Generation of ultrafast radially polarized pulses through chirp-assisted femtosecond optical parametric amplification

Radially polarized beams characterized by an axially symmetric polarization distribution can be sharply focused to produce strong longitudinal fields in the vicinity. Future applications of these beams will be facilitated by the availability of higher powers and shorter durations. Currently, the ultrafast radially polarized pulse is typically generated via wavefront reconstruction from conventional linearly polarized states. Achievable pulse duration and intensity limits are strictly dependent on extra-cavity optics. Herein, a chirp-assisted near-degenerate type-II parametric process is presented as a pulse-energy-scalable method of accessing ultrafast radially polarized pulses. In a proof-of-principle experiment, the broadband gain balance between the orthogonally polarized signal components was realized via controlling the chirp of the pump pulse. Through an analogous pulse-duration transfer effect, the radially polarized signal inherited the temporal and spectral characteristics of the pump pulse and maintained the radial polarization state of each frequency component of the signal. With a shorter pump pulse, the generation of few-cycle radially polarized pulses should be achievable, which may facilitate a wide range of ultrafast applications such as vacuum electron acceleration and high-harmonic generation.