We report the linear and nonlinear optical properties of single GaAs nanowires with polytypism effect. Electron transmission microscopy experiments show that the nanowires contain wurtzite segments as well as zinc blende segments with two different crystallographic orientations. Time-resolved photoluminescence spectroscopy of single nanowires shows ultrafast radiative recombination lifetimes in the range of 20−70 ps as a consequence of the charge scattering in the type-II band alignment between WZ and ZB segments and or by the high surface area of the nanowires. Polarization resolved second harmonic generation in the nanowires found to be highly sensitive to the crystallography of the nanowires. All three crystal axes orientations of the nanowires were determined. Furthermore, the volume fraction of the ZB segments oriented in the [0−11] and in the [01−1] directions was precisely determined for the first time by optical measurement besides the TEM technique.

We report the nonlinear optical properties of polaritons in a single quantum well GaAs microcavity. Single and double resonance cavities have been used to enhance the nonlinear effect of a nonlinear medium in high-Q quality factor cavities due to the cavity field amplification. On the other hand, such a cavity can generate polaritons, which condense (Bose− Einstein condensation) for determined conditions, such as cavity detuning and polariton density. Here, we present experimental observations of the second harmonic generation from polaritons in a GaAs microcavity, which exhibit a 2 order higher efficiency than those from the usual method. Furthermore, the efficiency is particularly high when the polariton is a photon-like particle and in a condensed state.

Researching optical effects in nanowires may require a high pump intensity which under ambient conditions can degrade nanowires due to thermal oxidation. In this work we investigated the photodegradation of a single Si-doped GaAs nanowire by laser heating in air. To understand the changes that occurred on the nanowire we carried out Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and photoluminescence spectroscopy in laser damaged regions as well as in non-affected ones. From Raman Stokes and anti-Stokes measurements we estimated the local temperature that the oxidation process of the nanowire (NW) surface starts at as 661 K, resulting in two new Raman modes at 200 cm À1 and 259 cm À1 . Scanning electron microscopy and energy dispersive X-ray spectroscopy measurements showed a significant loss of arsenic in the oxidized regions, but no erosion of the nanowire. Micro-photoluminescence measurements showed the near-band-edge emission of GaAs along the nanowire, as well as a new emission band at 755 nm corresponding to polycrystalline b-Ga 2 O 3 formation. Our results also indicate that neither amorphous As nor crystalline As were deposited on the surface of the nanowire. Combining different experimental techniques, this study showed the formation of polycrystalline b-Ga 2 O 3 by oxidation of the nanowire surface and the limits for performing spectroscopic investigations on individual GaAs NWs under ambient air conditions.

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