Xinyuan Fang

Multiple copies of orbital angular momentum states through second-harmonic generation in a two-dimensional periodically poled LiTaO3 crystal

We experimentally demonstrate multiple copies of optical orbital angular momentum (OAM) states through quasi-phase-matched (QPM) second-harmonic (SH) generation in a 2D periodically poled LiTaO3 (PPLT) crystal. Since the QPM condition is satisfied by involving different reciprocal vectors in the 2D PPLT crystal, collinear and noncollinear SH beams carrying OAMs of l2 are simultaneously generated by the input fundamental beam with an OAM of l1. The OAM conservation law (i.e., l2 = 2l1) holds well in the experiment, which can tolerate certain phase-mismatch between the interacting waves. Our results provide an efficient way to obtain multiple copies of the wavelength-converted OAM states, which can be used to enhance the capacity in optical communications.

Coupled orbital angular momentum conversions in a quasi-periodically poled LiTaO₃ crystal.

We experimentally demonstrate the orbital angular momentum (OAM) conversion by the coupled nonlinear optical processes in a quasi-periodically poled LiTaO3 crystal. In such a crystal, third-harmonic generation (THG) is realized by the coupled second-harmonic generation (SHG) and sum-frequency generation (SFG) processes, i.e., SHG is dependent on SFG and vice versa. The OAMs of the interacting waves are proved to be conserved in such coupled nonlinear optical processes. As we increase the input OAM in the experiment, the conversion efficiency decreases because of the reduced fundamental power density. Our results provide better understanding for the OAM conversions, which can be used to efficiently produce an optical OAM state at a short wavelength.

Conversion of the optical orbital angular momentum in a plasmon-assisted second-harmonic generation

We experimentally demonstrate the plasmon-assisted second-harmonic generation of an optical orbital angular momentum (OAM) beam. Because of the shape resonance, the plasmons in a periodic array of rectangular metal holes greatly enhance the nonlinear optical conversion of an OAM state. The OAM conservation (i.e., 2l1 = l2 with l1 and l2 being the OAM numbers of the fundamental and second-harmonic waves, respectively) holds well under our experimental configuration. Our results provide a potential way to realize nonlinear optical manipulation of an OAM mode in a nano-photonic device.

Multiple generations of high-order orbital angular momentum modes through cascaded third-harmonic generation in a 2D nonlinear photonic crystal

We experimentally demonstrate multiple generations of high-order orbital angular momentum (OAM) modes through third-harmonic generation in a 2D nonlinear photonic crystal. Such third-harmonic generation process is achieved by cascading second-harmonic generation and sum-frequency generation using the non-collinear quasi-phase-matching technique. This technique allows multiple OAM modes with different colors to be simultaneously generated. Moreover, the OAM conservation law guarantees that the topological charge is tripled in the cascaded third-harmonic generation process. Our method is effective for obtaining multiple high-order OAM modes for optical imaging, manipulation, and communications.

Conical third-harmonic generation in a hexagonally poled LiTaO3 crystal

We experimentally observed conical third harmonic generation (THG) in a two-dimensional periodically poled LiTaO3 nonlinear photonic crystal. The conical THG was attributed to cascaded quasi-phase matching (QPM) processes assisted by scattering fundamental waves in the nonlinear photonic crystal. The characteristics of the generated conical beam, such as the emit angle and beam size, varied with the fundamental wavelength because it changed the scattering-assisted QPM configuration. Our experimental results demonstrated that scattering light can be effectively involved in a cascade nonlinear optical process through QPM, which provides a possible way to produce short-wavelength conical beams to meet the demands of practical applications.

Examining second-harmonic generation of high-order Laguerre–Gaussian modes through a single cylindrical lens

We experimentally investigate the second-harmonic generation of a high-order Laguerre-Gaussian (LG) mode under the quasi-phase-matching (QPM) configuration. First, we introduce a simple method to observe the azimuthal (l) and radial (p) indices of the high-order LG modes. Based on the astigmatic transformation technique, l and p are revealed in the number of dark stripes of the converted pattern in the focal plane. Then, using this efficient method of measurement, we demonstrate in experiments a second-harmonic LG mode with its radial and azimuthal indices being twice those of the inputted fundamental wave through QPM in a periodically poled KTP crystal. Our results provide a feasible way to obtain simultaneously the LG modes with larger radial and azimuthal indices.

Tunable third harmonic generation of vortex beams in an optical superlattice

We report generation of tunable vortex beams in the blue spectral range, with a 3.3 nm spectral tuning range, by frequency tripling of the near-infrared (IR) wave at around 1.34 um in a LiTaO3 optical superlattice. The nonlinear crystal used in this work has a chirped dual-periodical structure which can provide two expanded reciprocal vectors for tunable performance of the cascaded third harmonic generation (THG). The maximum THG efficiency reaches about 1.4%.