Showing papers on "Group velocity published in 2022"
TL;DR: Based on the virtual time reversal technique, the dispersive wave packets adaptively back propagate according to their respective propagation distances so that the dispersion can be completely compensated and focusing signals are obtained.
29 citations
TL;DR: In this paper , the authors demonstrate the programming of ''rainbow trapping'' (slowing waves to a temporary stop, based on frequency) in an elastic waveguide for chosen spatial grading profiles.
Abstract: Stretch here, not there: Programmable tailoring of a wave's group velocity in space, achieved via synthetic impedance circuitry with digital signal processing, enables substantial design flexibility for devices. Here researchers demonstrate the programming of ``rainbow trapping'' (slowing waves to a temporary stop, based on frequency) in an elastic waveguide for chosen spatial grading profiles. The use of spatially graded resonators also significantly enhances the band gap's bandwidth, beyond that for a uniform resonator. This class of metamaterials enables simple on-demand programming of elastic wave trapping, spatial filtering, and attenuation through a digital interface.
16 citations
TL;DR: In this article , the authors synthesize space-time wave packets that travel in free space with arbitrary axial acceleration profiles, including group velocities that change with integer or fractional exponents of the distance, and realize a composite acceleration profile: the wave packet first accelerates from an initial to a terminal group velocity, decelerates back to the initial value, and then travels at a fixed group velocity.
Abstract: All known realizations of optical wave packets that accelerate along their propagation axis, such as Airy wave packets in dispersive media or wave-front-modulated X-waves, exhibit a constant acceleration; that is, the group velocity varies linearly with propagation. Here we synthesize space-time wave packets that travel in free space with arbitrary axial acceleration profiles, including group velocities that change with integer or fractional exponents of the distance. Furthermore, we realize a composite acceleration profile: the wave packet first accelerates from an initial to a terminal group velocity, decelerates back to the initial value, and then travels at a fixed group velocity. These never-before-seen optical-acceleration phenomena are all produced using the same experimental arrangement that precisely sculpts the wave packet's spatio-temporal spectral structure.
13 citations
TL;DR: In this paper , the authors investigate the consequences of introducing non-differentiable angular dispersion into a pulsed field and show that it is the crucial ingredient required to realize group velocities that deviate from c (the speed of light in vacuum) along the propagation axis.
Abstract: Conventional diffractive and dispersive devices introduce angular dispersion (AD) into pulsed optical fields, thus producing so-called 'tilted pulse fronts'. Naturally, it is always assumed that the functional form of the wavelength-dependent propagation angle[s] associated with AD is differentiable with respect to wavelength. Recent developments in the study of space-time wave packets - pulsed beams in which the spatial and temporal degrees of freedom are inextricably intertwined - have pointed to the existence of non-differentiable AD: field configurations in which the propagation angle does not possess a derivative at some wavelength. Here we investigate the consequences of introducing non-differentiable AD into a pulsed field and show that it is the crucial ingredient required to realize group velocities that deviate from c (the speed of light in vacuum) along the propagation axis in free space. In contrast, the on-axis group velocity for conventional pulsed fields in free space is always equal to c. Furthermore, we show that non-differentiable AD is needed for realizing anomalous or normal group-velocity dispersion along the propagation axis, while simultaneously suppressing all higher-order dispersion terms. We experimentally verify these and several other consequences of non-differentiable AD using a pulsed-beam shaper capable of introducing AD with arbitrary spectral profile. Non-differentiable AD is not an exotic phenomenon, but is rather an accessible, robust, and versatile resource for sculpting pulsed optical fields.
9 citations
TL;DR: In this paper , a controllable specific correlation between temporal frequencies and spatial frequencies of the Bessel modes was introduced to generate near-diffraction-free 2D space-time OAM wave packets with high mode purities.
Abstract: Novel forms of light beams carrying orbital angular momentum (OAM) have recently gained interest, especially due to some of their intriguing propagation features. Here, we experimentally demonstrate the generation of near-diffraction-free two-dimensional (2D) space-time (ST) OAM wave packets (ℓ = +1, +2, or +3) with variable group velocities in free space by coherently combining multiple frequency comb lines, each carrying a unique Bessel mode. Introducing a controllable specific correlation between temporal frequencies and spatial frequencies of these Bessel modes, we experimentally generate and detect near-diffraction-free OAM wave packets with high mode purities (>86%). Moreover, the group velocity can be controlled from 0.9933c to 1.0069c (c is the speed of light in vacuum). These ST OAM wave packets might find applications in imaging, nonlinear optics, and optical communications. In addition, our approach might also provide some insights for generating other interesting ST beams.
7 citations
TL;DR: In this article, a variety of possibilities to adjust the group velocity of an optical beam by means of designed angular dispersion are presented. But the results of these studies can be of interest to scientists working in the fields of optical beam engineering, light amplitude and intensity interferometry, ultrafast optics, and optical tweezers.
Abstract: The group velocity of an optical beam in free space is usually considered to be equal to the speed of light in vacuum. However, it has been recently realized that, by structuring the beam’s angular and temporal spectra, one can achieve well pronounced and controlled subluminal and superluminal propagation. In this work, we consider multifrequency Bessel beams that are known to propagate without divergence and show a variety of possibilities to adjust the group velocity of the beam by means of designed angular dispersion. We present several examples of multifrequency Bessel beams with negative and arbitrary positive group velocities, as well as longitudinally accelerating beams and beams with periodically oscillating local group velocities. The results of these studies can be of interest to scientists working in the fields of optical beam engineering, light amplitude and intensity interferometry, ultrafast optics, and optical tweezers.
7 citations
TL;DR: In this paper , the authors show how origami fused with artificial magnetism unveils a versatile pathway to topologically reconfigure magnetic polaritons and experimentally demonstrate that the three-dimensional origami deformation allows to reconfigure hyperbolic or elliptic topology of polariton dispersion and modulate group velocity.
Abstract: Hyperbolic polaritons in extremely anisotropic materials have attracted intensive attention due to their exotic optical features. Recent advances in optical materials reveal unprecedented dispersion engineering of polaritons, resulting in twistronics for photons, canalized phonon polaritons, shear polaritons, and tunable topological polaritons. However, the on-demand reconfigurability of polaritons, especially with magnetic anisotropic dispersions, is restricted by weak natural magnetic anisotropy and hence remains largely unexplored. Here, we show how origami fused with artificial magnetism unveils a versatile pathway to topologically reconfigure magnetic polaritons. We experimentally demonstrate that the three-dimensional origami deformation allows to reconfigure hyperbolic or elliptic topology of polariton dispersion and modulate group velocity. With group velocity transitioning from positive to negative directions, we further report reconfigurable origami polariton circuitry in which the polariton propagation and phase distribution can be tailored. Our findings provide alternative perspectives on on-chip polaritonics, with potential applications in energy transfer, sensing, and information transport.
6 citations
TL;DR: In this article, the authors reported on the diverse optical parameters of Mn2O3 thin films prepared by nebulized spray pyrolysis technique by the effect of spray solution concentration.
Abstract: In this work, we report on the diverse optical parameters of Mn2O3 thin films prepared by nebulized spray pyrolysis technique by the effect of spray solution concentration. X-ray diffraction study confirmed the Mn2O3 films belong to cubic structure. The Raman vibrational band observed at 636 cm−1 is related with the symmetric stretching of Mn–O bond of trivalent manganese ions (Mn3+). The surface morphological study showed that the nano-sized particles are homogenously distributed on the film surface. The films exhibit the highest transmittance in the near infrared region. The spray solution concentration induced reduction of Urbach energy (from 677 to 655 meV) indicates the decrease in degree of crystal defects in the film. The extinction coefficient of the film was obtained in the order of 10−2. It was observed that the optical dispersion threshold point (TODT) of Mn2O3 thin film is coincides with the refractive index threshold wavelength (λrtw) of the material. This was further corroborated from the intersection of phase velocity and group velocity curves at the point of λrtw or TODT. This indicates that no optical dispersion was taken place at that point and therefore the phase velocity is equal to the group velocity. The possible correlations among the data were made with the light of underlying concepts.
6 citations
TL;DR: A novel method to estimate group velocities from experimental data-driven models from experimental in-plane and out-of-plane FRFs of a one-dimensional structure and the vector-fitting (VF) algorithm is adopted.
6 citations
TL;DR: In this article , the existence of a frozen mode in a periodic serpentine waveguide with broken longitudinal symmetry is demonstrated numerically, and the frozen mode is associated with a stationary inflection point (SIP) of the Bloch dispersion relation, where three Bloch eigenmodes collapse on each other, as it is an exceptional point of order three.
Abstract: The existence of a frozen mode in a periodic serpentine waveguide with broken longitudinal symmetry is demonstrated numerically. The frozen mode is associated with a stationary inflection point (SIP) of the Bloch dispersion relation, where three Bloch eigenmodes collapse on each other, as it is an exceptional point of order three. The frozen mode regime is characterized by vanishing group velocity and enhanced field amplitude, which can be very attractive in various applications including dispersion engineering, lasers, and delay lines. Useful and simple design equations that lead to realization of the frozen mode by adjusting a few parameters are derived. The trend in group delay and quality factor with waveguide length that is peculiar of the frozen mode is shown. The symmetry conditions for the existence of exceptional points of degeneracy associated with the frozen mode are also discussed.
6 citations
TL;DR: In this article, the sum-frequency interaction of two high amplitude primary waves is used as a local probe of material non-linearity, and the coupling coefficients that measure the transfer rate of energy as a function of time from primary to secondary modes are derived.
TL;DR: In this article , the authors investigate the refraction of ST wave packets normally incident onto a planar interface between two dispersive, homogeneous, isotropic media and obtain a law of refraction that determines the change in their group velocity across the interface.
Abstract: Space-time (ST) wave packets are a class of pulsed optical beams whose spatiotemporal spectral structure results in propagation invariance, tunable group velocity, and anomalous refractive phenomena. Here, we investigate the refraction of ST wave packets normally incident onto a planar interface between two dispersive, homogeneous, isotropic media. We formulate a new, to the best of our knowledge, refractive invariant for ST wave packets in this configuration, from which we obtain a law of refraction that determines the change in their group velocity across the interface. We verify this new refraction law in ZnSe and CdSe, both of which manifest large chromatic dispersion at near-infrared frequencies in the vicinity of their band edges. ST wave packets can thus be utilized in nonlinear optics for bridging large group-velocity mismatches in highly dispersive scenarios.
TL;DR: In this paper , the axiparabola design is used to generate a plasma waveguide to overcome diffraction or for driving a dephasingless wakefield accelerator for laser-plasma acceleration.
Abstract: Abstract An axiparabola is a reflective aspherical optics that focuses a light beam into an extended focal line. The light intensity and group velocity profiles along the focus are adjustable through the proper design. The on-axis light velocity can be controlled, for instance, by adding spatio-temporal couplings via chromatic optics on the incoming beam. Therefore the energy deposition along the axis can be either subluminal or superluminal as required in various applications. This article first explores how the axiparabola design defines its properties in the geometric optics approximation. Then the obtained description is considered in numerical simulations for two cases of interest for laser-plasma acceleration. We show that the axiparabola can be used either to generate a plasma waveguide to overcome diffraction or for driving a dephasingless wakefield accelerator.
TL;DR: In this paper , a femtosecond pulsed beam is designed to realize dispersion cancellation symmetrically in either the normal or anomalous group velocity dispersion regimes by virtue of non-differentiable angular dispersion inculcated into the pulsed field.
Abstract: Angular dispersion can counterbalance normal group‐velocity dispersion (GVD) that increases the wave‐vector length in a dispersive medium. By tilting the wave vector, angular dispersion reduces the axial wave number in this case to match the pre‐GVD value. By the same token, however, angular dispersion fails to counterbalance anomalous GVD, which in contrast reduces the wave‐vector length. Consequently, GVD‐cancellation via angular dispersion has not been demonstrated to date in the anomalous dispersion regime. Here, structed femtosecond pulsed beams, known as ‘space‐time’ wave packets, are designed to realize dispersion‐cancellation symmetrically in either the normal‐ or anomalous‐GVD regimes by virtue of non‐differentiable angular dispersion inculcated into the pulsed field. Furthermore, GVD‐inversion is also verified by reversing the GVD sign experienced by the field with respect to that dictated by the chromatic dispersion of the medium itself.
TL;DR: In this article , two efficient analytical techniques, including the generalized exponential rational function method and the extended sinh-Gordon equation expansion method, are taken into account to construct optical solutions to a variety of nonlinear Schrödinger's equation with group velocity dispersion.
Abstract: Taking two efficient analytical techniques, including the generalized exponential rational function method and the extended sinh–Gordon equation expansion method, into account several optical solutions to a variety of nonlinear Schrödinger’s equation with group velocity dispersion are constructed. We are investigated several families of localized structures (bright solitons) for attractive nonlinearities including localized structures in both angular directions in addition to localized structures in one-directional and homogeneous in the other. To provide a better understanding of the obtained results, some numerical simulations for the obtained solutions are carried out. Further, the obtained solutions are represented graphically in order to clarify the vision and deep understanding of the mechanism of all nonlinear phenomena described by these solutions. The acquired results in this survey are new and have not been obtained in previous literature. Moreover, both utilized techniques can be assumed as an important tool for the explanation of some nonlinear physical phenomena such as the modulated envelope localized structures in plasma physics and fluid mechanics. Also, the obtained solutions can help researchers interested in studying the acoustic nonlinear modulated structures in different plasma models especially bright envelope solitons.
TL;DR: In this article , coupled exciton-surface plasmon polaritons (E-SPPs) were used to generate slow light with a 1300 fold decrease of the SPP group velocity.
Abstract: Spectrally narrow optical resonances can be used to generate slow light, i.e., a large reduction in the group velocity. In a previous work, we developed hybrid 2D semiconductor plasmonic structures, which consist of propagating optical frequency surface-plasmon polaritons interacting with excitons in a semiconductor monolayer. Here, we use coupled exciton-surface plasmon polaritons (E-SPPs) in monolayer WSe2 to demonstrate slow light with a 1300 fold decrease of the SPP group velocity. Specifically, we use a high resolution two-color laser technique where the nonlinear E-SPP response gives rise to ultra-narrow coherent population oscillation (CPO) resonances, resulting in a group velocity on order of 105 m/s. Our work paves the way toward on-chip actively switched delay lines and optical buffers that utilize 2D semiconductors as active elements.
TL;DR: In this paper , an automated algorithm was proposed to fit the phase of the Hankel function to a causal symmetric part of the CCF in order to determine phase velocity curves as continuous functions of frequency.
Abstract:
Ambient seismic noise tomography has, over the last two decades, developed into a well established tool for imaging seismic properties of the Earth’s crust. Fundamental mode Rayleigh and Love wave phase velocity dispersion curves can be measured from ambient noise cross correlation functions (CCF) either using a high-frequency approximation theory, or by fitting the spectrum of the CCF to a Bessel function. Here, we advance the latter approach and present an automated algorithm that fits the phase of the Hankel function to the phase of the causal symmetric part of the CCF in order to determine phase velocity curves as continuous functions of frequency. Synthetic tests verify the reliability of the proposed method in the presence of low signal-to-noise ratio (SNR). Moreover, usage of the phase allows for robust phase velocity measurements at longer periods than when using the zero crossings of the Bessel function only and is, therefore, particularly useful at short inter-station distances. In the frequency domain, acceptable bandwidths of smooth phase velocity curves are obtained in an automated procedure using a set of fine-tuned quality criteria. We apply the method to 2.5 years of continuous waveform data recorded by 58 temporary and permanent broad band seismic stations in northern Oman. We obtain 1072 and 670 phase velocity curves for Rayleigh and Love waves, respectively, in the period range of 2 - 40 s. The data are inverted for isotropic and azimuthally anisotropic period-dependent phase velocity maps. Synthetic reconstruction tests show that the phase velocity maps have a lateral resolution of ∼30 km. The results suggest distinctly different middle to lower crustal architecture between the northern and eastern Oman Mountains. Azimuthal anisotropy shows contrasting fast propagation orientations in the shallow and deep crust, which we attribute to stress-induced and structural anisotropy in the upper crust and to lattice-preferred orientation in the lower crust.
TL;DR: In this paper , a planar planar waveguide with hybrid guided space-time modes is considered, where dispersion is eliminated and the link between modal order and group index is severed, and direct measurements of the modal group delays confirm that the group index for low-loss, dispersion-free, hybrid space time modes can each be tuned away from the same-order conventional mode.
Abstract: The structure of an optical waveguide determines the characteristics of its guided modes, such as their spatial profile and group index. General features are shared by modes regardless of the waveguiding structure; for example, modal dispersion is inevitable in multimode waveguides, every mode experiences group-velocity dispersion, and higher-order modes usually travel at lower group velocities than their lower-order counterparts. We show here that such trends can be fundamentally altered in a multimode planar waveguide by exploiting hybrid guided space-time modes, whereupon dispersion is eliminated and the link between modal order and group index is altogether severed. Hybrid space-time modes are confined in one-dimension by the planar waveguide, and in the other by the underlying spatiotemporal spectral structure of the field itself. Direct measurements of the modal group delays confirm that the group index for low-loss, dispersion-free, hybrid space-time modes can each be tuned away from the group index of the same-order conventional mode, and that the transverse size of these hybrid modes can be varied independently of the modal order and group index. These findings are verified in a few-mode planar waveguide consisting of a 25.5 mm-long, 4-μm-thick silica film deposited on a MgF2 substrate.
TL;DR: Wang et al. as discussed by the authors proposed a method named DisperPicker to automatically extract fundamental mode group and phase velocity dispersion curves using a convolutional neural network (CNN), where the inputs to CNN include paired group and velocity disprograms in the period-velocity domains.
Abstract:
To image high-resolution crustal and upper-mantle structures, ambient noise tomography (ANT) has been widely used on local and regional dense seismic arrays. One of the key steps in ANT is to extract surface-wave group and phase velocity dispersion curves from cross-correlation functions of continuous ambient noise recordings. One traditional way is to manually pick the dispersion curves from dispersion spectrograms in the period-velocity domains, which is very labor intensive and time consuming. Another way is to automatically pick the dispersion curves using some predefined criteria, which are not reliable in many cases especially for phase velocity data. In this study, we propose a novel method named DisperPicker to automatically extract fundamental mode group and phase velocity dispersion curves using a convolutional neural network (CNN). The inputs to CNN include paired group and phase velocity dispersion spectrograms in the period-velocity domains, which are calculated from empirical surface-wave Green’s functions. In this way, group velocity dispersion curves can implicitly guide the extraction of phase velocity dispersion curves, which have large ambiguities to pick on the dispersion spectrograms. The labels or outputs of the network are the probability images converted from dispersion curves. The U-net architecture is adopted because it is powerful for image processing. We have assembled short-period surface-wave data from three different dense seismic arrays to train the network. The trained network is further tested and validated by two datasets close to Chao Lake, China. The tests show that DisperPicker has the generalization ability to efficiently and accurately extract dispersion curves of large datasets without new training.
TL;DR: In this article , the authors theoretically simulate the modulation of chirped Gaussian vector solitons with group-velocity dispersion at 1-μm under the condition of same central wavelength (1064 nm) and slightly different central wavelengths (1063 nm and 1065 nm).
TL;DR: In this paper , the authors proved that space-time wave packets are stable in dispersive media as well and free from the spread in time caused by material dispersion, and generalized the law of anomalous refraction for space time wave packets to weakly dispersive situations.
Abstract: Space–time wave packets can propagate invariantly in free space with arbitrary group velocity thanks to the spatio-temporal correlation. Here it is proved that the space–time wave packets are stable in dispersive media as well and free from the spread in time caused by material dispersion. Furthermore, the law of anomalous refraction for space–time wave packets is generalized to the weakly dispersive situation. These results reveal new potential of space–time wave packets for the applications in real dispersive media.
TL;DR: In this paper , the authors present a model of shear wave velocity for the botanical garden of the Benemérita Universidad Autónoma de Puebla (BUAP), using ambient seismic noise recorded on a geotechnical scale array.
TL;DR: In this paper, a tunable plasmon-induced transparency (PIT) metamaterial is presented for manipulating the group velocity of terahertz (THz) waves.
Abstract: We present a tunable plasmon-induced transparency (PIT) metamaterial for manipulating the group velocity of terahertz (THz) waves. The metamaterial is composed of metal split rings and photoconductive silicon strips. The strong PIT effect with slowing down THz waves is generated by the bright-bright mode coupling between the high-order plasmon mode and the lattice surface mode via electromagnetic destructive interference. By varying the conductivity of silicon strips, the group slowing performance is dynamically tunable. The group delay can achieve beyond 20 ps with the group index as high as 592, showing the promising application for THz signal manipulation.
TL;DR: In this paper , the possibility of simultaneous modal phase and group velocity matching was shown to be feasible with silica microstructured optical fibers with at least one ring of air holes in the cladding and a heavily Germanium doped core.
Abstract: Optical fibers provide a favorable medium for nonlinear optical processes owing to the small mode field size and concurrently high optical intensity combined with the extended interaction lengths. Second harmonic generation (SHG) is one of those processes that has been demonstrated in silica glass optical fibers. Since silica is centrosymmetric, generating SHG in an optical fiber requires poling of the glass. In addition and when one wants to use ultrashort pulses for SHG, achieving both phase and group velocity matching is crucial. Although fibers that feature either modal phase velocity or group velocity matching for SHG have been reported, the possibility of simultaneous modal phase and group velocity matching was never reported before. In this paper we address this challenge, and for the first time to our knowledge, we show that it is feasible to do so with silica microstructured optical fibers featuring at least one ring of air holes in the cladding and a heavily Germanium doped core (above 25 mol.%) by exploiting the LP01(ω) and LP02(2ω) modes at 1.06 µm pump and 0.53 µm second harmonic wavelengths. This finding can greatly impact applications requiring waveguide based SHG generation with ultrashort pulses, including microscopy, material characterization and nonlinear imaging.
TL;DR: In this article , a segment of polarization-maintaining fiber (PMF) was inserted inside the cavity of an er-doped fiber laser, and abundant dynamics of group velocity-locked vector solitons (GVLVSs) were investigated by using graphene oxide/polyvinyl alcohol (GO/PVA) film as a saturable absorber.
Abstract: Abstract With the insertion a segment of polarization-maintaining fiber (PMF) inside the cavity, abundant dynamics of group velocity locked vector solitons (GVLVSs) in Er-doped fiber laser have been investigated by using graphene oxide/polyvinyl alcohol (GO/PVA) film as a saturable absorber (SA). The generated Kelly sidebands in emission spectra reveal peak-valley or valley-peak alternation and slightly shift in two orthogonal components, which are the characteristics of GVLVSs. Through proper adjustment of polarization controllers (PCs) inside the EDFLs cavity, versatile vector soliton dynamics such as polarization locked GVLVSs (PL-GVLVSs), polarization rotation GVLVSs (PR-GVLVSs), dual wavelength GVLVSs, bound state GVLVSs, bunch GVLVSs and harmonic mode-locking GVLVSs (HML-GVLVSs) have been observed. The separation between two emission peaks from the dual wavelength GVLVSs was controlled by the Lyot filter and related to the insertion length of PMF inside the cavity. Unlike PL-GVLVSs, the period-doubling phenomenon has been found in two orthogonal components of the PR-GVLVSs. Besides, the bound state GVLVSs were generated showing strongly modulated interference fringes in emission spectrum. For the bunch and HML GVLVSs, the number of solitons inside the cavity increased with the pump power, and it showed the quintuple solitons and the 7th HML-GVLVSs at the highest pump power.
TL;DR: In this article , the authors used Legendre polynomial to predict the guided dispersion curves in pre-stressed functionally graded piezoelectric-piezomagnetic materials (FGPPM) with negative magnetic permeability.
18 May 2022
TL;DR: In this article , the authors theoretically proposed a uide (CCSW), which is composed of a zigzag edge-like structure based on C-4 symmetrical lattice, and achieved weak coupling between a sequence of higher order topological corner state (TCS).
Abstract: We theoretically propose a uide (CCSW), which is composed of a zigzag edge-like structure based on C-4 symmetrical lattice. CCSW mode is achieved by weak coupling between a sequence of higher order topological corner state (TCS). Based on the tight-binding approximation, the flat dispersion relation of CCSW mode is obtained, and suitable for slowing down light. The characteristics of slow light, including the group index, group velocity dispersion, normalized bandwidth and normalized delay-bandwidth product, are discussed in detail. At the Eigen frequency of individual TCS, the group velocity dispersion of CCSW mode is zero. Importantly, the CCSW mode shows strong robustness when introducing disorders, compared with the conventional Coupled-Resonator-Optical Waveguide based on photonic crystal defect cavities. Our findings may find topological slow light applications such as optical buffers, the processing of optical signals, optical delay lines and so on.