arXiv subjects

Ayman F. Abouraddy

Publications and source records attributed to Ayman F. Abouraddy.

At least 55 records · Page 3Linked to original sources

arXiv2022

Space-time wave packets propagating a kilometer in air

We report on the diffraction-free propagation of space-time wave packets (STWPs) -- a class of propagation-invariant pulsed beams -- for $\sim\!1$ km in an open-air laser range in a low-turbulence scenario. Making use of $\approx\!100$-fs pulses (bandwidth $\sim\!25$ nm) at a wavelength of $\approx\!1$ $μ$m, we construct an STWP with a transverse width of $\approx\!2$ mm that expands to $\approx\!3$ mm after $\sim\!500$ m, and another that expands from $\approx\!8$ mm to $\approx\!10$ mm after 1 km. The propagation of the STWPs is compared to Gaussian wave packets of the same transverse spatial width and bandwidth. We establish a theoretical model that accounts for the significant factors limiting the STWP propagation distance and suggests the path to further extending this distance.

physics.optics↗

arXiv2022

Non-differentiable angular dispersion as an optical resource

Introducing angular dispersion into a pulsed field associates each frequency with a particular angle with respect to the propagation axis. A perennial yet implicit assumption is that the propagation angle is differentiable with respect to the frequency. Recent work has shown that the existence of a frequency at which the derivative of the propagation angle does not exist -- which we refer to as non-differentiable angular dispersion -- allows for the optical field to exhibit unique and useful characteristics that are unattainable by endowing optical fields with conventional angular dispersion. Because these novel features are retained in principle even when the specific non-differentiable frequency is not part of the selected spectrum, the question arises as to the impact of the proximity of the spectrum to this frequency. We show here that operating in the vicinity of the non-differentiable frequency is imperative to reduce the deleterious impact of (1) errors in implementing the angular-dispersion profile, and (2) the spectral uncertainty intrinsic to finite-energy wave packets in any realistic system. Non-differential angular dispersion can then be viewed as a resource -- quantified by a Schmidt number -- that is maximized in the vicinity of the non-differentiable frequency. These results will be useful in designing novel phase-matching of nonlinear interactions in dispersive media.

physics.optics↗

arXiv2022

Relativistic transformations of quasi-monochromatic optical beams

A monochromatic plane wave recorded by an observer moving with respect to the source undergoes a Doppler shift and spatial aberration. We investigate here the transformation undergone by a generic, paraxial, spectrally coherent quasi-monochromatic optical \textit{beam} (of finite transverse width) when recorded by a moving detector. Because of the space-time coupling engendered by the Lorentz transformation, the monochromatic beam is converted into a propagation-invariant pulsed beam traveling at a group velocity equal to that of the relative motion, and which belongs to the recently studied family of `space-time wave packets'. We show that the predicted transformation from a quasi-monochromatic beam to a pulsed wave packet can be observed even at terrestrial speeds.

physics.optics↗

arXiv2022

Space-time wave packets localized in all dimensions

Optical wave packets that are localized in space and time, but nevertheless overcome diffraction and travel rigidly in free space, are a long sought-after field structure with applications ranging from microscopy and remote sensing, to nonlinear and quantum optics. However, synthesizing such wave packets requires introducing non-differentiable angular dispersion with high spectral precision in two transverse dimensions, a capability that has eluded optics to date. Here, we describe an experimental strategy capable of sculpting the spatio-temporal spectrum of a generic pulsed beam by introducing arbitrary radial chirp via two-dimensional conformal coordinate transformations of the spectrally resolved field. This procedure yields propagation-invariant `space-time' wave packets localized in all dimensions, with tunable group velocity in the range from $0.7c$ to $1.8c$ in free space, and endowed with prescribed orbital angular momentum. By providing unprecedented flexibility in sculpting the three-dimensional structure of pulsed optical fields, our experimental strategy promises to be a versatile platform for the emerging enterprise of space-time optics.

physics.optics↗

arXiv2022

Spatial resolution of omni-resonant imaging

Omni-resonance refers to the broadening of the spectral transmission through a planar cavity, not by changing the cavity structure, but by judiciously preconditioning the incident optical field. As such, broadband imaging can be performed through such a cavity with all the wavelengths simultaneously resonating. We examine here the spatial resolution of omni-resonant imaging and find that the spectral linewidth of the cavity resonance determines the spatial resolution. Surprisingly, the spatial resolution improves at longer wavelengths because of the negative angular dispersion intrinsic to Fabry-Perot resonances, in contrast to conventional diffraction-limited optical imaging systems where the spatial resolution improves at shorter wavelengths. These results are important for applications ranging from transparent solar windows to nonlinear resonant image processing.

physics.optics↗

arXiv2022

Vector space-time wave packets

Space-time wave packets (STWPs) are propagation-invariant pulsed beams whose characteristics stem from the tight association between their spatial and temporal degrees of freedom. Until recently, only scalar STWPs have been synthesized in the form of light sheets. Here we synthesize vector STWPs that are localized in all dimensions by preparing polarization-structured spatio-temporal spectra and unveil the polarization distribution over the STWP volume via time-resolved complex field measurements. Such vector STWPs are endowed with cylindrically symmetric polarization vector structures, which require joint manipulation of the spatial, temporal, and polarization degrees-of-freedom of the optical field. These results may be useful in particle manipulation, and in nonlinear and quantum optics.

physics.optics↗

arXiv2022

Spectral reorganization of space-time wave packets in presence of normal group-velocity dispersion

Space-time wave packets (STWPs) are pulsed beams that propagate invariantly (without diffraction or dispersion) in linear media. The behavior of STWPs in free space is now well-established, and recently their propagation invariance was confirmed in both the normal and anomalous dispersion regimes. However, yet-to-be-observed rich dynamics of spectral reorganization have been predicted to occur in the presence of normal group-velocity dispersion (GVD). Indeed, propagation invariance in the normal-GVD regime is compatible with spatio-temporal spectra that are X-shaped, hyperbolic, parabolic, elliptical, or even separable along the spatial and temporal degrees-of-freedom. These broad varieties of field structures can be classified in a two-dimensional space parameterized by the group velocity of the STWP and its central axial wave number. Here we observe the entire span of spectral reorganization for STWPs in the paraxial regime in normally dispersive ZnSe at a wavelength $\sim1$~$μ$m with STWPs of on-axis pulse width of $\sim\!200$~fs. By tuning the group velocity and central axial wave number of the STWP, we observe transitions in the structure of the spatio-temporal spectrum and verify the associated change in its intensity profile. These results lay the foundation for initiating novel phase-matched nonlinear processes using STWPs in dispersive media.

physics.optics↗

arXiv2022

Excitation of Surface Plasmon Polaritons by Diffraction-Free and Vector Beams

Surface plasmon polaritons (SPPs) are traditionally excited by plane waves within the Rayleigh range of a focused transverse magnetic (TM) Gaussian beam. Here, we investigate and confirm the coupling between SPPs and two-dimensional Gaussian and Bessel-Gauss wave packets, as well as one-dimensional light sheets and space-time wave packets. We encode the incoming wavefronts with spatially varying states of polarization then couple the respective TM components of radial and azimuthal vector beam profiles to confirm SPP polarization-correlation and spatial-mode selectivity. Our results do not require material optimization or multi-dimensional confinement via periodically corrugated metal surfaces to achieve coupling at greater extents. Hereby, outlining a pivotal, yet commonly overlooked, path towards the development of long-range biosensors and all-optical integrated plasmonic circuits.

physics.optics↗

arXiv2022

Propagation-invariant space-time supermodes in a multimode waveguide

When an optical pulse is spatially localized in a highly multimoded waveguide, its energy is typically distributed among a multiplicity of modes, thus giving rise to a speckled transverse spatial profile that undergoes erratic changes with propagation. It has been suggested theoretically that pulsed multimode fields in which each wavelength is locked to an individual mode at a prescribed axial wave number will propagate invariantly along the waveguide at a tunable group velocity. In this conception, an initially localized field remains localized along the waveguide. Here, we provide proof-of-principle experimental confirmation for the existence of this new class of pulsed guided fields, which we denote space-time supermodes, and verify their propagation invariance in a planar waveguide. By superposing up to 21 modes, each assigned to a prescribed wavelength, we construct space-time supermodes in a 170-micron-thick planar glass waveguide with group indices extending from 1 to 2. The initial transverse width of the field is 6 microns, and the waveguide length is 9.1 mm, which is 257x the associated Rayleigh range. A variety of axially invariant transverse spatial profiles are produced by judicious selection of the modes contributing to the ST supermode, including single-peak and multi-peak fields, dark fields (containing a spatial dip), and even flat uniform intensity profiles.

physics.optics↗

arXiv2022

Observation of ultrabroadband striped space-time surface plasmon polaritons

Because surface plasmon polaritons (SPPs) are surface waves characterized by one free transverse dimension, the only monochromatic diffraction-free spatial profiles for SPPs are cosine and Airy waves. Pulsed SPP wave packets have been recently formulated that are propagation-invariant and localized in the in-plane dimensions by virtue of a tight spectral association between their spatial and temporal frequencies, which have thus been dubbed `space-time' (ST) SPPs. Because of the spatio-temporal spectral structure unique to ST-SPPs, the optimal launching strategy of such novel plasmonic field configurations remains an open question. We present here a critical step towards realizing ST-SPPs by reporting observations of ultrabroadband striped ST-SPPs. These are SPPs in which each wavelength travels at a prescribed angle with respect to the propagation axis to produce a periodic (striped) transverse spatial profile that is diffraction-free. We start with a free-space ST wave packet that is coupled to a ST-SPP at a gold-dielectric interface, and unambiguously identify the ST-SPP via an axial beating detected in two-photon fluorescence produced by the superposition of incident ST wave packet and the excited surface-bound ST-SPP. These results highlight a viable approach for efficient and reliable coupling to ST-SPPs, and thus represent the first crucial step towards realization of the full potential of ST-SPPs for plasmonic sensing and imaging.

physics.optics↗

arXiv2022

Canceling and inverting normal and anomalous group-velocity dispersion using space-time wave packets

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. We synthesize here structured femtosecond pulsed beams known as `space-time' wave packets 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, we also verify GVD-inversion: reversing the GVD sign experienced by the field with respect to that dictated by the chromatic dispersion of the medium itself.

physics.optics↗

arXiv2022

Space-time wave packets

"Space-time" (ST) wave packets constitute a broad class of pulsed optical fields that are rigidly transported in linear media without diffraction or dispersion, and are therefore propagation-invariant in absence of optical nonlinearities or waveguiding structures. Such wave packets exhibit unique characteristics, such as controllable group velocities in free space and exotic refractive phenomena. At the root of these behaviors is a fundamental feature underpinning ST wave packets: their spectra are not separable with respect to the spatial and temporal degrees of freedom. Indeed, the spatio-temporal structure is endowed with non-differentiable angular dispersion, in which each spatial frequency is associated with a single prescribed wavelength. Furthermore, deviation from this particular spatio-temporal structure yields novel behaviors that depart from propagation invariance in a precise manner, such as acceleration with an arbitrary axial distribution of the group velocity, tunable dispersion profiles, and Talbot effects in space-time. Although the basic concept of ST wave packets has been known since the 1980's, only very recently has rapid experimental development emerged. These advances are made possible by innovations in spatio-temporal Fourier synthesis, thereby opening a new frontier for structured light at the intersection of beam optics and ultrafast optics.

physics.optics↗

arXiv2021

Refraction of space-time wave packets in a dispersive medium

Space-time (ST) wave packets are a class of pulsed optical beams whose spatio-temporal spectral structure results in propagation invariance, tunable group velocity, and fascinating refractive phenomena. Here, we investigate the refraction of ST wave packets at a planar interface between two dispersive, homogeneous, isotropic media. We formulate a new 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 a tool in nonlinear optics for bridging large group-velocity mismatches in highly dispersive scenarios.

physics.optics↗

arXiv2021

Severing the link between modal order and group index using hybrid guided space-time modes

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 are usually slower than their lower-order counterparts. We show here that such trends can be fundamentally altered -- altogether severing the link between modal order and group index hybrid and eliminating dispersion -- by exploiting hybrid guided space-time modes in a planar multimode waveguide. Such modes are confined in one-dimension by the waveguide and in the other by the spatio-temporal 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 be each tuned away from the group index of the conventional mode of same order, 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-$μ$m-thick silica film deposited on a MgF$_2$ substrate.

physics.optics↗

arXiv2021

A universal angular-dispersion synthesizer

We uncover a surprising gap in optics with regards to angular dispersion (AD) that has persisted for decades. A systematic examination of pulsed optical-field configurations classified according to their three lowest dispersion orders resulting from AD (the axial phase velocity, group velocity, and group-velocity dispersion) reveals that the majority of possible classes of fields have eluded optics thus far. This gap is due in part to the limited technical reach of the standard components that provide AD such as gratings and prisms, but due in part also to misconceptions regarding the set of physically admissible field configurations that can be accessed via AD. For example, it has long been thought that AD cannot yield normal group-velocity dispersion in free space. To rectify this state of affairs, we introduce a 'universal AD synthesizer': a pulsed-beam shaper that produces a wavelength-dependent propagation angle with arbitrary spectral profile, thereby enabling access to all physically admissible field configurations realizable via AD. This universal AD synthesizer is a versatile tool for preparing pulsed optical fields for dispersion compensation, optical signal processing, and nonlinear optics.

physics.optics↗

arXiv2021

The consequences of non-differentiable angular dispersion in optics: Tilted pulse fronts versus space-time wave packets

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 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 phase and group velocities are always equal in conventional scenarios. 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. These and several other consequences of non-differentiable AD are verified experimentally using a pulsed-beam shaper capable of introducing AD with arbitrary spectral profile. Rather than being an exotic phenomenon, non-differentiable AD is an accessible, robust, and versatile resource for sculpting pulsed optical fields.

physics.optics↗

arXiv2021

Arbitrarily accelerating space-time wave packets

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.

physics.optics↗

arXiv2021

Realizing normal group-velocity dispersion in free space via angular dispersion

It has long been thought that normal group-velocity dispersion (GVD) cannot be produced in free space via angular dispersion. Indeed, conventional diffractive or dispersive components such as gratings or prisms produce only anomalous GVD. We identify the conditions that must be fulfilled by the angular dispersion introduced into a plane-wave pulse to yield normal GVD. We then utilize a pulsed-beam shaper capable of introducing arbitrary angular-dispersion profiles to symmetrically produce normal and anomalous GVD in free space, which are realized here on the same footing for the first time.

physics.optics↗