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SEM images of a WZ-GaAs nanoneedle (a) from surface normal planar view and (b) 30°-tilt view. The crystal -axis lies along the longitudinal growth direction of the nanoneedle. The -axis is in the transverse direction. (c) Schematic detailing measurement conditions. The nanoneedle rests horizontally on a double-side polished sapphire substrate. is the angle between the laser’s polarization and the nanoneedle -axis.
Laser and SHG spectra for both fundamental and second-harmonic fields polarized along the nanoneedle’s -axis. The bandwidth of the SHG spectra represents the bandwidth of the 120 fs pulses at 806 nm used for excitation. Nonlinear optical coefficients are estimated for WZ-GaAs by comparing the intensity of SHG from a single nanoneedle to SHG from reference CdSe and CdS wafers.
Power dependence of the integrated second-harmonic intensity from a single nanoneedle plotted on a log-log scale. The data fits a slope of two and thereby follows a quadratic power dependence, confirming the second-order nature of the emitted light.
Blue circles and red diamonds show experimental excitation polarization dependence data for SHG polarized -axis and -axis, respectively. Solid lines show the respective theoretical fits. Relative intensity scales are arbitrary. (a) Fundamental field polarization dependence of SHG polarized along a single nanoneedle’s -axis. and contribute to this second-harmonic polarization. Maxima occur at 0°, 180°, and 360° revealing that is the stronger tensor element. The ratio between the maximum and minimum second-harmonic intensity is typically about 20:1. (b) Fundamental field polarization dependence of SHG polarized along a single nanoneedle’s -axis. For second-harmonic light polarized transverse to the nanoneedle, the only nonzero tensor element is . Thus, maxima are expected at 45°, 135°, 225°, and 315° as shown by the experimental data. From these measurements, the ratios of the tensor elements are estimated to be .
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