Resonant excitation and imaging of nonequilibrium exciton spins in single core-shell GaAs-AlGaAs nanowires

Nonequilibrium spin distributions in single GaAs/AlGaAs core-shell nanowires are excited using resonant polarized excitation at 10 K. At all excitation energies, we observe strong photoluminescence polarization due to suppressed radiative recombination of excitons with dipoles aligned perpendicular to the nanowire. Excitation resonances are observed at 1- or 2-LO phonon energies above the exciton ground states. Using rate equation modeling, we show that, at the lowest energies, strongly nonequilibrium spin distributions are present and we estimate their spin relaxation rate.     

Nonlinear optical processes in optically trapped InP nanowires

We report on the observation of nonlinear optical excitation and related photoluminescence from single InP semiconductor nanowires held in suspension using a gradient force optical tweezers. Photoexcitation of free carriers is achieved through absorption of infrared (1.17 eV) photons from the trapping source via a combination of two- and three-photon processes. This was confirmed by power-dependent photoluminescence measurements. Marked differences in spectral features are noted between nonlinear optical excitation and direct excitation and are related to band-filling effects. Direct observation of second harmonic generation in trapped InP nanowires confirms the presence of nonlinear optical processes.     

Field measurements of aerodynamic pressures in tunnels induced by high speed trains

The passage of high speed trains causes aerodynamic effects in tunnels, and considerable pressure transients are generated because of the restricted airspace within the tunnel. This leads to passenger discomfort, noise surrounding the tunnel, resistance to train movement, and possible damage to the train body and tunnel facilities. For the real assessment of the aerodynamic pressures in the tunnel induced by the passing trains of the high speed rail in Taiwan, a series of field measurements were performed near the portal and the shaft or the adit of the tunnel during normal operation. The measurements were conducted for several train speeds, including the maximum operation speed of the high speed rail in Taiwan, which is 300 km/h. Pressure sensors were deployed along the tunnel to investigate the propagation of pressure waves. The results show that the train nose entry/exit generated a compression wave propagating throughout the tunnel, resulting in a sharp increase in pressure. Conversely, the train tail entry/exit generated an expansion wave causing a pressure drop. The successive reflections of these pressure waves between both ends of the tunnel were observed. The pass-by of a train inside the tunnel also induced an immediate local pressure drop due to aerodynamic drag. Based on the measurement results, the spatial variation of the train-induced pressure peaks inside the tunnel is discussed. Furthermore, the relationship between the pressure peaks and the train speed is established, and the influence of the cross-sectional area of the tunnel is also presented.     

Effect of O2 plasma pretreatment on structural and optical properties of ZnO films on PES substrate by atomic layer deposition

ZnO films were deposited on the O₂ plasma treated polyethersulfone (PES) substrates by atomic layer deposition. X-ray diffraction (XRD) measurements reveals that the grains in ZnO films show strongly (0 0 2) preferential orientation, when the duration of plasma pretreatment increases. The decreased grain size and improved crystallinity results in the decreased surface roughness of ZnO films. In contrast, when the duration of plasma pretreatment increases to 60 min, the surface roughness increases again due to the increased grain size and worse crystallinity. In photoluminescence measurement, slight blue shift of near-band-edge emission occurs with increasing duration of plasma pretreatment up to 30 min.