Relative Modulation Depth Research Articles

Efficient polarization-insensitive quasi-BIC modulation by VO2 thin films.

Bound states in the continuum (BIC) offer great design freedom for realizing high-quality factor metasurfaces. By deliberately disrupting the inherent symmetries, BIC can degenerate into quasi-BIC exhibiting sharp spectra with strong light confinement. This transformation has been exploited to develop cutting-edge sensors and modulators. However, most proposed quasi-BICs in metasurfaces are composed of unit cells with Cs symmetry that may experience performance degradation due to polarization deviation, posing challenges in practical applications. Addressing this critical issue, our research introduces an innovative approach by incorporating metasurfaces with C4v unit cell symmetry to eliminate polarization response sensitivity. Vanadium Dioxide (VO2) is a phase-change material with a relatively low transition temperature and reversibility. Here, we theoretically investigate the polarization-insensitive quasi-BIC modulation in Si-VO2 hybrid metasurfaces. By introducing defects into metasurfaces with Cs, C4, and C4v symmetries, we enable the emergence of quasi-BICs characterized by strong Fano resonance in their transmission spectra. Via numerically calculating the multipole decomposition, distinct dominant multipoles for different quasi-BICs are identified. A comprehensive investigation into the polarization responses of these structures under varying directions of linearly polarized light reveals the superior polarization-independent characteristics of metasurfaces with C4 and C4v symmetries, a feature that ensures the maintenance of maximum resonance peaks irrespective of polarization direction. Utilizing the polarization-insensitive quasi-BIC, we thus designed two different Si-VO2 hybrid metasurfaces with C4v symmetry. Each configuration presents complementary benefits, leveraging the VO2 phase transition's loss change to facilitate efficient modulation. Our quantitative calculation indicates notable achievements in modulation depth, with a maximum relative modulation depth reaching up to 342%. For the first time, our research demonstrates efficient modulation using polarization-insensitive quasi-BICs in designed Si-VO2 hybrid metasurfaces, achieving identical polarization responses for quasi-BIC-based applications. Our work paves the way for designing polarization-independent quasi-BICs in metasurfaces and marks a notable advancement in the field of tunable integrated devices.

Near-infrared switch effect of polarization modulation induced by guided-mode resonance in dielectric grating

Light intensity modulation is crucial for the development of optical imaging, optical sensing, and optical switch. Light intensity modulation methods, such as changing structural parameters, external temperature, or external voltage, make the control process time consuming and complex. The plasmonic polarization modulation is an effective strategy to modulate the light intensity, but this method is limited by the excitation of surface plasmons with transverse magnetic (TM) polarized light. Herein, we report another polarization modulation method for light intensity based on guided mode resonance in a dielectric grating excited by transverse electric (TE) polarized light. The nanosystem comprises a Si grating and a TiN substrate. By adjusting the polarization states of the incident light from TE to TM, the proposed nanosystem exhibits an outstanding light intensity modulation performance with a relative modulation depth of 25833%. The presented method provides another way for modulating the light intensity, which has potential applications in optical switching, optical imaging, and optical anti-counterfeiting.

Ultrafast dynamic switching of optical response based on nonlinear hyperbolic metamaterial platform

The pursuit of high-speed and on-chip optical communication systems has promoted extensive exploration of all-optical control of light-matter interactions via nonlinear optical processes. Here, we have numerically investigated the ultrafast dynamic switching of optical response using tunable hyperbolic metamaterial (HMM) which consists of five pairs of alternating layers of indium tin oxide (ITO) and SiO2. The nonlinearity of the HMM is analyzed by the ultrafast dynamics of the hot electrons in the epsilon-near-zero (ENZ) ITO. Our approach allows large and broad all-optical modulation of the effective permittivity and topology of the HMM on the femtosecond time-scale. Based on the proposed HMM platform, we have shown considerable tunability in the extinction ratio and Purcell enhancement under various pump fluence. In addition, we have achieved all-optical control of the coupling strength through depositing plasmonic resonators on the HMM platform. A significant tuning of the coupled resonance is observed by changing pump fluence, which leads to a switching time within 213 fs at a specific wavelength with a relative modulation depth more than 15 dB.

Ultrafast Optical Modulation of Harmonic Generation in Two-Dimensional Materials.

The modulation of optical harmonic generation in two-dimensional (2D) materials is of paramount importance in nanophotonic and nano-optoelectronic devices for their applications in optical switching and communication. However, an effective route with ultrafast modulation speed, ultrahigh modulation depth, and broad operation wavelength range is awaiting a full exploration. Here, we report that an optical pump can dynamically modulate the third harmonic generation (THG) of a graphene monolayer with a relative modulation depth above 90% at a time scale of 2.5 ps for a broad frequency ranging from near-infrared to ultraviolet. Our observation, together with the real-time, time-dependent density functional theory (TDDFT) simulations, reveals that this modulation process stems from nonlinear dynamics of the photoexcited carriers in graphene. The superior performance of the nonlinear all-optical modulator based on 2D materials paves the way for its potential applications including nanolasers and optical communication circuits.

Tailoring Multi-Walled Carbon Nanotubes into Graphene Quantum Sheets.

Transformation of carbon nanotubes (CNTs) into sub-10 nm pieces is highly required but remains a great challenge. Herein, we report a robust strategy capable of mechanically tailoring pristine multi-walled carbon nanotubes (MWCNTs) into graphene quantum sheets (M-GQSs) with an extremely high yield of up to 44.6 wt %. The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation and therefore enables reproducible high-yield production of M-GQSs directly from MWCNTs. Remarkable solvent diversity and extraordinary solvability (up to 7 mg/mL) are demonstrated facilitating the solution processing of the M-GQSs. The M-GQSs are essentially monolayers with intrinsic curvature, which could be determinative to their outstanding performances in both dispersions and thin films. Besides the excitation wavelength-, concentration-, and solvent-dependent photoluminescence in dispersions, the solid-state fluorescence and exceptional nonlinear saturation absorption (NSA) in thin films are demonstrated. Particularly, NSA with relative modulation depth up to 46% and saturation intensity down to 1.53 MW/cm2 are achieved in M-GQS/poly(methyl methacrylate) hybrid thin films with a loading content of merely 0.2 wt %. Our method opens up a new avenue toward conversion and utilization of CNTs.

Ultrafast all-optical switching enabled by epsilon-near-zero-tailored absorption in metal-insulator nanocavities

Ultrafast control of light−matter interactions is fundamental in view of new technological frontiers of information processing. However, conventional optical elements are either static or feature switching speeds that are extremely low with respect to the time scales at which it is possible to control light. Here, we exploit the artificial epsilon-near-zero (ENZ) modes of a metal-insulator-metal nanocavity to tailor the linear photon absorption of our system and realize a nondegenerate all-optical ultrafast modulation of the reflectance at a specific wavelength. Optical pumping of the system at its high energy ENZ mode leads to a strong redshift of the low energy mode because of the transient increase of the local dielectric function, which leads to a sub-3-ps control of the reflectance at a specific wavelength with a relative modulation depth approaching 120%.

Tunable terahertz metamaterial absorber actuated by thermomechanical bimaterial microcantilevers.

We report a tunable terahertz (THz) metamaterial absorber (MA) actuated by thermomechanical bimaterial microcantilevers. The THz MA, which is suspended on a silicon substrate by the bimaterial microcantilevers, is a sandwich structure with a bottom Al ground plane, middle air and SiNx dielectric layers, and a top Al rotationally symmetric open split ring resonator. Upon application of a current, a Ti heating resistor integrated on the SiNx dielectric layer induces the bimaterial microcantilevers to bend, causing the air layer thickness to change, modulating the absorption of the THz MA. The tunable THz MA exhibited a relative modulation depth of absorption of 28.1% at 0.69 THz and a thermomechanical sensitivity of 0.12°/K. This tunable THz MA has potential applications in filtering, modulation, control, and THz imaging.

Nonlinear optical absorption of few-layer molybdenum diselenide (MoSe_2) for passively mode-locked soliton fiber laser [Invited

In this paper, both nonlinear saturable absorption and two-photon absorption (TPA) of few-layer molybdenum diselenide (MoSe2) were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge. Few-layer MoSe2 nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer, Raman spectroscopy, and atomic force microscopy. The obtained few-layer MoSe2 dispersion is further composited with a polymer material for convenient fabrication of MoSe2 thin films. Then, we investigated the nonlinear optical (NLO) absorption property of the few-layer MoSe2 film using a balanced twin-detector measurement technique. Both the saturable absorption and TPA effects of the few-layer MoSe2 film were found by increasing the input optical intensity. The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of ∼3.5%, corresponding to the relative modulation depth of 18%. The TPA effect occurred when the input optical intensity exceeds ∼260 MW/cm2. Furthermore, we experimentally exploit the saturable absorption of few-layer MoSe2 film to mode lock an all-fiber erbium-doped fiber laser. Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps. It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity. Our results indicate that layered MoSe2, as another two-dimensional nanomaterial, can provide excellent NLO properties (e.g., saturable absorption and TPA) for potential applications in ultrashort pulse generation and optical limiting.

Interaction of first- and second-order signals in the extraction of global-motion and optic-flow

The intention of this series of experiments was to determine the extent to which the pathways sensitive to first-order and second-order motion are independent of one another at, and above, the level of global motion integration. We used translational, radial and rotational motion stimuli containing luminance-modulated dots, contrast-modulated dots, or a mixture of both. Our results show that the two classes of motion stimuli interact perceptually in a global motion coherence task, and the extent of this interaction is governed by whether the two varieties of local motion signal produce an equivalent response in the pathways that encode each type of motion. This provides strong psychophysical evidence that global motion and optic flow processing are cue-invariant. The fidelity of the first-order motion signal was moderated by either reducing the luminance of the dots or by increasing the displacement of the dots on each positional update. The experiments were carried out with two different types of second-order elements (contrast-modulated dots and flicker-modulated dots) and the results were comparable, suggesting that these findings are generalisable to a variety of second-order stimuli. In addition, the interaction between the two different types of second-order stimuli was investigated and we found that the relative modulation depth was also crucial to whether the two populations interacted. We conclude that the relative output of local motion sensors sensitive to either first-order or second-order motion dictates their weight in subsequent cue-invariant global motion computations.

Transient nutation electron spin resonance spectroscopy on spin-correlated radical pairs: A theoretical analysis on hyperfine-induced nuclear modulations

The influence of anisotropic hyperfine interaction on transient nutation electron paramagnetic resonance (EPR) of light-induced spin-correlated radical pairs is studied theoretically using the density operator formalism. Analytical expressions for the time evolution of the transient EPR signal during selective microwave excitation of single transitions are derived for a model system comprised of a weakly coupled radical pair and one hyperfine-coupled nucleus with I=1/2. Zero-quantum electron coherence and single-quantum nuclear coherence are created as a result of the sudden light-induced generation of the radical pair state from a singlet-state precursor. Depending on the relative sizes of the nuclear Zeeman frequency and the secular and pseudo-secular parts of the hyperfine coupling, transitions between levels with different nuclear spin orientations are predicted to modulate the time-dependent EPR signal. These modulations are in addition to the well-known transient nutations and electron zero-quantum precessions. Our calculations provide insight into the mechanism of recent experimental observations of coherent nuclear modulations in the time-resolved EPR signals of doublets and radical pairs. Two distinct mechanisms of the modulations are presented for various microwave magnetic field strengths. The first modulation scheme arises from electron and nuclear coherences initiated by the laser excitation pulse and is “read out” by the weak microwave magnetic field. While the relative modulation depth of these oscillations with respect to the signal intensity is independent of the Rabi frequency, ω1, the frequencies of this coherence phenomenon are modulated by the effective microwave amplitude and determined by the nuclear Zeeman interaction and hyperfine coupling constants as well as the electron-electron spin exchange and dipolar interactions between the two radical pair halves. In a second mechanism the modulations are both created and detected by the microwave radiation. Here, the laser pulse merely defines the beginning of the microwave-induced coherent time evolution. This second mechanism appears the most consistent with current experimental observations.

Quantitative Effects of Global Tempo on Expressive Timing in Music Performance: Some Perceptual Evidence

This study examines whether global tempo and expressive timing microstructure are independent in the aesthetic judgment of music performance. Measurements of tone interonset intervals in pianists' performances of pieces by Schumann ("Traumerei") and Debussy ("La fille aux cheveux de lin") at three different tempi show a tendency toward reduced relative variation in expressive timing at both faster and slower tempi, relative to the pianist's original tempo. However, this could reflect merely the pianists' discomfort when playing at an unfamiliar tempo. Therefore, a perceptual approach was taken here. Experimental stimuli were created artificially by independently manipulating global tempo (three levels) and "relative modulation depth" of expressive timing (RMD, five levels) in MIDI-recorded complete performances of the Schumann and Debussy pieces. Skilled pianists rated the quality of the resulting two sets of 15 performances on a 10-point scale. The question was whether the same RMD would receive the highest rating at all three tempi, or whether an interaction would emerge, such that different RMDs are preferred at different tempi. A small but significant interaction was obtained for both pieces, indicating that the listeners preferred a reduced RMD when the tempo was increased, but the same or a larger RMD when the tempo was decreased. Thus, they associated an increase in tempo with a decrease in (relative) expressive timing variation, which, in general agreement with the performance data, suggests that the two temporal dimensions are not independent.

Does expressive timing remain relationally invariant under global tempo transformations?

Measurements of tone interonset intervals in pianists’ performances of Schumann’s ‘‘Träumerei’’ at three different tempi [B. H. Repp, Psychol. Res. (in press)] have suggested approximate relational invariance of expressive timing, but with a tendency towards reduced temporal contrast (relative modulation depth) at a faster tempo. In this study a perceptual test was assembled by manipulating global tempo (3 levels) and degree of temporal contrast (5 levels) independently in a single MIDI-recorded performance of ‘‘Träumerei.’’ Skilled pianist subjects rated the resulting 15 performances on a 10-point scale. The question was whether the same degree of temporal contrast would be preferred at all three tempi (suggesting relational invariance), or whether an interaction would emerge, such that smaller degrees of contrast are preferred at faster tempi. The results may speak to the adequacy of simple tempo controls on MIDI instruments or in performance synthesis. [Work supported by NIMH.]

Signal threshold as a function of the relative modulation depth between on‐frequency and flanking masker components

It is apparent that the mechanism underlying comodulation masking release (CMR) relies on the existence of temporal fluctuations in the masker envelope. This raises the question of how much fluctuation is necessary to facilitate CMR. The present experiment addresses this question by measuring psychometric functions relating signal threshold to depth of flanker band modulation using sinusoidally amplitude‐modulated pure tones as masker components. In the first set of conditions, the on‐frequency component had a constant modulation depth of 100%, while the depth of the flanking components was varied between 100% and 0%. As expected, signal threshold improved monotonically as depth of flanker band modulation increased. In the second set of conditions, the on‐frequency component had a constant modulation depth of 63%, while the depth of the flanking components was either 100%, 63%, or 0%. Results to date suggest that signal threshold is lowest when the flanking components have the same depth of modulation as the on‐frequency component. [Research supported by AFOSR.]