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 pulseduration 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.

Efficient idler broadening via oppositely dual-chirped difference frequency generation

Dual-chirped difference frequency generation(DFG)is an advantageous technique for generating the broadband midinfrared(IR)idler wave,which is inaccessible by a population-inversion-based laser system.In principle,the generated idler wave may even suffer a spectrum broadening compared with the driving pulsed lasers if the pump and signal waves are oppositely chirped.However,broadband phase-matching is always the determining factor for the resulting efficiency and the bandwidth of the generated idler wave.In this study,specific to an oppositely dual-chirped DFG scheme,we derive the precondition to realize broadband frequency conversion,wherein a negative(1/υp-1/υi)/(1/υs-1/υi),in terms of the correlation coefficient of the group velocity(σ),is necessary.However,most birefringence bulk crystals can only provide the required material dispersions in limited spectral regions.We show that the periodically poled lithium niobate crystal that satisfies an inactive Type-II(eo-o)quasi-phase-matching condition has a stable negativeσand exerts the expected broadband gain characteristic across an ultra-broad idler spectral region(1.7-4.0µm).Finally,we propose and numerically verify a promising DFG configuration to construct a tunable mid-IR spectrum broader based on the broadband phase-matched oppositely dual-chirped DFG scheme.