Haigang Liu

Nonlinear Raman-Nath second harmonic generation with structured fundamental wave.

We proposed and experimentally demonstrated that nonlinear Raman-Nath second harmonic can be achieved when a fundamental wave with the phase periodically modulated, termed as structured fundamental wave, incident in a homogeneous nonlinear medium.

Scattering-assisted second harmonic generation of structured fundamental wave

We study a scattering-assisted second harmonic generation process of a structured fundamental wave, whose phase front is periodically modulated, in a homogenous nonlinear medium. Scattering-assisted multiple-conical second harmonic in a one-dimensional structured fundamental wave and second harmonic belts in two-dimensional cases were observed. The experimental results are consistent with our analysis. This method not only allows dynamic control of the scattering-assisted second harmonic signal, but also has potential applications in various nonlinear frequency conversion processes.

Probe of symmetry reduction at domain walls by nonlinear Cherenkov measurement

We report on the investigation of symmetrical properties of lithium niobate (LiNbO3) domain walls utilizing the nonlinear Cherenkov radiation. Compared with LiNbO3 bulk crystals, new nonzero elements of the χ(2) tensor at domain walls are found by the Cherenkov second harmonic generation (CSHG) and Cherenkov sum frequency generation (CSFG) measurement. Experimentally, we demonstrate the symmetry reduction of domain walls, where the mirror inversion symmetry of LiNbO3 lattice is broken while the threefold rotational symmetry still remains.

Dynamically tailoring nonlinear Cherenkov radiation in PPLN by structured fundamental wave

We numerically and experimentally investigated the Cherenkov-type second harmonic generation of structured fundamental wave, whose phase was periodically modulated, in periodically poled nonlinear crystals. The Cherenkov-type second harmonic generation with different parameters of the structured fundamental wave was investigated. The experimental results are in good agreement with the theoretical analysis. This study provides a method of dynamically tailoring the Cherenkov-type second harmonic wave and also has potential application in other nonlinear frequency conversion processes.

Dynamic computer-generated nonlinear optical holograms in a non-collinear second-harmonic generation process

The nonlinear holography technique is a powerful tool for all-optical switching and manipulation of an arbitrary harmonic wave. The common method of realizing such nonlinear holography is by configuring the structure of nonlinear photonic crystals. However, it is a challenge to dynamically tune the harmonic wave pattern. To overcome the long-term existing non-dynamic property of such nonlinear holographs, we realize dynamic computer-generated nonlinear optical holograms in the non-collinear second-harmonic (SH) generation process in which only one infrared beam is modulated. Arbitrary patterns in both fundamental-frequency and second-harmonic wavebands can be generated at the same time. This Letter offers a flexible and dynamic method for arbitrary nonlinear wavefront shaping technology.

Second harmonic generation with full Poincaré beams

We report a concise yet efficient experiment to extend the study of full Poincaré beams to incorporate the nonlinear optical effect. The main feature of our scheme is the employment of Type-II phase-matching KTP crystal to implement the second harmonic generation with structured vector light from invisible to visible region. Of particular interest is the revelation and visualization of the hidden topological structures transferred from the input polarization state to the output observable intensity patterns. The experimental results are in good agreement with the numerical simulations. Our present work provides us with the insight into the interaction of full Poincaré beams with media in the nonlinear regime.

Nonlinear vortex beam array generation by spatially modulated fundamental wave

We experimentally demonstrated the generation of a dynamic nonlinear vortex beam array by utilizing a fundamental wave with a modulated phase structure, which was incident into a homogeneous nonlinear medium. In our experiment, one-dimensional and two-dimensional second harmonic vortex beam arrays were investigated, and the topological charge of second harmonic vortex beam of different order was measured. This study presents a method of dynamic control of the nonlinear vortex beam array, which may have applications in multiple-particles optical trapping, optical communication, and so on.

Linear Cherenkov radiation in ferroelectric domain walls

Nonlinear Cherenkov radiation plays an important role in detecting the internal structure of the ferroelectric crystal. However, linear Cherenkov radiation in the ferroelectric crystal was not observed before. Based on mirror symmetry reduction at the domain walls, we theoretically predicted that the linear Cherenkov radiation could be generated in periodically poled ferroelectric crystals according to the coupled wave equation. Then, we experimentally demonstrated the generation of such Cherenkov radiation in domain walls. Compared with the domain region, new nonzero elements χ13, χ23, χ31 and χ32 appear in the linear susceptibility tensor. The experimental results are consistent with our theoretical prediction.

On-chip tunable optofluidic dye laser

We demonstrate a chip-scale tunable optofluidic dye laser with Au-coated fibers as microcavity. The chip is fabricated by soft lithography. When the active region is pumped, a relatively low threshold of 6.7  μJ/mm2 is realized with multimode emission due to good confinement of the cavity mirrors, long active region, as well as total reflectivity. It is easy to tune the lasing emission wavelength by changing the solvent of laser dye. In addition, the various intensity ratios of multicolor lasing can be achieved by controlling flow rates of two fluid streams carried with different dye molecules. Furthermore, the convenience in fabrication and directional lasing emission outcoupled by the fiber make the tunable optofluidic dye laser a promising underlying coherent light source in the integrated optofluidic systems.