Effect Of Temporal Coherence Research Articles

Broadband ptychography using curved wavefront illumination.

We examine the interplay between spectral bandwidth and illumination curvature in ptychography. By tailoring the divergence of the illumination, broader spectral bandwidths can be tolerated without requiring algorithmic modifications to the forward model. In particular, a strong wavefront curvature transitions a far-field diffraction geometry to an effectively near-field one, which is less affected by temporal coherence effects. The relaxed temporal coherence requirements allow for leveraging wider spectral bandwidths and larger illumination spots. Our findings open up new avenues towards utilizing pink and broadband beams for increased flux and throughput at both synchrotron facilities and lab-scale beamlines.

Performance in an Audiovisual Selective Attention Task Using Speech-Like Stimuli Depends on the Talker Identities, But Not Temporal Coherence.

Audiovisual integration of speech can benefit the listener by not only improving comprehension of what a talker is saying but also helping a listener select a particular talker's voice from a mixture of sounds. Binding, an early integration of auditory and visual streams that helps an observer allocate attention to a combined audiovisual object, is likely involved in processing audiovisual speech. Although temporal coherence of stimulus features across sensory modalities has been implicated as an important cue for non-speech stimuli (Maddox et al., 2015), the specific cues that drive binding in speech are not fully understood due to the challenges of studying binding in natural stimuli. Here we used speech-like artificial stimuli that allowed us to isolate three potential contributors to binding: temporal coherence (are the face and the voice changing synchronously?), articulatory correspondence (do visual faces represent the correct phones?), and talker congruence (do the face and voice come from the same person?). In a trio of experiments, we examined the relative contributions of each of these cues. Normal hearing listeners performed a dual task in which they were instructed to respond to events in a target auditory stream while ignoring events in a distractor auditory stream (auditory discrimination) and detecting flashes in a visual stream (visual detection). We found that viewing the face of a talker who matched the attended voice (i.e., talker congruence) offered a performance benefit. We found no effect of temporal coherence on performance in this task, prompting an important recontextualization of previous findings.

RaDiO: An efficient spatiotemporal radiation diagnostic for particle-in-cell codes

This work describes a novel radiation algorithm designed to capture the three-dimensional, space-time resolved electromagnetic field structure emitted by large ensembles of charged particles. The algorithm retains the full set of degrees of freedom that characterize electromagnetic waves by employing the Liénard-Wiechert fields to retrieve radiation emission. Emitted electric and magnetic fields are deposited in a virtual detector using a temporal interpolation scheme. This feature is essential to accurately predict field amplitudes and preserve the continuous character of radiation emission, even though particle dynamics is known only in a discrete set of temporal steps. Our algorithm retains and accurately captures, by design, full spatial and temporal coherence effects. We demonstrate that our numerical approach recovers well known theoretical radiated spectra in standard scenarios of radiation emission. We show that the algorithm is computationally efficient by computing the full spatiotemporal radiation features of High Harmonic Generation through a plasma mirror in a Particle-In-Cell (PIC) simulation.

Partial spectral and temporal coherence of plane-wave pulse trains in second-harmonic generation

We study the spectral and temporal coherence effects in the passage of a Gaussian Schell-model (GSM) scalar, plane-wave pulse train through a slab of nonlinear optical crystal exhibiting second-harmonic generation. We show that due to the nonlinear interaction the temporal and spectral degrees of coherence of the fundamental (F) and second-harmonic (SH) pulse trains at the exit facet may deviate markedly from the GSM and the global degree of coherence of the SH wave generally decreases with increasing incident F beam intensity. In addition, we find that due to the partial coherence of the incident GSM field the transmitted SH wave may show a double-peaked intensity distribution.

Three-Dimensional Transfer Functions of Interference Microscopes

Three-dimensional transfer functions (3D TFs) are generally assumed to fully describe the transfer behavior of optical topography measuring instruments such as coherence scanning interferometers in the spatial frequency domain. Therefore, 3D TFs are supposed to be independent of the surface under investigation resulting in a clear separation of surface properties and transfer characteristics. In this paper, we show that the 3D TF of an interference microscope differs depending on whether the object is specularly reflecting or consists of point scatterers. In addition to the 3D TF of a point scatterer, we will derive an analytical expression for the 3D TF corresponding to specular surfaces and demonstrate this as being most relevant in practical applications of coherence scanning interferometry (CSI). We additionally study the effects of temporal coherence and disclose that in conventional CSI temporal coherence effects dominate. However, narrowband light sources are advantageous if high spatial frequency components of weak phase objects are to be resolved, whereas, for low-frequency phase objects of higher amplitude, the temporal coherence is less affecting. Finally, we present an approach that explains the different transfer characteristics of coherence peak and phase detection in CSI signal analysis.

Coherent white emission of graphene

The coherence properties of broadband white light emitted by graphene foam after irradiation with intense light of laser diodes were investigated. The interference fringes assigned to spatial and temporal coherence were observed for graphene white-light emission excited by two different excitation wavelengths of 975 nm and 808 nm of laser diodes. Both light sources were spectrally filtered to a narrow bandwidth in order to exclude the effect of temporal coherence. It was found that the coherence degree of white light emission of graphene differs depending on the excitation wavelength; however, no excitation power effect on the coherence degree was observed. The origin of coherence of laser-induced white emission of graphene is the stimulated emission due to the inversion population.

Effects of temporal coherence and signal-frequency uncertainty for tone detection in a random-frequency multi-tonal masker

Detection thresholds tend to be lower when the spectral and/or temporal characteristics of the signal are predictable than unpredictable, particularly in an unpredictable masker. The present study evaluated the detrimental effect of frequency uncertainty and the beneficial effect of temporal coherence for a pure-tone signal presented in a multi-tonal masker with unpredictable frequency sequences. Stimuli were composed of 80-ms tone bursts. The signal was 1, 2, 4, or 8 sequential tone bursts, and burst frequency was either fixed across trials or randomly varied across trials but fixed across bursts. The masker was composed of four streams of tone bursts; the frequency of each burst was randomly drawn from a uniform distribution 250-4000 Hz, with the caveat that synchronous masker and signal bursts were separated by 1/5 oct or more. Signal and masker bursts were synchronously gated, and the masker played continuously throughout a threshold estimation track. Thresholds tended to improve with increasing numbers of signal bursts and worsen with increasing signal frequency uncertainty. These factors appeared to interact; signal frequency uncertainty was less detrimental when the signal was composed of larger numbers of bursts. Preliminary models of detection will be discussed.

Temporal coherence effects on target-based phasing of laser arrays.

This paper studies how temporal coherence (in particular, linewidth broadening introduced to suppress stimulated Brillouin scattering) affects target-based phasing of fiber laser arrays. A radio-frequency modulated array whose elements are fed by a broadband laser source phasing on a remote step target is theoretically analyzed. An expression for the detector plane irradiance, ultimately used to phase the array on the target, is derived and discussed in detail. Simulation results of a seven-element hexagonal array phasing on a distant step target with scattering surfaces separated by many coherence lengths are presented to validate the theoretical findings.

Evidence of partial temporal coherence effects in the linear autocorrelation of extreme ultraviolet laser pulses.

We study how the degree of temporal coherence of plasma-based extreme ultraviolet lasers operated in the amplification of the spontaneous emission mode is encoded in the shape of the linear autocorrelation function, which is obtained from the variation of the fringe visibility while varying the delay in a variable path-difference interferometer. We discuss the implications of this effect when the technique is used to infer the spectral properties of the source. Our numerical simulations, based on a partial coherence model developed by other authors for x-ray free electron lasers, are in good agreement with previously reported sets of measurements, illustrating similar statistical properties for both sources.

Assessing the effects of temporal coherence on auditory stream formation through comodulation masking release

Recent studies of auditory streaming have suggested that repeated synchronous onsets and offsets over time, referred to as "temporal coherence," provide a strong grouping cue between acoustic components, even when they are spectrally remote. This study uses a measure of auditory stream formation, based on comodulation masking release (CMR), to assess the conditions under which a loss of temporal coherence across frequency can lead to auditory stream segregation. The measure relies on the assumption that the CMR, produced by flanking bands remote from the masker and target frequency, only occurs if the masking and flanking bands form part of the same perceptual stream. The masking and flanking bands consisted of sequences of narrowband noise bursts, and the temporal coherence between the masking and flanking bursts was manipulated in two ways: (a) By introducing a fixed temporal offset between the flanking and masking bands that varied from zero to 60 ms and (b) by presenting the flanking and masking bursts at different temporal rates, so that the asynchronies varied from burst to burst. The results showed reduced CMR in all conditions where the flanking and masking bands were temporally incoherent, in line with expectations of the temporal coherence hypothesis.

Temporal Coherence Effects on Coherent Diffractive Imaging of a Binary Sample by a High Harmonic Source

Coherent Diffractive Imaging (CDI) is performed with single and multiple harmonics from an ultrafast HHG source. The effect of HHG source bandwidth on the effectiveness of the reconstruction algorithms is compared. A low quality reconstruction from broadband data is achieved assuming full coherence in the algorithm.

Spatial and temporal coherence effects in interference microscopy and full‐field optical coherence tomography

AbstractLow‐coherence optical microscopy or optical coherence microscopy uses light with short coherence length. The well‐known case is: “white‐light interferometry”, which became recently more known as: “optical coherence tomography”. However, when lenses and microscope objectives are used to create interferometric images, in what is known classically as “interference microscopy” or today as “full‐field optical coherence tomography” the spatial coherence starts to play a critical role. In this article the coherence effects in low‐coherence optical microscopy are reviewed. As this technology is becoming increasingly publicized due to its importance in three‐dimensional imaging, particularly of scattering biological media and optical metrology, the understanding of the fundamental physics behind it is essential. The interplay between longitudinal spatial coherence and temporal coherence and the effects associated with them are discussed in detail particularly when high numerical apertures are used. An important conclusion of this study is that a high‐contrast, high‐resolution system for imaging of multilayered samples is the one that uses narrowband illumination and high‐NA objectives with an index‐matching fluid. Such a system, when combined with frequency‐domain operation, can reveal nearly real‐time three‐dimensional images, and is thus competitive with confocal microscopy.

Learning sequence of views of three-dimensional objects: The effect of temporal coherence on object memory

How humans recognize objects remains a contentious issue in current research on high-level vision. Here, I test the proposal by Wallis and Bulthoff (1999 Trends in Cognitive Sciences3 22–31) suggesting that object representations can be learned through temporal association of multiple views of the same object. Participants first studied image sequences of novel, three-dimensional objects in a study block. On each trial, the images were from either an orderly sequence of depth-rotated views of the same object (SS), a scrambled sequence of those views (SR), or a sequence of different objects (RR). Recognition memory was assessed in a following test block. A within-object advantage was consistently observed—greater accuracy in the SR than the RR condition in all four experiments, greater accuracy in the SS than the RR condition in two experiments. Furthermore, spatiotemporal coherence did not produce better recognition than temporal coherence alone (similar or less accuracy in the SS compared to the SR condi...

Measurement of effective source distribution and its importance for quantitative interpretation of STEM images

We review the manner in which lens aberrations, partial spatial coherence, and partial temporal coherence affect the formation of a sub-Å electron probe in an aberration-corrected transmission electron microscope. Simulations are used to examine the effect of each of these factors on a STEM image. It is found that the effects of partial spatial coherence (resulting from finite effective source size) are dominant, while the effects of residual lens aberrations and partial temporal coherence produce only subtle changes from an ideal image. We also review the way in which partial spatial and temporal coherence effects are manifest in a Ronchigram. Finally, we provide a demonstration of the Ronchigram method for measuring the effective source distribution in a probe aberration-corrected 300 kV field-emission gun transmission electron microscope.

Learning Sequence of Views of Three-Dimensional Objects: The Effect of Temporal Coherence on Object Memory

How humans recognize objects remains a contentious issue in current research on high-level vision. Here, I test the proposal by Wallis and Bülthoff (1999 Trends in Cognitive Sciences 3 22-31) suggesting that object representations can be learned through temporal association of multiple views of the same object. Participants first studied image sequences of novel, three-dimensional objects in a study block. On each trial, the images were from either an orderly sequence of depth-rotated views of the same object (SS), a scrambled sequence of those views (SR), or a sequence of different objects (RR). Recognition memory was assessed in a following test block. A within-object advantage was consistently observed --greater accuracy in the SR than the RR condition in all four experiments, greater accuracy in the SS than the RR condition in two experiments. Furthermore, spatiotemporal coherence did not produce better recognition than temporal coherence alone (similar or less accuracy in the SS compared to the SR condition). These results suggest that the visual system can use temporal regularity to build invariant object representations, via the temporal-association mechanism.

Visibility in differential phase-contrast imaging with partial coherence source

This paper gives theoretical analysis of visibility of fringes, which is influenced by distances, temporal and spatial coherence of source, in hard x-ray differential phase-contrast imaging with microfocus x-ray source. According to the character of longitudinal periodicity of the interferogram, the setup is insensitive to mechanical drift and vibrations. The effect of temporal coherence of x-ray source is investigated and its related bandwidth is derived. Based on the theory of partially coherent light, it shows that the requirement for the spatial coherence of x-ray source is not strict and can be met by the general microfocus x-ray tube for x-ray differential phase-contrast imaging.

Use of laser diode in joint transform correlator

The use of a laser diode in a joint transform correlator (JTC) is proposed. Effects of temporal coherence, spatial coherence, astigmatism, and elliptical beams are analyzed. Laser diodes can be used in JTC, and also can be used for the correlation process in a Vander Lugt correlator and reading a hologram. Even a light-emitting diode (LED) may be used for the purposes mentioned, if its exit facet is sufficiently small. Two methods for correcting a laser diode beam using a cylindrical lens or a pair of cylindrical lenses are also reviewed. A corrected laser diode can be used in JTC to totally eliminate the effect of astigmatism.

The role of temporal coherence in imaging with extreme ultraviolet lithography optics

Being based on reflective elements, extreme ultraviolet (EUV) lithography optics can support much larger spectral bandwidths then do modern refractive lithography optics. Moreover, EUV source power limitations dictate the use of the full bandwidth supported by the reflective EUV multilayers. A typical four mirror EUV optic supports an operating bandwidth of approximately 360 pm. This large bandwidth, combined with the off-axis nature of EUV lithography optics leading to significant pathlength differences across the field, raises the issue of temporal coherence effects on the imaging properties of EUV systems. Here we present analysis and computer simulation of the temporal coherence effect in EUV lithography optics. The analysis shows that under typical lithographic operating conditions, temporal coherence effects can still be safely ignored. However, non-negligible effects could arise in cases of higher-coherence imaging (small partial coherence factor, σ).

Vibrational coherence in nonadiabatic dynamics

In this paper we explore temporal vibrational coherence effects in nonadiabatic radiationless transitions between two electronic states in a large molecule or in the condensed phase, accounting explicitly for the role of the (intramolecular and/or medium) vibrational quasicontinuum of the final states. Our treatment of the time evolution of the wave packet of states and of coherence effects in the nonradiative population probabilities of the reactants and the products rests on the diagonalization of the Hamiltonian of the entire multimode system, with supplementary information being inferred from the effective Hamiltonian formalism. New features of the vibrational Franck–Condon quasicontinuum, which originate from weak, but finite, correlations between off-diagonal coupling terms, were established. The state dependence of the off-diagonal couplings Vsα between the doorway states manifold {|s〉} and the quasicontinuum {|α〉} was quantified by the correlation parameters ηss=〈VsαVαs〉/[〈Vsα2〉〈Vsα2〉]1/2, where 〈 〉 denotes the average over the relevant energy range. Calculations were conducted for a Franck–Condon four-mode system consisting of ns=100 doorway states and nα=3000 quasicontinuum states. The correlation parameters for all pairs of doorway states are considerably lower than unity (|ηss|≲0.4), obeying propensity rules with the highest values of |ηss| corresponding to a single vibrational quantum difference, while for multimode changes between |s〉 and |s〉 very low values of |ηss| are established. Quantum beats in the population probabilities of products and reactants in nonadiabatic dynamics are characterized by an upper limit for their modulation amplitudes ξ≅(Γ/ΔE)η (for ΔE/2πΓ⩾1), where Γ is the decay width of the doorway states and ΔE is their energetic spacing. These low ξ values originate from a small (∼Γ/ΔE) contribution to the off-diagonal matrix elements of the nonradiative decay matrix in conjunction with low correlation parameters. The amplitudes of the quantum beats in nonradiative temporal dynamics provide dynamic information on the larger correlation parameters ηss. Our theoretical and numerical analysis was applied for temporal coherence effects in nonadiabatic electron transfer dynamics in a Franck–Condon quasicontinuum of Mulliken charge transfer complexes [K. Wynne, G. Reid, and R. M. Hochstrasser, J. Chem. Phys. 105, 2287 (1996)]. This accounts for the “preparation” (signature of coherent excitation), for the low amplitudes of coherent temporal modulation of reactants and products (ξ≅0.05–0.06 determined by the ηss parameters) and for the dominating contributions to temporal coherence (subjected to propensity rules).

Self-oscillation in BaTiO_3 using a multimode laser

The effect of temporal coherence on the phenomena of self-oscillation in photorefractive materials is analyzed using multilongitudinal-mode argon-ion and helium-neon lasers. Observations demonstrate that self-oscillation is possible when the mirror-crystal separation is large compared to the laser coherence length. The observations are shown to agree with predictions based on the recurrence in the temporal coherence of a multilongitudinal-mode laser.