Demonstration of space-time wave packets localized in all dimensions*

Space-time (ST) wave packets are pulsed beams endowed with tight spatio-temporal spectral correlations, resulting in propagation invariance at arbitrary group velocities. We previously introduced an experimental methodology to synthesize ST light sheets based on a 2D pulse shaper. Here, we introduce an experimental method to synthesize ST wave packets localized in all dimensions, including both transverse dimensions and time. This approach for synthesizing 3D ST relies on volume chirped-Bragg-gratings to spatially resolve the spectrum, followed by a Polar-to-Cartesian coordinate transformation to introduce a one-to-one correlation between wavelength and the radial transverse spatial frequency. Modulating this polar-to-Cartesian transformation enables dynamical tuning of the group velocity of the 3D ST wave packets over a wide range of values, whether subluminal or superluminal. We characterize the synthesized 3D ST wave packets in the Fourier domain to confirm the spatio-temporal structure and in the physical domain to verify the propagation invariance, and tune the group velocity of wave packets from 0.8c to 1.8c (c is the speed of light in vacuum). Finally, we assign a helical phase into the ST wave packets and observe diffraction-free ST-OAM wave packets traveling at 1.2c in free space.