Xiaopeng Hu

Cerenkov third-harmonic generation via cascaded χ (2) processes in a periodic-poled LiTaO 3 waveguide

We report a type of quasi-phase-matched (QPM) Cerenkov third-harmonic generation (CTHG) in a periodic-poled LiTaO₃ waveguide. The CTHG results from a guided-to-guided second-harmonic generation cascaded with a guided-to-radiated sum-frequency generation (SFG) in the waveguide. In the guided-to-radiated SFG process, nonlinear interactions with participating and nonparticipating reciprocal vectors would lead to different CTHG radiations. In addition, the power and temperature detuning characters of QPM CTHG were studied. Theoretical predictions were in good agreement with experimental results.

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., l2u2009=u20092l1) 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.

Diffraction Interference Induced Superfocusing in Nonlinear Talbot Effect

We report a simple, novel subdiffraction method, i.e. diffraction interference induced superfocusing in second-harmonic (SH) Talbot effect, to achieve focusing size of less than λSH/4 (or λpump/8) without involving evanescent waves or subwavelength apertures. By tailoring point spread functions with Fresnel diffraction interference, we observe periodic SH subdiffracted spots over a hundred of micrometers away from the sample. Our demonstration is the first experimental realization of the Toraldo di Francias proposal pioneered 62 years ago for superresolution imaging.

Mode-coupling Cerenkov sum-frequency-generation in a multimode planar waveguide

We present experimental and theoretical studies of the mode-coupling Cerenkov sum-frequency radiations in a multimode LiNbO3 planar waveguide. The radiations result from the coupling of different guided modes of the fundamental wave, which have the same optical frequencies but different propagation constants. At the same time, scattering-involved Cerenkov sum-frequency-generation was also observed and discussed. Our theoretical predictions are in well accordance with the experimental results.

Cerenkov second-harmonic arc from a hexagonally poled LiTaO3 planar waveguide

We demonstrate a new type of arc-shaped Cerenkov second-harmonic generation in a hexagonally poled LiTaO3 planar optical waveguide. Scattering of fundamental light, arising from domain walls and other imperfections on the surface and inside the waveguide, is found to be involved in such an optical parametric process, which is enhanced in the modulated domain structure by quasi-phase matching. One can use the Cerenkov second-harmonic arc to study the structure information of a nonlinear optical waveguide and to characterize the scattering in the waveguide.

Integrated noncollinear red–green–blue laser light source using a two-dimensional nonlinear photonic quasicrystal

We report on a noncollinear red–green–blue (RGB) laser light source using a two-dimensional nonlinear photonic quasicrystal. The red and blue lights result from a green light pumped optical parametric generation process cascading two frequency doubling processes in a single-pass setup. Together with the residual green light, two sets of RGB lights were observed in a wide temperature range, which indicates a practical method for constructing a compact multiwavelength laser light source.

Superposed second-harmonic Talbot self-image from a PPLT crystal

We experimentally demonstrate the superposed second-harmonic Talbot self-image in a z-cut periodically-poled LiTaO3 crystal. The generated second-harmonic (SH) waves in the positive and negative domains have the same intensity but different phases (a phase shift of π) due to the opposite poling directions, i.e. a second-harmonic phase pattern is generated from the crystal. By introducing a reference SH wave, we can selectively study the self-imaging originating from the SH patterns with different phases. In the integer and fractional Talbot planes, the two patterns interfere with each other and form superposed self-imaging patterns.

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.

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.