Simultaneous spatial and temporal focusing of vortex and vector beams (Conference Presentation)

Abstract

In simultaneous spatial and temporal focusing (SSTF) a wide bandwidth pulse with transverse spatial chirp is focused, resulting in a pulse that is temporally compressed only near the focal plane. The pulse also has a pulse front tilt angle that depends on the amount of initial transverse chirp. In this work, we explore computationally and experimentally the properties of SSTF vortex and vector beams. To analyze the beam propagation, we build on the concept that a spatially chirped beam is a superposition of Gaussian beams with a position or angle that depends on frequency. We extend this to superpositions of Hermite-Gauss high-order modes to describe the singular beams. At focus, the beams of the ultrashort pulses are tilted versions of the familiar doughnut beams. Away from focus, however, where the spectral components do not fully overlap, we find that vortex and vector beams result in strikingly different mapping of the singularity mapping in the spatio-temporal domain. The use of higher-order modes increases the focal spot size without reducing the already short SSTF depth of focus. Experimentally, we use spiral phase plates to produce vortex beams and a linear to radial polarization converter for the vector beams. The vector beam is not distorted by the polarization-insensitive transmission gratings. The spatial chirp compressor is improved over our previous work to vary the chirp positive and negative. The sensitivity of the singular beam focus to grating misalignment can actually be used to optimize the compressor alignment.