Volume 110, Issue 9, 01 November 2011
Index of content:
- Lasers, Optics, and Optoelectronics
Properties of defected one-dimensional terahertz plasmonic crystal films in a metal air-gap waveguide110(2011); http://dx.doi.org/10.1063/1.3658255View Description Hide Description
We investigated properties of localized modes of one-dimensional terahertz plasmonic crystal films with defects in a metal air-gap waveguide by terahertz time-domain spectroscopy experiments and finite-difference time-domain simulations. A defect was created by varying the width of an air slit at the center of the plasmonic crystal film. The donor (accept)-like defect modes were observed when the width of the defect air slit increases (decreases) from an initial width. The quality factor increases abruptly as the air-gap size decreases, while the defect mode frequency slightly decreases. The high quality-defect mode was examined for terahertz sensor applications.
110(2011); http://dx.doi.org/10.1063/1.3658247View Description Hide Description
An optical switch based on liquid crystal dielectric loaded surface plasmon polariton waveguides is proposed and theoretically analyzed. The infiltration of the plasmonic structure with a nematic liquid crystalline material serving as the dielectric loading is shown to allow for extensive electrical tuning of its waveguiding characteristics. Both the electrical switching and optical properties of the proposed waveguide are investigated in the context of designing a directional coupler optical switch, which is found to combine efficient voltage control, low power consumption, high extinction ratio, and relatively low insertion losses.
110(2011); http://dx.doi.org/10.1063/1.3658248View Description Hide Description
We investigated the photoluminescence(PL) of CdTe doped with indium using above- and below-bandgap excitation at temperatures of 4.5−20 K. We recorded and measured the selectively excited PL arising from the recombination of donor-acceptor (D–A) pairs with the A-center acceptor in the spectral region of the 1.4-eV PL band for different excitation photonenergies,. Sharp, strong PL lines that shifted with over the total contour of the D–A pair band represented the selective pair luminescence(SPL). The energy difference of ∼125 meV between the excited- and ground-state of the charged D–A pair is very close to the 6-longitudinal-optical phononenergy in CdTe. This multiplicity favors the relaxation of an excited hole to the ground state of an acceptor, and increases the probability of recombination in the D–A pair. The SPL line quenches with temperature, characteristically with energy of 6–14 meV for D–A pairs with different D-A distances. The temperature shift of the 1.4-eV band supposedly is caused by the redistribution of occupied- and empty-shallow donors neighboring the A-center.
Performance enhancement of blue light-emitting diodes with AlGaN barriers and a special designed electron-blocking layer110(2011); http://dx.doi.org/10.1063/1.3651393View Description Hide Description
In this study, the characteristics of the nitride-based blue light-emitting diode (LED) with AlGaN barriers are analyzed numerically and experimentally. The emission spectra, carrier concentrations in the quantum wells (QWs), energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results indicate that the LED with AlGaN barriers has a better hole-injection efficiency and an enhanced carrier confinement in its active region over the conventional counterpart with GaN barriers. The results also show that the AlGaN electron-blocking layer (EBL) with a gradual variation of Al mole fraction has a significantly enhanced electron blocking capability as well as a greatly improved hole-injection efficiency. When Al0.08Ga0.92N QW barriers and the special designed EBL are used, the electroluminescence emission intensity is increased greatly by 69% at 200 A/cm2 and the efficiency droop is reduced markedly to 8.7% from 85% at 400 A/cm2 compared with those of the conventional LED.
110(2011); http://dx.doi.org/10.1063/1.3658257View Description Hide Description
Bent-waveguide laser emitting around 1550 nm was tuned by external grating (in Littrow configuration) at temperatures from 300 K down to 80 K and at hydrostatic pressures up to 2.2 GPa. The tuning range achieved with grating was significantly reduced at lower temperatures. The tuning range achieved by pressure and grating was 390 nm (from 1220 to 1610 nm). In this range the emission power of the laser was above 30 mW with narrow emission line and the side-mode suppression ratio above 30 dB. Our results were interpreted by calculating the pressure and temperature dependence of gain spectra determined from a nonparabolic 8×8 kp model.
110(2011); http://dx.doi.org/10.1063/1.3658849View Description Hide Description
104 times higher Er3+:2.7 μm fluorescence was observed when Ho3+ was added into an Er3+doped fluorophosphate glass under 976 nm excitation. Obviously changed J-O parameters and the calculated spectroscopic properties of Er3+ prove that Ho3+ influences the local environment of Er3+ greatly. Increased upconversion luminescence around 550 nm and 658 nm shows that heat generation rises with Ho3+ introduction. Complicated energy transfer processes exist in this codoping situation and interactions among the excessive Ho3+ions are considered as the dominant energy loss mechanisms, otherwise, the sensitizing effect from Ho3+ to Er3+ is very high if Ho3+:Er3+ ratio below 4. All the calculated results coincide well with the measured ones. Excellent 2.7 μm spectroscopic property, Raman analysis, together with the outstanding glass forming ability identify that this Er3+/Ho3+ codoped fluorophosphate glass can be effectively used for Er3+:2.7 μm fiber laser.
110(2011); http://dx.doi.org/10.1063/1.3660270View Description Hide Description
The recent realization of silicon core optical fibers has the potential for novel low insertion loss rack-to-rack optical interconnects and a number of other uses in sensing and biomedical applications. To the best of our knowledge, incoherent light source based rapid photothermal processing (RPP) was used for the first time to anneal glass-clad silicon core optical fibers.X-ray diffraction examination of the silicon core showed a considerable enhancement in the length and amount of single crystallinity post-annealing. Further, shifts in the Raman frequency of the silicon in the optical fiber core that were present in the as-drawn fibers were removed following the RPP treatment. Such results indicate that the RPP treatment increases the local crystallinity and therefore assists in the reduction of the local stresses in the core, leading to more homogenous fibers. The dark current-voltage characteristics of annealedsiliconoptical fiber diodes showed lower leakage current than the diodes based on as-drawn fibers. Photons in UV and vacuum ultraviolet (VUV) regions play a very important role in improving the bulk and carrier transport properties of RPP-treated siliconoptical fibers, and the resultant annealing permits a path forward to in situ enhancement of the structure and properties of these new crystalline core optical fibers.
Influence of local atomic configuration in AlGdN phosphor thin films on deep ultra-violet luminescence intensity110(2011); http://dx.doi.org/10.1063/1.3658845View Description Hide Description
We investigated the narrowband ultraviolet emission properties of Al0.94Gd0.06N phosphorthin films pumped by an electron beam. An extremely narrow luminescence line, which was less than 1 nm from the intra-orbital f-f transition in Gd3+ ions, was confirmed at 318 nm. The corresponding emission efficiency was improved by decreasing the growth temperature. The extended X-ray absorption fine structure analysis of the local atomic structure revealed that a low-temperature growth led to the formation of a uniform atomic configuration around Gd, which was found to play a key role in improving the luminescence intensity of the films.
110(2011); http://dx.doi.org/10.1063/1.3660207View Description Hide Description
We report a simulation of quantum cascade lasers based on the integration of a number of optoelectronic models on both microscopic and macroscopic scales. On the microscopic scale, quantum mechanical computation was performed to find the quantization states and a rate equation approach was used to compute the optical gain. On the macroscopic scale, we solved the drift-diffusion equations with modification of current density to account for long-range carrier transport, including quantum tunneling, mini-band tunneling, and hot carrier transport. Multiple lateral optical modes were computed by solving a scalar wave equation as an eigenvalue problem. Finally, multiple lateral mode laser cavity photon rate equations were solved with the drift-diffusion equations in a self-consistent manner to predict the lasing characteristics of a quantum cascade laser. The simulation compared the integrated models with experimental data from a number of AlInGaAs/InP systems with variable quantum wells and at different temperatures. Reasonable agreements with experiments have been obtained for both electrical and lasing characteristics.
110(2011); http://dx.doi.org/10.1063/1.3660230View Description Hide Description
In this work, terahertz (THz) temperature-dependentproperties of defect mode in a defective semiconductor-dielectric photonic crystal (SDPC) are theoretically investigated based on the calculated transmittance spectrum. Two different defective PCs, the symmetric structure of (Si/SiO2) N InSb(SiO2/Si) N and the asymmetric one of (Si/SiO2) N InSb(Si/SiO2) N , will be considered. With a strongly temperature-dependentpermittivity in defect layer InSb, the defect mode can be thermally tuned, that is, the defect frequency will be shifted to higher frequency as the temperature increases. With the inherent loss in InSb, the strength of defect mode will be strongly depressed at a higher temperature. We use the condition of impedance match to explain the presence of defect mode. The understanding of properties of defect mode could be of technical use in the terahertz optoelectronic applications.
Raman and photoluminescence characterization of focused ion beam patterned InGaN/GaN multi-quantum-wells nanopillar array110(2011); http://dx.doi.org/10.1063/1.3658866View Description Hide Description
High crystal quality GaNnanopillar arrays containing InGaN/GaN multi-quantum wells (MQWs) have been fabricated by focused ion beam followed by wet etch treatments to remove the ion damage. The first order Raman spectra reveal a well-built additional peak when the diameter of the nanopillars is less than 220 nm. This peak is also observed in the GaN pillars without MQW and is clearly assigned to the surface optical (SO) mode originating from the A1phonon in wurtzite GaN. The frequency of this SO mode is found to be sensitive with the diameter and surface roughness of the nanopillars. Temperature-variable photoluminescencemeasurements show that a broadband emission in the as-grown sample split into the two well-resolved bands for nanopillars and the emission band at the higher energy side quickly thermally quenched.
110(2011); http://dx.doi.org/10.1063/1.3658847View Description Hide Description
We have developed a comprehensive theory about optical control of p – dexchange interaction between spins of hole and Mn 2+ in single-manganese dopedGaAs material irradiated by a monochromatic, linearly polarized, intense pulsed laser field (PLF) under nonresonant conditions. The p – dexchange interaction leads to formation of magnetic polaron. While the PLF induces a dressed acceptor Coulomb potential, which transforms single center problem into the one with two virtual positively charged centers, resembling hydrogen molecule ion . The dichotomy of hole wave functions, determined by the laser-intensity, affects strongly the p – dexchange interaction as well as binding energy of magnetic polaron. Increasing the laser intensity reduces the magnetic polaron binding energy. At larger excitation intensity, the magnetic polaron can be completely dissolved.
- Plasmas and Electrical Discharges
Electron-emission yield under electron impact of ceramics used as channel materials in Hall-effect thrusters110(2011); http://dx.doi.org/10.1063/1.3653820View Description Hide Description
We report measurement of electron-emission yield (EEY) under the impact of electrons on materials of Hall-effect-thruster (HET) interest: BN, BN–SiO2, and Al2O3. The effects of the material aging (under electron irradiation) on the yield of BN and Al2O3 are investigated. The EEY of BN grows with electron exposure, whereas that of Al2O3 reduces. A simple analysis of our experimental results indicates that these variations are most likely because of surface and near surface composition changes caused by the electron beam. The representativeness of EEY measurements on ceramics that have not suffered from the specific environment of a HET (ion and electron bombardment) is discussed.
110(2011); http://dx.doi.org/10.1063/1.3658812View Description Hide Description
A two-dimensional magneto-hydrodynamic (MHD) model of an ablation-dominated capillary discharge is developed. Special attention is paid to the interaction between the plasma and the capillary in two-dimensional geometry. The mass exchange process is taken into account by adopting a kinetic ablation model and a revised depositionmodel. The momentum and energy exchange processes associated with the ablated and deposited mass are also considered. By solving this model, key plasma parameters are calculated and presented in the paper. It is shown that distinct two-dimensional features exist in the distribution of these plasma parameters. Further, the calculated temperature results are consistent with previously presented measurements
110(2011); http://dx.doi.org/10.1063/1.3660386View Description Hide Description
Long electrical arcs that are created using high voltage impulses in air gaps require average electric fields (AEFs) in the order of 100 to 1000 kV/m. In this paper, a method is presented where arcs can be created using AEFs of just 4.5 kV/m. The results from experiments up to 60 m long are used to form a mathematical model that predicts the occurrence of arc formation. The parameters of the energy supply, namely the voltage and capacitance, are found to determine the length of the arc that can be obtained. This model can be used by other research groups to reliably create long arcs; it is shown that many high voltage laboratories already have equipment capable of producing arcs several hundred meters long by using this method.
110(2011); http://dx.doi.org/10.1063/1.3660764View Description Hide Description
The process of electron beam interaction with metal targets was characterized using electrical and optical diagnostics. Electron beams with current density of 5–10 A/cm2, electron energy up to 120 keV, pulse duration up to 200 μs, and cross-sectional area of 8–30 cm2 at the target surface were generated by GESA I and GESA II facilities. Streak imaging of the target surface specular reflectivity was used to determine the onset of melting and re-solidification of the target surface. Using time- and space-resolved schlieren imaging, the evolution of surface irregularities was studied. Experimental and numerical investigations of the neutral flow evaporated from the target surface showed a neutral density of ∼1019 cm−3 in the vicinity of the target and neutral velocities up to 2 × 105 cm/s. Framing and streak images of visible light emission were used to study the temporal evolution of the target surfaceplasma and vapors. Time- and space-resolved spectroscopy was applied to determine the surfaceplasma density and temperature, which were found to be ∼1014 cm−3 and ≤1 eV, respectively. Because of this small plasma density, electric fields in the plasma sheath are not sufficient to cause electrohydrodynamic instability of the liquid target surface. However, hydrodynamic instabilities due to the intense neutral flow observed in experimental and numerical studies are likely to be responsible for the growth of wavelike irregularities.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
110(2011); http://dx.doi.org/10.1063/1.3652761View Description Hide Description
Growths of Co epifilms on GaN(0001)-“1 × 1” and (1 × 1) surfaces were studied, where the structural properties of the crystals and the interfaces are compared. Stacking faults are seen to be abundant in epitaxial Co films grown on excess Ga covered GaN(0001)-“1 × 1” surface. Such stacking defects are effectively suppressed in Co films grown on less excess Ga covered GaN(0001)-(1 × 1) surfaces. The hetero-interface between Co and GaN(0001) is characterized by a disordered or amorphous region, and diffusion of Ga and N from the substrate into Co is suggested.
110(2011); http://dx.doi.org/10.1063/1.3658259View Description Hide Description
Gallium (Ga) implantation induced self-atom mixing in crystalline and amorphousgermanium(Ge) is investigated utilizing isotopically controlled Gemultilayerstructures grown by molecular beam epitaxy. The distribution of the Ga ions and the ion-beam induced depth-dependent mixing of the isotopestructure was determined by means of secondary ion mass spectrometry. Whereas the distribution of Ga in the crystalline and amorphousGe is very similar and accurately reproduced by computer simulations based on binary collision approximation (BCA), the ion-beam induced self-atom mixing is found to depend strongly on the state of the Gestructure. The experiments reveal stronger self-atom mixing in crystalline than in amorphousGe. Atomistic simulations based on BCA reproduce the experimental results only when unphysically low Ge displacement energies are assumed. Analysis of the self-atom mixing induced by siliconimplantation confirms the low displacement energy deduced within the BCA approach. This demonstrates that thermal spike mixing contributes significantly to the overall mixing of the Geisotopestructures. The disparity observed in the ion-beam mixing efficiency of crystalline and amorphousGe indicates different dominant mixing mechanisms. We propose that self-atom mixing in crystalline Ge is mainly controlled by radiation enhanced diffusion during the early stage of mixing before the crystalline structure turns amorphous, whereas in an already amorphous state self-atom mixing is mediated by cooperative diffusion events.
Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN110(2011); http://dx.doi.org/10.1063/1.3656987View Description Hide Description
We report on fundamental structural and optical properties of lateral polarity junctions in GaN.GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopymeasurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescencemeasurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices.
Ab-initio aprroach to the electronic, structural, elastic, and finite-temperature thermodynamic properties of Ti2AX (A = Al or Ga and X = C or N)110(2011); http://dx.doi.org/10.1063/1.3652768View Description Hide Description
In this work, the electronic, structural, elastic, and thermodynamic properties of Ti2AX MAX phases (A = Al or Ga, X = C or N) were investigated using density functional theory(DFT). It is shown that the calculations of the electronic, structural, and elastic properties of these structures, using local density approximation (LDA) and generalized gradient approximation (GGA) coupled with projected augmented-wave (PAW) pseudopotentials, agree well with experiments. A thermodynamic model, which considers the vibrational and electronic contributions to the total free energy of the system, was used to investigate the finite-temperature thermodynamic properties of Ti2AX. The vibrational contribution was calculated using the supercell method, whereas the electronic contribution resulted from one-dimensional integration of electronic density of states (DOSs). To verify the model, the specific heats of pure elements were calculated and compared to experimental data. The DFT-D2 technique was used to calculate the heat capacity of graphite, taking into account the van der Waals (vdW) effect. Good agreement between the calculations and experiments for the specific heats of graphite and other pure elements lends validity to the approach used. The calculated results for the specific heats of Ti2AlC and Ti2AlN match well with experimental data. These strengthen the results of specific heats of Ti2GaC and Ti2GaN as well as other calculated thermodynamic properties, including the energies of formation and thermal expansion coefficient.