, but will soon hit the channel wall because they have a velocity component perpendicular to the channel axis. Since they were accelerated, they have enough kinetic energy to release multiple secondary electrons from the channel surface. This process repeats until the electron cloud reaches the end of the channel. Typically, an amplification of a, Microchannel plates can be thought of as arrays of in Refs. ,9.Because a high voltage is applied between front and rear side, the resistance of an MCP must be high and is typically in the order of 10 to 100 MThe size of an MCP is typically ranging from a few millimeters up to several centimetres. The channel density crease ρ . An evenAn essential advantage of MCPs is the low noise compared to other photon imaging devices such as silicon photomultipliers or charge-coupled device (CCD) cameras. This ensures an acceptable signal-to-noise ratio for low real signal rates in the low Hz or even below Hz regime, particularly when the excitation source has a comparably large repetition rate. In these instances, CCDs require either long exposure times or frequent read-outs, which add significant amounts of noise.x and y are here defined as the axes of a two-dimensional right-angled coordinate system). In order to retrieve the position information, position sensitive anodes are used. For optimal utilization, the active area of the anode should be at least of the same size as the MCPs. The principles of three commonly used anode types are described briefly in the following paragraphs.The information of the position of a single event is conserved during the amplification process and the charge cloud leaves the MCP centered around the same A, B, and C. Part A are strips, equally spaced, but with increasing width towards higher x values. B resembles equally spaced wedges along the y-direction, with their base at larger y and their apex at lower y. Part C is the area in between A and B and is called meander. If normalized to the sum A and B can be transferred into the position f:With wedge and strip anodes, the position information is obtained by measuring the amount of charge (amplified to voltage pulses of different heights) reaching three different anode parts, each isolated with respect to each other . The anof. If, however, spatial imperfections shall be corrected to a very high degree, a transformation of higher order might be necessary. Typically, a resolution of 2% of the detector area can be achieved. Because the total amount of charge is the measure of interest using wedge and strip anodes, the raw anode pulses are amplified by an integrating amplifier and then read out by an analog-to-digital converter (ADC).In explicit applications, it is typically sufficient to use a linear transformation for The working principle of delay line anodes is shown in elations :(2)x=2ux parallel to U, V and W disposed by ± 120 degrees. Delay-line anodes provide resolution of about 0.5% of the detector area.The small letters A, B, C, and D of one anode is recorded, which are located on the bisectrix of the With resistive anodes, the signal height reaching four different points e system . They coe system (3)x=B+CThe working principle of resistive anodes is shown in These three anode types are commonly used in MCP-based detector systems. A certain anode type might be suited best for a specific application, but, in principle, all types can be used for single photon counting as described here. While delay-line anodes usually perform best with respect to spatial and temporal resolution and are potentially capable of multi-hit detection, wedge and strip anodes and resistive anodes can be operated using somewhat simpler electronics and data processing.f. If, however, spatial imperfections shall be corrected to a very high degree, a transformation of higher order might be necessary. Typically, a resolution of 2% of the detector area can be achieved. Because the total amount of charge is the measure of interest using wedge and strip anodes, the raw anode pulses are amplified by an integrating amplifier and then read out by an analog-to-digital converter (ADC).In explicit applications, it is typically sufficient to use a linear transformation for The working principle of delay line anodes is shown in elations :(4)x=2ux parallel to U, V and W disposed by ±120 degrees. Delay-line anodes provide resolution of about 0.5% of the detector area.The small letters A, B, C, and D of one anode is recorded, which are located on the bisectrix of the With resistive anodes, the signal height reaching four different points e system . They coe system (5)x=B+CThe working principle of resistive anodes is shown in These three anode types are commonly used in MCP-based detector systems. A certain anode type might be suited best for a specific application, but, in principle, all types can be used for single photon counting as described here. While delay-line anodes usually perform best with respect to spatial and temporal resolution and are potentially capable of multi-hit detection, wedge and strip anodes and resistive anodes can be operated using somewhat simpler electronics and data processing.If the source of excitation is pulsed, MCP based detectors offer the possibility of time resolved photon detection. The typical time resolution of MCPs of about 50 ps allows the measurement of lifetimes of most radiative states, which are usually on the nanosecond time scale. A weak voltage pulse can be coupled out capacitively from the short breakdown of the high voltage supply of the MCP when an event is detected. To explicitly determine the lifetime of a radiatively decaying state, the time between excitation and detection has to be measured for each event and all such obtained event times are listed in a histogram. If the exciting pulse is short compared to the decay, the lifetime can be extracted by fitting an exponential decay. The time distance between subsequent excitation pulses is required to be sufficiently long compared to the lifetime of the decaying state observed by photon emission. If both are within the same order of magnitude, the lifetime can still be estimated by considering the signal pile up from several pulses. As an example, in Time resolved event detection also enables the application of coincidence techniques. We developed an apparatus for photon-photon coincidence experiments . HoweverThe total detector efficiency Photek MCP240 detector system , as it is used in our set-up, is shown. This detector is equipped with a bialkali photocathode deposited on a fused silica window substrate.The quantity tage see . The chaThe detector efficiency The technique of position resolved photon detection is especially powerful if combined with photon spectrometers. In such a spectrometer, an optical grating is typically used to disperse and thereby separate photons of different wavelengths along an axis. If no spatial detection is available, emission spectra must be recorded by monochromating the dispersed light with an exit slit and detecting the intensity by scanning the selected wavelength. Using position sensitive detection, however, a partial spectrum can be detected at once with its range depending on the magnitude of dispersion and the active detector area. This way, the time required for recording emission spectra is reduced drastically.R back onto the circle, as illustrated in The grating and the detector are mostly used in the Rowland geometry . Here, aThe properties of the optical grating determine the dispersion magnitude and the spectral range with highest reflectivity and should be chosen to match the sensitivity range of the detector. For enhancing the reflectivity in a certain spectral range, the grating is usually coated and blazed. The latter means that the grating surface is tilted locally with respect to the surface normal, thereby increasing the reflectivity under a certain angle. The fraction of photons L between source volume and grating, the grating area n of diffraction, the blaze wavelength Q of the whole set-up, i.e., the probability that a photon emitted from a source volume is detected as an event is given byHere, cess see . The parting see . The refThe resolving power of the apparatus depends on several parameters—firstly, by the width of the source volume or entrance slit to the spectrometer, which is projected onto the detector and determines the apparative width of spectral lines ; secondly, by the line density of the grating. The resolving power is proportional to the line density and the order of diffraction. However, with increasing line density the absolute range of the partial spectrum which fits onto the detector area is narrowed. Thirdly, by the resolving power of the detector itself. Depending on the application, the combination of the three mentioned parameters should be optimized for count rate and time efficient measurement or highest resolution. If optimized for, typically a resolution of about 1 Å can be achieved.To record emission spectra in a wider emission wavelength range, the grating, and hence the Rowland circle, may be reoriented inside the chamber. By varying the direction with respect to the incident photons, the partial spectrum on the detector area can be chosen. The full spectrum in a range of interest can be obtained by measuring several partial spectra subsequently and merging them afterwards. For this procedure, it is recommended to record the partial spectra with sufficient overlap to identify the correct offset when merging.In general, the emission spectrum wavelength axis is a function of the arbitrary projection the data acquisitioning system generates when reading out the position. A coarse calibration of the wavelength axis can be achieved by the position of the zeroth diffraction order, i.e., the direct reflection  up to the soft X-ray regime of several hundred eV. The source of choice for photons in this energy range is synchrotron radiation, which offers complete tunability and surpasses other light sources by orders of magnitude with respect to photon flux and bandwidth . In the Depending on the sample of interest, different target delivery systems can be used in combination with the photon spectroscopy set-up. For gaseous samples, typically a gas cell is used. The cell is mounted inside the interaction chamber and has a constant gas flow provided from outside the vacuum chamber. By using flow regulating valves, the pressure inside the chamber can be adjusted and kept constant. The only openings of the cell are apertures in the order of few millimeters as entrance and exit for the exciting radiation. If photons beyond the vacuum ultraviolet spectral range are detected, a narrow slit towards the spectrometer is used. For detection in all spectral ranges with longer wavelengths than about 120 nm, window materials see  may be uFor experiments involving weakly bound clusters gas cells are not a viable option. The formation of clusters can be achieved by supersonic expansion of gases with high pressure  or soft X-ray light provided by synchrotron radiation facilities. A differential pumping section is placed between the chamber and the source to compensate several orders of magnitude in pressure . DiffereThe investigation of gaseous atomic and molecular species is essential for the exploration of fundamental light–matter interaction and to provide benchmark data for theoretical models. The detection of luminescence from photoexcited molecules and from excited products of photodissociation enables the determination of relative, and in some cases absolute, production cross sections of molecules or fragments in distinct quantum states because photon emission is both species and state selective.2 [2 [2, NO [2O [2O [n was quantified. In addition, the investigation of atomic species can reveal relative or absolute cross sections of state selective photoionization and autoionization processes, as it was demonstrated mostly for noble gas targets  in neon clusters. The details of this mechanism are discussed in Ref. . Briefly3 solution (b) in the vicinity of the water and nitrogen K-shell edge, respectively. By comparing the intensity behavior across the edges, the emission can be assigned to the different processes. For details, see Ref. [The combination of a liquid microjet with a luminescence spectrometer opens the unique possibility to observe the response of the inner bulk of a liquid to X-ray irradiation. This region is inaccessible to electron spectroscopy due to the short mean free path of charged particles in this dense medium. Experiments on neat liquid water revealed the occurrence of three distinct pathways leading to photon emission: (I) the excitation of molecular water in the gaseous halo of the jet by direct irradiation with X-rays; (II) the excitation of molecular water in the gaseous halo of the jet by impact of fast electrons escaping from the jet-vacuum interface; and (III) extremely broad emission bands from the bulk liquid. Processes II and III have not been observed by other techniques before. All three processes are illustrated in see Ref. .We showed that single photon detection is a very versatile tool to track fundamental light matter interaction in various sample environments. The MCP based detection strategy allows to record nearly background free detection of the weakest signals, especially in combination with photon spectrometers. The method is not limited to the synchrotron radiation based experiments presented here, but can be advanced to any excitation scheme and sample delivery. Moreover, as the detection of photons does not require any applied fields, it can be readily combined to other spectroscopic methods, like electron or ion detection.


The material bone has attracted the attention of material scientists due to its fracture resistance and ability to self-repair. A mechanoregulated exchange of damaged bone using newly synthesized material avoids the accumulation of fatigue damage. This remodeling process is also the basis for structural adaptation to common loading conditions, thereby reducing the probability of material failure. In the case of fracture, an initial step of tissue formation is followed by a mechanobiological controlled restoration of the pre-fracture state. The present perspective focuses on these mechanobiological aspects of bone remodeling and healing. Specifically, the role of the control function is considered, which describes mechanoregulation as a link between mechanical stimulation and the local response of the material through changes in structure or material properties. Mechanical forces propagate over large distances leading to a complex non-local feedback between mechanical stimulation and material response. To better understand such phenomena, computer models are often employed. As expected from control theory, negative and positive feedback loops lead to entirely different time evolutions, corresponding to stable and unstable states of the material system. After some background information about bone remodeling and healing, we describe a few representative models, the corresponding control functions, and their consequences. The results are then discussed with respect to the potential design of synthetic materials with specific self-repair properties. Bone is an archetypical example for an organ that completely regenerates through a healing process ,2,3 and This perspective focuses on bone healing and remodeling. Both are described as mechanobiological processes ,9, i.e.,From a theoretical point of view, the crucial material characteristic is the material’s response to a mechanical stimulation. This characteristic is of higher complexity than typical material properties. A first step from a static to a more dynamic description of material behavior is to consider possible influences of environmental conditions. Taking the example of electrical conductance, the material can respond to a change in temperature by an increase of the resistance. In this way, the material can become a responsive material, since the temperature can be used to influence the resulting current , middle.Bone remodeling and healing provide examples of mechanoregulation via negative and positive feedback loops, respectively. In theoretical models of these processes, the mechanoregulation is implemented by functions that link a local mechanical stimulus to a local change in the structure or properties of bone material. These so-called control functions are typically motivated from biology and challenging to measure experimentally. Consequently, computer models play an essential role in the exploration of such systems. In computer models the following two steps are run through iteratively: Firstly, a mechanical assessment is performed under external loading conditions considering the current mechanical properties of the material. The obtained local strains and stresses in the material are then used to calculate the mechanical stimulus. Secondly, the control function converts the stimulus into local changes of structural or material properties, which are consequently updated. With the so changed properties of the material, a new iteration starts.Due to the biomedical importance of bone healing and remodeling, the mechanobiological aspect of these processes have been studied extensively using computer simulations. The scientific literature provides excellent reviews about computational work addressing the mechanobiological aspects of bone  and specThe paper is organized into two parts. The first part deals with bone remodeling and the second with bone healing. In each part, the biological background of the process is given first. This description prescinds from all molecular and cell biological details and should only provide the background from a materials science viewpoint to understand the model approaches. After some theoretical considerations about the processes, the computer models and their used control functions are presented. The perspective concludes by discussing mechanoregulation in the context of the design of synthetic materials with self-repair properties.The processes of bone modeling and remodeling are thought to reduce the fracture risk of bone. Remodeling refers to a renewal of the material by exchanging old, damaged bone with new bone. The aim of remodeling is, therefore, the maintenance of bone. Modeling refers to adding or removal of bone as response to mechanical needs. Here, the aim is an adaptation of the bone structure to a change in the mechanical environment ,6. For eOsteoblasts and osteoclasts can form and resorb bone only at free bone surfaces. The foamy structure of trabecular bone provides ample surface and easy access for bone cells. In the dense cortical bone, the surface has to be created by the cells themselves . OsteoclTo allow for an adaptation of the bone structure to changes in the mechanical loading, a mechanosensor has to have a controlling role in bone remodeling. Although the sensing mechanism is still debated, there is general agreement that osteocytes play a key role in bone mechanosensation ,23. OsteFirst, ideas about structural adaption of bone and the mechanoregulation of bone remodeling date back to the late 19th century, with the work of Julius Wolff and Wilhelm Roux. A modern formulation of what is known as Wolff’s law, which governs bone remodeling, reads: bone is locally deposited wherever mechanically needed and is resorbed where it is not needed. Harold Frost described the adaptation of the total bone mass to changes in the mechanical loading using a mechanical feedback model. In his mechanostat model, he introduced an upper threshold of the mechanical stimulation above which the bone mass starts to increase and a lower threshold corresponding to mechanical disuse below which the bone reacts by reducing its mass. Between these two thresholds, a “lazy zone” describes an equilibrium situation with an unchanged bone mass within a range of intermediate mechanical stimulation . A schemRecently, an important experimental breakthrough was achieved, leading toward a more quantitative description of Wolff’s law. This has been made possible by using in vivo microcomputed tomography (µCT) on small rodents like mice to monitor the progress of bone remodeling . SeveralThe description of the mechanoregulation of bone remodeling using remodeling rules for bone formation and resorption b allows Application of the computer model showed a natural coarsening of the trabecular architecture with age, i.e., the number of trabeculae decreased with the remaining trabeculae becoming thicker so that the bone volume stayed virtually constant . TestingIn contrast to the model just presented, where the adding or removal of bone form the surface resulted indirectly in a local stiffening or softening of the material on a larger length scale, the model proposed by Rik Huiskes et al. describes bone remodeling more explicitly as a change in local mechanical properties. Again, the bone structure is mapped on a cubic lattice. However, the variable describing the structure is not a binary variable , but a relative bone density fined as :(1)dmE. In contrast, fibrous tissues are more suited to resist shape changes and therefore would form under shear stresses . Even clOnly the essence of the computer models should be described here, avoiding all the technical details. The starting point of mechanobiological simulations of fracture healing is often the situation after the initial inflammation phase and the disappearance of the hematoma. At this stage, the fracture callus consists of soft tissue and is loaded via the broken bone ends. Each voxel of tissue within the callus is then characterized by three variables: (i) the local loading conditions characterized by the mechanical stimulus; (ii) the tissue type that is present in the voxel  ,55; and In the case of bone healing, the control function coupled the mechanical stimulus with the formed tissue type. Bone does not only have a much higher stiffness than cartilage  the size of the fracture gap ,59, (ii)The scenario of a stiff fractured material that is embedded in a mechanoresponsive material and heals under mechanical stimulation was systematically investigated in  Figure a. The heirection a, E0 theThe simulations demonstrated that the course of healing can be easily manipulated by the range of mechanical stimuli, in which the material shows a healing response. For an intermediate choice of the exponent , and can include thresholds of the mechanical stimulation. In addition, different assumptions are often made about how the material responds locally to mechanical stimulation: if the material can undergo structural changes by addition or removal of material, it can increase or decrease its stiffness or it can change the local amount of damage. Similar damage models as presented here for bone are used, such as in the context of self-healing cementitious materials .Finally, the mechanoregulation of adaptive responses in synthetic materials within the framework of the control function is introduced above. Specifically, we discussed two material classes with mechanoresponsive building blocks with very different sizes: mechanical metamaterials and polymers including mechanophores.The idea behind metamaterials is the use of a designed arrangement of tailored building blocks to obtain a material with unexpected effective properties. In an inverse design process, the desired material properties are pre-defined and the adequate microstructure is then searched for ,66. WithHowever, we think that a clear-cut distinction between hardware and software is not the appropriate view on programmable materials. Preferable seems a viewpoint that considers different layers of control starting from more basic layers to top layers responsible for the fine-tuning of material behavior. A possible feedback between the different control layers does not allow to describe these control layers in the form of a hierarchical stacking. In returning to the biological example of bone remodeling, the most basic layer would be the control rule for bone formation and resorption, as shown in Polymeric materials with mechanophores are a second example in which mechanical stimulation results not only in structural changes but in a change of mechanical properties. Mechanophores are force reactive groups that are embedded along the polymer backbone or within cross-links. The function of the mechanophore is that a local overloading does not result in molecular scission events, but in a constructive response with a local strengthening of the material  Figure c. On the