We report angle-resolved second-harmonic generation (SHG) measurements from suspensions of centrosymmetric micron-size polystyrene spheres with surface-adsorbed dye (malachite green). The second-harmonic scattering profiles differ qualitatively from linear light scattering profiles of the same particles. We investigated these radiation patterns using several polarization configurations and particle diameters. We introduce a simple Rayleigh-Gans-Debye model to account for the SHG scattering anisotropy. The model compares favorably with our experimental data. Our measurements suggest scattering anisotropy may be used to isolate particle nonlinear optics from other bulk nonlinear optical effects in suspension.

We have performed ultrafast second-harmonic generation spectroscopy of GaN/Al 2 O 3 . A formalism was developed to calculate the nonlinear response of thin nonlinear films excited by an ultrashort laser source (Ti:Al 2 O 3 ), and then used to extract zxx(2) (ϭ2 o ) and xzx (2) (ϭ2 o ) from our SHG measurements over a two-photon energy range of 2.6-3.4 eV. The spectra are compared to theory ͓J. L. P. Hughes, Y. Wang, and J. E. Sipe, Phys. Rev. B 55, 13 630 ͑1997͔͒. A weak sub-band-gap enhancement of zxx(2) (ϭ2 o ) was observed at a two-photon energy of 2.80 eV; it was not present in xzx(2) (ϭ2 o ). The enhancement, which may result from a three-photon process involving a midgap defect state, was independent of the carrier concentration, intentional doping, and the presence of the ''yellow luminescence band'' defects. In addition, we determined sample miscuts by rotational SHG; the miscuts did not generate observable strain induced nonlinearities. The linear optical properties of GaN from 1.38 to 3.35 eV were also determined.

Spatially separated second-harmonic generating and photocarrier exciting light beams were employed to study transverse diffusion of carriers confined near GaAs(001) surfaces. The measurements utilize the intrinsic sensitivity of second-harmonic generation to surface charge density in order to probe these processes. Carrier transport was found to be difFusive with small efFective difFusion coefIicients compared to those of carriers in the bulk. Several models are considered to explain these results.Many years ago an elegant experiment by Haynes and

Second-order nonlinear optical spectroscopies reveal midgap interface states at buried metal/GaAs junctions, and demonstrate that these states are sensitive to interface preparation.In As-rich (Garich) Au/GaAs n-type samples one (two) midgap resonance was observed. Similar resonances were exhibited in As/GaAs n-type samples, but were not present in metal/GaAs p-type systems. The sharp spectral features provide compelling evidence for the existence and symmetry of atomic displacementinduced defect states just below the buried interface. PACS numbers: 73.20.r, 42.65.Ky, 73.30.+y, 78.66.w Interface states at metal/semiconductor junctions play an important role in controlling semiconductor device performance. For instance, Fermi level pinning and Schottky barrier heights are determined by these interface states [1,2]. In this paper we focus on the states associated with the buried metal/GaAs interface. Although there are a number of experimental observations consistent with the existence of midgap states in this system, there are very few direct spectroscopic measurements of interface energy levels. Some spectroscopic evidence for interface states has been derived from cathodoluminescence (CLS) [3] and inverse photoemission (IPS) [4], but these spectra are broad, and the measurements are not intrinsically sensitive to the buried interface. Other experiments have probed GaAs surfaces For exam. ple, scanning tunneling microscopy (STM) was used to study As-related atomic defects on GaAs surfaces [5], and ultraviolet-photoemission spectra (UPS) have been used to study metal adsorbate-induced states in GaAs [6]. The lack of clean spectra from the buried interface, however, makes it difficult to challenge microscopic models about growth and defects in these and other systems.In this Letter, we report investigations of interface states at metal/GaAs junctions by second-harmonic (SH) and sum-frequency (SF) spectroscopies [7]. Our experiments reveal interface states at metal/GaAs junctions and demonstrate that these states are sensitive to interface preparation. In contrast to CLS and IPS measurements, sharp resonant features were observed. The intrinsic interface sensitivity of second-order nonlinear optical spectroscopies enable us to suppress spectral contributions from the adjoining bulk media, resulting in more specific assignments of the spectral features, and a narrowing of the features by comparison to those observed with competing spectroscopies. Our observations provide strong evidence for the existence of atomic displacement-induced defect states at the buried metal/GaAs interface, and offer energy level and symmetry information that can be used to check theoretical predictions about this class of interface state [8].In particular, Au/GaAs n-type systems exhibit a single midgap state in As-rich interfaces and two closely spaced midgap states in Ga-rich interfaces. As/GaAs n-type samples were observed to exhibit the same resonance as the As-rich, Au/GaAs n-type sample. On the other hand, no resonances were observ...

We report experimental observations of second-harmonic generation ͑SHG͒ from single micrometer-size polystyrene, silica, and polymethylmethacrylate spheres on flat substrates by SHG microscopy. At low input light intensities the SH signals depend quadratically on the intensity of the excitation beam, but at larger input intensities some of the SH signals increase exponentially with increasing input intensity. This exponential enhancement depends on particle size and sphere composition. We describe the experiments, and parametrize the observations.