Second-harmonic generation in periodic fork-shaped χ^((2)) gratings at oblique incidence

Three-dimensional(3D) nonlinear photonic crystals have received intensive interest as an ideal platform to study nonlinear wave interactions and explore their applications. Periodic fork-shaped gratings are extremely important in this context because they are capable of generating second-harmonic vortex beams from a fundamental Gaussian wave, which has versatile applications in optical trapping and materials engineering. However, previous studies mainly focused on the normal incidence of the fundamental Gaussian beam, resulting in symmetric emissions of the second-harmonic vortices. Here we present an experimental study on second-harmonic vortex generation in periodic fork-shaped gratings at oblique incidence, in comparison with the case of normal incidence. More quasi-phase-matching resonant wavelengths have been observed at oblique incidence, and the second-harmonic emissions become asymmetric against the incident beam.These results agree well with theoretic explanations. The oblique incidence of the fundamental wave is also used for the generation of second-harmonic Bessel beams with uniform azimuthal intensity distributions. Our study is important for a deeper understanding of nonlinear interactions in a 3D periodic medium. It also paves the way toward achieving highquality structured beams at new frequencies, which is important for manipulation of the orbital angular momentum of light.

Nonlinear photonic quasi-periodic spiral

The design of nonlinear photonic Vogel's spiral based on quasi-crystal theory was demonstrated.Two main parameters of Vogel's spiral were arranged to obtain multi-reciprocal circles.Typical structure was fabricated by the near-infrared femtosecond laser poling technique,forming a nonlinear photonic structure,and multiple ring-like nonlinear Raman±Nath second-harmonic generation processes were realized and analyzed in detail.The structure for the cascaded thirdharmonic generation process was predicted.The results could help deepen the understanding of Vogel's spiral and quasi-crystal and pave the way for the combination of quasi-crystal theory with more aperiodic structures.

High-peak-power passively Q-switched 1.3 μm Nd:YAG/V^(3+):YAG laser pumped by a pulsed laser diode

We demonstrate a high-pulse-energy, short-pulse-width passively Q-switched（PQS） Nd：YAG∕V3t：YAG laser at 1.3 μm, which is end-pumped by a pulsed laser diode. During the PQS regime, a maximum total output pulse energy of 3.3 m J is obtained under an absorbed pump pulse energy of 21.9 m J. Up to 400 μJ single-pulse energy is realized with the shortest pulse width of 6.16 ns and a pulse repetition frequency of 34.1 k Hz,corresponding to a peak power of 64.9 k W. The high-energy laser pulse is operated in the dual wavelengths of 1319 and 1338 nm, which is a potential laser source for THz generation.