Isochronous space-time wave packets

TL;DR

This paper demonstrates the existence of isochronous space-time wave packets that maintain a fixed group delay across various incident angles, enabling angle-independent propagation in optical slabs.

Contribution

It introduces and experimentally confirms a new class of wave packets with angle-invariant group delay, expanding control over light propagation in optical materials.

Findings

Isochronous space-time wave packets exist in optical slabs.

These wave packets maintain fixed group delay over wide incident angles.

Observed in materials with indices from 1.38 to 2.5.

Abstract

The group delay incurred by an optical wave packet depends on its path length. Therefore, when a wave packet is obliquely incident on a planar homogeneous slab, the group delay upon traversing it inevitably increases with the angle of incidence. Here we confirm the existence of isochronous `space-time' (ST) wave packets: pulsed beams whose spatio-temporal structure enables them to traverse the layer with a fixed group delay over a wide span of incident angles. This unique behavior stems from the dependence of the group velocity of a refracted ST wave packet on its angle of incidence. Isochronous ST wave packets are observed in slabs of optical materials with indices in the range from 1.38 to 2.5 for angles up to $50^{\circ}$ away from normal incidence.