We experimentally demonstrate the formation of GaN nanotip pyramids by selective and anisotropic etching of N-polar GaN in KOH solution. For samples grown with adjacent Ga-and N-polar regions on the same wafer, the KOH solution was found to selectively etch only the N-polar surface while leaving the Ga-polar surface intact. An aggregation of hexagonal pyramids with well defined ͕10 1 1 ͖ facets and very sharp tips with diameters less than ϳ20 nm were formed. The density of the pyramids can be controlled by varying the KOH concentration, solution temperature or the etch duration. The GaN etching activation energy is estimated to be E a Ϸ0.587 eV. Dense GaN pyramids with sharp tips have applications in both electronic and photonic devices.

We report the experimental demonstration of second-harmonic generation in periodically poled GaN by first-order quasiphase matching. The periodically poled structure was grown by plasma-assisted molecular-beam epitaxy. We observed about 9 μW of second-harmonic power from a fundamental input laser wavelength of 1658.6 nm with a normalized conversion efficiency of 12.76% W−1 cm−2. The ability to perform nonlinear wavelength conversion by periodic poling and the fact that GaN has a very wide window of transparency, pave the way for GaN to be used for nonlinear optical devices in telecommunications as well as a nonlinear light source for biochemical detection in the far infrared and deep ultraviolet.

We show that long wave-short wave resonance can be achieved in a second-order nonlinear negative refractive index medium when the short wave lies on the negative index branch. With the medium exhibiting a second-order nonlinear susceptibility, a number of nonlinear phenomena such as solitary waves, paired solitons, and periodic wave trains are possible or enhanced through the cascaded second-order effect. Potential applications include the generation of terahertz waves from optical pulses.

No abstract

We present a theoretical analysis for simultaneous optical wavelength interchange and isolation of a pair of collinear input optical signals by use of two concurrent difference-frequency-generation processes in a two-dimensional second-order nonlinear photonic crystal. We have derived a set of relations, including a general nonlinear Bragg condition, that we use to determine the parameters of the nonlinear lattice, given the input wavelengths and desired exit angles of the wavelength-interchanged outputs.