Near-Field Imaging of Nonlinear Optical Mixing in Single Zinc Oxide Nanowires

Abstract

The nonlinear optical response of semiconductor nanowires has potential application for frequency conversion in nanoscale optical circuitry. Here, second- and third-harmonic generation (SHG, THG) are imaged on single zinc oxide (ZnO) nanowires using near-field scanning optical microscopy (NSOM). The absolute magnitudes of the two independent (2) elements of a single wire are determined, and the nanowire SHG and THG emission patterns as a function of incident polarization are attributed to the hexagonal nanowire geometry and (2) tensor symmetry. Semiconductor nanowires are of current interest because of their unique electrical and optical properties. 1-3 In particular, their nonlinear optical properties suggest potential applications as frequency converters or logic/routing elements in nanoscale optoelectronic circuitry. A linear optical property of nanowires, photoluminescence (PL) polarization, has recently been studied in single indium phosphide nanowires. 2 In that case, the PL polarization is based upon the classical electromagnetic properties of a dielectric cylinder and averages ca. 91%. In contrast, coherent nonlinear optical phenomena, such as second- and third-harmonic generation (SHG and THG, respectively), depend explicitly on the crystal lattice structure of the medium, which could yield a very high (nearly 100%) polarization selectivity. In addition, the temporal response of the nonresonant harmonic generation is similar to the pulse width of the pump laser, in some cases 20 fs, 4 while incoherent processes are at least 2-4 orders of magnitude slower. Moreover, nonresonant SHG is essentially independent of wavelength below the energy band gap of semiconductor materials, most often including the 1.3-1.5 Im wavelength region typically used in optical fiber