Yan Sheng

Broadband efficient second harmonic generation in media with a short-range order

The authors demonstrate broadband efficient quasiphase matched second harmonic generation (SHG) in nonlinear LiNbO3 crystals. The distribution of ferroelectric domains in a short-range order provides the possibility for broadband second harmonic generation in the visible range. This process is similar to the broadband SHG in random media, but it results in much higher conversion efficiency. The unique distribution of reciprocal vectors provided by the structural short-range order plays an important role in the enhancement. The achieved broadband SHG with high conversion efficiency will aid in the development of optics integration and multiple-channel devices.

Quasiphase matched harmonic generation in a two-dimensional octagonal photonic superlattice

We have fabricated LiNbO3 nonlinear photonic crystals with a two-dimensional octagonal quasilattice. The effective collinear quasiphase matched second-harmonic orange and green lights, with conversion efficiencies of ∼38% and 23%, were realized. The noncollinear harmonic processes at multiple wavelengths were also observed with the conversion efficiency of around 5%. All the-same results can be obtained when the crystal is rotated by the integral multiples of π∕4.

Giant enhancement of second harmonic generation in poled ferroelectric crystals

The authors propose a simple one-dimensional inhomogeneous system composed of periodically alternating stacked air and poled LiNbO3 crystal layers. This structure combines the advantages of photonic band gap (PBG) structures and periodically poled ferroelectric crystals, so the high density of modes and quasiphase matching can be achieved simultaneously. They find that compared with simple quasiphase-matched and PBG structures, the conversion efficiency of second harmonic generation in this structure can be significantly enhanced when the pump and second harmonic fields are both tuned to the band edge.

Temperature and angle tuning of second harmonic generation in media with a short-range order

The authors theoretically study the nonlinear optical characterization of the short-range ordered structure and deduce the analytical expression of the effective nonlinear optical coefficient. The dependences of the second harmonic conversion efficiency on crystal temperature, incident angle, and reversed domain cycle are measured. They find that for a substantial range of parameters, the short-range ordered structure has a broader tuning response than the periodic or the quasiperiodic structures. The experimental results accord well with the theory.

Anisotropy of domain broadening in periodically poled lithium niobate crystals

We experimentally study the dependence of reversed domain broadening of lithium niobate (LiNbO3) wafers on their poling electrode configurations. Experiments show that the reversed domain area is anisotropic and has 60° symmetry. The slightest domain broadening takes place as the poling stripe electrode parallels to the diagonals of hexagonal domain walls. We suppose the anisotropy of domain expansions is directly related to the symmetry of LiNbO3 crystal lattices, and it can be understood by considering the anisotropoic distributions of tangential component of electric field and the spontaneous polarization on domain walls.

Enhanced second-harmonic generation in an electro-optic controlled periodically poled ferroelectric crystal

We present a novel design of enhanced second-harmonic generation in a periodically poled ferroelectric crystal. It profits from enhanced light intensity and slowed group velocity, which occur near photonic band edges through electro-optic modulation. The designed structure, with 108 periods and a length of 322 microm, can generate a second-harmonic beam whose conversion efficiency is 2 orders of magnitude larger than the one produced by an equivalent length of quasi-phase-matching bulk medium. This result might be beneficial to the development of optics integration and compact devices.