Ultrabroadband second-harmonic generation via spatiotemporal-coupled phase matching

We propose a spatially chirped quasi-phase-matching[QPM]scheme that enables ultrabroadband second-harmonic-gen-eration[SHG]by using a fan-out QPM grating to frequency-convert a spatially chirped fundamental wave.A"zero-dispersion"4f system maps the spectral contents of ultrabroadband fundamental onto different spatial coordinates in the Fourier plane,where the fundamental is quasi-monochromatic locally in picosecond duration,fundamentally canceling high-order phase mismatch.A fan-out QPM grating characterized by a linear variation of the poling period along the trans-verse direction exactly supports the QPM of the spatially chirped beam.We theoretically demonstrate the SHG of an 810-nm,12.1-fs pulse into a 405-nm,10.2-fs pulse with a conversion efficiency of 77％.