Volume 107, Issue 1, 01 January 2010
Index of content:
- Lasers, Optics, and Optoelectronics
Surface plasmon-enhanced emission from Ag-coated Ce doped thin films phosphor capped with a dielectric layer of107(2010); http://dx.doi.org/10.1063/1.3277015View Description Hide Description
The effect of a surface plasmon (SP) on the yellow emission of Ce doped (YAG:Ce) phosphorthin films was studied. The coating of YAG:Ce thin films with silver nanoislands on a 3-nm-thick spacer significantly enhanced their photoluminescence(PL). The PL enhancement is caused by the resonant coupling of photoemission in YAG:Ce into SP and the electric field of incident light at the metal interfaces. Varying the mass thickness and the annealing condition of the Ag layer indicated that the PL intensity was highest at an Ag mass thickness of 20 nm. Therefore, yellow emission was enhanced by tuning the matching conditions of thin film phosphor-SP coupling by controlling the structure of silver nanoislands. The PL intensity can be further and remarkably enhanced by capping Ag islands with a 10-nm-thick layer as a dielectric medium.
107(2010); http://dx.doi.org/10.1063/1.3277042View Description Hide Description
Temperature-dependent modulation characteristics of InAs/GaAs quantum dot(QD) lasers under small signals have been carefully studied at various bias currents. Based on experimental observations, it is found that the modulation bandwidth significantly increases when excited state(ES) lasing emerges at high temperature. This is attributed to additional photons emitted by ES lasing which contribute to the modulation response. A rate equationmodel including two discrete electron energy levels and the level of wetting layer has been used to investigate the temperature-dependent dynamic behavior of the QD lasers. Numerical investigations confirm that the significant jump for the small signal modulation response is indeed caused by ESphotons. Furthermore, we identify how the electron occupation probabilities of the two discrete energy levels can influence the photon density of different states and finally the modulation rate. Both experiments and numerical analysis show that the modulation bandwidth of QD lasers at high temperature can be increased by injecting more carriers into the ES that has larger electron state degeneracy and faster carrier’s relaxation time than the ground state.
107(2010); http://dx.doi.org/10.1063/1.3276156View Description Hide Description
We have systematically investigated the impact of device size scaling on the light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes(LEDs). Devices with diameters in the range have been studied. It is shown that smaller LED pixels can deliver higher power densities (despite the lower absolute output powers) and sustain higher current densities. Investigations of the electroluminescencecharacteristics of differently sized pixels against current density reveal that the spectral shift is dominated by blueshift at the low current density level and then by redshift at the high current density level, owing to the competition between the bandgap shrinkage caused by self-heating and band-filling effects. The redshift of the emission wavelength with increasing current density is much faster and larger for the bigger pixels, suggesting that the self-heating effect is also size dependent. This is further confirmed by the junction-temperature rise measured by the established spectral shift method. It is shown that the junction-temperature rise in smaller pixels is slower, which in turn explains why the smaller redshift of the emission wavelength with current density is present in smaller pixels. The measured size-dependent junction temperature is in reasonable agreement with finite element method simulation results.
107(2010); http://dx.doi.org/10.1063/1.3276160View Description Hide Description
Based on an ensemble Monte Carlo analysis, we show that Coulomb interactions play a dominant role in bound-to-continuum terahertz quantum cascade lasers and thus require careful modeling. Coulomb interactions enter our simulation in the form of space charge effects as well as Coulomb scattering events. By comparison to a full many-subband Coulomb screening model, we show that simplified approaches produce considerable deviations for such structures. Also the spin dependence of electron-electron scattering has to be adequately considered. Moreover, we demonstrate that iterative Schrödinger–Poisson and carrier transport simulations are necessary to correctly account for space charge effects.
107(2010); http://dx.doi.org/10.1063/1.3270410View Description Hide Description
The electrical tuning of negative dielectricanisotropy cholesteric liquid crystals (CLCs) under the influence of ac and dcelectric fields was studied. Unlike CLCs, these materials align their helical axis along the applied electric field, allowing the preservation of the optical band gap. Our results show band gap shifts greater than 20% of the original notch position with little change in the quality of the band gap, including the bandwidth and notch depth. These results can be understood by using an electromechanical model, which shows that the Maxwell’s stresses are sufficiently strong to distort the ITO glass substrates. Simple beam and plate elasticity theory is shown to adequately describe the observed behavior. The electromechanical effect is then used to create CLC cells, which can both red and blue tune. This mechanism may have interesting applications in tunable optical filters, optical pressure/stress sensors, and tunable laser technologies.
107(2010); http://dx.doi.org/10.1063/1.3276222View Description Hide Description
Experiments performed on Nb substrates coated with thin films of CsBr indicate a substantial enhancement of 150 to 800 times of the photoyield at 257 nm relative to the uncoated substrates. Results are presented for several power density illuminations and sample thickness. Further enhancement of photoyield was observed when the laser illumination was interrupted for a short time in samples with 5–10 nm thick CsBr coatings.
The optical gain and radiative current density of GaInNAs/GaAs/AlGaAs separate confinement heterostructure quantum well lasers107(2010); http://dx.doi.org/10.1063/1.3277019View Description Hide Description
The optical gain and radiative current density of GaInNAs/GaAs/AlGaAs separate confinement heterostructure quantum well(QW) lasers with an emission wavelength of have been theoretically investigated. The effect of carrier leakage from the GaInNAs QW to the GaAswaveguide layer is studied, and its influence on the optical gain and radiative current density is identified. The hole filling caused by an injected carrier has a strong impact on the optical gain and radiative current density, while the effect of electron filling is negligible, reflecting the smaller band-gap discontinuity in the valence band than in the conduction band. Hole occupation in the waveguide layer decreases the optical gain, and increases the radiative and threshold current densities of the laser. Our calculated threshold current density at is in good agreement with the experimental value reported in literature [R. Fehse et al., IEEE J. Sel. Top. Quantum Electron.8, 801 (2002)].
107(2010); http://dx.doi.org/10.1063/1.3247544View Description Hide Description
In this paper, the photonic band-gap(PBG) maps of fundamental photonic crystal(PhC) lattices are presented, and discussed. Two fundamental types of lattice: square and hexagonal (triangular, graphite or honeycomb and kagome) with circular, square, and hexagonal hole shapes are considered. Because they show the largest area gap map and because of the relative ease of fabrication of circular holes, it can be asserted that square and triangular lattices of circular holes offer the best choice of two dimensionally periodic photonic crystal(PhC)structure.Graphite and kagome lattices of circular holes in GaAs also show large area gap maps (and the largest gap map is for the second or third higher PBG region). So graphite and kagome lattices are also relevant for exploitation in PhC devices. At the scale required, fabrication process limitations are a significant problem for the realization of hole shapes other than circular.
Propagation length of surface plasmon polaritons determined by emission from introduced surface discontinuities107(2010); http://dx.doi.org/10.1063/1.3273480View Description Hide Description
Flexible far-field microscopy methods suitable for directly measuringsurface plasmonpolariton propagation along optically thick or buried waveguides are introduced. The methods monitor the local intensity of surface plasmonpolaritons by imaging the light scattered when the plasmons encounter discontinuities in the form of (i) the terminal end of the guide, (ii) randomly dispersed nanoparticles, and (iii) nanoholes drilled through the guide. Measurements by these three methods give consistent values to within of for the propagation length along -wide Au-stripe waveguides deposited on an oxidized silicon wafer and excited at a wavelength of 860 nm. This range is due to varying losses associated with the introduction of the nanoholes and nanoparticles. These losses are quantified and could be reduced with realistic experimental improvements. Finite-element computations find that propagation in these optically thick (107 nm) guides is intrinsically limited not only by Ohmic losses, but also by radiation emitted into the substrate from the stripe edges. The radiative loss depends on the slope of the edge sidewall and on the wafer oxide thickness, both of which must be considered when reconciling experiment with theory.
107(2010); http://dx.doi.org/10.1063/1.3280038View Description Hide Description
The generation of broadband terahertz pulses on the facet of waveguides is presented as an alternative to widely used coupling techniques. Dielectric loaded subwavelength waveguidestructures with lateral confinement are investigated with respect to propagating modes and waveguide losses. The results show the terahertz waveguide emitter to be a promising tool for terahertz spectroscopy in the near field and for the probing of microstructured devices such as quantum cascade lasers.
- Plasmas and Electrical Discharges
107(2010); http://dx.doi.org/10.1063/1.3268462View Description Hide Description
The ion emission of a Sn-based discharge produced extreme ultraviolet producing plasma is characterized with the combined use of different time-of-flight techniques. An electrostatic ion spectrometer is employed to measure the average charge distribution of the emitted Sn ions. A dedicated Faraday cup configuration is used to measure the total ion flux from the source for different discharge energies. High-energy Sn ions emitted by the plasma with energies up to 100 keV have been identified. The number of high-energy ions increases for higher electrical input energy into the plasma while the signal associated with the expanding plasma ions does not show such dependence. The ion energy distribution for a bulk of detected ions is calculated based on the Faraday cup measurements and compared with theoretical plasma expansion dynamics.
107(2010); http://dx.doi.org/10.1063/1.3273499View Description Hide Description
An experimental study of the recently developed version of the ferromagneticinductively coupled plasmasource has shown that under certain circumstances its input impedance becomes almost independent of the delivered rf driving power and (therefore) of the produced plasma density. This plasma source consists of a large ferromagnetic core, which is fully immersed in plasma. This core is surrounded by a primary winding and plasma appears due to gas discharge driven by an rf voltage applied to this primary winding. We have found values of parameters which determine the input impedance in such an “independent” regime and derived a quantitative theory which is in good agreement with the measured impedance values.
107(2010); http://dx.doi.org/10.1063/1.3275869View Description Hide Description
Two different wall charge evolution models, i.e., reversion and accumulation models, were proposed incorporating the phenomenon of secondary electron emission during the glow discharges of alternating current-PDPs. Based on the wall charge evolution models, theoretical analyses on exoelectron emission kinetics were conducted, and the effects of initial electron concentration trapped and temperature on the exoelectron currents were predicted. The theoretical prediction was compared with the currents measured experimentally. The theoretical analyses and experimental observations suggest that wall charge evolution model and the first- and second-order reaction kinetics may represent the wall charge formation and exoelectron emission from MgO layer of ac-PDPs.
Molecular dynamics simulation of hyperthermal neutrals generated by energetic ion impact on a metal plate107(2010); http://dx.doi.org/10.1063/1.3276097View Description Hide Description
A hyperthermal neutral beam (HNB) source is one of candidate methods to reduce plasma-induced damage problems. The HNB is generated by vertical collisions between energetic ions and a reflector composed of a tungsten plate. We perform a HNB generation simulation using a molecular dynamics algorithm. The roughness of the reflector surface is experimentally measured and the surface structure is taken into consideration in the simulation. The energy and angular distributions of the HNB are obtained by the simulation and the energy yield of the reflected neutral particles is found to be in good agreement with experimental data.
107(2010); http://dx.doi.org/10.1063/1.3273412View Description Hide Description
Hydrogenated amorphous carbon (a-C:H) deposited from an expanding thermal plasma chemical vapor deposition (ETP-CVD) is reported. The downstream plasma region of an ETP is characterized by a low electron temperature, which leads to an ion driven chemistry and negligible physical effects, such as ion bombardment (ion energy ) on the depositing surface. The material properties in ETP-CVD can be controlled by varying the plasma chemistry. In this article we investigate the change in a-C:H material properties by varying the gas flow ratio over a wide range (1.33–150), with emphasis on low gas flow ratios (1.33–5). By changing the gas flow ratio, the gas residence time in the ETP expansion can be tuned, which in turn defines the chemistry of the ETP-CVD. Soft polymerlike a-C:H to moderately hard a-C:H films have been deposited by lowering the gas flow ratio. Recently, under very low gas flow ratios, a hard graphitelike a-C:H material has been deposited. The striking feature of this material is the infrared absorptionspectrum in the stretching region , which is a distinct narrow bimodal spectrum evolving from a broad spectrum for the moderately hard a-C:H. This transition was attributed to the absence of end groups ( and ), which favors an enhanced cross-linking in the film in a similar effect to elevated ion bombardment or annealing. Moreover, the hard graphitelike film has an increased refractive index as high as 2.5 at 633 nm with a corresponding mass density of .
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
107(2010); http://dx.doi.org/10.1063/1.3275048View Description Hide Description
Valence electron spectroscopic imaging (VESI) techniques, taking advantages of the energy-losses suffered by inelastic scattering of the fast electrons in the transmission electron microscope, offer an inherently high spatial resolution to characterize the electronic structure of materials close to the Fermi level. Here we demonstrate that the combination of an electron monochromator and a highly dispersive imaging energy filter, which has become available only recently, allows reliable measurements of local bandgaps on the nanometer scale. In addition, the correlations of structural, chemical, and optical properties can be revealed via VESI using monochromated electrons with a high spatial resolution.
107(2010); http://dx.doi.org/10.1063/1.3273493View Description Hide Description
Nonpolar GaN crystal on lattice-matched  substrate was grown by a newly designed chemical vapor deposition(CVD) reactor. Following the CVDgrowth,x-ray diffraction indicated that the GaNfilm was oriented in the nonpolar m-plane with orientation. Further structural characterizations and defect analysis of nonpolar GaN material was performed using transmission electron microscope. Low-temperature photoluminescence was dominated by neutral donor bound excitons and the yellow luminescence was negligible. Raman spectroscopy showed that the as-grown GaN epilayer on  substrate were indeed of good quality. Compared to the previous report, nonpolar GaN with an improved quality was demonstrated by modifying the inner structure of the CVD reactor.
107(2010); http://dx.doi.org/10.1063/1.3273498View Description Hide Description
Surface chemistry of as-prepared nanoparticles obtained by electrochemical etching of bulk substrates was studied. Chemical environment was found to influence strongly the photoinduced electronic transitions in the nanoparticles. The influence of different interfacial chemical environments of the nanoparticles, such as surface chemistry, solvent nature, and surface charges on the photoinduced absorption and luminescence of the nanoparticles at room temperature, is described and discussed in detail. For example, oxidation induced passivation of the radiative band gap states allows visualization of the transitions between energy levels in the nanoparticles in which photogenerated charge carriers are quantumly confined. Electrostatic screening of the radiative band gap states by highly polar solvent media leads to a blueshift and a decrease in the width at half maximum of the photoluminescence spectra of the nanoparticles. As for the surface charges, they govern band bending slope and thus influence strongly the radiative transitions via energy states in the band gap.
107(2010); http://dx.doi.org/10.1063/1.3275425View Description Hide Description
We have shown that thermoelastic losses give the significant contribution to the total mechanical damping in the low loss quartz even for the bulk samples if the thickness of the sample is smaller than other dimensions. We have developed a model that describes experimental data of mechanical losses in a round quartz plate with the diameter 7.48 cm and the thickness 1.2 cm at temperatures 5–25 K in the range of eigenfrequencies 11–300 kHz. The model takes into account both the contribution of thermoelastic losses and the contribution due to the interaction of the acoustic wave with thermal phonons (Akhieser damping). Thermoelastic processes determine losses below 120 kHz. At larger eigenfrequencies, the Akhieser damping dominates.
107(2010); http://dx.doi.org/10.1063/1.3269723View Description Hide Description
Elastic and anelastic properties of a ceramic sample of elasticoluminescent have been characterized as a function of temperature by resonant ultrasound spectroscopy. Both the bulk and shear moduli show changes attributable to the influence of the sequence of structural phase transitions. Softening of and stiffening of at the transition is consistent with weak strain/order parameter coupling and tricritical character. In marked contrast, the first order transition near is accompanied by stiffening such that is larger at room temperature than the value for a structure extrapolated directly from high temperatures and is larger. Softening of at high temperatures is consistent with the existence of a soft acoustic mode. Both the and phases show an increasing acoustic dissipation with increasing temperature and there is an additional peak in the dissipation behavior below . Landau theory has been used to analyze the overall strain and elastic behavior of . The stability of the structure is considered in terms of coupling between order parameters with and symmetry, though it could also be stabilized by pairwise coupling of order parameters corresponding to irreps , , , and . Twin walls in this material should have interesting properties, which are quite distinct from those of the bulk material and could contribute differently to the elasticoluminescentproperties.