Temporal Carrier Research Articles

Visualizing Carrier Diffusion in Cs‐Doping FAPbI3 Perovskite Thin Films Using Transient Absorption Microscopy

AbstractThe determination of the semiconducting materials performance heavily relies on the diffusion coefficient and length of the carrier. Recently, significant progress is made in enhancing solar cell efficiency through improved carrier diffusion in perovskite thin films. However, the spatial‐temporal mechanisms underlying carrier transport remain unclear. Recent advancements in utilizing transient absorption microscopy (TAM) offer promising opportunities to directly visualize the carrier transport dynamics within perovskite films. Here, the wide‐field imaging TAM combined with X‐ray diffraction and scanning electron microscopy is employed to investigate the spatial‐temporal carrier transport dynamics in FA1−xCsxPbI3 perovskites with varying Cs doping ratios. The experimental results indicate that the diffusion constant remains consistent regardless of the excitation power. Moreover, a decrease in the Cs doping ratio leads to an increase in the diffusion length within FA1−xCsxPbI3 perovskites. The measurements reveal a highest diffusion coefficient of up to 0.085 cm2 s−1 and a maximum diffusion length of ≈1.4 µm in FA0.97Cs0.03PbI3. Comparative analysis of short‐circuit current density, open‐circuit voltage, fill factor, and power conversion efficiency demonstrates that FA0.97Cs0.03PbI3 exhibits superior device efficiency. The TAM visualizes spatial/ temporal carrier diffusion dynamics, showing a significant correlation with device efficiency and thus providing valuable insights for further enhancing device performance.

High-fidelity correspondence imaging in complex media with varying thresholds and 1-bit compressive sensing

Here, we report high-fidelity correspondence imaging (CI) in complex media. By introducing varying thresholds to binarize single-pixel light intensities recorded in complex media, dynamic scaling factors can be eliminated. Then, the binarized light intensities and illumination patterns can be fed into a modified 1-bit compressive sensing algorithm to realize high-fidelity object reconstruction. The proposed method can implement object reconstruction with high fidelity in complex media without extra temporal carriers. It is experimentally verified that the method can effectively eliminate dynamic scaling factors and realize high-fidelity object reconstruction in complex media where conventional CI methods could fail. Experimental results demonstrate that the proposed method broadens a potential application of CI in complex media, e.g., turbid water, biological tissues, and dynamic smoke.

Charge carrier dynamics in conducting polymer PEDOT using abinitio molecular dynamics simulations.

As conducting polymers become increasingly important in electronic devices, understanding their charge transport is essential for material and device development. Various semi-empirical approaches have been used to describe temporal charge carrier dynamics in these materials, but there have yet to be any theoretical approaches utilizing abinitio molecular dynamics. In this work, we develop a computational technique based on abinitio Car-Parrinello molecular dynamics to trace charge carrier temporal motion in archetypical conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). Particularly, we analyze charge dynamics in a single PEDOT chain and in two coupled chains with different degrees of coupling and study the effect of temperature. In our model we first initiate a positively charged polaron (compensated by a negative counterion) at one end of the chain, and subsequently displace the counterion to the other end of the chain and trace polaron dynamics in the system by monitoring bond length alternation in the PEDOT backbone and charge density distribution. We find that at low temperature (T = 1K) the polaron distortion gradually disappears from its initial location and reappears near the new position of the counterion. At the room temperature (T = 300K), we find that the distortions induced by polaron, and atomic vibrations are of the same magnitude, which makes tracking the polaron distortion challenging because it is hidden behind the temperature-induced vibrations. The novel approach developed in this work can be used to study polaron mobility along and between the chains, investigate charge transport in highly doped polymers, and explore other flexible polymers, including n-doped ones.

A novel scheme for ultrashort terahertz pulse generation over a gapless wide spectral range: Raman-resonance-enhanced four-wave mixing

Ultrashort energetic terahertz (THz) pulses have created an exciting new area of research on light interactions with matter. For material studies in small laboratories, widely tunable femtosecond THz pulses with peak field strength close to MV cm−1 are desired. Currently, they can be largely acquired by optical rectification and difference frequency generation in crystals without inversion symmetry. We describe in this paper a novel scheme of THz pulse generation with no frequency tuning gap based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, particularly diamond. We show that we could generate highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles and carrier frequency tunable from 5 to >20 THz. They had a stable and controllable carrier-envelop phase and carried ~15 nJ energy per pulse at 10 THz (with a peak field strength of ~1 MV cm−1 at focus) from a 0.5-mm-thick diamond. The measured THz pulse characteristics agreed well with theoretical predictions. Other merits of the scheme are discussed, including the possibility of improving the THz output energy to a much higher level.

Charge collection efficiency, underlying recombination mechanisms, and the role of electrode distance of vented ionization chambers under ultra-high dose-per-pulse conditions

PurposeInvestigating and understanding of the underlying mechanisms affecting the charge collection efficiency (CCE) of vented ionization chambers under ultra-high dose rate pulsed electron radiation. This is an important step towards real-time dosimetry with ionization chambers in FLASH radiotherapy. MethodsParallel-plate ionization chambers (PPIC) with three different electrode distances were build and investigated with electron beams with ultra-high dose-per-pulse (DPP) up to 5.4 Gy. The measurements were compared with simulations. The experimental determination of the CCE was done by comparison against the reference dose based on alanine dosimetry. The numerical solution of a system of partial differential equations taking into account charge creations by the radiation, their transport and reaction in an applied electric field was used for the simulations of the CCE and the underlying effects. ResultsA good agreement between the experimental results and the simulated CCE could be achieved. The recombination losses found under ultra-high DPP could be attributed to a temporal and spatial charge carrier imbalance and the associated electric field distortion. With ultra-thin electrode distances down to 0.25 mm and a suitable chamber voltage, a CCE greater than 99 % could be achieved under the ultra-high DPP conditions investigated. ConclusionsWell-guarded ultra-thin PPIC are suited for real-time dosimetry under ultra-high DPP conditions. This allows dosimetry also for FLASH RT according to common codes of practice, traceable to primary standards. The numerical approach used allows the determination of appropriate correction factors beyond the DPP ranges where established theories are applicable to account for remaining recombination losses.

Sign determination to measure non-monotonous displacements in temporal speckle pattern interferometry without a temporal carrier

In this paper, we present a generalized function to automatically detect the correct sign of non-monotonous displacements in temporal speckle pattern interferometry without the introduction of a temporal carrier in the optical setup. This approach can be used with any phase retrieval technique. Here, when the phase change is evaluated, the sign ambiguity is resolved due to a function that determines the correct sign of the displacement between two consecutive speckle interferograms. The errors introduced by the proposed function are determined using computer-simulated speckle interferograms, and its performance is analyzed using three phase recovery techniques and different phase distributions and average speckle sizes. A relation between the sliding window used to compute the correct sign and the speckle sizes is also presented. Finally, the presentation is illustrated with an experimental application of the proposed method to detect the correct sign of the out-of-plane displacement field of a metal plate.

Sign Determination to Measure Non-Monotonous Displacements in Temporal Speckle Pattern Interferometry Without a Temporal Carrier

Sign Determination to Measure Non-Monotonous Displacements in Temporal Speckle Pattern Interferometry Without a Temporal Carrier

Analysis of temporal carrier build‐up in reconfigurable field‐effect transistor

Based on the analysis of the carrier density change of a symmetric gate reconfigurable field-effect transistor that can operate p- or n-type transistors in an integrated circuit (IC), its unique limiting factor, carrier build-up time, is quantitatively derived for operating speed in addition to conventional resistance and capacitance (RC) and transit time effect. Originating from the characteristic of carrier confinement in a channel between two Schottky potential barriers, the carrier build-up time for the operation could take up to ∼1000 times longer than the transit time across the channel.

Nonseparable modulation strategy for channeled spatiotemporal Stokes polarimeters.

Spatiotemporally modulated polarimeters have shown promising imaging performance by leveraging the tradeoff between spatial bandwidth and temporal bandwidth to outperform polarimeters that use spatial or temporal modulation alone. However, the existing separable modulation strategy, in which the spatial carriers are generated independently from the temporal carriers, makes such devices sensitive to the systematic errors of the rotation element inevitably. In this paper, we propose two novel strategies that have spatiotemporal modulation that is inherently mixed. The method enables different elements of the Mueller matrix to be used to create the carriers, reducing the effects of systematic errors in different ways. We present the indepth comparison of the channel structure and the reconstruction accuracy of each modulation strategy in various bandwidth scenarios under the presence of systematic error. Simulation results show that the nonseparable modulation can provide higher reconstruction accuracy of polarimetric information as compared to the separable strategy.

A Brücke-Bartley effect for contrast.

Accurate derivation of the psychophysical (a.k.a. transducer) function from just-notable differences requires accurate knowledge of the relationship between the mean and variance of apparent intensities. Alternatively, a psychophysical function can be derived from estimates of the average between easily discriminable intensities. Such estimates are unlikely to be biased by the aforementioned variance, but they are notoriously variable and may stem from decisional processes that are more cognitive than sensory. In this paper, to minimize cognitive pollution, we used amplitude-modulated contrast. As the spatial or temporal (carrier) frequency increased, estimates of average intensity became less variable across observers, converging on values that were closer to mean power (i.e. contrast2) than mean contrast. Simply put, apparent contrast increases when physical contrast flickers. This result is analogous to Brücke's finding that brightness increases when luminance flickers. It implies an expansive transduction of contrast in the same way that Brücke's finding implies an expansive transduction of luminance.

Channeled spatio-temporal Stokes polarimeters.

We present the analysis and design of spatio-temporal channeled Stokes polarimeters. We extend our recent work on optimal pixelated polarizer arrays by utilizing temporal carrier generation, resulting in polarimeters that achieve super-resolution via the tradeoff between spatial bandwidth and temporal bandwidth. Utilizing the channel space description, we present a linear-Stokes design and two full-Stokes imaging polarimeter designs that have the potential to operate at the full frame rate of the imaging sensor of the system by using hybrid spatio-temporal carriers. If the objects are not spatially bandlimited, the achievable temporal bandwidth is more difficult to analyze; however, a spatio-temporal tradeoff still exists.

Printable organometallic perovskite enables large-area, low-dose X-ray imaging.

Medical X-ray imaging procedures require digital flat detectors operating at low doses to reduce radiation health risks. Solution-processed organic-inorganic hybrid perovskites have characteristics that make them good candidates for the photoconductive layer of such sensitive detectors. However, such detectors have not yet been built on thin-film transistor arrays because it has been difficult to prepare thick perovskite films (more than a few hundred micrometres) over large areas (a detector is typically 50 centimetres by 50 centimetres). We report here an all-solution-based (in contrast to conventional vacuum processing) synthetic route to producing printable polycrystalline perovskites with sharply faceted large grains having morphologies and optoelectronic properties comparable to those of single crystals. High sensitivities of up to 11 microcoulombs per air KERMA of milligray per square centimetre (μC mGyair-1 cm-2) are achieved under irradiation with a 100-kilovolt bremsstrahlung source, which are at least one order of magnitude higher than the sensitivities achieved with currently used amorphous selenium or thallium-doped cesium iodide detectors. We demonstrate X-ray imaging in a conventional thin-film transistor substrate by embedding an 830-micrometre-thick perovskite film and an additional two interlayers of polymer/perovskite composites to provide conformal interfaces between perovskite films and electrodes that control dark currents and temporal charge carrier transportation. Such an all-solution-based perovskite detector could enable low-dose X-ray imaging, and could also be used in photoconductive devices for radiation imaging, sensing and energy harvesting.

Measurement of directional range components of nuclear recoil tracks in a fiducialised dark matter detector

We present results from the first measurement of axial range components of fiducialized neutron induced nuclear recoil tracks using the DRIFT directional dark matter detector. Nuclear recoil events are fiducialized in the DRIFT experiment using temporal charge carrier separations between different species of anions in 30:10:1 Torr of CS2:CF4:O2 gas mixture. For this measurement, neutron-induced nuclear recoil tracks were generated by exposing the detector to 252Cf source from different directions. Using these events, the sensitivity of the detector to the expected axial directional signatures were investigated as the neutron source was moved from one detector axis to another. Results obtained from these measurements show clear sensitivity of the DRIFT detector to the axial directional signatures in this fiducialization gas mode.

Human Frequency Following Responses to Vocoded Speech.

Vocoders offer an effective platform to simulate the effects of cochlear implant speech processing strategies in normal-hearing listeners. Several behavioral studies have examined the effects of varying spectral and temporal cues on vocoded speech perception; however, little is known about the neural indices of vocoded speech perception. Here, the scalp-recorded frequency following response (FFR) was used to study the effects of varying spectral and temporal cues on brainstem neural representation of specific acoustic cues, the temporal envelope periodicity related to fundamental frequency (F0) and temporal fine structure (TFS) related to formant and formant-related frequencies, as reflected in the phase-locked neural activity in response to vocoded speech. In experiment 1, FFRs were measured in 12 normal-hearing, adult listeners in response to a steady state English back vowel /u/ presented in an unaltered, unprocessed condition and six sine-vocoder conditions with varying numbers of channels (1, 2, 4, 8, 16, and 32), while the temporal envelope cutoff frequency was fixed at 500 Hz. In experiment 2, FFRs were obtained from 14 normal-hearing, adult listeners in response to the same English vowel /u/, presented in an unprocessed condition and four vocoded conditions where both the temporal envelope cutoff frequency (50 versus 500 Hz) and carrier type (sine wave versus noise band) were varied separately with the number of channels fixed at 8. Fast Fourier Transform was applied to the time waveforms of FFR to analyze the strength of brainstem neural representation of temporal envelope periodicity (F0) and TFS-related peaks (formant structure). Brainstem neural representation of both temporal envelope and TFS cues improved when the number of channels increased from 1 to 4, followed by a plateau with 8 and 16 channels, and a reduction in phase-locking strength with 32 channels. For the sine vocoders, peaks in the FFRTFS spectra corresponded with the low-frequency sine-wave carriers and side band frequencies in the stimulus spectra. When the temporal envelope cutoff frequency increased from 50 to 500 Hz, an improvement was observed in brainstem F0 representation with no change in brainstem representation of spectral peaks proximal to the first formant frequency (F1). There was no significant effect of carrier type (sine- versus noise-vocoder) on brainstem neural representation of F0 cues when the temporal envelope cutoff frequency was 500 Hz. While the improvement in neural representation of temporal envelope and TFS cues with up to 4 vocoder channels is consistent with the behavioral literature, the reduced neural phase-locking strength noted with even more channels may be because of the narrow bandwidth of each channel as the number of channels increases. Stronger neural representation of temporal envelope cues with higher temporal envelope cutoff frequencies is likely a reflection of brainstem neural phase-locking to F0-related periodicity fluctuations preserved in the 500-Hz temporal envelopes, which are unavailable in the 50-Hz temporal envelopes. No effect of temporal envelope cutoff frequency was seen for neural representation of TFS cues, suggesting that spectral side band frequencies created by the 500-Hz temporal envelopes did not improve neural representation of F1 cues over the 50-Hz temporal envelopes. Finally, brainstem F0 representation was not significantly affected by carrier type with a temporal envelope cutoff frequency of 500 Hz, which is inconsistent with previous results of behavioral studies examining pitch perception of vocoded stimuli.

Experimental evaluation of a 3D wavelet-based phase recovery method in temporal speckle pattern interferometry.

We test the performance of a phase recovery method based on a three-dimensional directional wavelet transform applied to the intensity signal measured by temporal speckle pattern interferometry (TSPI). We present and discuss several sources of uncertainty by analyzing experimental datasets recorded for an in-plane interferometer without introducing a temporal carrier. The dynamic phase data measured with the proposed method are compared with those obtained from the well-known one-dimensional Fourier transform phase recovery technique. In the Fourier method, the filtered Fourier transform for each intensity pixel is evaluated along the temporal direction. In contrast, the three-dimensional directional wavelet transform method uses the information of adjacent pixels and then increases the performance of the recovered dynamic phase results. The advantages and limitations of the three-dimensional directional wavelet transform approach are discussed, and a summary of conclusions from the analysis of TSPI data is also given.

Role of carrier signal types in perception of Thai phonemes: Implications for cochlear implant recipients

It is well known that segmental (consonants and vowels) and suprasegmental (e.g., tone and stress) speech sounds provide intrinsically different kinds of perceptual information. These differences suggest that phoneme perception might be improved in cochlear implant strategy by employing a carrier signal that is most compatible to each type of speech sounds. However, all speech vocoder strategies for cochlear implant (CI) available in most of works use one carrier signal due to their simplicity to implement. In this paper, we investigated the role of five different carrier signals for Thai speech perception on different types of phonemes, i.e., sine-carrier with/without band-pass filter (TF/TNF), noise-carrier with/without band-pass filter (NF/NNF), and temporal fine structure carrier (TFS). Each type of carrier signal was used to synthesize speech stimuli using the continuous interleaved sampling (CIS) strategy as basic and commonly used for CI. Four different psychoacoustic tests for initials, finals, vowels were performed on 20 Thai normal hearing (NH) listeners, while 13 NH listeners were used for lexical tones. Experimental results showed that sine-carriers provided the highest intelligibility scores for initials, finals, and vowels, while NNF provided the highest intelligibility scores for lexical tones. The results suggested that the way to provide an optimal speech perception for tonal language is to use two parallel carriers in speech vocoder strategy for CI.

Phase-resolved heterodyne shearographic vibrometer for observation of transient surface motion: theory and model.

A phase-resolved heterodyne shearing interferometer concept is under development for high-rate, whole field observations of transient surface motion. The sensor utilizes frequency and polarization multiplexing with two temporal carrier frequencies to separate each segment of a shearing Mach-Zehnder interferometer. Post-processing routines have been developed to recombine the segments by extracting the scattered object phase from Doppler shifted intermediate carrier frequencies. The processing routines provide quantitative relative phase changes and information required to generate phase resolved shearographic fringe patterns without temporal or spatial phase shifting. Separation of each segment allows for adjustment of shearing distance and direction as well as simultaneous whole field Doppler velocity (LDV) measurements. This paper presents background theory and numerical model results leading to a sensor concept.

In-vivo retinal imaging with off-axis full-field time-domain optical coherence tomography.

With a simple setup, mainly composed of a low coherence light source and a camera, full-field optical coherence tomography (FF-OCT) allows volumetric tissue imaging. However, fringe washout constrains its use in retinal imaging. Here, we present a novel motion-insensitive approach to FF-OCT, which introduces path-length differences between the reference and the sample light in neighboring pixels using an off-axis reference beam. The temporal carrier frequency in scanned time-domain OCT is replaced by a spatial carrier frequency. Volumetric in-vivo FF-OCT measurements of the human retina were acquired in only 1.3 s, comparable to the acquisition times of current clinically used OCT devices.

Intense, carrier frequency and bandwidth tunable quasi single-cycle pulses from an organic emitter covering the Terahertz frequency gap.

In Terahertz (THz) science, one of the long-standing challenges has been the formation of spectrally dense, single-cycle pulses with tunable duration and spectrum across the frequency range of 0.1–15 THz (THz gap). This frequency band, lying between the electronically and optically accessible spectra hosts important molecular fingerprints and collective modes which cannot be fully controlled by present strong-field THz sources. We present a method that provides powerful single-cycle THz pulses in the THz gap with a stable absolute phase whose duration can be continuously selected between 68 fs and 1100 fs. The loss-free and chirp-free technique is based on optical rectification of a wavelength-tunable pump pulse in the organic emitter HMQ-TMS that allows for tuning of the spectral bandwidth from 1 to more than 7 octaves over the entire THz gap. The presented source tunability of the temporal carrier frequency and spectrum expands the scope of spectrally dense THz sources to time-resolved nonlinear THz spectroscopy in the entire THz gap. This opens new opportunities towards ultrafast coherent control over matter and light.

Measurement of non-monotonous phase changes in temporal speckle pattern interferometry using a correlation method without a temporal carrier

Recently, a phase evaluation method was proposed to measure nanometric displacements by means of digital speckle pattern interferometry when the phase change introduced by the deformation is in the range [0,π) rad. This method is based on the evaluation of a correlation coefficient between two speckle interferograms generated by both deformation states of the object. In this paper, we present a novel technique to measure non-monotonous displacements in temporal speckle pattern interferometry using a correlation method without a temporal carrier. In this approach, the sign ambiguity is resolved automatically due to the introduction of a function that determines the correct sign of the displacement between two consecutive speckle interferograms. The rms phase errors introduced by the proposed method are determined using computer-simulated speckle interferograms. An application of the phase retrieval method to process experimental data is also presented.