Index of content:
Volume 113, Issue 21, 07 June 2013
- Lasers, Optics, and Optoelectronics
Quantum cascade laser absorption spectroscopy with the amplitude-to-time conversion technique for atmospheric-pressure plasmas113(2013); http://dx.doi.org/10.1063/1.4808261View Description Hide Description
The NO2 concentration, i.e., density, in a small plasma of a nitrogen oxide (NOx) treatment reactor has been measured by highly sensitive laser absorption spectroscopy. The absorption spectroscopy uses a single path of a quantum cascade laser beam passing through a plasma whose dimension is about 1 cm. The high sensitivity of spectroscopy is achieved by the amplitude-to-time conversion technique. Although the plasma reactor is designed to convert NO in the input gas to NO2, it has been demonstrated by this highly sensitive absorption spectroscopy that NO2 in a simulated exhaust gas that enters the reactor is decomposed by the plasma first and then NO2 is formed again, possibly more than it was decomposed, through a series of gas-phase reactions by the time the gas exits the reactor. The observation is consistent with that of an earlier study on NO decomposition by the same type of a plasma reactor [T. Yumii et al., J. Phys. D 46, 135202 (2013)], in which a high concentration of NO2 was observed at the exit of the reactor.
Investigations of afterpulsing and detection efficiency recovery in superconducting nanowire single-photon detectors113(2013); http://dx.doi.org/10.1063/1.4807833View Description Hide Description
We report on the observation of a non-uniform dark count rate in Superconducting Nanowire Single Photon Detectors (SNSPDs), specifically focusing on an afterpulsing effect present when the SNSPD is operated at a high bias current regime. The afterpulsing exists for real detection events (triggered by input photons) as well as for dark counts (no laser input). In our standard set-up, the afterpulsing is most likely to occur at around 180 ns following a detection event, for both real counts and dark counts. We characterize the afterpulsing behavior and speculate that it is not due to the SNSPD itself but rather the amplifiers used to boost the electrical output signal from the SNSPD. We show that the afterpulsing indeed disappears when we use a different amplifier with a better low frequency response. We also examine the short-lived enhancement of detection efficiency during the recovery of the SNSPD due to temporary perturbation of the bias and grounding conditions.
Electrically pumped random lasing in ZnO-based metal-insulator-semiconductor structured devices: Effect of ZnO film thickness113(2013); http://dx.doi.org/10.1063/1.4808445View Description Hide Description
In our previous report [Ma et al., Appl. Phys. Lett. 91, 251109 (2007)], electrically pumped random lasing (RL) from polycrystalline ZnO films has been realized by means of metal-insulator-semiconductor (MIS) structures based on ZnO films on silicon substrate. Herein, we investigate the effect of ZnO film thickness on the threshold current and output power of RL from the ZnO-based MIS structured devices. It is found that the RL threshold current increases with the increase of ZnO film thickness. Moreover, the output power of RL decreases with the increase of ZnO film thickness at small injection current, while it increases with the ZnO film thickness at large injection current. The mechanisms underlying the above-mentioned results have been tentatively explored in terms of the two ingredients of RL, i.e., multiple light scattering and optical gain.
The effect of shape anisotropy on the spectroscopic characterization of the magneto-optical activity of nanostructures113(2013); http://dx.doi.org/10.1063/1.4808449View Description Hide Description
How to measure magnetic field induced magneto-optical (MO) activity of nonmagnetic elliptical plasmonic nanodisks which rest on a dielectric substrate remains a challenge since the substrate contribute most of the overall MO which varies with light polarization with respect to the orientation of the nanodisks. Here, we present a spectroscopic characterization. We find that only when light is incident from the nanostructures' side with polarization aligned with one of the two symmetry axes, one can subtract the MO contribution from the substrate by an amount equal to that of a bare one. By a detailed polarizing transmittance measurement, we determine the orientation of the two symmetry axes of the nanodisks. Light polarization is then aligned along the axes, enabling measurement of the intrinsic MO activity of gold nanodisks, which is the overall MO activity subtracted by that of a bare glass substrate. The narrow line widths of the plasmonic resonance features in the MO spectra imply a potential application in refractive index sensing.
113(2013); http://dx.doi.org/10.1063/1.4808291View Description Hide Description
Based on the studies of the GaAs photocathode, the surface model of the Ga0.37Al0.63As photocathode is investigated and the energy distributions of electrons reaching the surface charge region, reaching the surface and emitting into vacuum are calculated. The (Cs, O) adsorption and photoemission characteristics of the Ga0.37Al0.63As photocathode are studied according to the experiments. We use the quantum efficiency formula to fit the experimental curve, and obtain the performance parameters of the photocathode and the surface barrier parameters. The results show that the surface barrier of the Ga0.37Al0.63As photocathode is similar to that of the GaAs photocathode. The prepared reflection-mode Ga0.37Al0.63As photocathode responds to the blue-green light, while the transmission-mode Ga0.37Al0.63As photocathode is sensitive to the 532 nm light.
113(2013); http://dx.doi.org/10.1063/1.4808455View Description Hide Description
Laser ablation of brass in air, water, and ethanol was investigated using a femtosecond laser system operating at a wavelength of 785 nm and a pulse width less than 130 fs. Scanning electron and optical microscopy were used to study the efficiency and quality of laser ablation in the three ablation media at two different ablation modes. With a liquid layer thickness of 3 mm above the target, ablation rate was found to be higher in water and ethanol than in air. Ablation under water and ethanol showed cleaner surfaces and less debris re-deposition compared to ablation in air. In addition to spherical particles that are normally formed from re-solidified molten material, micro-scale particles with varying morphologies were observed scattered in the ablated structures (craters and grooves) when ablation was conducted under water. The presence of such particles indicates the presence of a non-thermal ablation mechanism that becomes more apparent when ablation is conducted under water.
113(2013); http://dx.doi.org/10.1063/1.4808458View Description Hide Description
Laser ignition of energetic material composites was studied for initiation with heating rates from 9.5 × 104 to 1.7 × 107 K/s. This is a unique heating rate regime for laser ignition studies because most studies employ either continuous wave CO2 lasers to provide thermal ignition or pulsed Nd:YAG lasers to provide shock ignition. In this study, aluminum (Al) and molybdenum trioxide (MoO3) nanoparticle powders were pressed into consolidated pellets and ignited using a Nd:YAG laser (1064 nm wavelength) with varied pulse energy. Results show reduced ignition delay times corresponding to laser powers at the ablation threshold for the sample. Heating rate and absorption coefficient were determined from an axisymmetric heat transfer model. The model estimates absorption coefficients from 0.1 to 0.15 for consolidated pellets of Al + MoO3 at 1064 nm wavelength. Ablation resulted from fracturing caused by a rapid increase in thermal stress and slowed ignition of the pellet.
113(2013); http://dx.doi.org/10.1063/1.4809574View Description Hide Description
We propose and simulate a device structure of resonant cavity-enhanced quantum-dot infrared photodetector (RCE-QDIP). The RCE-QDIP consists of a conventional n-i-n QDIP sandwiched by a bottom GaAs/Al2O3 distributed Bragg reflector and a top mirror of Ge/SiO2 sub-wavelength grating. Aiming for detecting mid-infrared at 8 μm, the total thickness of the device is only ∼7.7 μm. According to our simulation, the external quantum efficiencies of RCE-QDIP could be as high as 59%-78% with the enhancement factors of 7–30, compared with a conventional QDIP. The proposed RCE-QDIP is highly feasible as the various fabrication parameters are considered.
Nonspecific detection of lead ions in water using a simple integrated optical polarimetric interferometer113(2013); http://dx.doi.org/10.1063/1.4809646View Description Hide Description
Real-time detection of heavy metal ions in water was implemented by using a composite optical waveguide (COWG) based polarimetric interferometer. The COWG was made by local deposition of a tapered nanometric layer of high-index materials onto a single-mode slab glass waveguide, and it is a low-cost robust waveguide with a locally large modal birefringence. The COWG-based polarimetric interferometer operates with a single incident laser beam and uses the transverse electric and transverse magnetic modes as the sensing and reference beams, respectively, and it can easily detect 0.1 ppm lead(II) ions in water via nonspecific adsorption on the tapered layer of TiO2. The excellent linearity was obtained between the lead(II) concentration and the ratio of concentration to the phase-difference change (Δϕ), suggesting that adsorption of lead(II) ions on the TiO2 film follows the Langmuir isotherm model. The saturation adsorption leads to Δϕ max = 7.485π. By use of the eigenvalue equations for a homogeneous waveguide to fit the measured refractive-index (RI) sensitivity of the interferometer, the equivalent thickness of T eq = 26.05 nm for the tapered TiO2 layer used was achieved. With T eq = 26.05 nm and Δϕ max = 7.485π and the thickness of 0.264 nm for the lead(II) adlayer, the adlayer RI was derived to be nad ≈ 1.945 at the maximum coverage.
Recording of self-induced waveguides in lithium niobate at 405 nm wavelength by photorefractive–pyroelectric effect113(2013); http://dx.doi.org/10.1063/1.4808321View Description Hide Description
We characterize the process of soliton waveguides (SWGs) recording at 405 nm wavelength using pyroelectric effect in lithium niobate (LN) crystals. We experimentally study and discuss the influence of the input irradiance, the polarization of the signal beam, and the crystal temperature change on the waveguide writing time and mode-profile. These characteristics significantly change when changing the recording wavelength. The advantages of recording SWGs in LN by using blue-violet light and pyroelectric field are emphasised. The generation of radiation at 405 nm wavelength by inexpensive laser diodes, the fast recording at this wavelength, and the convenient way to produce a static electric field inside the crystal by heating it with few degrees leads to a next step in the soliton waveguides recording process with applications in 3D integrated optical circuits.
The use of mould-templated surface structures for high-quality uniform-lying-helix liquid-crystal alignment113(2013); http://dx.doi.org/10.1063/1.4808341View Description Hide Description
The chiral-flexoelectrooptic effect in a Uniform Lying Helix (ULH) configuration provides a sub-millisecond in-plane rotation of the optic axis with the application of a transverse field. This enables displays with a wide viewing angle without costly in-plane-type electrodes. The salient challenge is one of alignment of the ULH, which is not topologically compatible with uniform alignment surface treatments. Here, we create a micro-grooved surface structure with features on the micron scale by using a replica-moulding technique. When the cell is assembled, the micro-grooves create channels, and using surface-energy considerations, we explain how and show experimentally that the channels align a cholesteric material in the ULH geometry with the helicoidal axis oriented parallel to the channels. The resultant alignment provides a high level of contrast between crossed polarizers and exhibits an electrooptic response with a switching time of the order of tens of microseconds.
113(2013); http://dx.doi.org/10.1063/1.4809045View Description Hide Description
A series of InAs x Sb1− x ternary thin films (x = 0-0.4) has been studied in a far-infrared reflection experiment over the range of 50-4000 cm−1 at room temperature. The obtained spectra are fitted using a multi-oscillator model. Two types of lattice vibration modes, InSb-like and InAs-like, plus one plasmon mode have been identified in x > 0 samples. The lattice vibration in these ternary alloy films shows a typical two-mode behavior. Within the studied fraction range, the InSb transverse-optical (TO) phonon frequency decreases with x, while the InAs TO frequency increases. A random-element-isodisplacement model has been employed to describe the phonon frequency changes. The fitted plasmon parameters have been used to extract the carrier concentrations and mobility. The carrier concentration increases monotonously with the increase of As fraction and is attributed to the bandgap narrowing effect. The mobility decreases with x, indicating an increased scattering.
- Plasmas and Electrical Discharges
113(2013); http://dx.doi.org/10.1063/1.4807298View Description Hide Description
A CH3F:O2 (50%:50%) inductively coupled discharge, sustained in a compact plasma reactor, was investigated as a function of power (20–400 W) and pressure (9–30 mTorr), using optical emission spectroscopy and Langmuir probe measurements. The electron density increased linearly with power but only weakly with pressure. The effective electron temperature was nearly independent of power and pressure. The gas temperature, obtained from the rotational structure of N2 (C → B) optical emission, increased from 400 to 900 K as a function of inductive mode power between 75 and 400 W at 10 mTorr. For constant feed gas flow, the absolute H, F, and O atom densities, estimated by optical emission rare gas actinometry, increased linearly with power. The absolute number density ratios H/Ar, F/Ar, and O/Ar, increased, decreased, and remained constant, respectively, with pressure. The H-atom density was estimated to be 5.4 × 1013 cm−3 at 400 W and 10 mTorr (gas temperature = 900 K), implying a high degree of dissociation of the CH3F feedstock gas. The F and O atom number densities were much lower (8.3 × 1012 cm−3 and 5.9 × 1012 cm−3, respectively) for the same conditions, suggesting that most of the fluorine and oxygen is contained in reaction products HF, CO, CO2, H2O, and OH. The relative number densities of HF, CO, and CO2 were observed to first rapidly increase with power, and then reach a plateau or decay slightly at higher power. Reaction mechanisms were proposed to explain the observed behavior of the number density of F and HF vs. power and pressure.
Temporally, spatially, and spectrally resolved barrier discharge produced in trapped helium gas at atmospheric pressure113(2013); http://dx.doi.org/10.1063/1.4809764View Description Hide Description
Experimental study was made on induced effects by trapped helium gas in the pulsed positive dielectric barrier discharge (DBD) operating in symmetrical electrode configuration at atmospheric pressure. Using fast photography technique and electrical measurements, the differences in the discharge regimes between the stationary and the flowing helium are investigated. It was shown experimentally that the trapped gas atmosphere (TGA) has notable impact on the barrier discharge regime compared with the influence of the flowing gas atmosphere. According to our experimental results, the DBD discharge produced in trapped helium gas can be categorized as a multi-glow (pseudo-glow) discharge, each discharge working in the sub-normal glow regime. This conclusion is made by considering the duration of current pulse (few μs), their maximum values (tens of mA), the presence of negative slope on the voltage-current characteristic, and the spatio-temporal evolution of the most representative excited species in the discharge gap. The paper focuses on the space-time distribution of the active species with a view to better understand the pseudo-glow discharge mechanism. The physical basis for these effects was suggested. A transition to filamentary discharge is suppressed in TGA mode due to the formation of supplementary source of seed electrons by surface processes (by desorption of electrons due to vibrationally excited nitrogen molecules, originated from barriers surfaces) rather than volume processes (by enhanced Penning ionisation). Finally, we show that the pseudo-glow discharge can be generated by working gas trapping only; maintaining unchanged all the electrical and constructive parameters.
113(2013); http://dx.doi.org/10.1063/1.4809766View Description Hide Description
The main processes related to discharges between pin and plate electrodes in hydrocarbon liquid (heptane) are modelled for micro-gap (from 10 to 100 μm) conditions. When a plasma channel hits the surface, a micro-crater is created. The different phenomena controlling the geometry (shape and dimension) of a single crater are described and included in a theoretical model developed for the specific case of pure aluminium. The influence of the most important parameters affecting the geometry of the crater is discussed. Among them, one finds the pressure exerted by the plasma on the liquid metal. It is found that the distribution of the pressure applied on the liquid pool changes significantly the way the plasma shapes the pool. It is assumed that at high charges, the pressure profile is tilted from the channel axis, leading to the formation of a central protrusion. On the other hand, we demonstrate that Thomson-Marangoni forces play an important role for crater diameters smaller than 5 μm. Then, the choice of the first derivative of the surface tension with respect to the temperature is a key factor. This effect is strongly related to the way convection displaces matter in the liquid pool. Finally, the quenching step is sufficiently fast to freeze the liquid shape as soon as the plasma vanishes.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
113(2013); http://dx.doi.org/10.1063/1.4804935View Description Hide Description
We report the near through mid-infrared (MIR) optical absorption spectra, over the range 0.05–1.3 eV, of monocrystalline silicon layers hyperdoped with chalcogen atoms synthesized by ion implantation followed by pulsed laser melting. A broad mid-infrared optical absorption band emerges, peaking near 0.5 eV for sulfur and selenium and 0.3 eV for tellurium hyperdoped samples. Its strength and width increase with impurity concentration. Its strength decreases markedly with subsequent thermal annealing. The emergence of a broad MIR absorption band is consistent with the formation of an impurity band from isolated deep donor levels as the concentration of chalcogen atoms in metastable local configurations increases.
The impact of film thickness and substrate surface roughness on the thermal resistance of aluminum nitride nucleation layers113(2013); http://dx.doi.org/10.1063/1.4808238View Description Hide Description
Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1 ± 2.8 m2K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). Atomic resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR.
113(2013); http://dx.doi.org/10.1063/1.4807786View Description Hide Description
Diamond anvil cell (DAC) experiments have been used in various studies to determine plastic flow strength in ductile metals at high pressure. To gain insight into the experiments and assess how accurately the material's strength at pressure can be determined, finite element simulations of DAC experiments are performed. In the analyses, constitutive responses are assumed for the diamonds and the vanadium test specimen; within the constitutive models, the pressure dependence of the strength is prescribed. The quantities typically measured during experiments are extracted from the simulations and analyzed in an identical manner as the experimental data would be to obtain the pressure dependent flow strength. This computed pressure dependent strength is then compared with the prescribed input, allowing the accuracy and sensitivities of the experimental technique to be evaluated. Recommendations are made to improve the accuracy of strength determinations.
Structure-stress-resistivity relationship in WTi alloy ultra-thin and thin films prepared by magnetron sputtering113(2013); http://dx.doi.org/10.1063/1.4808240View Description Hide Description
WTi thin films were prepared from an alloyed target (W:Ti ∼ 70:30 at. %) by magnetron sputtering. Body-centered cubic solid solutions with a fiber texture and columnar grains have been produced with . The sub-stoichiometry of Ti is shown to result from atom transport and, to a lesser extent, from resputtering. The stress-free lattice parameter values of the films are shown to be close to the bulk lattice parameter of α-W. The electrical resistivity of the produced WTi thin films is about , depending on the film thickness and microstructure (sputtering conditions). For both ultra-thin (9.5 nm) and thin (180 nm) films, a stress transition from compressive to tensile is observed as the working pressure increases. The process-structure-property relations of the WTi ultra-thin and thin films are discussed in relation with the state of the art.
Magneto-optical Kerr effect spectroscopy based study of Landé g-factor for holes in GaAs/AlGaAs single quantum wells under low magnetic fields113(2013); http://dx.doi.org/10.1063/1.4808302View Description Hide Description
Results from a magneto-optical Kerr effect (MOKE) spectroscopy study of ground state heavy-hole and light-hole excitons in GaAs/Al0.3Ga0.7As single quantum wells, with widths ranging from 4.3 nm to 14 nm, are presented. A novel setup and signal analysis procedure was adopted whereby polar MOKE measurements in magnetic fields up to 1.8 T could be performed with a conventional H-frame electromagnet. A first principles simulation based procedure used for simultaneously analyzing both the measured Kerr rotation and Kerr ellipticity spectral lineshapes is described in detail. The Zeeman splitting obtained from the above analysis helped to determine the longitudinal Landé g-factors. The hole g-factors were found to vary with well width, ranging from −0.6 to 1.1 for heavy-holes and 6.5 to 8.6 for light-holes. While the heavy-hole g-factor values are in fair agreement with values expected from k·p perturbation theory, no evidence is found to support theoretical predictions of possible well width dependent giant light-hole g-factors in GaAs/Al x Ga1–x As single quantum wells.