Broadband Modulation Research Articles

A frequency-dependent ground-motion spatial correlation model of within-event residuals for Fourier amplitude spectra

Ground motion time series recorded at stations separated by up to about 50 km show a frequency-dependent spatial coherency structure, and the corresponding ground motion intensity measures are found to be correlated. As omitting this correlation can result in underestimation of seismic losses in risk analysis, it is critical to quantify the spatial correlation structure for ground motion Fourier spectra estimated at different sites during a single event within a region. Toward this goal, we have developed an empirical frequency-dependent spatial correlation model for the within-event residuals of effective Fourier amplitude spectra from the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation (NGA) West2 database. The correlation model shows slower decrease of the spatial correlation with distance at lower frequencies compared with higher frequencies, in agreement with the underlying ground motion data, and no significant dependence on the magnitude of the earthquakes is observed. We use this empirical model to incorporate frequency-dependent spatial correlation into a hybrid deterministic-stochastic broadband ground motion generation module, which successfully generates synthetic time series for seven western US earthquakes with frequency-dependent spatial correlation that closely mimics that of the empirical model. Furthermore, the method also significantly improves the correlation for spectral accelerations, cumulative absolute velocities, and Arias intensities, compared with that derived from the original broadband module.

Evaluation of the effectiveness of using broadband modulation currents in medical rehabilitation of patients after acute cerebrovascular accident

To study the effect of broadband modulation currents on the manifestations of spasticity and on cognitive functions in patients after an ischemic stroke in the late recovery period. The article presents data on the effectiveness of the use of broadband modulation currents in the late recovery period of medical rehabilitation of patients with ischemic stroke. The study included 80 patients who underwent ischemic stroke with movement disorders in the form of hemiparesis and increased muscle tone by the type of spasticity in the late recovery period. Control group patients (40 people) underwent standard drug therapy and medical rehabilitation, patients of the main group (40 people) received broadband modulation currents against the background of standard drug therapy and medical rehabilitation. The severity of spasticity and impaired motor function of the arm and leg was assessed according to the modified Ashfort Spasticity Scale (MAS) and goniometry parameters; the analysis of indicators of impairment of cognitive functions according to the data of the Montreal Cognitive Assessment Scale (MoCA) was carried out. After the course of treatment in patients of the main group, at all control points, there was a statistically significant positive dynamics of a decrease in spasticity, both in the proximal and in the distal parts of the upper limb, according to the modified Ashforth Spasticity Scale (MAS), while in the patients of the control group, only positive trend. After the course of treatment, a significant advantage of the influence of the developed method on cognitive impairment of functions in patients of the main group, who received broadband modulation currents, was revealed. This was confirmed by the increase in the total indicator of the MoCA scale to 26.7±0.6 points, which corresponded to the reference values. In patients of the control group, less pronounced results were obtained, the total indicator of the MoCA scale was 25.1±1.0 points, which was below the norm. The data obtained indicate that the inclusion of broadband modulation currents in the medical rehabilitation program contributes to a significant decrease in the severity of spasticity, increases the range of motion in the affected limb, and has a positive effect on the cognitive functions of patients.

High-Contrast Dynamic Reflecting System Based on Pneumatic Micro/Nanoscale Surface Morphing.

Cephalopods offer a fascinating dynamic reflecting system to create desired colors and patterns through contracting and releasing their soft skins in response to environmental stimuli. Inspired by this natural display strategy, we designed a novel dynamic reflecting system based on pneumatic micro/nanoscale surface morphing. This system consists of a thin metal skin/elastomer bilayer modulated by a microfluidic-based gas injector. Benefited from the "wrinkled-specular" transition of the metal's surface under a small pneumatic actuation (4 kPa), an unprecedented reflectance contrast of 93 for broad-band (500-750 nm) modulation is achieved. This remarkable response also has excellent cycle stability (>2500 times) and fast response time (∼0.2 s). These advantages enable a robust and ultrasensitive optical gas pressure sensor with a sensitivity of 178 kPa-1, which is 3-4 orders of magnitude higher than those of conventional optical gas pressure sensors based on either a Fabry-Pérot interferometer or a Mach-Zehnder interferometer. Moreover, as proof-of-concept applications, we also experimentally demonstrated a curvature-variable convex mirror and noniridescent dynamic display, suggesting that our pneumatically dynamic reflecting system will potentially broaden the applications in adaptive optical devices, sensors, and displays.

Ultra-broadband spatial light modulation with dual-resonance coupled epsilon-near-zero materials

It has been found that the dielectric constants of transparent conductive oxides (TCOs) can be adjusted in an extremely large range by tuning the carrier density. Due to the remarkable light confinement property of the epsilon-near-zero (ENZ) effect of TCOs, it has attracted extensive interests of light modulation. However, the operation wavelength bandwidth is usually limited by optical resonance that is applied to enhance the light-TCOs interaction. In this work, a dual-resonance light coupling scheme is proposed to expand the modulation depth-bandwidth product with almost one order-of-magnitude improvement. In a metallic subwavelength grating structure with deep trenches backed by a ground plane, the ENZ mode can be coupled to both magnetic resonance and Fabry-Perot resonance respectively by tuning the bias. Decent light modulation can be obtained in a large operation wavelength band covering two resonances by optimizing the dual-resonance configuration. Such a reconfigurable efficient broadband modulation is important for robust communication link and possesses remarkable capacity for wavelength division multiplexing.

Theoretical design of a reconfigurable broadband integrated metamaterial terahertz device.

An actively reconfigurable broadband terahertz (THz) metamaterial functional device based on the phase-change material vanadium dioxide (VO2) and two-dimensional graphene material is theoretically proposed and demonstrated. The device has excellent tolerance under oblique incidence. When the VO2 is in the metallic state, and the Fermi energy of graphene is fixed at 0.1 eV, the designed device acts as a broadband THz absorber in the transverse magnetic (TM) polarization mode. The absorptance bandwidth exceeds 0.55 THz with a complete absorption intensity of more than 90%. In this state, the absorber operates as a broadband modulator with the total modulation depth exceeding 91.5% as the continually decreased conductivity of VO2 from 200000 S/m to 10 S/m. In the transverse electric (TE) polarization process, the structure behaves as a dual-band absorber with two perfect absorption peaks. When the conductivity of VO2 is changed, the tunable absorber can also be regarded as an absorptance modulator, with a maximum modulation intensity of 92.1%. Alternatively, when VO2 behaves as an insulator at room temperature in the TE polarization mode, a strong broadband electromagnetically induced transparency (EIT) window is obtained, with a bandwidth exceeding 0.42 THz in the transmittance spectrum. By varying the Fermi energy of graphene from 0 to 0.9 eV, the EIT-like window or broadband transmission spectrum (in TM mode) can be switched. The results indicate that the device can also be operated as a modulator in the transmission mode. The impedance matching theory is used, and electric field distributions are analyzed to quantify the physical mechanism. An advantage of the manipulation of the polarization angle is that the modulation performance of the proposed multi-functional THz device can be regulated after fabricated.

Multitone Multiharmonic Scattering Parameters for the Characterization of Nonlinear Networks

This work describes a generalized multitone multiharmonic scattering parameters ( $\mathbf {M^{2}S}$ -parameters) measurement approach suitable for multiport nonlinear network characterization in the wave-variable domain. Based on the incommensurate (or offset) frequency concept, the $\mathbf {M^{2}S}$ -parameters are directly measurable quantities independent of the incident wave phases and display a mild nonlinear dependence on the incident wave amplitudes. They feature a well-defined relationship with the frequency-domain nonlinear transfer functions derived from the Volterra theory, representing a generalization to the multiinput multiharmonic case of the widely used single-input single-frequency describing function approach. The proposed framework allows to capture the nonlinear steady-state behavior from a reduced set of measurements and can be effectively exploited for performance prediction in application-like scenarios. The approach is here applied to RF transistor and power amplifier load-pull measurements, as well as to the case of broadband modulation, showing that the performance assessment of the device can be obtained in a straightforward way from the $\mathbf {M^{2}S}$ -parameters description.

Glued diffraction optical elements with broadband and a large field of view.

High diffraction efficiency is an important requirement for hybrid diffractive-refractive optical systems with a wide field of view. The issue is that diffractive optical elements cannot maintain high diffraction efficiency across a designed waveband and range of incident angles simultaneously. Glued diffractive optical elements (GDOEs) consist of two single-layer diffractive elements, and optical adhesives are presented to address the problem. Two diffractive optical elements are glued together to reduce the straylight scattered into unwanted diffraction orders. The parameters of diffractive optical elements are optimized to achieve broadband high diffraction efficiency and modulation transfer function over a wide-incident-angle range. The GDOEs enable the system to realize a diffraction efficiency of over 90% when the incident angle is no more than 58°. Through gluing two single-layer diffractive optical elements together, we can minimize the inner reflection and refraction. Diffraction efficiency losses can be compensated by the optical adhesives layer, and image quality can be improved. Our design method could make possible the use of diffraction elements in different kinds of optical systems.

Narrowband Modulation Two-Dimensional Mass Spectrometry and Label-Free Relative Quantification of Histone Peptides.

Two-dimensional mass spectrometry (2D MS) on a Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer allows for tandem mass spectrometry without requiring ion isolation. In the ICR cell, the precursor ion radii are modulated before fragmentation, which results in modulation of the abundance of their fragments. The resulting 2D mass spectrum enables a correlation between the precursor and fragment ions. In a standard broadband 2D MS, the range of precursor ion cyclotron frequencies is determined by the lowest mass-to-charge (m/z) ratio to be fragmented in the 2D MS experiment, which leads to precursor ion m/z ranges that are much wider than necessary, thereby limiting the resolving power for precursor ions and the accuracy of the correlation between the precursor and fragment ions. We present narrowband modulation 2D MS, which increases the precursor ion resolving power by reducing the precursor ion m/z range, with the aim of resolving the fragment ion patterns of overlapping isotopic distributions. In this proof-of-concept study, we compare broadband and narrowband modulation 2D mass spectra of an equimolar mixture of histone peptide isoforms. In narrowband modulation 2D MS, we were able to separate the fragment ion patterns of all 13C isotopes of the different histone peptide forms. We further demonstrate the potential of narrowband 2D MS for label-free quantification of peptides.

Epsilon near-zero all-optical terahertz modulator

We propose an efficient optically actuated THz modulator based on an ultrathin epsilon-near-zero (ENZ) slab photogenerated in an InAs semiconductor. We experimentally demonstrate a modulation depth of 90% at 1 THz obtained with a continuous laser at irradiation lower than 10 W cm−2. Beyond the strong attenuation of the THz transmission provided by the ENZ absorption effect, we also report a broadband modulation of the THz waves from 1 to 10 THz. In addition, our experimental results show that the cut-off frequency of 3 dB attains 2 MHz in the dynamic modulation regime.

Low-voltage, broadband graphene-coated Bragg mirror electro-optic modulator at telecom wavelengths.

We demonstrate a graphene based electro-optic free-space modulator yielding a reflectance contrast of 20% over a strikingly large 250nm wavelength range, centered in the near-infrared telecom band. Our device is based on the original association of a planar Bragg reflector, topped with an electrically contacted double-layer graphene capacitor structure employing a high work-function oxide shown to confer a static doping to the graphene in the absence of an external bias, thereby reducing the switching voltage range to +/-1V. The device design, fabrication and opto-electric characterization is presented, and its behavior modeled using a coupled optical-electronic framework.

Broadband Light Amplitude Tuning Characteristics of SnSe2 Coated Microfiber

Broadband light amplitude tuning characteristics are demonstrated in a 2-D tin diselenide (SnSe2) nanosheets coated microfiber (MF) structure (SnSe2CMF). Due to the strong absorption characteristics of SnSe2 in broadband wavelengths, the transmitted optical power (TOP) of SnSe2CMF can be tuned by external pump lights with different wavelengths (405, 532, and 660 nm) over a broadband spectrum (1520–1620 nm). Five wavelengths are extracted in order to investigate the tuning performance. The TOP of SnSe2CMF increases with the pump light power and a maximum tuning efficiency of ∼0.3074 dB/mW is achieved. In terms of tuning speed, the minimum response time is ∼3.7/3.9 ms. The experimental results have qualitatively good agreement with the simulations predicted by FEM models. The demonstrated structure has the advantages of broad tuning bandwidth and high tuning efficiency. Therefore, it might be a promising candidate for building high performance broadband fiber-optic sensors and modulators.

Bandwidth Enhancement of Planar Terahertz Metasurfaces via Overlapping of Dipolar Modes

We present enhancement of operational bandwidths of planar terahertz metasurfaces by incorporating a complex unit cell that consists of a pair of concentric ring resonators. The inner resonator is a closed ring, while the outer resonator contains a pair of split gaps introduced symmetrically into the two opposite arms. The enhancement of the bandwidth is attributed to the spectral overlapping of the resonant responses arising from both the resonators. A total of five metasurface samples (MM1 to MM5) have been designed, fabricated, and characterized using terahertz time domain spectroscopy (THz-TDS). Resonance broadening along with the blue shifting of higher order resonance mode is being observed while introducing the split gaps in the outer ring and increasing the split gap length (MM1 to MM5) successively. We have further performed detailed numerical investigations in order to explain the experimental observations which support the experimental results. Such metasurfaces can pave the way to realize versatile photonic applications including broadband modulators and band stop filters in the comparatively lesser explored yet technologically relevant terahertz region.

Ultrasensitive and Broadband All‐Optically Controlled THz Modulator Based on MoTe2/Si van der Waals Heterostructure

AbstractDespite the impressive progresses in terahertz (THz) sources and detection, there is still a big challenge for high performance active optoelectronic THz devices such as THz modulators. All‐optically controlled THz modulators with large modulation depth (MD) and wide modulation bandwidth are of great importance for the THz technology. Herein, a MoTe2/Si van der Waals (vdWs) heterostructure is rationally designed as all‐optical THz modulator by taking advantage of their similar band alignment and easy integration with Si complementary metal‐oxide‐semiconductor (CMOS). The MoTe2/Si modulator presents an ultrasensitive THz modulation performance with a MD of 99.9% under a low illumination power of 300 mW at 1064 nm. This is a record MD for TMDCs‐based all‐optically controlled THz modulators to date. Moreover, the MoTe2/Si modulator exhibits broadband modulation performance with a wide frequency range from 0.3 to 2.0 THz. The high modulation performance under low illumination power is beneficial for practical application with low energy consumption and easy heat dissipation, which is advantageous to modulator chip. This work validates a facile protocol for fabricating high performance THz modulators and paves the way for their practical applications in THz technology.

Low voltage, high modulation depth graphene THz modulator employing Fabry–Perot resonance in a metal/dielectric/graphene sandwich structure

In this article, a novel THz modulator for modulating signals in the frequency range of 0.5–10 THz is designed and numerically investigated with finite difference time domain method. The structure is composed of a graphene layer sandwiched between two complementary metallic gratings. The proposed structure has Fabry–Perot like behavior at which the resonant condition depends on the geometrical parameters of the device and graphene plasmonic mode characteristics. A major advantage of the presented modulator is its capability to function in various regimes such as resonant and non-resonant as well as transmission and reflection. Based on the proposed structure, a broadband modulator for THz signals in the frequency range of 0.5–3 THz is proposed with modulation depth of 90% (36%) at 0.5 THz (3 THz). Insertion loss of the modulator is 1.2 dB. Electro-optic bandwidth is more than 2 GHz depending on the device surface area. In addition to this, several low voltage and relatively narrowband modulators are designed either at central frequencies of 4.5, 5.5, and 6.5 THz with modulation depth of more than 75% and insertion loss of less than 2.5 dB at their central frequency.

CMOS-compatible all-optical modulator based on the saturable absorption of graphene

Graphene resting on a silicon-on-insulator platform offers great potential for optoelectronic devices. In the paper, we demonstrate all-optical modulation on the graphene–silicon hybrid waveguides (GSHWs) with tens of micrometers in length. Owing to strong interaction between graphene and silicon strip waveguides with compact light confinement, the modulation depth reaches 22.7% with a saturation threshold down to 1.38 pJ per pulse and a 30-μm-long graphene pad. A response time of 1.65 ps is verified by a pump–probe measurement with an energy consumption of 2.1 pJ. The complementary metal-oxide semiconductor compatible GSHWs with the strip configuration exhibit great potential for ultrafast and broadband all-optical modulation, indicating that employing two-dimensional materials has become a complementary technology to promote the silicon photonic platform.

Modulation change detection in human auditory cortex: Evidence for asymmetric, non-linear edge detection.

Changes in modulation rate are important cues for parsing acoustic signals, such as speech. We parametrically controlled modulation rate via the correlation coefficient (r) of amplitude spectra across fixed frequency channels between adjacent time frames: broadband modulation spectra are biased toward slow modulate rates with increasing r, and vice versa. By concatenating segments with different r, acoustic changes of various directions (e.g., changes from low to high correlation coefficients, that is, random-to-correlated or vice versa) and sizes (e.g., changes from low to high or from medium to high correlation coefficients) can be obtained. Participants listened to sound blocks and detected changes in correlation while MEG was recorded. Evoked responses to changes in correlation demonstrated (a) an asymmetric representation of change direction: random-to-correlated changes produced a prominent evoked field around 180ms, while correlated-to-random changes evoked an earlier response with peaks at around 70 and 120ms, whose topographies resemble those of the canonical P50m and N100m responses, respectively, and (b) a highly non-linear representation of correlation structure, whereby even small changes involving segments with a high correlation coefficient were much more salient than relatively large changes that did not involve segments with high correlation coefficients. Induced responses revealed phase tracking in the delta and theta frequency bands for the high correlation stimuli. The results confirm a high sensitivity for low modulation rates in human auditory cortex, both in terms of their representation and their segregation from other modulation rates.

All-optical modulation with 2D layered materials: status and prospects

Abstract Optical modulation technique plays a crucial role in photonics technologies, and there is an ever-increasing demand for broadband and ultrafast optical modulation in the era of artificial intelligence. All-optical modulation is known to be able to operate in an ultrafast way and has a broadband response, showing great potential in applications for ultrafast information processing and photonic computing. Two-dimensional (2D) materials with exotic optoelectronic properties bring tremendous new opportunities for all-optical modulators with excellent performance, which have attracted lots of attention recently. In this review, we cover the state-of-art all-optical modulation based on 2D materials, including graphene, transitional metal dichalcogenides, phosphorus, and other novel 2D materials. We present the operations mechanism of different types of all-optical modulators with various configurations, such as fiber-integrated and free-space ones. We also discuss the challenges and opportunities faced by all-optical modulation, as well as offer some future perspectives for the development of all-optical modulation based on 2D materials.

Broadband modulation of subwavelength topological interface states in a one-dimensional acoustic system

The acoustic analogy of topological insulators is a hot field of materials research. On one-dimensional acoustic systems, many researchers have lately paid their attention to the applications of the Su-Schrieffer-Heeger (SSH) model, which can support topologically nontrivial phases. In this paper, we design a supercell composed of two identical hollow cylinders with a side split immersed in the air background. The supercell is arranged in a line to form a SSH model, which has three bandgaps including two zone-folding-induced gaps and a local resonant gap in the subwavelength region. By analyzing the eigenstates and calculating the Zak phases, we find that a topological phase transition takes place only in the zone-folding-induced gaps when we rotate the split-cylinders. Thus, a finite-size array, made of two sublattices with distinct topological properties, inevitably produces topological interface states on their interface. In addition, we demonstrate that the topological interface states can be adjusted in a wide frequency range by rotating the cylinders that control the coupling strength between two split-cylinders in one supercell. These tunable topological interface states may have potential application prospects in wave filtering, wave detecting, and so on.

Ultrafast and Broadband Terahertz Modulator With Polarization Selectivity

We demonstrate a broadband and polarization selective terahertz modulator with ultrafast modulation speed, based on the Si grating metasurface on the Sapphire substrate. The Si layer is implanted with B+ and subsequently annealed, which significantly reduces the lifetime of carriers and increases the carrier density, and thus realizes ultrafast and efficient modulation. Our modulator achieves modulation depth up to 60% for the x -polarized THz wave and less than 1% for the y -polarized THz wave in the frequency ranging from 0.5 to 2.5 THz, with switch-on and switch-off time of modulation in 20 ps and around 300 ps, respectively. The carrier dynamics of the fabricated Si grating is also studied to illustrate the ultrafast modulated mechanism. Our results pave the way for ultrafast manipulation of terahertz spectrum and shows great potential in terahertz communication.

Dielectric waveguides with embedded graphene nanoribbons for all-optical broadband modulation

All-optical processing offers low power consumption and fast operation speed and is a promising approach to high-bit-rate communication. Realization of all-optical integrated photonics requires core materials that strongly mediate photon–photon interaction. Recently, it was shown that, in the long-wavelength limit, graphene nanoribbons (GNRs) have a very strong Kerr optical nonlinearity in the telecom wavelength range (1.3−1.6μm). We propose a dielectric waveguide with embedded GNRs for all-optical self-amplitude-modulation applications. By implanting a van der Waals (vdW) heterostructure consisting of GNRs and hexagonal boron nitride into a rib silicon waveguide, we maximize the optical concentration near the GNRs and enhance nonlinear optical effects. Different-width GNRs incorporated in the vdW heterostructure provide strong self-sustaining broadband modulation over the telecom frequency range, without a need for dynamical tuning. The compact footprint and self-sustaining, broad-bandwidth saturable absorption make the proposed device a suitable component for ultrafast nanophotonic applications.