Rui Ni

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.

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.

Topological charge transfer in frequency doubling of fractional orbital angular momentum state

Nonlinear frequency conversion is promising for manipulating photons with orbital angular momentum (OAM). In this letter, we investigate the second harmonic generation (SHG) of light beams carrying fractional OAM. By measuring the OAM components of the generated second harmonic (SH) waves, we find that the integer components of the fundamental beam will interact with each other during the nonlinear optical process; thus, we figure out the law for topological charge transfer in frequency doubling of the fractional OAM state. Theoretical predictions by solving the nonlinear coupled wave equations are consistent with the experimental results.

Tunable diffraction-free array in nonlinear photonic crystal

Diffraction-free beams have attracted increasing research interests because of their unique performances and broad applications in various fields. Although many methods have been developed to produce such beams, it is still challenging to realize a tunable non-diffracting beam. Here, we report the generation of a tunable diffraction-free array through second-harmonic generation in a nonlinear photonic crystal, i.e., a 2D periodically-poled LiTaO3 crystal. In such a crystal, the second-harmonic wave is engineered by properly designing the domain structure based on the Huygens-Fresnel principle. The characteristics of the generated diffraction-free array including its period, propagation length, and wavelength can be tuned by simply changing the input wavelength. Our observation not only enriches the diffraction-free optics, but also has potential applications for photolithography and imaging.

Nonlinear Cherenkov difference-frequency generation exploiting birefringence of KTP

In this letter, we demonstrate the realization of nonlinear Cherenkov difference-frequency generation (CDFG) exploiting the birefringence property of KTiOPO4 (KTP) crystal. The pump and signal waves were set to be along different polarizations, thus the phase-matching requirement of CDFG, which is, the refractive index of the pump wave should be smaller than that of the signal wave, was fulfilled. The radiation angles and the intensity dependence of the CDFG on the pump wave were measured, which agreed well with the theoretical ones.

Nonlinear Cherenkov radiations modulated by mode dispersion in a Ti in-diffused LiNbO 3 planar waveguide

We report nonlinear Cherenkov radiations (NCRs) in a Ti in-diffused LiNbO3 planar waveguide. The radiations were modulated exploiting different polarizations and orders of the guided modes, the fundamental wavelengths and the working temperatures. Some characteristics related to NCRs, such as radiation angles and relative intensities were investigated in detail. The experimental results matched well with theoretical calculations.

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%.

Realization of nonlinear Cherenkov frequency down-conversion by phase velocity modulation

Nonlinear Cherenkov radiations (NCR) have attracted increasing research interest in the past years. Similar to the traditional Cherenkov radiation effect in particle physics, NCR requires that the nonlinear polarization wave propagates faster than the harmonic waves. Cherenkov frequency up-conversions can be automatically realized in normal dispersion materials and extensive work on Cherenkov second-harmonic generation, sum-frequency generation and high-order harmonic generation have been reported in both bulk and waveguide systems. However, Cherenkov frequency down-conversion process is still a challenge for the restriction of phase velocity threshold. Recently, we have proposed two schemes to realize the degenerated Cherenkov difference-frequency generation (CDFG) by modulating the phase velocity of the nonlinear polarization wave using both quasi-phase-matching (QPM) and birefringence-phasematching (BPM) respectively. For the first scheme, we used the QPM method. The reciprocal vectors provided by the domain engineered structure can modulate the phase velocity of the nonlinear polarization wave, and particularly, the backward vectors can accelerate the nonlinear polarization wave. If the backward reciprocal vector is large enough, Cherenkov DFG can be realized. In experiment, a proton exchanged LiTaO3 planar waveguide with a poling period of 4.5 μm was used and the wavelengths of pump and signal were 532 and 1064nm respectively. At a radiation angle close to the theoretical one, the infrared CDFG spot can be observed with a CCD. For the second scheme, the birefringence property of KTP was utilized. For birefringent crystal, light with different polarization has different refractive index. By proper choice of the polarization states of the interacting waves, the refractive index of the signal wave can be larger than that of the pump wave, thus the phase velocity of the nonlinear polarization wave of the DFG process will exceed that of the harmonic wave. In experiment, we set the polarization of 532-nm pump and the 1064-nm signal waves along x and z axis of KTP respectively. CDFG was realized and some cascaded Cherenkov processes were observed as well. The method based on BPM can reduce the requirements for fabricating short-period domain structures and can be extended to mode-matching in waveguides for realizing nonlinear Cherenkov radiations.