arXiv subjects

Ayman F. Abouraddy

Publications and source records attributed to Ayman F. Abouraddy.

At least 37 records · Page 2Linked to original sources

arXiv2025

Resonance-free Fabry-Pérot cavity via unrestricted orbital-angular-momentum ladder-up

Introducing elements into an optical cavity that modify the transverse spatial field structure can also impact the cavity spectral response. In particular, an intra-cavity spatial mode-converter is expected to induce modal runaway: unrestricted ladder-up in the modal order, concomitantly thwarting coherent field interference, thereby altogether suppressing the resonant response - a phenomenon that has yet to be observed in an optical cavity. Here we show that a single intra-cavity holographic phase mask placed in a compact free-standing planar Fabry-Pérot cavity renders the cavity spectral response resonance-free. By acting as a mode-converter on a basis of Laguerre-Gaussian (LG) modes, an incident broadband fundamental Gaussian mode exits the cavity in the form of a superposition of a large number of collinearly propagating broadband LG modes of fixed parity whose spectra coincide with that of the input. Crucially, the resonance-free spectral response is maintained while changing the cavity length by $\sim350\%$, raising the prospect of stable resonant optical sensors whose performance is impervious to length perturbations.

physics.optics↗

arXiv2025

Universal structure of propagation-invariant optical pulses

Space-time structuring of light - where spatial and temporal degrees of freedom are deliberately coupled and controlled - is an emerging area of optics that enables novel configurations of electromagnetic fields. Of particular importance for applications are optical pulses whose peak intensity travels at an arbitrary, tunable velocity while maintaining its spatiotemporal profile. Space-time wave packets and the ideal flying focus are two prominent realizations of these pulses. Here, we show that these realizations share an identical spatiotemporal field structure, and that this structure represents a universal solution for constant-velocity, propagation-invariant pulses.

physics.optics↗

arXiv2025

Ultrafast space-time optical merons in momentum-energy space

Skyrmions, topologically non-trivial localized spin structures, are fertile ground for exploring emergent phenomena in condensed matter physics and next-generation magnetic-memory technologies. Although magnetics and optics readily lend themselves to two-dimensional realizations of spin texture, only recently have breakthroughs brought forth three-dimensional (3D) magnetic skyrmions, whereas their optical counterparts have eluded observation to date because their realization requires precise control over the spatiotemporal spectrum. Here, we demonstrate the first 3D-localized optical skyrmionic structures with a non-trivial topological spin profile by imprinting meron spin texture on open and closed spectral surfaces in the momentum-energy space of an ultrafast optical wave packet. Precise control over the spatiotemporal spin texture of light - a key requisite for synthesizing 3D optical merons - is the product of synergy between novel methodologies in the modulation of light jointly in space and time, digital holography, and large-area birefringent metasurfaces. Our work advances the fields of spin optics and topological photonics and may inspire new developments in imaging, metrology, optical communications, and quantum technologies.

physics.optics↗

arXiv2025

Observation of space-time surface plasmon polaritons

Surface plasmon polaritons (SPPs) are surface-bound waves at metal-dielectric interfaces that exhibit strong out-of-plane field confinement, a key feature for applications is nano-scale sensing and imaging. However, this advantage is offset by diffractive spreading during in-plane propagation, leading to transverse spatial delocalization. Conventional strategies to combat diffraction through spatial structuring are not applicable for dimensionally restricted SPPs -- except for cosine plasmons that are not localized or Airy plasmons that propagate along a curved trajectory. Here, we report the first realization of space-time SPPs (ST-SPPs), ultrashort (16 fs) diffraction-free SPPs that propagate in a straight line, whose unique propagation characteristics stem from precise sculpting of their spatiotemporal spectra. By first synthesizing a spatiotemporally structured field in free space, we couple the field to an axially invariant ST-SPP at a metal-dielectric surface via an ultra-broadband nanoslit coupling mechanism, further enabling control over the ST-SPP group velocity and propagation characteristics. Time-resolved two-photon fluorescence interference microscopy enables reconstructing the surface-bound field in space and time, thereby verifying their predicted phase-tilted spatiotemporal wave-front and diffraction-free propagation. Our work opens new avenues for combining spatiotemporally structured light with the field-localization associated with nanophotonics, and may thus enable novel applications in surface-enhanced sensing and nonlinear optical interactions.

physics.optics↗

arXiv2024

Synthesis and characterization of space-time light sheets: A tutorial

Space-time wave packets (STWPs) are a new class of pulsed optical beams with many unique and intriguing attributes, including propagation invariance and tunable group velocity in linear optical media. STWPs are a form of spatiotemporally structured light, so their synthesis poses challenges that are not shared by conventional monochromatic structured light fields. We present here a detailed description of the synthesis of STWPs that are localized along one transverse dimension and uniform along the other; i.e., space-time light sheets. We also describe the main characterization schemes needed for benchmarking the unique properties of space-time light sheets.

physics.optics↗

arXiv2024

Abrupt X-to-O-wave structural field transition in presence of anomalous dispersion

All linear, propagation-invariant, paraxial pulsed beams are spatiotemporally X-shaped (conical waves) in absence of group-velocity dispersion (GVD), or in presence of normal GVD. It is known, however, that such conical waves become O-shaped in presence of anomalous GVD, resulting in a field profile that is circularly symmetric in space and time. To date, experiments generating conical waves in which the wavelength of a high-energy pump laser is tuned across the zero-dispersion wavelength of a nonlinear medium have not revealed the expected X-to-O-wave structural field transition. We report here unambiguous observation of a fixed-wavelength X-to-O-wave structural field transition occurring in linear dispersion-free wave packets in the anomalous GVD regime -- without needing to change the sign or magnitude of the GVD. Instead, by tuning the group velocity of a space-time wave packet (STWP) across a threshold value that we call the `escape velocity', we observe an abrupt transition in the STWP from an O-shaped to an X-shaped spatiotemporal profile. This transition is associated with an abrupt change in the associated spatiotemporal spectrum of the STWP: from closed elliptical spatiotemporal spectra below the escape velocity to open hyperbolic spectra above it. These results may furnish new opportunities for engineering the phase-matching conditions in nonlinear and quantum optics.

physics.optics↗

arXiv2024

Iso-entropy partially coherent optical fields that cannot be inter-converted unitarily

For partially coherent optical fields in which a single binary degree of freedom (DoF) is relevant, such as polarization, entropy uniquely identifies the class of optical fields that can be converted into each other via unitary transformations. However, when multiple DoFs are taken into consideration, entropy no longer serves this purpose. We investigate the structure of the family of iso-entropy partially coherent optical fields defined by two binary DoFs (polarization and two spatial modes) and described by a 4x4 coherence matrix G. We find that the rank of G (the number of its non-zero eigenvalues) plays a critical role in this context: whereby any pair of iso-entropy rank-2 fields can be converted into each other unitarily, this is not necessarily the case for a pair of rank-3 or rank-4 fields. Furthermore, unitary transformations between iso-entropy fields of different ranks are strictly forbidden. Instead, such conversions require entropy-maintaining non-unitary transformations that potentially combine filtering projections and randomizing operations. We experimentally synthesize partially coherent iso-entropy optical fields of all ranks, and tomographically reconstruct their coherence matrices. Moreover, we steer the coherence matrix over iso-entropy trajectories that maintain a fixed rank (intra-rank conversion) or that involve changes in the rank (inter-rank conversion). These findings offer a new perspective for the potential utility of partially coherent light in optical communications and sensing.

physics.optics↗

arXiv2023

Locked entropy in partially coherent fields

We introduce a taxonomy for partially coherent optical fields spanning multiple degrees of freedom (DoFs) based on the rank of the associated coherence matrix (the number of non-zero eigenvalues). When DoFs comprise two spatial modes and polarization, a fourfold classification emerges, with rank-1 fields corresponding to fully coherent fields. We demonstrate theoretically and confirm experimentally that these classes have heretofore unrecognized different properties. Specifically, whereas rank-2 fields can always be rendered separable with respect to its DoFs via a unitary transformation, rank-3 fields are always non-separable. Consequently, the entropy for a rank-2 field can always be concentrated into a single DoF (thus ridding the other DoF of statistical fluctuations), whereas some entropy is always 'locked' in one DoF of a rank-3 field.

physics.optics↗

arXiv2023

Omni-resonant imaging across the visible

Resonant field enhancement in optical cavities is provided over only narrow linewidths and for specific spatial modes. Consequently, spectrally restrictive planar Fabry-P{é}rot cavities have not contributed to date to white-light imaging, which necessitates a highly multimoded broadband field to satisfy the resonance condition. Here we show that introducing judicious angular-dispersion circumvents the fundamental trade-off between cavity linewidth and finesse in a Fabry-P{é}rot cavity by exciting a 130-nm-bandwidth achromatic resonance across the visible spectrum, which far exceeds the finesse-limited linewidth (0.5~nm), and even exceeds the free spectral range (45~nm). This omni-resonant configuration enables broadband color-imaging over a 100-nm-bandwidth in the visible with minimal spherical and chromatic aberrations. We demonstrate omni-resonant imaging using coherent and incoherent light, and spatially extended and localized fields comprising stationary and moving objects. This work paves the way to harnessing broadband resonant enhancements for spatially structured fields, as needed for example in solar windows.

physics.optics↗

arXiv2023

Experimental realization of Lorentz boosts of space-time wave packets

It is now well-understood that a Lorentz boost of a spatially coherent monochromatic optical beam yields a so-called space-time wave packet (STWP): a propagation-invariant pulsed beam whose group velocity is determined by the relative velocity between the source and observer. Moreover, the Lorentz boost of an STWP is another STWP, whose group velocities are related by the relativistic law for addition of velocities typically associated with massive particles. We present an experimental procedure for testing this prediction in both the subluminal and superluminal regimes that makes use of spatio-temporal Fourier synthesis via a spatial light modulator. Our approach enables realizing the change in temporal bandwidth, the invariance of the spatial bandwidth, the concomitant change in the spatio-temporal wave-packet envelope, and the change in group velocity that all accompany a Lorentz boost of a monochromatic optical beam. The only consequence of the Lorentz boost not captured by this methodology is the Doppler shift in the optical carrier. This work may provide an avenue for further table-top demonstration of relativistic transformations of optical fields.

physics.optics↗

arXiv2023

Compact dual-band spectral analysis via multiplexed rotated chirped volume Bragg gratings

Chirped Bragg volume gratings (CBGs) offer a useful alternative for spectral analysis, but increasing the bandwidth necessitates increasing the device area. In contrast, recently developed rotated CBGs (r-CBGs), in which the Bragg structure is rotated by $45^{\circ}$ with respect to the device facets, require increasing only the device length to extend the bandwidth, in addition to the convenience of resolving the spectrum at normal incidence. Here, we multiplex r-CBGs in the same device to enable spectral analysis in two independent spectral windows without increasing the system volume. This new device, which we term an X-CBG, allows for compact multi-band spectroscopy in contiguous or separated spectral windows in the visible and near-infrared for applications in nonlinear microscopy and materials identification and sensing.

physics.optics↗

arXiv2023

Observation of optical de Broglie-Mackinnon wave packets

de Broglie wave packets accompanying moving particles are dispersive and lack an intrinsic length scale dictated solely by the particle mass and velocity. Mackinnon proposed almost 45~years ago a localized non-dispersive wave packet constructed out of dispersive de Broglie phase waves via a Copernican inversion of the roles of particle and observer, whereupon an intrinsic length scale emerges by accounting for every possible observer -- rather than by introducing an \textit{ad hoc} uncertainty in the particle velocity. The de Broglie-Mackinnon (dBM) wave packet has nevertheless remained to date a theoretical entity. Here, we report the observation of optical dBM wave packets using paraxial space-time-coupled pulsed laser fields in presence of anomalous group-velocity dispersion. Crucially, the bandwidth of dBM wave packets has an upper limit that is compatible with the wave-packet group velocity and equivalent mass. In contrast to previously observed linear propagation-invariant wave packets whose spatio-temporal profiles at any axial plane are X-shaped, those for dBM wave packets are uniquely O-shaped (circularly symmetric with respect to space and time). By sculpting their spatio-temporal spectral structure, we produce dispersion-free dBM wave packets in the dispersive medium, observe their circularly symmetric spatio-temporal profiles, and tune the field parameters corresponding to particle mass and velocity that uniquely determine the wave-packet length scale.

physics.optics↗

arXiv2023

Transverse spin angular momentum of space-time surface plasmon polariton wave packet

In addition to longitudinal spin angular momentum (SAM) along the axis of propagation of light, spatially structured electromagnetic fields such as evanescent waves and focused beams have recently been found to possess transverse SAM in the direction perpendicular to the axis of propagation. In particular, the SAM of SPPs with spatial structure has been extensively studied in the last decade after it became clear that evanescent fields with spatially structured energy flow generate threedimensional spin texture. Here we present numerical calculations of the space-time surface plasmon polariton (ST-SPP) wave packet, a plasmonic bullet that propagates at an arbitrary group velocity while maintaining its spatial distribution. ST-SPP wave packets with complex spatial structure and energy flow density distribution determined by the group velocity are found to propagate with accompanying three-dimensional spin texture and finite topological charge density. Furthermore, the spatial distribution of the spin texture and topological charge density determined by the spatial structure of the SPP is controllable, and the deformation associated with propagation is negligible. ST-SPP wave packets, which can stably transport customizable three-dimensional spin textures and topological charge densities, can be excellent subjects of observation in studies of spinphotonics and optical topological materials.

physics.optics↗

arXiv2022

Theory of space-time supermodes in planar multimode waveguides

When an optical pulse is focused into a multimode waveguide or fiber, the energy is divided among the available guided modes. Consequently, the initially localized intensity spreads transversely, the spatial profile undergoes rapid variations with axial propagation, and the pulse disperses temporally. Space-time (ST) supermodes are pulsed guided field configurations that propagate invariantly in multimode waveguides by assigning each mode to a prescribed wavelength. ST supermodes can be thus viewed as spectrally discrete, guided-wave counterpart of the recently demonstrated propagation-invariant ST wave packets in free space. The group velocity of an ST supermode is tunable independently -- in principle -- of the waveguide structure, group-velocity dispersion is eliminated or dramatically curtailed, and the time-averaged intensity profile is axially invariant along the waveguide in absence of mode-coupling. We establish here a theoretical framework for studying ST supermodes in planar waveguides. Modal engineering allows sculpting this axially invariant transverse intensity profile from an on-axis peak or dip (dark beam), to a multi-peak or flat distribution. Moreover, ST supermodes can be synthesized using spectrally incoherent light, thus paving the way to potential applications in optical beam delivery for lighting applications.

physics.optics↗

arXiv2022

Ultra-compact synthesis of space-time wave packets

Space-time wave packets (STWPs) are pulsed fields in which a strictly prescribed association between the spatial and temporal frequencies yields surprising and useful behavior. However, STWPs to date have been synthesized using bulky free-space optical systems that require precise alignment. We describe a compact system that makes use of a novel optical component: a chirped volume Bragg grating that is rotated by 45 degrees with respect to the plane-parallel device facets. By virtue of this grating's unique structure, cascaded gratings resolve and recombine the spectrum without free-space propagation or collimation. We produce STWPs by placing a phase plate that spatially modulates the resolved spectrum between such cascaded gratings, with a device volume of 25x25x8 mm3, which is orders of magnitude smaller than previous arrangements.

physics.optics↗

arXiv2022

Rotated chirped volume Bragg gratings for compact spectral analysis

We introduce a new optical component - a rotated chirped volume Bragg grating (r-CBG) - that spatially resolves the spectrum of a normally incident light beam in a compact footprint and without the need for subsequent free-space propagation or collimation. Unlike conventional chirped Bragg volume gratings in which both the length and width of the device must be increased to increase the bandwidth, by rotating the Bragg structure we sever the link between the length and width of an r-CBG, leading to a significantly reduced device footprint for the same bandwidth. We fabricate and characterize such a device in multiple spectral windows, we study its spectral resolution, and confirm that a pair of cascaded r-CBGs can resolve and then recombine the spectrum. Such a device can lead to ultra-compact spectrometers and pulse modulators.

physics.optics↗

arXiv2022

Exciting space-time surface plasmon polaritons by irradiating a nanoslit structure

Space-time (ST) wave packets are propagation-invariant pulsed optical beams that travel freely in dielectrics at a tunable group velocity without diffraction or dispersion. Because ST wave packets maintain these characteristics even when only one transverse dimension is considered, they can realize surface-bound waves (e.g., surface plasmon polaritons at a metal-dielectric interface, which we call ST-SPPs) that have the same unique characteristics of their freely propagating counterparts. However, because the spatio-temporal spectral structure of ST-SPPs is key to their propagation invariance on the metal surface, their excitation methodology must be considered carefully. We show here using finite-difference time-domain (FDTD) simulations that an appropriately synthesized ST wave packet in free space can be couples to a ST-SPP via a single nano-scale slit inscribed in the metal surface. Our calculations confirm that this excitation methodology yields surface-bound ST-SPPs that are locarized in all dimensions (and can thus be considered as plasmonic 'bullets'), which travel rigidly at the metal-dielectric interface without diffraction or dispersion at a tunable group velocity.

physics.optics↗

arXiv2022

Roadmap on spatiotemporal light fields

Spatiotemporal sculpturing of light pulse with ultimately sophisticated structures represents the holy grail of the human everlasting pursue of ultrafast information transmission and processing as well as ultra-intense energy concentration and extraction. It also holds the key to unlock new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are always treated as spatiotemporally separable wave packet as solution of the Maxwell's equations. In the past decade, however, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This roadmap intends to highlight the recent advances in the creation and control of increasingly complex spatiotemporally sculptured pulses, from spatiotemporally separable to complex nonseparable states, with diverse geometric and topological structures, presenting a bird's eye viewpoint on the zoology of spatiotemporal light fields and the outlook of future trends and open challenges.

physics.optics↗