Spectral Contrast Research Articles

Multispectral Contrast of Archaeological Features: A Quantitative Evaluation

This study provides an evaluation of spectral responses of hollow ways in Upper Mesopotamia. Hollow ways were used for the transportation of animals, carts, and other moving agents for centuries. The aim is to show how the success of spectral indices varies in describing topologically simple features even in a seemingly homogeneous geographic unit. The variation is further highlighted under the changing precipitation regime. The methodology begins with an exploration of the relationship between the date of a multispectral scene and the visibility of hollow ways. The next step is to evaluate the impact of rainfall levels on numerous indices and to quantify spectral contrast. The contrast between a hollow way and its background is evaluated with Welch’s t-test and the association between precipitation regime and spectral responses of hollow ways are investigated with Correspondence Analysis and Fisher’s test. Results highlight an intrinsic relationship between the precipitation regime and the ways in which archaeological features reflects and/or emits electromagnetic energy. Next, the categorization of spectral indices based on different rainfall levels can be used as a guidance in future studies. Finally, the study suggests contrast becomes an even more fruitful concept as one moves from the spatial domain to the spectral domain.

Particle Size Effects on Mid‐Infrared Spectra of Lunar Analog Minerals in a Simulated Lunar Environment

AbstractMid‐infrared spectroscopic analysis of the Moon and other airless bodies requires a full accounting of spectral variation due to the unique thermal environment in airless body regoliths and the substantial differences between spectra acquired under airless body conditions and those measured in an ambient environment on Earth. Because there exists a thermal gradient within the upper hundreds of microns of lunar regolith, the data acquired by the Diviner Lunar Radiometer Experiment are not isothermal with wavelength. While this complication has been previously identified, its effect on other known variables that contribute to spectral variation, such as particle size and porosity, have yet to be well characterized in the laboratory. Here we examine the effect of particle size on mid‐infrared spectra of silicates common to the Moon measured within a simulated lunar environment chamber. Under simulated lunar conditions, decreasing particle size is shown to enhance the spectral contrast of the Reststrahlen bands and transparency features, as well as shift the location of the Christiansen feature to longer wavelengths. This study shows that these variations are detectable at Diviner spectral resolution and emphasizes the need for simulated environment laboratory data sets, as well as hyperspectral mid‐infrared instruments on future missions to airless bodies.

Tuning Plasmon Induced Reflectance with Hybrid Metasurfaces

Electrically tunable metasurfaces with graphene offer design flexibility to efficiently manipulate and control light. These metasurfaces can be used to generate plasmon-induced reflectance (PIR), which can be tuned by electrostatic doping of the graphene layer. We numerically investigated two designs for tunable PIR devices using the finite difference time-domain (FDTD) method. The first design is based on two rectangular antennas of the same size and a disk; in the second design, two parallel rectangular antennas with different dimensions are used. The PIR-effect was achieved by weak hybridization of two bright modes in both devices and tuned by changing the Fermi level of graphene. A total shift of ∼362 nm was observed in the design with the modulation depth of 53% and a spectral contrast ratio of 76%. These tunable PIR devices can be used for tunable enhanced biosensing and switchable systems.

Hybrid Metal Graphene-Based Tunable Plasmon-Induced Transparency in Terahertz Metasurface.

In this paper, we look at the work of a classical plasmon-induced transparency (PIT) based on metasurface, including a periodic lattice with a cut wire (CW) and a pair of symmetry split ring resonators (SSR). Destructive interference of the ‘bright-dark’ mode originated from the CW and a pair of SSRs and resulted in a pronounced transparency peak at 1.148 THz, with 85% spectral contrast ratio. In the simulation, the effects of the relative distance between the CW and the SSR pair resonator, as well as the vertical distance of the split gap, on the coupling strength of the PIT effect, have been investigated. Furthermore, we introduce a continuous graphene strip monolayer into the metamaterial and by manipulating the Fermi level of the graphene we see a complete modulation of the amplitude and line shape of the PIT transparency peak. The near-field couplings in the relative mode resonators are quantitatively understood by coupled harmonic oscillator model, which indicates that the modulation of the PIT effect result from the variation of the damping rate in the dark mode. The transmitted electric field distributions with polarization vector clearly confirmed this conclusion. Finally, a group delay of 5.4 ps within the transparency window is achieved. We believe that this design has practical applications in terahertz (THz) functional devices and slow light devices.

Earlier music biases subsequent musical instrument categorization

Perception of sounds occurs in the context of surrounding sounds. When spectral properties differ between earlier (context) and later (target) sounds, categorization of later sounds becomes biased through spectral contrast effects (SCEs). Past research has shown SCEs to bias categorization of speech and music alike. Additionally, the magnitudes of SCEs are not all-or-none but vary continuously in both speech and music categorization. Recently, the natural spectral composition of (unfiltered) sentences biased speech categorization via SCEs (Stilp and Assgari, under review). Here, we tested whether natural (unfiltered) music would similarly bias categorization of French horn and tenor saxophone targets. Preceding contexts were either solo performances of the French horn or tenor saxophone (unfiltered) or a string quintet processed to emphasize frequencies in the horn or saxophone (filtered). Categorization was influenced by SCEs in both unfiltered and filtered conditions, with more “saxophone” responses following horn / horn-like contexts and vice versa. Since unfiltered music produced SCEs in musical instrument categorization, this extends their influence to everyday listening conditions.

Variability in talkers' fundamental frequencies shapes context effects in speech perception.

The perception of any given sound is influenced by surrounding sounds. When successive sounds differ in their spectral compositions, these differences may be perceptually magnified, resulting in spectral contrast effects (SCEs). For example, listeners are more likely to perceive /ɪ/ (low F1) following sentences with higher F1 frequencies; listeners are also more likely to perceive /ɛ/ (high F1) following sentences with lower F1 frequencies. Previous research showed that SCEs for vowel categorization were attenuated when sentence contexts were spoken by different talkers [Assgari and Stilp. (2015). J. Acoust. Soc. Am. 138(5), 3023-3032], but the locus of this diminished contextual influence was not specified. Here, three experiments examined implications of variable talker acoustics for SCEs in the categorization of /ɪ/ and /ɛ/. The results showed that SCEs were smaller when the mean fundamental frequency (f0) of context sentences was highly variable across talkers compared to when mean f0 was more consistent, even when talker gender was held constant. In contrast, SCE magnitudes were not influenced by variability in mean F1. These findings suggest that talker variability attenuates SCEs due to diminished consistency of f0 as a contextual influence. Connections between these results and talker normalization are considered.

Tunable multiple Fano resonance employing polarization-selective excitation of coupled surface-mode and nanoslit antenna resonance in plasmonic nanostructures

Modeling and tailoring of multispectral Fano resonance in plasmonic system employing nanoslit-antenna array is demonstrated and investigated. Efficient control of the multiple Fano profile can be manipulated, where the overall spectral is achieved by the separate contributions from the fundamental subgroups plasmonic resonance eigenstates. A polarization-selective strategy on nano-antennas resonance is proposed to shed light on the efficient manipulation of the multiple Fano resonances. Theory prediction of TM−1 surface mode excited in the system and thorough dispersion analysis of the supported Bloch modes provides evidence for understanding the origin of the transmission spectra. Compact nanophotonics planar optical linear-polarizer in the proposed nanostructure is investigated and demonstrated, where flexible Fano resonance control over the profile, linewidth and spectral contrast is appealing for applications such as sensing, switches and multifunctional nanophotonics devices.

Theoretical investigation of active modulation and enhancement of Fano resonance in THz metamaterials

We have theoretically proposed electrical modulation and enhancement of Fano resonance in THz metamaterials. In order to practically realize Fano resonance based devices, resonance dips should be sharp and to be clearly detected. However, realization of Fano resonances with simultaneously high quality factor (Q) and higher spectral contrast remains a major challenge. Therefore we propose a metamaterial based device to overcome the above limitation with active tuning capability. Our simulated results show a large enhancement of surface current and electric field, which results in sharp, extremely narrow bandwidth Fano type resonances. Furthermore, a higher degree of tunability has been achieved for Fano resonance as compared to dipole resonance, which can be useful to realize advanced dynamically reconfigurable devices.

Spectral Weighting Underlies Perceived Sound Elevation

The brain estimates the two-dimensional direction of sounds from the pressure-induced displacements of the eardrums. Accurate localization along the horizontal plane (azimuth angle) is enabled by binaural difference cues in timing and intensity. Localization along the vertical plane (elevation angle), including frontal and rear directions, relies on spectral cues made possible by the elevation dependent filtering in the idiosyncratic pinna cavities. However, the problem of extracting elevation from the sensory input is ill-posed, since the spectrum results from a convolution between source spectrum and the particular head-related transfer function (HRTF) associated with the source elevation, which are both unknown to the system. It is not clear how the auditory system deals with this problem, or which implicit assumptions it makes about source spectra. By varying the spectral contrast of broadband sounds around the 6–9 kHz band, which falls within the human pinna’s most prominent elevation-related spectral notch, we here suggest that the auditory system performs a weighted spectral analysis across different frequency bands to estimate source elevation. We explain our results by a model, in which the auditory system weighs the different spectral bands, and compares the convolved weighted sensory spectrum with stored information about its own HRTFs, and spatial prior assumptions.

Musical instrument categorization is highly sensitive to spectral properties of earlier sounds.

Auditory perception is shaped by spectral properties of surrounding sounds. For example, when spectral properties differ between earlier (context) and later (target) sounds, this can produce spectral contrast effects (SCEs; i.e., categorization boundary shifts) that bias perception of later sounds. SCEs affect perception of speech and nonspeech sounds alike (Stilp Alexander, Kiefte, & Kluender in Attention, Perception, & Psychophysics, 72(2), 470-480, 2010). When categorizing speech sounds, SCE magnitudes increased linearly with greater spectral differences between contexts and target sounds (Stilp, Anderson, & Winn in Journal of the Acoustical Society of America, 137(6), 3466-3476, 2015; Stilp & Alexander in Proceedings of Meetings on Acoustics, 26, 2016; Stilp & Assgari in Journal of the Acoustical Society of America, 141(2), EL153-EL158, 2017). The present experiment tested whether this acute context sensitivity generalized to nonspeech categorization. Listeners categorized musical instrument target sounds that varied from French horn to tenor saxophone. Before each target, listeners heard a 1-second string quintet sample processed by filters that reflected part of (25%, 50%, 75%) or the full (100%) difference between horn and saxophone spectra. Larger filter gains increased spectral distinctness across context and target sounds, and resulting SCE magnitudes increased linearly, parallel to speech categorization. Thus, a highly sensitive relationship between context spectra and target categorization appears to be fundamental to auditory perception.

Fano resonances in symmetric gold nanorod trimers

A symmetrical gold nanorod trimer structure consisting of a short center nanorod and two long nanorods on both sides is proposed. The scattering spectra, electromagnetic field and current density vector distributions across the central cross section of the nanorod trimer are calculated by the finite difference time domain method, and the effects of structural parameters and dielectric environment on Fano resonance characteristics are theoretically investigated in detail. The results show that the Fano resonance can be generated mainly due to the interference between the bonding electric dipole mode in lower energy and the antibonding electric dipole mode or antiphase magnetic dipole mode in higher energy. The Fano dip is blue-shifted with the decrease in the short nanorod length, the size of whole trimer structure with constant displacement, or the refractive index of dielectric medium in the gaps between the central nanorod and two side nanorods; the resonance intensity on both sides of the Fano dip also changes. Meanwhile, the bonding mode on the red side of the Fano dip is gradually dominated by the electric dipole mode of two side nanorods, and the spectral intensity increases, while the antibonding mode on the blue side gradually evolves into the short nanorod-dominated antiphase magnetic dipole mode, and the spectral intensity becomes weaker. The increase in the inter-rod spacing also leads the Fano dip to be blue-shifted, and a similar change in the spectral intensity occurs on both sides of the Fano dip, due to the degeneration of bonding and antibonding modes caused by the decrease of near-field coupling between the short nanorod and two side nanorods, which finally degenerate into the electric dipole modes generated by the short nanorod or the two side nanorods, respectively. In addition, the Fano dip is insensitive to the change of the side nanorod length, but the relative resonance intensity on both sides of the Fano dip also changes. Furthermore, it is found that the spectral contrast ratio of the Fano resonance first increases and then decreases by varying the above-mentioned structural parameters or dielectric environment. These results are expected to be used for guiding the design of Fano controllable nanostructures and also for developing the applications of specific micro-nano photonics.

Parameterisation of auto-spectral density of earthquake induced strong ground motions

This paper explores the possibility of a new parametric representation of auto-spectral density (ASD) that enables extraction of several meaningful insights. Recorded translational ground motions are considered first and the resulting parametric model is capable to address the scenario when seismic energy is distributed over multiple frequency bands. A slightly different form is developed to address the rotational ground motion. Variation of ASD over the footprint of seismic arrays, both large (SMART1) and dense (LSST), is studied. The assessment is based on spectral similitude using spectral contrast angles and distance correlation. Possible correlation of the parameters of the proposed ASD model against site and event characteristics is investigated based on selected ground motions from PEER database. Model parameters thus computed are used in synthetically predicting the ASD of ground motions at the LSST array and that shows good resemblance against the recorded events.

Electrically controllable plasmon induced reflectance in hybrid metamaterials

The tunable plasmon induced reflectance (PIR) effect has been numerically investigated and experimentally realized by hybrid metal-graphene metamaterials. The PIR effect is produced by two parallel strips of gold (Au) and controlled electrically by applying the gate voltage to the graphene layer. The PIR response is generated by the weak hybridization of two bright modes of the gold strips and tuned by changing the Fermi level (Ef) of the graphene. The total shift of 211.7 nm was achieved in the reflection peak by applying only 3 V. This concept of real time electrical tuning of PIR, with a modulation depth of ∼49% and a spectral contrast ratio of 66.6%, can be used for designing optical switches, optical modulators, and tunable sensors.

Hyperspectral wide-field time domain single-pixel diffuse optical tomography platform.

We present the design and comprehensive instrumental characterization of a time domain diffuse optical tomography (TD-DOT) platform based on wide-field illumination and wide-field hyperspectral time-resolved single-pixel detection for functional and molecular imaging in turbid media. The proposed platform combines two digital micro-mirror devices (DMDs) to generate structured light and a spectrally resolved multi-anode photomultiplier tube (PMT) detector in time domain for hyperspectral data acquisition over 16 wavelength channels based on the time-correlated single-photon counting (TCSPC) technique. The design of the proposed platform is described in detail and its characteristics in spatial, temporal and spectral dimensions are calibrated and presented. The performance of the system is further validated through a phantom study where two absorbers in glass tubes with spectral contrast are mapped in a turbid medium of ~20 mm thickness. The method presented here offers the potential of accelerating the imaging process and improving reconstruction results in TD-DOT and thus facilitates its wide spread use in preclinical and clinical in vivo imaging scenarios.

Rock Sample Surface Preparation Influences Thermal Infrared Spectra

High-resolution laboratory-based thermal infrared spectroscopy is an up-and-coming tool in the field of geological remote sensing. Its spatial resolution allows for detailed analyses at centimeter to sub-millimeter scales. However, this increase in resolution creates challenges with sample characteristics, such as grain size, surface roughness, and porosity, which can influence the spectral signature. This research explores the effect of rock sample surface preparation on the thermal infrared spectral signatures. We applied three surface preparation methods (split, saw, and polish) to determine how the resulting differences in surface roughness affects both the spectral shape as well as the spectral contrast. The selected samples are a pure quartz sandstone, a quartz sandstone containing a small percentage of kaolinite, and an intermediate-grained gabbro. To avoid instrument or measurement type biases we conducted measurements on three TIR instruments, resulting in directional hemispherical reflectance spectra, emissivity spectra and bi-directional reflectance images. Surface imaging and analyses were performed with scanning electron microscopy and profilometer measurements. We demonstrate that surface preparation affects the TIR spectral signatures influencing both the spectral contrast, as well as the spectral shape. The results show that polished surfaces predominantly display a high spectral contrast while the sawed and split surfaces display up to 25% lower reflectance values. Furthermore, the sawed and split surfaces display spectral signature shape differences at specific wavelengths, which we link to mineral transmission features, surface orientation effects, and multiple reflections in fine-grained minerals. Hence, the influence of rock surface preparation should be taken in consideration to avoid an inaccurate geological interpretation.

Iridescent colouration of male Anna\u2019s hummingbird (Calypte anna) caused by multilayered barbules

The male Anna’s hummingbird features a brightly reddish-pink reflecting gorget, due to large stacks of melanosomes in the feather barbules, arranged in layers separated by keratin. Direct observations together with detailed scatterometry demonstrated that the barbules reflect incident light in an approximately specular manner. The structural colouration is iridescent, i.e. varies with a changing angle of light incidence. Spectrophotometrical measurements of the barbule reflectance and absorbance can be well interpreted with calculated spectra obtained with a transfer matrix method for optical multilayers, using anatomical data and measured refractive index spectra. The organization of the reflectors as a Venetian blind presumably functions to create a high spectral contrast of the male’s plumage during courtship.

Background-deflection Brillouin microscopy reveals altered biomechanics of intracellular stress granules by ALS protein FUS

Altered cellular biomechanics have been implicated as key photogenic triggers in age-related diseases. An aberrant liquid-to-solid phase transition, observed in in vitro reconstituted droplets of FUS protein, has been recently proposed as a possible pathogenic mechanism for amyotrophic lateral sclerosis (ALS). Whether such transition occurs in cell environments is currently unknown as a consequence of the limited measuring capability of the existing techniques, which are invasive or lack of subcellular resolution. Here we developed a non-contact and label-free imaging method, named background-deflection Brillouin microscopy, to investigate the three-dimensional intracellular biomechanics at a sub-micron resolution. Our method exploits diffraction to achieve an unprecedented 10,000-fold enhancement in the spectral contrast of single-stage spectrometers, enabling, to the best of our knowledge, the first direct biomechanical analysis on intracellular stress granules containing ALS mutant FUS protein in fixed cells. Our findings provide fundamental insights on the critical aggregation step underlying the neurodegenerative ALS disease.

Optimising NDWI supraglacial pond classification on Himalayan debris-covered glaciers

The ability of medium-resolution (10–30 m) satellite imagery to delineate the size and persistence of ponds on debris-covered glaciers is a topic of recent interest as it has become apparent through the use of fine-resolution products that smaller ponds have often been neglected. In this study, we performed a quantitative analysis of pond detection using a normalised difference water index (NDWI) applied to several widely used satellite sensors, which offer multispectral information at high radiometric precision. These data include: RapidEye (5 m spatial resolution), Sentinel-2 Multispectral Instrument (MSI) (10–20 m), and Landsat 8 Operational Land Imager (OLI) multispectral imagery (30 m). We demonstrate a method to derive an optimum NDWI value for pond classification using a subset reference dataset of 285 ponds classified using fine-resolution (0.5 m) imagery. We then applied the optimised NDWI (NDWI-O) to the remaining images to assess pond classification accuracy against a broader reference dataset of 898 ponds. NDWI values calculated using Sentinel-2 imagery showed the best spectral contrast between water and surrounding debris cover, and the strongest relationship with pixel water content (R2 = 0.56), followed by the RapidEye NDWI (R2 = 0.45). We conclude that RapidEye and Sentinel-2 imagery is best suited for accurate pond classification using a multispectral classification approach, which is important for quantifying their role in glacier ablation, meltwater regulation, and lake development. By comparison, the impact of using coarse-resolution Landsat 8 imagery to characterise surface water dynamics is minimised when applied to large glacier lakes, where the area-to-perimeter ratio is greater.

Bringing the Nonlinearity of the Movement System to Gestural Theories of Language Use: Multifractal Structure of Spoken English Supports the Compensation for Coarticulation in Human Speech Perception.

Coarticulation is the tendency for speech vocalization and articulation even at the phonemic level to change with context, and compensation for coarticulation (CfC) reflects the striking human ability to perceive phonemic stability despite this variability. A current controversy centers on whether CfC depends on contrast between formants of a speech-signal spectrogram—specifically, contrast between offset formants concluding context stimuli and onset formants opening the target sound—or on speech-sound variability specific to the coordinative movement of speech articulators (e.g., vocal folds, postural muscles, lips, tongues). This manuscript aims to encode that coordinative-movement context in terms of speech-signal multifractal structure and to determine whether speech's multifractal structure might explain the crucial gestural support for any proposed spectral contrast. We asked human participants to categorize individual target stimuli drawn from an 11-step [ga]-to-[da] continuum as either phonemes “GA” or “DA.” Three groups each heard a specific-type context stimulus preceding target stimuli: either real-speech [al] or [a], sine-wave tones at the third-formant offset frequency of either [al] or [aɹ], and either simulated-speech contexts [al] or [aɹ]. Here, simulating speech contexts involved randomizing the sequence of relatively homogeneous pitch periods within vowel-sound [a] of each [al] and [aɹ]. Crucially, simulated-speech contexts had the same offset and extremely similar vowel formants as and, to additional naïve participants, sounded identical to real-speech contexts. However, randomization distorted original speech-context multifractality, and effects of spectral contrast following speech only appeared after regression modeling of trial-by-trial “GA” judgments controlled for context-stimulus multifractality. Furthermore, simulated-speech contexts elicited faster responses (like tone contexts do) and weakened known biases in CfC, suggesting that spectral contrast depends on the nonlinear interactions across multiple scales that articulatory gestures express through the speech signal. Traditional mouse-tracking behaviors measured as participants moved their computer-mouse cursor to register their “GA”-or-“DA” decisions with mouse-clicks suggest that listening to speech leads the movement system to resonate with the multifractality of context stimuli. We interpret these results as shedding light on a new multifractal terrain upon which to found a better understanding in which movement systems play an important role in shaping how speech perception makes use of acoustic information.

Notches in sentence spectra bias subsequent phoneme categorization

Spectral differences across earlier (context) and later (target) sounds are perceptually magnified, resulting in spectral contrast effects (SCEs) that bias categorization of later sounds. Most studies added spectral peaks to context sounds in order to produce SCEs, but noise contexts with spectral notches at speech frequencies biased phoneme categorization in a complementary manner (Coady et al., 2003 JASA). We tested whether this approach generalized to speech contexts with spectral notches. On each trial, a context sentence preceded the target phoneme (Experiment 1: /ɪ/-/ɛ/; Experiment 2: /d/-/g/). Sentences were processed by notch filters that attenuated energy in the same frequency regions that, when amplified to add spectral peaks, produced SCEs (Experiment 1: 100–400/550–850 Hz, as in Stilp et al., 2015 JASA; Experiment 2: 1700–2700/2700–3700 Hz, as in Stilp & Assgari, 2017 JASA). Notch depths ranged from −5 to −20 dB in 5-dB steps. In both experiments, notch-filtered sentences biased phoneme categorization in complementary directions to SCEs, with bias magnitudes increasing at larger notch depths. Whether earlier sounds have spectral peaks or notches, speech categorization is highly sensitive to the magnitudes of spectral differences across context and target sounds.Spectral differences across earlier (context) and later (target) sounds are perceptually magnified, resulting in spectral contrast effects (SCEs) that bias categorization of later sounds. Most studies added spectral peaks to context sounds in order to produce SCEs, but noise contexts with spectral notches at speech frequencies biased phoneme categorization in a complementary manner (Coady et al., 2003 JASA). We tested whether this approach generalized to speech contexts with spectral notches. On each trial, a context sentence preceded the target phoneme (Experiment 1: /ɪ/-/ɛ/; Experiment 2: /d/-/g/). Sentences were processed by notch filters that attenuated energy in the same frequency regions that, when amplified to add spectral peaks, produced SCEs (Experiment 1: 100–400/550–850 Hz, as in Stilp et al., 2015 JASA; Experiment 2: 1700–2700/2700–3700 Hz, as in Stilp & Assgari, 2017 JASA). Notch depths ranged from −5 to −20 dB in 5-dB steps. In both experiments, notch-filtered sentences biased phoneme catego...