Speaker
Description
In recent decades, considerable attention has been paid to the study of structured light with transverse angular momentum. For example, transverse spin angular momentum arises in tightly focused circularly polarized Gaussian beams and evanescent fields. The concept of the formation of transverse orbital angular momentum (OAM) is gaining relevance, which consist in development of spatiotemporal optical vortices with vortex phase in spatiotemporal plane [1]. The question arises of the theoretical development of methods for generating such wave packets with transverse OAM, robust to dispersion and diffraction during propagation both in free space and in optically inhomogeneous media.
Spatiotemporal wave packets (STWP) is a special class of propagation invariant impulse fields [1-2], in which the spatial and temporal degrees of freedom are related through a specially formed spectral correlation. Unlike a conventional pulsed beam, where spatial and spectral coordinates can be considered as independent variables, in STWP each spectral frequency (ω) corresponds to a certain transverse spatial frequency ($k_x$ or $k_y$). This non-separability of spatial and spectral degrees of freedom that determines a number of unusual properties of STWP, in particular the possibility of controlling the group velocity and forming diffraction-free, propagation invariant modes [2].
A variety of STWP are spatiotemporal optical vortices (STOV), for which the fundamental spatial-spectral correlation is supplemented by a phase singularity. The phase vortex in the x-t coordinates is due to the modulation of the spectral phase in $k_x$-ω space. However, conventional Gaussian STOVs are not propagation invariant with respect to their vortex structure and the wave packet envelope [1]. Recent investigations show, that Bessel-Gaussian STOVs [3] are more robust, allowing the conservation of the wave packet structure during propagation in free space and an anomalously dispersive medium over a much longer distances. However, Bessel-Gaussian STOVs formed by modulating the spectral phase in the frequency domain are not inherently invariant, as they have separate temporal and spatial degrees of freedom.
The most successful way to modulate the propagation-invariant STWP is the spatio-temporal Fourier synthesis [2], which provides phase modulation in the spatial-spectral (x-ω) plane. Combination of the conditions for the propagation-invariant STWP formation and the vortex phase in the spatial-temporal coordinates by means of phase (and possibly amplitude) modulation in the x-ω domain (Fig. 1) allows to form propagation-invariant STOVs. It is expected to find conditions for the preserving of such wavepackets characteristics (amplitude envelope, phase vortex in x-t, topological charge, transverse OAM, etc.) during propagation both in free space and dispersive medium, with the elimination of spectral uncertainty [2]. The next expected step is to find ways to increase the robustness of such pulses when propagating in a turbid (multiple scattering) and/or turbulent medium.
[1] Tripathi N, Hancock SW, Milchberg HM. Transverse orbital angular momentum of spatiotemporal optical vortices: setting the record straight. Opt Express 2025;33:41766-41776. doi: 10.1364/OE.568833
[2] Yessenov M, Hall LA, Schepler KL, Abouraddy AF. Space-time wave packets. Adv Opt Photon. 2022;14(3):455-570. doi: 10.1364/AOP.450016
[3] Chen W, et al. Time diffraction-free transverse orbital angular momentum beams. Nat Commun. 2022;13:4021. doi: 10.1038/s41467-022-31623-7
| Type of presence | Presence online |
|---|---|
| Figure caption | Scheme of the spatiotemporal Fourier synthesis for propagation invariant optical wave packets formation [2] |