[en] We report precision measurements of the refractive indices of dry air, N2, O2, Ar, and CO2, performed by using a frequency comb as the light source in a Mach-Zehnder interferometer setup. Improved dispersion formulas for all gases are derived with a sensitivity level of 10^-9. These results are valid for a wavelength range from 740 to 860 nm and are in good agreement with measurements from other groups

[en] The energy-level alignment at metal/organic interfaces has traditionally been studied using ultraviolet photoelectron spectroscopy (UPS) in ultra-high vacuum (UHV). However, since most devices are fabricated in high vacuum (HV), these studies do not accurately reflect the interfaces in real devices. We demonstrate, using UPS measurements of samples prepared in HV and UHV and current-voltage measurements of devices prepared in HV, that the small amounts of residual gases that are adsorbed on the surface of clean Cu, Ag, and Au (i.e., the noble metals) in HV can significantly alter the energy-level alignment at metal/organic interfaces.

[en] Silicon/SiO₂ superlattices (SLs) are nanostructured thin films bearing multiple Si/SiO₂ interfaces. In such materials, silicon is walled by its oxide in order to generate multiple quantum wells. Here, the structural and electronic properties of a structurally-relaxed Si/SiO₂ SL model are studied using a first principles approach; the Si/SiO₂ interfaces contain all suboxide Si atoms (Si¹⁺, Si²⁺, and Si³⁺). The valence and conduction band offsets (VBO and CBO) are evaluated from the relative shift between densities of states (DOSs) of Si atoms in bulk SiO₂ and in the SL. The CBO is shown to be reduced compared to the VBO. The DOSs of the three suboxide Si atoms are also calculated. It is shown that there are contributions from all suboxides at the threshold of the gap

[en] The fundamental magnetism of the Fe film of 4 ML thick epitaxially grown on GaAs(100) was studied using ex situ ferromagnetic resonance (FMR) at room temperature. In parallel geometry the resonance fields were found to be strikingly different along [0 bar 11] (2170 Oe) and [011] (3360 Oe) direction, exhibiting a large uniaxial anisotropy with the easy and hard directions along [0bar 11] and [011], respectively. For perpendicular geometry the data of FMR show an abnormally low resonance field of 5050 Oe. It implies a pronounced reduction of effective magnetization. This can be explained by a combination of the shape anisotropy of the nanoclusters with their large perpendicular anisotropy. The g factor of these nanoclusters is even smaller than the bulk value. The linewidth is also obviously anisotropic, 220 Oe in easy direction, [0bar 11] and smaller by 50% in hard direction, [011]. [copyright] 2001 American Institute of Physics

[en] We report a new method in the precision measurement of the refractive index of air using a highly unbalanced Michelson interferometer with a femtosecond optical frequency comb as the light source. Standard dry air is filled into a 30 m multipass cell, serving as the long arm of the interferometer, while a short arm acts as the reference path. Both time and frequency domain interferograms are recorded to measure the refractive index of air. The deviation of our experimental results with Edlen's formula is 1.4x10^-9 at 800 nm. Our experiment has a standard error of 5.2x10^-9 at fixed parameters (pressure and temperature). This is achieved by putting the multipass cell into a temperature-stabilized box, and also by locking the interferometer path length with a He-Ne laser. We achieved a temperature stabilization of 0.8 mK for 25 h. This corresponds to 0.4 μm multipass cell length change. The locking of the He-Ne interferometer enables us to achieve 7 nm path-length change outside the multipass cell. Combined with accurate measurement of temperature and pressure, we were able to achieve an accuracy of 7.7x10^-9

[en] Interferometric filter-based planar Doppler velocimetry is used in conjunction with imaging fibre bundles to make time-averaged three-component velocity measurements using a single imaging head. The Doppler frequency shifts of light scattered by particles entrained into the flow to be measured are transduced to intensity variations using a Mach–Zehnder interferometer. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. Three-component measurements are made possible by porting different views of the measurement plane to a single imaging head using the imaging fibre bundles. A comparison of three different image-processing techniques is presented and analysed with the aid of modelled images. Results are presented here for time-averaged measurements of a rotating disc with maximum velocities of ∼ ±34 m s⁻¹ in the field of view with the computed measurement error in the orthogonal velocity components being (0.89, 0.68, 1.42) m s⁻¹ for the measurement geometry used. Three-component velocity measurements were also made on a seeded air jet with a nozzle diameter of 20 mm and an exit velocity of ∼85 m s⁻¹

[en] A planar Doppler velocimetry system to measure flow velocity fields is described. The technique uses a Mach-Zehnder interferometric filter to convert Doppler frequency shifts into intensity variations. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. An experimental arrangement is described that incorporates a phase-locking system designed to stabilise the interferometric filter. Two methods to process the interference fringe images are presented; the first uses the shift of the fringe pattern to determine the Doppler shift along profiles. The second provides a full-field measurement of the Doppler shift by determining the phase at each pixel in the images. Results are presented here for measurements of velocity fields on a rotating disc with maximum velocities, at the edge, of ±70m/s. Measurements on a seeded air jet with a nozzle diameter of 20mm and an exit velocity of ∼85m/s are also presented

[en] Aluminum doped zinc oxide (AZO) was grown via magnetron sputtering as a low-cost alternative to indium tin oxide (ITO) for organic photovoltaics (OPVs). Postdeposition ozone treatment resulted in devices with lower series resistance, increased open-circuit voltage, and power conversion efficiency double that of devices fabricated on untreated AZO. Furthermore, cells fabricated using ozone treated AZO and standard ITO displayed comparable performance.

[en] The polarization characteristics of the signal wave produced in Rb vapor by difference-frequency, parametric four-wave mixing (FWM) has been investigated for either ultrafast (∼150 fs) or nanosecond time-scale excitation of the 5s→→5d, 7s two photon transitions. The electronic configurations of the 5d ²D_5/2 and 7s ²S_1/2 states of Rb, as well as their energy separation (∼608 cm^-1), offers the opportunity to examine separately the resonantly enhanced 5s→→7s, 5d→6p→5s FWM pathways on the nanosecond time scale and then to drive both channels simultaneously with an ultrafast pulse of sufficient spectral width. As expected, dye laser (∼10 ns) excitation of the 5s→→5d (J=5/2) transition produces a signal wave (λ_s∼420 nm) having the same ellipticity as the driving optical field. Two photon excitation of Rb (7s) on the same time scale, however, generates an elliptically polarized signal when the pump is linearly polarized (ε=1), a result attributed to 7s→6p, 5p amplified spontaneous emission at ∼4 μm and ∼741 nm, respectively. Simultaneous excitation of the 5s→→7s, 5d transitions with ∼150 fs pulses centered at ∼770 nm yields polarization characteristics that can be approximated as a superposition of those for the individual transitions, thus displaying weak coupling between the two FWM channels. Also, the influence of molecular contributions to the FWM signal is observed for Rb number densities above ∼5x10¹⁴ cm^-3

[en] An aluminium ion optical clock is under development at Huazhong University of Science and Technology. The ²⁵Mg⁺ ion is chosen as logic ion to sympathetically cool an Al⁺ ion and to detect its states. The ²⁵Mg⁺ ion is cooled to the motional ground state through Raman sideband cooling as the first step for quantum logic spectroscopy. Ultra-stable lasers for the interrogation of the clock transition are developed. The instability of the laser beat frequency is 1.2 x 10^-15 at 1 s, which is close to the thermal noise limit of the reference cavity. (paper)