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

TL;DR

This paper reports the first direct observation of an abrupt X-to-O wave structural transition in linear wave packets within anomalous dispersion regimes, achieved by tuning the group velocity across an escape velocity, with implications for nonlinear and quantum optics.

Contribution

It demonstrates a novel, velocity-controlled X-to-O wave transition in linear wave packets without changing dispersion sign, revealing new spectral and structural dynamics.

Findings

Observed abrupt X-to-O wave transition in wave packets

Transition linked to a change from elliptical to hyperbolic spectra

Transition occurs at a specific 'escape velocity' threshold

Abstract

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.