Volume 96, Issue 14, 05 April 2010
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- interdisciplinary and general physics
Index of content:
Scanning tunneling microscopy(STM),atomic force microscopy(AFM), lateral force microscopy (LFM), and conductive AFM (cAFM) are employed to characterize epitaxialgraphene on SiC(0001). Of particular interest are substrates that possess single-layer and bilayer graphene domains, which form during thermal decomposition of silicon from SiC(0001). Since these samples are often partially graphitized, characterization techniques are needed that can distinguish domains of epitaxialgraphene from the adjacent reconstructed SiC(0001) surface. The relative merits of STM,AFM, LFM, and cAFM for this purpose are outlined, thus providing nanometer-scale strategies for identifying and characterizing epitaxialgraphene.
- LASERS, OPTICS, AND OPTOELECTRONICS
96(2010); http://dx.doi.org/10.1063/1.3367725View Description Hide Description
By laser-induced Zndoping,hole concentration in the p-type GaN contact layer of conventional GaN-based light-emitting diodes(LEDs) is increased and improvement of the LED property is confirmed. Compared with LED with no use of laser-induced doping, the forward voltage under 20 mA current is decreased from 3.33 to 3.13 V and the thermal resistance of the chip is decreased from 18.6 to 9.7 K/W. In addition, the lifetime of the device is increased about 41%. These results are attributed to the improvement of the p-type Ohmic contact due to laser-induced doping of Zn to the p-GaN contact layer.
Utilizing the interface adsorption of nitrogen for the growth of high-quality GaInAsN/GaAs quantum wells by metal organic chemical vapor deposition for near infrared applications96(2010); http://dx.doi.org/10.1063/1.3360216View Description Hide Description
We have investigated the composition and optical properties of GaInAsN/GaAs single quantum wellsgrown using metal organic chemical vapor epitaxy at . Using time-of-flight secondary ion mass spectrometry and photoluminescencespectroscopy, we have shown the presence of a 1–2 nm thick nitrogen-rich interfacial layer at the first interface grown. The inhomogeneous asymmetric distribution of nitrogen atoms along the growth direction is attributed to the dominance of surface kinetics, nonlinear dependence of N incorporation on In content, and the strain gradient effect on the effective diffusion of N. We have utilized this finding to grow high quality quantum wells.
96(2010); http://dx.doi.org/10.1063/1.3374404View Description Hide Description
We examine the terahertz (THz) performance of an ErAs:GaAs photoconductive switch under varying bias conditions and optical drive power. Despite THz power up to , saturation effects were not seen. In addition, the THz power spectra were measured with a Fourier transform infrared spectrometer, and the roll-off was found to be invariant to bias voltage and consistent with a THz pulsewidth of 1.59 ps and a peak power of 3.1 W. These results are confirmed by a large-signal, high-frequency circuit model that suggests that further increase in THz power and efficiency are possible through an increase in the mode-locked laser power and reduction in its pulse width. The model is useful in designing both the laser and photoconductive switches to maximize available power and efficiency.
96(2010); http://dx.doi.org/10.1063/1.3371696View Description Hide Description
With advances in nanotechnology, obtaining circularly and elliptically polarized optical spots beyond the diffraction limit is an emerging need for plasmonic applications. Two techniques are suggested to obtain circularly and elliptically polarized near-field radiation using subwavelength apertures. It is demonstrated that a square aperture can mediate diffraction limited circularly or elliptically polarized radiation into an optical spot with circular or elliptical polarization beyond the diffraction limit. Linearly polarized diffraction limited radiation is converted into a circularly or an elliptically polarized optical spot beyond the diffraction limit by creating asymmetry in the subwavelength aperture.
96(2010); http://dx.doi.org/10.1063/1.3377008View Description Hide Description
A quantum cascade laser design for wide voltage-tuning, emitting at , is presented based on a diagonal bound-to-continuum design. The relatively short period length and the diagonal nature of the laser transition guarantees a wide tuning of the emission due to the linear Stark shift effect. Tuning of both the spontaneous and stimulated emission is presented over almost . In spite of the large tuning, laser performance are comparable with the best results present in literature in this spectral range. In particular, continuous wave operation up to 450 mW and pulsed wall plug efficiencies up to 11.5% were measured at 300 K. A transport model, based on the density matrix formalism, was used to simulate spontaneous and stimulated emission as function of the applied field. Same model was also used to predict light-current-voltage characteristics of the lasers.
96(2010); http://dx.doi.org/10.1063/1.3377912View Description Hide Description
We investigate an amplified spontaneous emission phenomenon and lasing in a two-layer system: a layer of modified deoxyribonucleic acid (DNA)polymeric matrix containing dye which is superimposed on a periodic relief structure formed in photochromicpolymer. The luminescent layer was prepared from DNA blended with cationic surfactant molecule cetyltrimethyl-ammonium chloride (CTMA) and a Rhodamine (Rh6G) laser dye. This layer covered a specially designed photochromicpolymer layer in which a surface relief grating was inscribed by holographic method in order to form a Bragg reflector for photons. Thin film of the DNA-CTMA:Rh6G/photochromic polymer was excited with pulsed laser.
96(2010); http://dx.doi.org/10.1063/1.3378997View Description Hide Description
We propose a design of near field plate for deep subwavelength super-focusing at optical frequency. The method used antisymmetric surface plasmon mode to generate abrupt phase change within small fraction of a wavelength, while varied thin metallic film thickness to control the amplitude profile of the near field distribution. The numerical simulation result shows a resolution at focus plane for the designed near field plate at the wavelength of 1550 nm, which is due to the superposition of the near field phase and amplitude distribution.
96(2010); http://dx.doi.org/10.1063/1.3374401View Description Hide Description
We characterize superlattices as substrates for photoconductive terahertz emitters excited at . The bandwidth of the emitted radiation is studied as a function of the superlattice period (or equivalently the electron lifetime) and the applied bias field. The results show that a variation in the electron lifetime from 0.2 to 6.3 ps does not considerably influence the bandwidth of the emitted radiation. However, the bandwidth increases linearly from 2.6 to 3.0 THz as the applied bias field is increased from 7 to 30 kV/cm. At higher bias fields, saturation is observed. The largest measured bandwidth is 3.1 THz.
96(2010); http://dx.doi.org/10.1063/1.3383220View Description Hide Description
This paper reports the enhanced photocurrent and relative quantum efficiency of cesium iodide (CsI) films on magnesium oxide (MgO)-coated multiwall carbon nanotubes(MWCNTs) on a silica substrate, i.e., CsI/MgO/MWCNTs/Si, when illuminating with 147 nm photons under an external electric field. The incorporation of MWCNTs resulted in significant enhancement of the photocurrent by several orders of magnitude compared to that of a conventional CsI. An analysis of the photoelectron energy spectrum attributed the phenomena to the creation of a very high electric field through the MgO/CsI film with the subsequent generation of avalanchesecondary electrons.
96(2010); http://dx.doi.org/10.1063/1.3385778View Description Hide Description
Quantum cascade lasers(QCLs) emitting at wavelengths as short as are demonstrated. The InAs/AlSb QCL design was optimized to weaken carrier leakage into the L-valley by reducing coupling between the active InAsquantum wells. The lasers with HR-coated facets operated up to 175 K.
96(2010); http://dx.doi.org/10.1063/1.3378266View Description Hide Description
We show that the broadband conical emission associated with filaments in air extends down to the radiofrequency region. This rf emission which originates from the longitudinal oscillation of charged ions formed during filamentation is strongly enhanced by the presence of a longitudinal static electric field.
Low-threshold 303 nm lasing in AlGaN-based multiple-quantum well structures with an asymmetric waveguide grown by plasma-assisted molecular beam epitaxy on c-sapphire96(2010); http://dx.doi.org/10.1063/1.3373834View Description Hide Description
We report on AlGaN multiple-quantum-well separate confinement laser heterostructures grown by plasma-assisted molecular-beam epitaxy directly on c-sapphire at low temperatures . Threading dislocation density was reduced down to owing to both intentionally introduced strained AlGaN/AlN superlattices and self-organized blocking structures in the AlGaN step-graded buffer layers. The quantum wells were fabricated by a submonolayer digital alloying technique. Calculations of the optical gain and confinement in the optically pumped laser structures yielded its optimum design comprising an asymmetric waveguide. Lasing at 303 nm with the relatively low threshold excitation density of at 295K has been achieved.
96(2010); http://dx.doi.org/10.1063/1.3374335View Description Hide Description
The authors describe electrical beam steering of mid-infrared quantum cascade lasers with a monolithically integrated Y-coupled cavity and electrically separated branches. The deflection of the laser beam is achieved without any additional components such as optic or mechanic systems. This is done by injecting additional direct current into one of the two emitting branches, locally increasing the temperature. We estimate that the required temperature difference between left and right branch is approximately 12 K to achieve 2° of the beam steering. This value is in a good agreement with heat transfer simulations.
96(2010); http://dx.doi.org/10.1063/1.3378690View Description Hide Description
A fine control of a photonic molecule is obtained by nanofluidic techniques. The coupling condition between the modes of two photonic crystal nanocavities is modified by spectrally tuning each single resonator. Clear mode anticrossing and transition from localized to delocalized states are observed. The detuning induced by disorder, always present in real device, is experimentally compensated by locally modifying the photonic environment of the cavity.
96(2010); http://dx.doi.org/10.1063/1.3393997View Description Hide Description
Wide-bandwidth random lasing action is observed from the randomly assembled ZnS/ZnO biaxial nanobeltheterostructures under optical excitation. This is because optical gain at ultraviolet regime can be obtained from the near-band-edge radiative recombination of ZnS and ZnO. Surface defects related radiative recombination centers of ZnS and ZnOnanostructures also contribute to the visible optical gain. Hence, a broadband optical gain is obtained from the ZnS/ZnO biaxial nanobeltheterostructures. Moreover, a wide bandwidth coherent optical feedback can be achieved from the randomly assembled biaxial nanobelts due to high refractive index contrast between the nanobelts and air.
96(2010); http://dx.doi.org/10.1063/1.3380825View Description Hide Description
A broadband antireflective coating for silicon was fabricated by tailoring the compositions of and during conventional plasma enhanced chemical vapor deposition. The coating’s refractive index was quasicontinuously graded, e.g., from 3.22 to 1.44 at 1550 nm. Over the 280–3300 nm wavelength range, the reflectance was below 8% peak and 4.3% average. The deposited stack was composed of dense dielectric materials. This enables patterning and processing into robust devices after coating deposition. Using single layer ellipsometry data, the transfer matrix method was applied to predict the multilayer coating’s reflectance spectra. The results showed good agreement with experimental data.
96(2010); http://dx.doi.org/10.1063/1.3385159View Description Hide Description
We present a study on the facet damage profile of quantum cascade lasers(QCLs). Conspicuous cascade half-loop damage strips on front facet are observed when QCLs catastrophically failed. Due to the large difference on thermal conductivities between active region and the substrate, dominant heat is compulsively driven to the substrate. Abundant heat accumulation and dissipation on substrate build large temperature gradient and thermal lattice mismatch. Thermal-induced stress due to sequential mismatch leads to the occurrence of the multistep damages on front facet. Good agreement is achieved between the observed locations of damaged strips and the calculated results.
- PLASMAS AND ELECTRICAL DISCHARGES
96(2010); http://dx.doi.org/10.1063/1.3374888View Description Hide Description
The experimental results of a thermionic cusp electron gun, to drive millimeter and submillimeter waveharmonic gyrodevices, are reported in this paper. Using a “smooth” magnetic field reversal formed by two coils this gun generated an annular-shaped, axis-encircling electron beam with 1.5 A current, and an adjustable velocity ratio of up to 1.56 at a beam voltage of 40 kV. The beam cross-sectional shape and transported beam current were measured by a witness plate technique and Faraday cup, respectively. These measured results were found to be in excellent agreement with the simulated results using the three-dimensional code MAGIC.
96(2010); http://dx.doi.org/10.1063/1.3383240View Description Hide Description
The electrical breakdown in a spark gap for repetitive switching has been a long research interest. For this purpose, microplasma discharge is implemented in the spark gap which is further integrated inside a coaxial transmission line. This work addresses important physical properties and insights of the microplasma discharge, to be optimized, such as plasma generation in a spark channel, dielectric recovery process, and residual plasma in the postspark discharge period. Although understanding the microplasma discharge is the primary goal, considerable attention has been focused on an external circuit scheme to drive the discharge system at a high repetition rate.
96(2010); http://dx.doi.org/10.1063/1.3385393View Description Hide Description
Inhomogeneous cold beams undergo wave breaking as they move along the axis of a magnetic focusing system. All the remaining control parameters fixed, the earliest wave breaking is a sensitive function of the inhomogeneity parameter: the larger the inhomogeneity, the sooner the breaking. The present work analyzes the role of envelope size mismatches in the wave breaking process. The analysis reveals that the wave breaking time is also very susceptible to the mismatch; judiciously chosen mismatches can largely extend beam lifetimes. The work is extended to include recently discussed issues on the presences of fast and slow regimes of wave breaking, and the theory is shown to be accurate against simulations.