Volume 113, Issue 11, 21 March 2013
Index of content:
- Lasers, Optics, and Optoelectronics
Optical bistability via dual electromagnetically induced transparency in a coupled quantum-well nanostructure113(2013); http://dx.doi.org/10.1063/1.4795282View Description Hide Description
We investigate optical bistability (OB) behavior in an asymmetric three-coupled quantum well structure inside a unidirectional ring cavity. By controlling the assisting coherent driven field and the frequency detunings of the two control laser fields, we find that the appearance and disappearance of OB can easily be controlled by adjusting the positions of the dual electromagnetically induced transparency windows. Analysis in the dressed-state picture is also given. Our scheme may be used for building more efficient all-optical switches and logic-gate devices for optical computing and quantum information processing.
Near-ultraviolet light-emitting diodes with transparent conducting layer of gold-doped multi-layer graphene113(2013); http://dx.doi.org/10.1063/1.4795502View Description Hide Description
We report on gold (Au)-doped multi-layer graphene (MLG), which can be used as a transparent conducting layer in near-ultraviolet light-emitting diodes (NUV-LEDs). The optical output power of NUV-LEDs with thermally annealed Au-doped MLG was increased by 34% compared with that of NUV-LEDs with a bare MLG. This result is attributed to the reduced sheet resistance and the enhanced current injection efficiency of NUV-LEDs by the thermally annealed Au-doped MLG film, which shows high transmittance in NUV and UV regions and good adhesion of Au-doped MLG on p- GaN layer of NUV-LEDs.
113(2013); http://dx.doi.org/10.1063/1.4795272View Description Hide Description
We demonstrate three-dimensional position and force detection of single gold nanosphere (GNP) and gold nanorod (GNR) particles in optical trap by combining backscattered light detection and dark field imaging. The trapping stiffness of the GNPs and GNRs for all three dimensions is measured. The results show that the spring constants in the propagation direction of the trapping laser are somewhat weaker than in other two directions for GNPs. While for GNRs, the spring constants in the polarization direction of the trapping laser are a little weaker than in other two directions. The effect of trapping laser polarization on the particles yields different spring constants in the transverse plane which is perpendicular to the propagation direction. And this effect is larger on GNRs than GNPs.
113(2013); http://dx.doi.org/10.1063/1.4794807View Description Hide Description
We present here a study of hard X-ray irradiation of soda-lime glasses performed in situ and in real time. For this purpose, we have used a Au thin film grown on glass and studied the excitation of its surface plasmon resonance (SPR) while irradiating the sample with X-rays, using a recently developed experimental setup at a synchrotron beamline [Serrano et al., Rev. Sci. Instrum. 83, 083101 (2012)]. The extreme sensitivity of the SPR to the features of the glass substrate allows probing the modifications caused by the X-rays. Irradiation induces color centers in the soda-lime glass, modifying its refractive index. Comparison of the experimental results with simulated data shows that both, the real and the imaginary parts of the refractive index of soda-lime glasses, change upon irradiation in time intervals of a few minutes. After X-ray irradiation, the effects are partially reversible. The defects responsible for these modifications are identified as non-bridging oxygen hole centers, which fade by recombination with electrons after irradiation. The kinetics of the defect formation and fading process are also studied in real time.
113(2013); http://dx.doi.org/10.1063/1.4795584View Description Hide Description
We present a simple and accurate optical model for simulation of the optical properties of top-emitting organic light emitting diodes (OLEDs). Based on the model, we investigated the optical properties of top-emitting OLEDs and optimized the structure of a blue top-emitting OLED based on 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl. The device optimization process focused on matching the optical properties of each functional layer. The out-coupling efficiency of device with optimized structure can be enhanced by 80%; in addition, the color purity of blue emission is improved. Experimental results show excellent agreement with the simulation results.
Chirp structure measurement of a supercontinuum pulse based on transient lens effect in tellurite glass113(2013); http://dx.doi.org/10.1063/1.4795587View Description Hide Description
We demonstrated the controllable acquisition of optical gated spectra from a chirped supercontinuum (SC) pulse based on ultrafast transient lens (TrL) effect. Comparing with CS2, the gated spectra had much narrower spectral bandwidths using tellurite glass (Te glass) as the nonlinear medium due to its ultrafast nonlinear response. Experimental results showed that the chirp structure of the SC pulse measured by TrL method was quite accordant with that measured by femtosecond optical Kerr gate method.
113(2013); http://dx.doi.org/10.1063/1.4796099View Description Hide Description
Plasmon resonances and the plasmon-induced field enhancement (FE) in sodium nanoring dimers are investigated by time-dependent density functional theory. For larger separations, the optical absorption, the induced charge response and the frequency dependent current demonstrate that there are two capacitive coupling plasmon modes. One feature of FE is that, in the surface region of the nanoring, it has a very large maximum. Another feature of FE is that, along the perpendicular bisector of the line segment joining the two nanoring center points in the middle region of the nanoring dimers, it has maxima. With the decrease of the gap distance, because of the electrons tunneling across the dimer junction and screening, collective excitation modes are changed, and the charge transfer plasmon modes emerge in the nanoring dimers. FE induced by any plasmon modes decreases in the gap region. Moreover, corresponding to different gap distances, the high-energy plasmon resonance peak almost does not shift, because this plasmon mode is mainly the collective excitation as a result of interactions among degenerate individual electronic states.
Dember type voltage and nonlinear series resistance of the optical confinement layer of a high-power diode laser113(2013); http://dx.doi.org/10.1063/1.4795586View Description Hide Description
An analytical model is developed for the carrier density distribution and the associated Dember type electric field/voltage in the waveguide layer of a high-power semiconductor laser for arbitrary levels of doping and injection. Nonlinear resistance of the waveguide layer is analysed; it is shown that at a very high injection level, doping the waveguide layer leads to almost negligible decrease in its effective resistance.
Effect of oscillator strength and intermediate resonance on the performance of resonant phonon-based terahertz quantum cascade lasers113(2013); http://dx.doi.org/10.1063/1.4795614View Description Hide Description
We experimentally investigated the effect of oscillator strength (radiative transition diagonality) on the performance of resonant phonon-based terahertz quantum cascade lasers that have been optimized using a simplified density matrix formalism. Our results show that the maximum lasing temperature (T max) is roughly independent of laser transition diagonality within the lasing frequency range of the devices under test (3.2–3.7 THz) when cavity loss is kept low. Furthermore, the threshold current can be lowered by employing more diagonal transition designs, which can effectively suppress parasitic leakage caused by intermediate resonance between the injection and the downstream extraction levels. Nevertheless, the current carrying capacity through the designed lasing channel in more diagonal designs may sacrifice even more, leading to electrical instability and, potentially, complete inhibition of the device's lasing operation. We propose a hypothesis based on electric-field domain formation and competition/switching of different current-carrying channels to explain observed electrical instability in devices with lower oscillator strengths. The study indicates that not only should designers maximize T max during device optimization but also they should always consider the risk of electrical instability in device operation.
Terahertz quantum cascade lasers with thin resonant-phonon depopulation active regions and surface-plasmon waveguides113(2013); http://dx.doi.org/10.1063/1.4795606View Description Hide Description
We report three-well, resonant-phonon depopulation terahertz quantum cascade lasers with semi-insulating surface-plasmon waveguides and reduced active region (AR) thicknesses. Devices with thicknesses of 10, 7.5, 6, and 5 μm are compared in terms of threshold current density, maximum operating temperature, output power, and AR temperature. Thinner ARs are technologically less demanding for epitaxial growth and result in reduced electrical heating of devices. However, it is found that 7.5-μm-thick devices give the lowest electrical power densities at threshold, as they represent the optimal trade-off between low electrical resistance and low threshold gain.
- Plasmas and Electrical Discharges
113(2013); http://dx.doi.org/10.1063/1.4795269View Description Hide Description
This paper describes simulations of nanosecond pulse plasma formation between planer electrodes covered by dielectric barriers in air at atmospheric pressure and 340 K. The plasma formation process starts as electrons detach from negative ions of molecular oxygen that are produced from the previous discharge pulse. An ionization front is found to form close to the positively biased electrode and then strengthens and propagates towards the grounded electrode with increasing gap voltage. Charge accumulation and secondary emission from the grounded electrode eventually lead to sheath collapse. One interesting feature is a predicted reversal in gap potential due to the accumulated charge, even when there is no reversal in applied potential. The simulation results are compared to recent measurement of mid-gap electric field under the same discharge conditions [Ito et al., Phys. Rev. Lett. 107, 065002 (2011)].
113(2013); http://dx.doi.org/10.1063/1.4794849View Description Hide Description
Particle-induced electron emission (PIE) is modeled in a simplified, well-characterized plasma Test Cell operated at UCLA. In order for PIE to be a useful model in this environment, its governing equations are first reduced to lower-order models which can be implemented in a direct simulation Monte Carlo and Particle-in-Cell framework. These reduced-order models are described in full and presented as semi-empirical models. The models are implemented to analyze the interaction of low- and high-energy (∼1–2 keV) xenon ions and atoms with the stainless steel electrodes of the Test Cell in order to gain insight into the emission and transport of secondary electrons. Furthermore, there is a lack of data for xenon-stainless steel atom- and ion-surface interactions for similar environments. Using experimental data as a reference, both total yields and emitted electron energy distribution functions can be deduced by observing sensitivities of current collection results to these numerical models and their parameters.
113(2013); http://dx.doi.org/10.1063/1.4795604View Description Hide Description
A model is developed to describe the electrostatic boundary layer in a positively biased magnetic filter in filtered arcs with low collisionality. The set of equations used includes the electron momentum equation, with an anomalous collision term due to micro-instabilities leading to Bohm diffusion, electron mass conservation, and Poisson equation. Analytical solutions are obtained, valid for the regimes of interest, leading to an explicit expression to determine the electron density current to the filter wall as a function of the potential of the filter and the ratio of electron density at the plasma to that at the filter wall. Using a set of planar and cylindrical probes it is verified experimentally that the mentioned ratio of electron densities remains reasonably constant for different magnetic field values and probe bias, which allows to obtain a closed expression for the current. Comparisons are made with the experimentally determined current collected at different sections of a positively biased straight filter.
113(2013); http://dx.doi.org/10.1063/1.4795765View Description Hide Description
Currently, further clarification of pre-breakdown phenomena in water such as propagation mechanisms of primary and secondary streamers are needed because applications of aqueous plasma to environmental and medical treatments are increasing. In this study, a series of primary streamer propagations in ultrapure water was visualized at 100-Mega frames per second (100 Mfps) in the range of 400 μm square using an ultra high-speed camera with a microscope lens when a single-shot pulsed positive voltage was applied to a needle electrode placed in a quartz cell. Every observation was synchronized with the waveforms of the applied voltage and the discharge current. The primary streamer, having many filamentary channels, started to propagate semi-spherically with a velocity of about 2 km/s when the pulsed currents occurred. Although most filamentary channels disappeared 400 ns after the beginning of the primary streamer, a few of them continued propagating with almost the same velocity (about 2 km/s) as long as the repetitive pulsed currents flowed. Shock waves were iteratively generated and streamer channels were formed while the repetitive pulsed currents were flowing. Thus, we concluded that the positive primary streamer in water propagates progressively with each repetitive pulsed current.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Formation of a ZnO2 layer on the surface of single crystal ZnO substrates with oxygen atoms by hydrogen peroxide treatment113(2013); http://dx.doi.org/10.1063/1.4792941View Description Hide Description
Formation of a ZnO 2 layer by H2O2 treatment for single crystal ZnO (0001) substrates was studied. X-ray diffraction (XRD) peaks of ZnO 2 with a pyrite structure were observed in XRD 2θ-ω scan patterns of the O-face of single crystal ZnO (0001) substrates with H2O2 treatment, but these peaks were not observed in patterns of the Zn-face of ZnO (0001) substrates with H2O2 treatment. XRD ω scan patterns of the ZnO (0002) plane of the O-face of single crystal ZnO (0001) substrates were broadened at the tail of the pattern by H2O2 treatment, but such broadening was not observed in that plane of the Zn-face. Grain structure of ZnO 2 layers was clearly observed in atomic force microscopy (AFM) images for the O-face of ZnO (0001) substrates with H2O2 treatment. Spectra of X-ray photoelectron spectroscopy (XPS) of the O-face of ZnO (0001) substrates with H2O2 treatment showed a definite peak shift of the O 1s peak. It is thought that a pyrite structure of ZnO 2 is easily formed around an O atom of the O-face of ZnO (0001) substrates. Results of XRD measurements, the AFM image, and XPS measurement of the H2O2-treated single crystal ZnO ( ) substrate that has oxygen atoms on the surface appeared to be the same as those of the O-face of ZnO (0001) substrates.
113(2013); http://dx.doi.org/10.1063/1.4795504View Description Hide Description
The structural phase transitions in Sm2O3 with mixed phases of cubic and monoclinic as starting material were studied by in situ high-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements up to 40.1 GPa and 41.0 GPa, respectively. The x-ray diffraction data indicate that the monoclinic and cubic phases begin to transform to a hexagonal phase at 2.5 and 4.2 GPa, respectively. The hexagonal phase is stable up to at least 40.1 GPa and could not be quenched to ambient conditions. These phase transitions have also been confirmed by Raman spectroscopy. A third-order Birch-Murnaghan fit based on the observed pressure-volume data yields zero pressure bulk moduli B0 = 149(2), 153(7), and 155(5) GPa for cubic, monoclinic, and hexagonal phases, respectively, when their first pressure derivatives ( ) were fixed as 4. The pressure coefficients of Raman peaks and the mode Grüneisen parameters of different Raman modes were also obtained. Coupled with previous results, we conclude that the transition pressure of medium rare-earth sesquioxides from the cubic and monoclinic to the hexagonal phase increase with the decreasing of the cation radius.
Vibrational, elastic, and structural properties of cubic silicon carbide under pressure up to 75 GPa: Implication for a primary pressure scale113(2013); http://dx.doi.org/10.1063/1.4795348View Description Hide Description
We present results of concomitant measurements of synchrotron x-ray diffraction (XRD), Brillouin, and Raman spectroscopy on the single crystal samples of cubic silicon carbide (3C-SiC) under quasi-hydrostatic pressures up to 65 GPa, as well as x-ray diffraction and Raman spectroscopy up to 75 GPa. We determined the equation of state of 3C-SiC and pressure dependencies of the zone-center phonon, elastic tensor, and mode Gruneisen parameters. Cubic SiC lattice was found to be stable up to 75 GPa, but there is a tendency for destabilization above 40 GPa, based on softening of a transverse sound velocity. By applying the concomitant density and elasticity measurements, we determined the pressure on the SiC sample without referring to any other pressure scale thus establishing a new primary pressure scale with a 2%–4% precision up to 65 GPa. We proposed corrections to the existing ruby and neon pressure scales, and also calibrated cubic SiC as a pressure marker for the x-ray diffraction and Raman experiments.
113(2013); http://dx.doi.org/10.1063/1.4795263View Description Hide Description
Cerium ions were implanted into wurtzite GaN/sapphire thin films at doses 3 × 1014 and 2 × 1015 cm−2. The samples were annealed at 900 °C and studied using photoluminescence (PL), Raman, optical transmission spectroscopy and Hall measurement techniques. Near band emission signs were found absent for implanted samples and two new luminescence centers were recorded. Detailed temperature dependence PL measurements were carried out to find possible origin of these centers. The observed luminescence lines are assigned to the radiative recombination of electrons bound to the complex CeGa-VN and isoelectronic CeGa traps with holes from valance band.
113(2013); http://dx.doi.org/10.1063/1.4795268View Description Hide Description
An experimental and theoretical study of the effect of excimer laser annealing (ELA) on B redistribution and electrical activation in Ge is reported. We performed detailed structural, chemical, and electrical characterizations of Ge samples implanted with B (20 keV, 1 × 1015, or 1 × 1016 B/cm2) and processed by ELA (λ = 308 nm) with multiple pulses (1, 3, or 10). We also developed a diffusion model, in order to simulate the B redistribution induced by the ELA process. We found an anomalous impurity redistribution in the molten phase, which causes a dopant incorporation during the melt-growth at the maximum melt depth. The investigated samples showed a partial electrical activation of the B dopant. The inactivation of B in the samples implanted with 1 × 1015 B/cm2 was correlated to an oxygen contamination, while the poor electrical activation of B in the samples implanted with 1 × 1016 B/cm2 was related to the precipitation of the dopant, in good agreement with the experimental and theoretical results.
113(2013); http://dx.doi.org/10.1063/1.4795510View Description Hide Description
We investigate the impact of isovalent (in particular lead (Pb)) doping on the production and thermal stability of the vacancy-related (VO) and the interstitial-related (CiOi and CiCs) pairs in 2 MeV electron irradiated Si samples. We compare the Cz-Si samples with high and low carbon concentration, as well as with Pb-C and Ge-C codoped samples. Using Fourier Transform Infrared Spectroscopy (FTIR), we first determine that under the examined conditions the production of VO decreases with the increase of the covalent radius of the prevalent dopant. Moreover, the production of the VO, CiOi, and CiCs pairs is quite suppressed in Pb-doped Si. In addition, we conclude to an enhanced trapping of both Ci and Cs by Pb impurity under irradiation. The results are further discussed in view of density functional theory calculations. The relative thermodynamic stability of carbon and interstitial related complexes was estimated through the calculations of binding energies of possible defect pairs. This allows to investigate the preferred trapping of vacancies in Pb-doped samples and interstitials in the Ge-doped samples. The different behavior is revealed by considering the analysis of the ratio of vacancy-related to interstitial-related clusters derived from the FTIR measurements. The presence of PbV complexes is confirmed due to the mentioned analysis.