S. Gwo

Direct measurement of the band gap and Fermi level position at InN(112¯0)

A nonpolar stoichiometric InN(112¯0) surface freshly cleaved inside UHV was investigated by scanning tunneling microscopy and spectroscopy. Due to the absence of intrinsic surface states in the band gap, scanning tunneling spectroscopy yields directly the fundamental bulk band gap of 0.7±0.1 eV. The Fermi energy is pinned 0.3 eV below the conduction band minimum due to cleavage induced defect states. Thus, intrinsic electron accumulation can be excluded for this surface. Electron accumulation is rather an extrinsic effect due to surface contamination or material decomposition, but not an intrinsic material property of InN.

Determination of the electron effective mass of wurtzite InN by coherent upper-branch A1(LO) phonon-plasmon coupling mode

Coherent A1(LO) phonon and its coupling with photoexcited plasmon in wurtzite InN were generated and detected with time-resolved second-harmonic generation. The experimental results directly reveal that the plasma damping time constant is about 60∼120fs, which depends on the photoexcited plasma density in InN. The frequency of the upper-branch A1(LO) phonon-plasmon coupling mode shifts as a function of the photoexcited plasma density. This frequency shift can be fitted consistently with different InN films by solving the InN dielectric response function and leads to the determination of the electron effective mass m‖*=(0.033±0.003)me, parallel to the c axis of wurtzite InN.

Influence of structural anisotropy to anisotropic electron mobility in a-plane InN

This study reports on the anisotropic electron transport properties and a correlation between the electron mobility (μ) and the stacking faults (SFs) in the a-plane InN film. Electron mobilities measured by terahertz time-domain spectroscopy and Hall effect measurement along the in-plane [1¯100] (c⊥) orientation were much higher than those of the in-plane [0001] (c∥) orientation. This result shows a sharp contrast to higher defect density for the c⊥ orientation as measured by x-ray diffraction. The electrons transporting through the planar SFs aligned along the c⊥ direction are expected to experience more scattering by defects, resulting in lower μ for the c∥ orientation.

Efficient single photon sources based on nanodiamonds on a plasmonic platform

We report on an efficient room-temperature source of single photons based on single nitrogen-vacancy centers in nanodiamonds (NDs) placing on a large-area plasmonic platform formed by crystalline gold flakes covered with a thin dielectric layer. Due to the strongly confined plasmonic fields in the thin dielectric layer, the NDs show a large enhancement in the fluorescence intensity without significant degradation in single photon purity. According to the deduced fluorescence lifetime from g(2) measurements, the Purcell factor is estimated to be ~1.4. The plasmonic platform is very ideal for large-area fluorescence enhancement without the needs of sophisticated nanopositioning schemes for locating NDs at local hot spots in conventional plasmonic nanostructures.

Photoluminescence from InN nanorod arrays with a critical size

Abnormal temperature dependence of peak energy and bandwidth of photoluminescence spectra was observed for InN nanorods with a diameter <;30 nm, which is of the same order of the thickness of surface electron accumulation layer.

Nonlinear absorption in InN under resonant- and non-resonant excitation

We report the wavelength-dependent nonlinear absorption (NLA) of InN film grown on anr-plane sapphire by molecular beam epitaxy technique. In order to understand the nonlinear optical properties of InN, the Z-scan measurement was performed at two different wavelengths, the photon energies of which are near (resonant excitation) and much higher (non-resonant excitation) than the bandgap of InN, respectively. Under non-resonant excitation, band-filling effect leads the dominant saturable absorption, while under resonant excitation, reverse saturable absorption dominates the nonlinear absorption. From the open-aperture Z-scan measurement under resonant excitation, we found that InN exhibits more than one nonlinear absorption process simultaneously. Particularly, under relatively weak resonant excitation, the transformation from saturable absorption to 2PA through the intermediate excitation state absorption was observed as the sample approaches the beam focus. The close-aperture Z-scan signals of InN show valley-peak response, implying that the nonlinear refraction in InN is caused by the self-focusing of the Gaussian laser beam. Using the Z-scan theory, the corresponding nonlinear parameters, such as saturation intensity, 2PA coefficient, and nonlinear refractive index, of InN were estimated.

GaN nanorod-based subwavelength optical media

Vertically self-aligned gallium nitride nanorod arrays grown by plasma-assisted molecular-beam epitaxy are shown to behave as subwavelength optical media in both their discrete and integrated forms, which have important implications for optoelectronic applications.