Efficient broadband second harmonic generations in periodic 90°-domain structures of $KNbO_3$

Abstract

This dissertation reports a new electrodes scheme for the fabrication of high quality $90\deg$-domain inverted periodic structures of $KNbO_3$ and an efficient broadband frequency doubling of ultra-short laser pulses in the telecommunication L-band at room temperature by satisfying both quasi-phase-matching and group-velocity-matching conditions. We devise multiple-line-shaped electrodes to fabricated a periodically poled $90\deg$-domain structures in $KNbO_3$ (PP90KN) by applying an electric field, which gives better uniformity of periodic domains than the conventional one and new electric poling scheme for preventing crystal loss. Theoretical investigation for various types of second-harmonic generation under simultaneous QPM and GV matching conditions in PP90KN were accomplished and an efficient broadband frequency doubling of ultra-short laser pulses in the telecommunication L-band at room temperature was demonstrated using a fabricated 8.3 mm long PP90KN of $20.25 \microm$ period. Second-harmonic pulses showed no significant temporal and spectral distortion and had a pulse duration of about 144 fs, which agreed well with the theoretical value of 142 fs. The conversion efficiency of the second-harmonic generation was 28.7% at the fundamental pulse energy of 0.92 nJ, which is about 60% of the theoretical value.