Index of content:
Volume 109, Issue 4, 15 February 2011
- Lasers, Optics, and Optoelectronics
Spatial hole burning in high-power edge-emitting lasers: A simple analytical model and the effect on laser performance109(2011); http://dx.doi.org/10.1063/1.3549155View Description Hide Description
A simple analytical model for the longitudinal distributions of photon and carrier densities in edge-emitting semiconductor lasers significantly above threshold is presented. It is shown that under the conditions considered, the shape of these distributions does not depend on pumping current. Good agreement with previous numerical and seminumerical investigations is obtained. It is shown that any direct effect of longitudinal spatial hole burning on the output power is seen only at very low output mirror reflectances and even then is weak, implying that the main effects of longitudinal nonuniformity are indirect ones, through carrier leakage.
109(2011); http://dx.doi.org/10.1063/1.3549157View Description Hide Description
Red, green, and blue stimulated emissions have been achieved in Ho3+doped oxyfluoride glass ceramic at room temperature. The material shows three emission bands at the red (650 nm), green (545 nm), and blue (488 nm) regions under infrared excitation at 750 nm. These emission bands are caused by a photonavalanche upconversion process previously reported. A pump and probe experimental setup has been designed to show stimulated emissions at the three bands. The pump power threshold for positive gain in the 490 nm band has been estimated around 2.7 kW/cm2. Higher thresholds are expected for the other bands.
Raman and emission characteristics of a-plane InGaN/GaN blue-green light emitting diodes on r-sapphire substrates109(2011); http://dx.doi.org/10.1063/1.3549160View Description Hide Description
Raman and emission properties of a nonpolar a-plane InGaN/GaN blue-green light emitting diode(LED) on an r-sapphire substrate are investigated and compared with a conventional c-plane blue-green LED. The output power of the a-plane LED was 1.4 mW at 20 mA. The c-plane LED has higher EQE, but it reaches the maximum at a lower forward current and the droop is faster than the a-plane counterpart. As the reverse bias increased, a blueshift in the PL spectra was not observed in the a-plane structure, which is indicative of an absence of quantum confined Stark effects. However, a strong blueshift in the electroluminescence spectra was still present, which means the In localization effects are relevant in nonpolar InGaN/GaN quantum wells. In the Raman spectra, a strong anisotropy of E 2(high) phonon modes was observed. By comparing the frequency of the E 2(high) modes, we demonstrate that the residual compressive strain in an a-plane LED is significantly smaller than in the polar counterpart.
109(2011); http://dx.doi.org/10.1063/1.3549732View Description Hide Description
Since titanium nitride appears golden and has a high conductivity, the possibility that it can be used in surfaceplasma wave applications in a manner similar to gold but with very strong scratch-resistance, is of interest. This work considers this possibility using the Kretschmann configuration, measuring the angle-dependent reflectivity as well as the wavelength-dependent reflectivity. Both sets of results demonstrate the excitation of a surfaceplasma wave at the TiN/air interface by an incident p-wave. The thickness of TiN that most efficiently couples the incident p-wave to the surfaceplasma wave is around 35 nm. All of the experiments are accompanied by corresponding numerical simulations.
109(2011); http://dx.doi.org/10.1063/1.3549731View Description Hide Description
The electrical derivative characteristics of quantum cascade lasers(QCLs) are investigated to test the QCL threshold, leakage current, and possibly explore carrier transport. QCL thresholds can be identified by searching for the slope peak of the first derivative of the I-V curves and can be further confirmed with its alignment to the peak of the second derivative of the I-V curves. Leakage current in QCLs with oxide-blocked ridge waveguides and buried heterostructure (BH) waveguides are studied and compared. The oxide-blocking structures provide the lowest leakage current although the capped-mesa-BH (CMBH) QCLs provide the toughest durability under highly stressful operations. The leakage current of CMBH QCLs are also compared at different temperatures.
109(2011); http://dx.doi.org/10.1063/1.3549836View Description Hide Description
The up-conversion (UC) photoluminescence and ferroelectric properties of Bi4− x Er x Ti3O12 (BErT) thin films were studied in terms of annealing temperature and Er3+doping concentration. The thin films were prepared by chemical solution deposition method. There are two green emission bands centered at 527 and 548 nm, and a red emission band centered at 663 nm in UC luminescence spectra measured under a 980 nm laser excitation at room temperature, which correspond to the radiative transitions from 2H11/2, 4S3/2, and 4F9/2 to 4I15/2, respectively. The quenching concentration of Er3+ions for green emission was as high as 20 mol % for Bi3.2Er0.8Ti3O12thin films. The large Er3+ quenching concentration and efficient energy transfer between two neighboring Er3+ions result in the improved UC emission. The dependence of UC emission intensity on pumping power indicated a two-photon UC emission process in the thin films. The combination of UC emission and ferroelectricity was realized in the capacitors of Pt/Bi3.25Er0.75Ti3O12/Pt/TiO2/SiO2/Si. The UC photoluminescent BErT ferroelectric thin films could be potentially applied to integrated optoelectronic devices.
High-Q microcavities in low-index one-dimensional photonic crystal slabs based on modal gap confinement109(2011); http://dx.doi.org/10.1063/1.3553450View Description Hide Description
Recently, various high quality factor photonic crystal microcavities have been demonstrated theoretically and experimentally with only one-dimensional periodicity. However, in most cases high-index materials such as silicon were chosen for easily achievable large photonic bandgap and elaborate refractive index modulation or taper structure is required for reducing radiation loss. Here, we present a design of high-Q microcavities in one-dimensional multilayer polystyrene photonic crystal slab structures with a low-index contrast of 1.59:1. Microcavities are introduced by simply decreasing the thickness of layers at the center region to form a double-heterostructure. A resonant mode with a quality factor up to 20 000 is obtained and found to originate from the modal gap confinement by comparing with a Fabry–Perot cavity. The dependence of the maximal quality factor on the cavity length further reveals that the small group velocity of light within the heterostructure cavity contributes significantly to the high-Q. In terms of the high quality factor, ease of fabrication, and large Kerr nonlinearity of polystyrene, our double-heterostructure microcavities will find potential application in realizing all-optical modulation devices.
109(2011); http://dx.doi.org/10.1063/1.3548896View Description Hide Description
In view of the emergence of wide ranging applications in the areas such as environmental monitoring, medical diagnostics, defense, security and sensing etc., it is indispensable to develop resourceful mid-infrared photodetectors. In this article, we present potential design considerations exploiting plasmonic effects in the conventional heterojunction mid-infrared detectors, optimized for their operation in 8–14 μm spectral range. Design issues concerning GaAs-AlGaAs based plasmonicphotodetectors are investigated using modal expansion method (MEM) in conjunction with Rayleigh expansion. Simple but insightful fitting expressions useful for designing practical photodetectors are presented. The effects of crucial design parameters on the photodetector performance are discussed in detail. Using metallic grating based plasmonic element, about 20 fold absorption enhancement is predicted, which is comparable or greater than that recently reported for InAs (Quantum Dots) and GaInNAs (Quantum Well) detectors. Photodetector designs showing considerable improvement in the responsivity and the specific detectivity, compared to their nonplasmonic but otherwise identical counterpart are presented.
109(2011); http://dx.doi.org/10.1063/1.3552914View Description Hide Description
A Zr47.7Cu31Ni9Al12.3 bulk metallic glass was irradiated directly by KrF excimer laser pulses with wavelength 248 nm and duration 10 ns. Scanning electronic microscope photographs indicated that many ripples in micro-nano scale would be generated on the edge of the irradiated area under the action of the higher intensity laser pulse. Detailed observation demonstrated that the ripples exhibited fluidity and became closer and closer out from interior. Theoretical analysis revealed the formation mechanism of the ripples, including melting, subsequent propagation of capillary waves and final solidification.
- Plasmas and Electrical Discharges
Current sheath behavior and its velocity enhancement in a low energy Mather-type plasma focus device109(2011); http://dx.doi.org/10.1063/1.3549017View Description Hide Description
The dynamics of the plasma sheath layer and its velocity enhancement have been studied in a low energy (4.9 kJ) Mather-type plasma focus device. Experiments were performed to study the effect of the Lorentz force variation on the current sheath expansion and movement, as well as the existence of traction between all parts of the sheath layer. Two different shape of anodes (cylindrical and step) along with an axial magnetic probe were used to investigate the effects of various experimental conditions, namely different charging voltages and gas pressures. In order to explore the upper limit of the current sheath velocity, a comparison has been made between the experimental data gathered by the probe and the Lee’s computational model. The limitations governing the enhancement of the current sheath velocity that can lead to the deterioration of a good focusing phenomenon were also investigated. The increase of the current sheath velocity due to the usage of the step anode on ion generation and hard x-ray emissions have been demonstrated by means of an ion collector and a hard x-ray detector.
109(2011); http://dx.doi.org/10.1063/1.3549159View Description Hide Description
This work reports the study of ion dynamics produced by ablation of Al,Cu,Ag,Au, and Bi targets using nanosecond laser pulses at 193 nm as a function of the laser fluence from threshold up to 15 J cm−2. An electrical (Langmuir) probe has been used for determining the ion yield as well as kinetic energy distributions. The results clearly evidence that ablation of Al shows unique features when compared to other metals. The ion yield both at threshold (except for Al, which shows a two-threshold-like behavior) and for a fixed fluence above threshold scale approximately with melting temperature of the metal. Comparison of the magnitude of the yield reported in literature using other wavelengths allows us to conclude its dependence with wavelength is not significant. The evolution of the ion yield with fluence becomes slower for fluences above 4–5 J cm−2 with no indication of saturation suggesting that ionization processes in the plasma are still active up to 15 J cm−2 and production of multiple-charged ions are promoted. This dependence is mirrored in the proportion of ions with kinetic energies higher than 200 eV. This proportion is not significant around threshold fluence for all metals except for Al, which is already 20%. The unique features of Al are discussed in terms of the energy of laser photons (6.4 eV) that is enough to induce direct photoionization from the ground state only in the case of this metal.
109(2011); http://dx.doi.org/10.1063/1.3549733View Description Hide Description
The effect of low-pressure He plasma on properties of nanoporous organosilicate glasses low-k films with 24% and 33% open porosity is studied. The influence of ions,VUVradiation, and metastable atoms are extracted separately using a special experimental system designed for this purpose. The low-k filmstreated in He plasma were exposed to O or H atoms in the downstream of high-pressure or rf discharge. The changes in chemical composition and structure occurring in low-k films were measured before and after all treatments. The loss probabilities of oxygen and hydrogen atoms on the low-k film surface were measured for both treated and pristine films. It is shown that the film pretreatment in He plasma leads to the noticeable densification of the top surface layer up to complete sealing all the films studied. The sealing layer prevents O atoms from deep penetration to the film bulk and carbon extraction. The sealing mechanism related to the joint impact of low-energy ions and VUVphotons with metastable atoms in He plasma is discussed in detail.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
109(2011); http://dx.doi.org/10.1063/1.3549170View Description Hide Description
In this study, the topography evolution of fused silicasurfaces during low-energy ion beamerosion has been investigated depending on the ion incidence angle and with focus on the importance of the initial surfacetopography. Ripple prepattern, also prepared by ion beamerosion, that exhibits an anisotropic surface with adjustable surface amplitudes and gradients was utilized. Based on experimental results that confirm smoothing and patterning behavior, gradient-dependent sputtering is identified being the dominant topography evolution mechanism.
Infrared spectroscopic study of pulsed laser deposited Fe3O4 thin film on Si (111) substrate across Verwey transition temperature109(2011); http://dx.doi.org/10.1063/1.3549237View Description Hide Description
We present low-temperature infrared measurements of magnetite (Fe3O4) thin films on Si (111) substrate across the Verwey transition temperature (TV). The line parameters of the most intense t1u mode observed in the Fourier transform infra red spectrum of the film is studied as a function of temperature. We observe that mode frequency increases abruptly at 130 K, and full width at half maxima of the mode increases abruptly at 113 K. The observations point out that structuraltransition may start earlier at 130 K than the actual Verwey transition (121 K) and that complete at 113 K. These results are consistent with the higher transition temperature as observed in resistivity and magnetization measurements.
109(2011); http://dx.doi.org/10.1063/1.3548936View Description Hide Description
The melting curve of NiTi alloy was predicted by using molecular dynamics simulations combining with the embedded atom model potential. The calculated thermal equation of state consists well with our previous results obtained from quasiharmonic Debye approximation. Fitting the well-known Simon form to our T m data yields the melting curves for NiTi: 1850(1 + P/21.938)0.328 (for one-phase method) and 1575(1 + P/7.476)0.305 (for two-phase method). The two-phase simulations can effectively eliminate the superheating in one-phase simulations. At 1 bar, the melting temperature of NiTi is 1575 ± 25 K and the corresponding melting slope is 64 K/GPa.
109(2011); http://dx.doi.org/10.1063/1.3549614View Description Hide Description
The use of ZnO bulk and especially nanolayer and nanowire structures for novel device applications has led to a renewal of interest in high-electron-density processes in ZnO, such as those occurring during lasing in ZnO. Using a pump-probe reflectometry technique, we investigate the ultrafast exciton dynamics of bulk ZnO under femtosecond laser excitation close to lasing conditions. Under intense excitation by 266-nm femtosecond (fs) pump pulses, the exciton resonance becomes highly damped and does not recover for several picoseconds. This slow recovery indicates a significant screening of the Coulomb interaction. Even below the lasing thresholds typically found for ZnO nanolayers and nanowires, we observe damping of the exciton resonance for several picoseconds, which indicates that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is electron-hole plasma recombination.
Broadband terahertz characterization of the refractive index and absorption of some important polymeric and organic electro-optic materials109(2011); http://dx.doi.org/10.1063/1.3549120View Description Hide Description
We report broad bandwidth, 0.1–10 THz time-domain spectroscopy of linear and electro-optic polymers. The common THz optical component materials high-density polyethylene, polytetrafluoroethylene, polyimide (Kapton), and polyethylene cyclic olefin copolymer (Topas) were evaluated for broadband THz applications. Host polymers polymethyl methacrylate, polystyrene, and two types of amorphous polycarbonate were also examined for suitability as host for several important chromophores in guest-host electro-optic polymer composites for use as broadband THz emitters and sensors.
Intersubband transition energy and linewidth modified by a submonolayer AlAs insertion into GaAs quantum wells109(2011); http://dx.doi.org/10.1063/1.3549126View Description Hide Description
We demonstrate a modification of the energy and linewidth of intersubband transitions by the hetero-insertion of a submonolayer into semiconductor quantum wells(QWs). Different changes in the transition energy and absorptionlinewidth are observed with systematically varied positions of a 0.92-monolayer AlAs insertion into 7.0-nm-wide GaAsQWs. The experimental results are well reproduced by theoretical calculations taking into account energy shifts and scattering processes in electron subbands due to the hetero-insertion. Our findings suggest that the combination of the insertion position and well width can be used to separately tune the transition energy and absorption/emission linewidth for optoelectronic devices.
Structural characterization of ZnO nanopillars grown by atmospheric-pressure metalorganic chemical vapor deposition on vicinal 4H-SiC and SiO2/Si substrates109(2011); http://dx.doi.org/10.1063/1.3549140View Description Hide Description
The structural characteristics of ZnOnanocrystals epitaxially grown on p-type (0001) 4H-SiC substrates were studied by transmission electron microscopy(TEM). The nanocrystallites were grown by atmospheric-pressure metalorganic chemical vapor deposition. The ZnOnanocrystals were formed at terraces introduced by vicinal 4H-SiC substrates toward the  direction. They had the shape of hexagonal nanopillars, with their edges parallel to the 〈〉 directions and a top c-plane facet, reflecting the crystal symmetry of ZnO. The free surface between the hexagonal nanopillars was covered by a very thin and highly defected epitaxialZnO film, which strongly suggests the Stranski-Krastanov mode of growth. The ZnO/SiC interface was systematically studied by plane view TEM and cross sectional high resolution TEM. The residual strain in the thin continuous film as well as in the nanopillars was estimated from Moiré patterns and by geometrical phase analysis. ZnO was also deposited on the SiO2/Si substrate for comparison. The films were polycrystalline exhibiting strong preferred orientation, with the c-axes of the grains almost perpendicular to the substrate resulting in the formation of nanopillars. The differences of nanopillar formation in the two substrates, 4H-SiC and SiO2 is also discussed.
109(2011); http://dx.doi.org/10.1063/1.3549735View Description Hide Description
Tears in any material act as barriers to phonon transport. In this study, molecular dynamics simulations are employed to investigate thermal transport around tears in graphene. Specifically, thermal boundary conductance across different tear orientations and lengths is computed. Analysis of vibrational density of states suggests that long-wavelength acoustic phonons within the spectrum range 0–700 cm−1 are vital to thermal transport across the tears. Different phononscattering phenomena are observed for both tear orientations. It is proposed that the dissimilitude of the scattering processes encountered by phonons carrying energy around the tears to the opposite end explains why thermal transport is generally more efficient for longitudinal tears in our simulations.