Vortex beams carrying optical orbital angular momentum (OAM) hold potential applications in various research fields, such as optical trapping field, quantum computation, quantum imaging and increasing the capacity of optical communications. In recent years, nonlinear generation and manipulation of such vortex beams, which enable vortex beams to be obtained at a new waveband, have become another hot topic. Traditionally, the nonlinear vortex generation methods can be mainly divided into intracavity and outcavity types. In the intracavity type, such nonlinear vortex beam can be directly generated in a microchip laser, which has high efficiency but less flexibility. On the contrary, the flexibility can be improved in case of outcavity type, which has four different approaches to generate such nonlinear vortex beam, such as two-step method by an SLM and a nonlinear crystal, compact nonlinear photonic crystals (NPCs) with special designed quadratic nonlinear structure, nonlinear metasurfaces, and special designed nonlinear crystal using three-dimensional (3D) direct laser writing.

Fig. 1. (a-c) Confocal microscopic images and central cutout of fork grating on the LN facet at different etching depth, (a) 1.5 μm ; (b) 2.5 μm; (c) 4 μm. (d) The curve of etching depth of LN at different etching time.

Fig. 2. (a-f) Far-field SH vortex beam generation by using such nonlinear fork-grating; (g-l) The transformed patterns of the SH vortex beam by using a cylindrical lens.

Fig. 3. Comparison of the nonlinear conversion efficiency be-tween experimental (circle) and theoretical (solid curves) results: (a) Total SH output power dependence on FF input power; (b) First diffraction order of SH vortex beam power dependence on FF input power.

However, previous nonlinear vortex beams generation suffer from either low efficiency or low-level integration. Therefore, finding a way to overcome the above problems is highly desirable. In our work, we propose the technique of ultraviolet photolithography assisted ICP etching to realize a compact nonlinear fork grating for high efficiency nonlinear vortex beam generation. We fabricate nonlinear fork-gratings with topological charge of l=1,2,3 at different depth by controlling etching time. Then, we characterize the etching depth by microscopy as shown in Figure. 1. We record far-field nonlinear vortex beams generation and measure the topological charge of SH by using a cylindrical lens in Figure. 2. We can see that the number of dark strips in the transformed patterns correspond to the values of generated by the fabricated fork grating. The number of dark stripes marked by the red solid line. The sign of the topological charge is distinguished by the orientation of the dark stripes. As shown in Figs. 2(g)–2(l), when the dark stripe along top left corner, the topological charge is positive. On the contrary, the topological charge is negative when the dark stripe along top right corner. Besides, we measure the total and the first diffraction order power of SH beam with the changing of FFW intensity in Figure. 3. The nonlinear frequency conversion efficiency of such SH vortex beam is up to 189% W^-1cm^-2. Our method not only provides an efficient and compact method for nonlinear beam shaping, but also suit for large area fabricating nonlinear functional device.

This research is published in “Yangfeifei Yang, Hao Li, Haigang Liu, and Xianfeng Chen, Highly efficient nonlinear vortex beam generation by using compact nonlinear fork grating, Optics Letters, 48(24), 6376-73791 (2023)”.

Link: https://opg.optica.org/ol/abstract.cfm?URI=ol-48-24-6376