Volume 103, Issue 1, 01 January 2008
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
103(2008); http://dx.doi.org/10.1063/1.2826748View Description Hide Description
An approach to accessing air holes in a structured optical fiber with a distributed-feedback (DFB) laser based on higher order mode lasing is proposed and demonstrated. A narrow linewidth DFBfiber laser is fabricated in rare-earth-doped structured optical fiber. A higher order mode is shown to lase. Dual laser operation in both fundamental and higher order modes is also achieved. Numerical simulation of the mode profiles within the fiber using the adjustable boundary conditions-Fourier decomposition method supports the experimental results. Laser performance for each mode is characterized including imaging the emission of pump and lasing mode intensity profiles.
103(2008); http://dx.doi.org/10.1063/1.2828158View Description Hide Description
single crystal grown by the Czochralski technique was oriented and the orientation relationship between the principal axes of the optical indicatrix (, , ) and the crystallographic axes (, , ) was determined. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curve of the crystal were measured at room temperature. The Judd-Ofelt theory and the Fuchtbauer-Landenburg formula were applied to analyze the spectroscopic parameters. The relation between the spectral properties and the crystal structure was discussed. A primary laser output of about at with the slope efficiency of 25% has been obtained under a Ti:sapphire laser pumping.
103(2008); http://dx.doi.org/10.1063/1.2825647View Description Hide Description
Radiometry for the next generation of high-efficiency, high-power industrial lasers requires thermal management at optical power levels exceeding 10 kW. Laser damage and thermal transport present fundamental challenges for laser radiometry in support of common manufacturing processes, such as welding, cutting, ablation, or vaporization. To address this growing need for radiometry at extremely high power densities, we demonstrate multiwalled carbon nanotube(MWCNT)coatings with damage thresholds exceeding and absorption efficiencies over at . This result demonstrates specific design advantages not possible with other contemporary high-power laser coatings. Furthermore, the results demonstrate a performance difference between MWCNTs and single-walled carbon nanotubecoatings, which is attributed to the lower net thermal resistance of the MWCNTcoatings. We explore the behavior of carbon nanotubes at two laser wavelengths ( and ) and also evaluate the optical-absorption efficiency and bulk properties of the coatings.
Temperature-dependent photoluminescence properties of synthesized schistoselike organic nanostructures103(2008); http://dx.doi.org/10.1063/1.2826941View Description Hide Description
Schistoselike organic nanostructures were synthesized via a simple hydrothermal method. Optical properties of the nanostructures have been investigated by temperature-dependent photoluminescence(PL) spectroscopy. Three PL peaks were observed and their positions can be tuned by temperature from visible to ultraviolet region. The Huang-Rhys factor of the PLspectrum was calculated and an extended empirical model of the Bose-Einstein statistical factor concerning phonon emission and absorption was applied to investigate the PL behavior. The emission dynamic behaviors of the organic nanostructures were studied by PL decay curve, indicating two lifetime components of 0.80 and at 300 K.
103(2008); http://dx.doi.org/10.1063/1.2817811View Description Hide Description
A metal-oxide-semiconductor tunneling diode is used to emit electroluminescence from a heterojunction. Besides the and infrared emission from the band edges of Si and SiGe, respectively, infrared emission is also observed due to the radiative recombination between the electrons in the Si conduction band and the holes in the SiGe valence band. This type II recombination can emit photons whose energy is below the SiGe band gap to extend the emission range of Si/Ge-based light-emitting devices. The emission line shape can be fitted by the electron-hole-plasma recombination model.
Infrared transient grating measurements of the dynamics of hydrogen local mode vibrations in amorphous silicon-germanium103(2008); http://dx.doi.org/10.1063/1.2822337View Description Hide Description
We report on picosecond, time-resolved measurements of the vibrational relaxation and decay pathways of the Si–H and Ge–H stretching modes in hydrogenated amorphous silicon-germanium thin films . It is demonstrated that the decay of both modes has a nonexponential shape, attributable to the local environment of the Si–H and Ge–H bonds. Temperature dependent measurements of the ensemble averaged population decay time are used to demonstrate that the stretch modes relax to Si(Ge)-H bending modes and that the excess energy is dissipated into a combination of bulk vibrations. The influence of the mixed character Si-Ge bulk vibrations upon the relaxation dynamics is discussed.
Dependence of high order harmonics intensity on laser focal spot position in preformed plasma plumes103(2008); http://dx.doi.org/10.1063/1.2825570View Description Hide Description
The dependence of the high-order harmonic intensity on the laser focal spot position in laser produced plasma plumes is experimentally studied. High order harmonics up to the 59th order were generated by focusing 48 fs laser pulses from a Ti:sapphire laser system in silverplasma plume produced using 300 ps uncompressed laser radiation as the prepulse. The intensity of harmonics nearly vanished when the best focus was located in the plume center, whereas it peaked on either side with unequal intensity. The focal spot position corresponding to the peak harmonic intensity moved away from the plume center for higher order harmonics. The results are explained in terms of the variation of phase mismatch between the driving laser beam and harmonics radiation produced, relativistic drift of electrons, and defocusing effect due to radial ionization gradient in the plasma for different focal spot positions.
Internal loss, modal characteristics, and bend loss of asymmetric cladding ridge waveguide lasers at 850 nm103(2008); http://dx.doi.org/10.1063/1.2824887View Description Hide Description
The upper cladding and residual guide thickness have been varied to develop an optimal asymmetric cladding ridge waveguide geometry for use in photonicintegrated circuits (PICs). Measured values of the intrinsic loss, threshold current, and lateral modal characteristics allowed the determination of the geometry that simultaneously achieved good active device performance and a strong lateral index step. The bend loss was found by fabricating curved-waveguide -branch lasers. These characteristics are important for fabricating low-loss routing structures in PICS. This experimental work analyzed an asymmetric cladding ridge waveguide for use with photonicintegrated circuits; however, the method presented here generally can be used for the experimental optimization of ridge waveguide lasers.
Quasi-guided electromagnetic beam propagation in one-dimensional photonic crystal with a left-handed metamaterial103(2008); http://dx.doi.org/10.1063/1.2828023View Description Hide Description
We show that due to consecutive phase front conjugation in one-dimensional photonic crystals consisting of layers of conventional right-handed dielectrics and left-handed metamaterials with simultaneously negative dielectricpermittivity and magnetic permeability, the wide two-dimensional electromagnetic beams can exhibit quasi-guided propagation completely restoring its initial transverse structure over each period of photonic crystal. We find the conditions of quasi-guided beam propagation for both transverse electric and transverse magnetic polarization and corresponding dispersion relations for such a waveguide. We also demonstrate that waveguide modes can be forward as well as backward at different values of permittivity and permeability of the left-handed layers.
103(2008); http://dx.doi.org/10.1063/1.2829819View Description Hide Description
We have studied the properties of Mg-doped GaN epilayers grown by molecular beam epitaxy(MBE) with ammonia as nitrogen source. GaN homojunctions has been developed to determine the optoelectronic characteristics of the junctions as a function of the -type GaN growth conditions. It is shown that the electrical characteristics strongly depend on the Mg flux and the growth temperature. As a result, the junction characteristics have been drastically improved and state of the art MBE-grown -type layers have been obtained: the hole concentration, the mobility, and the resistivity are , , and , respectively. These characteristics lead to an increase of the homojunction light emitting diode(LED) optical output power by two orders of magnitude. To further assess the quality of these MBE-grown -type layers, we have prepared a hybridLED which consists of an quantum well active structure grown by metal organic vapor phase epitaxy followed by a -type region grown by MBE. An optical power in the milliwatt range at is demonstrated confirming thereby the quality of low temperature MBE-grown -type layers.
103(2008); http://dx.doi.org/10.1063/1.2830711View Description Hide Description
Refractive index gradient formation in liquid crystal cells during the application of an electric field across patternedelectrodes was studied. The cells were characterized by measuring the intensity profile of a parallel beam of light passing through the cells. Most of the measurements were performed using transparent electrodes comprising periodic straight-line patterns with a conducting linewidth of and various pitches in the range of . The intensity profiles through such cells were studied and it was observed that the intensity showed oscillations as a function of angle. Director profiles were calculated in order to estimate the effective refractive index distribution within the cells. Using this input in theoretical models based on wave theory as well as geometrical optics, intensity profiles could be estimated. The limits of each theory were illustrated and it was shown that the oscillations in the intensity profile could be explained by the wave theory. The effects of the orientation of the molecules with respect to the electrode lines and cell surfaces and its influence on twist of the molecules leading to depolarization of light were investigated. Various other parameters such as the pitch and the cell thickness on the refractive index gradient formation and the resulting light distribution through the cells were also studied.
103(2008); http://dx.doi.org/10.1063/1.2830826View Description Hide Description
-based conductive crystal lines composed of closely compacted fine particles were patterned on the (mol %) glass surface by a continuous wave Yb fiber laserirradiation using the nickel atom heat processing method. Nickel (II) ion was proved to be an efficient optical active absorbent for Yb fiber laser irradiation. Homogeneous and smooth crystal lines with a uniform width of were obtained with a laser power of . X-ray diffraction, micro-Raman spectra, optical microscope, and impedance measurements were used to characterize the laser patterned glass. The conductivity value of the laser patterned glass was nearly two times as large as that of the glass matrix due to the formation of -based conductive crystal lines by Yb laser irradiation.
103(2008); http://dx.doi.org/10.1063/1.2830980View Description Hide Description
Laser-assisted chemical vapor deposition (LCVD), in combination with three-dimensional (3D) self-assembly of colloidalsilica particles, was used to fabricate 3D core-shell photonic band gap(PBG)structures.Self-assembled multilayer silica particles were formed on silicon substrates using the isothermal heating evaporation approach. A continuous-wave laser ( wavelength) was used as the energy source in the LCVD to fabricate a silica-core–silicon-shell PBGstructure. This technique is capable of fabricatingstructures with various PBGs by adjusting the silica particle size and Si-shell thickness using different LCVD parameters. This capability enables us to engineer positions and widths of PBGs by flexibly controlling the particle size and shell thicknesses. In the fabricatedPBGstructures, face-centered cubic structures consist of silica-core–silicon-shell “effective atoms.” A series of PBGstructures with designed PBGs was obtained under different experimental conditions. Incidence-angle-resolved spectroscopic ellipsometry was used to identify specific PBGs. The refractive indices of the effective atoms with different Si-shell thicknesses were calculated using the Bruggeman composite model. The plain-wave expansion method was used to simulate the photonic dispersion diagrams, which supported the experimental results.
103(2008); http://dx.doi.org/10.1063/1.2831225View Description Hide Description
We employ a cavity-length study to determine the temperature variation of the internal loss and gain per unit current density in a ten-stage interband cascade laser that operated cw up to with an emission wavelength of . The characteristic temperature for the gain per unit current density is , which is slightly lower than of the threshold current and is consistent with dominance by Auger recombination. The internal loss for the -wide mesa devices increased from at at .
103(2008); http://dx.doi.org/10.1063/1.2828176View Description Hide Description
In this contribution, the basic concepts underlying the physics of magnetoinductive (MI) lenses are developed and the main practical limitations to their performances are analyzed. Strategies to overcome such limitations are proposed. Possible applications of MI lenses in magnetic resonance imaging are discussed.
- PLASMAS AND ELECTRICAL DISCHARGES
103(2008); http://dx.doi.org/10.1063/1.2825046View Description Hide Description
The plasma display panel efficacy is known to decrease at high luminance, due to both phosphor and plasma saturation. Especially the default green phosphor Willemite is sensitive to saturation. It is shown that an alternative green phosphor,, is less sensitive to saturation than Willemite. Also plasma saturation is decreased in a “high efficiency discharge mode,” for design and driving conditions that govern a fast and spatially distributed discharge development. High Xe-content panels, with somewhat higher drive voltages, are especially suited for operation in this discharge mode. Further, in the high efficiency discharge mode a high luminance is obtained, enabling high resolution designs with a relatively small emissive area. For proper designs, operating in the high efficiency discharge mode and using less saturating phosphors, the combination of a high efficacy and a high luminance is achieved. For VGA resolution 5 lm/W and and for XGA resolution 4 lm/W concurrent with a luminance of has been realized in low cost stripe-type barrier rib structures, that were sand-blasted in soda-lime glass. Finally, it is shown that the increasing wall charge loss for increasing discharge confinement at higher resolution causes mainly higher operation voltages, while the influence on discharge efficiency is relatively small.
103(2008); http://dx.doi.org/10.1063/1.2821152View Description Hide Description
Cesium iodide (CsI) coated carbon fiber cathodes have shown promise as a cold cathode for microwave and x-ray devices. In particular, the cathodes have demonstrated over 1 000 000 shots lifetime at operating voltages at or in excess of 165 kV and current densities greater than . While the vacuum emission characteristics have been well studied, the materialscharacteristics of the cathodes themselves, particularly after operation, have received little attention. Furthermore, while researchers at University of Wisconsin have demonstrated a reduction in work function of carbon due to the CsI coating, the details of the emission mechanism remain poorly understood. This article gives results of a series of materials diagnostics investigating the cathodesurface morphology as well as the changes in the carbon fiber structure with cathode shot history. We demonstrate that the cathodesurface undergoes several changes in relation to the bond line along the fiber-substrate interface as well as at the fiber tips. While the exact mechanisms leading to these changes have not been determined, we offer several possible explanations for the changes, as well as the means by which these mechanisms can be ascertained.
103(2008); http://dx.doi.org/10.1063/1.2827477View Description Hide Description
Extreme ultraviolet(EUV) discharge-based lamps for EUV lithography need to generate extremely high power in the narrow spectrum band of . A simplified time-dependent collisional-radiative model and radiative transfer solution were utilized to investigate the wavelength-integrated Lyman- line light outputs in a hydrogen-like lithium ion. The study reveals in particular that a steady-state or magnetically confined lithium plasma radiates in the desired spectrum band not less than 1 kW in even at an ion density region as low as .
103(2008); http://dx.doi.org/10.1063/1.2828150View Description Hide Description
The study presents the electrodynamical description of surface-wave-sustained discharges contracted in filamentary structures. The results are for the spatial distribution of the wave field and for the wave propagationcharacteristics obtained from a two-dimensional model developed for describing surface-wave behavior in plasmas with an arbitrary distribution of the plasma density. In accordance with the experimental observations of filamentary discharges, the plasma density distribution considered is completed by cylindrically shaped gas-discharge channels extended along the discharge length and positioned in the out-of-center region of the discharge, equidistantly in an azimuthal direction. Due to the two-dimensional inhomogeneity of the plasma density of the filamentary structure, the eigen surface mode of the structure is a hybrid wave, with all—six—field components. For identification of its behavior, the surfacewaveproperties in the limiting cases of a plasma ring and a single filament—both radially inhomogeneous—are involved in the discussions. The presentation of the results is for filamentary structures with a decreasing number of filaments (from 10 to 2) starting with the plasma ring, the latter supporting propagation of an azimuthally symmetric wave. Due to the resonance absorption of the surface waves, always present because of the smooth variation of the plasma density, the contours of the critical density are those guiding the surfacewave propagation. Decreasing number of filaments in the structure leads to localization of the amplitudes of the wave-field components around the filaments. By analogy with the spatial distribution of the wave field in the plasma ring, the strong resonance enhancement of the wave-field components is along that part of the contour of the critical density which is far off the center of the filamentary structure. The analysis of the spatial distribution of the field components of the filamentary structure shows that the hybrid wave is an eigenmode of the whole structure, i.e., the wave field does not appear as a superposition of fields of eigenmodes of the separated filaments completing it. It is stressed that the spatial distribution of the field components of the eigen hybrid mode of the filamentary structure has an azimuthally symmetric background field.
103(2008); http://dx.doi.org/10.1063/1.2827484View Description Hide Description
A plasma actuator has been studied using a self-consistent multibody system of quiescent air, plasma, and dielectric.Equations governing the motion of charged and neutral species have been solved with Poisson’s equation. Based on first principles analysis, a functional relationship between electrodynamic force and electrical and physical control parameters has been approximated and numerically tested for air. The magnitude of approximated force increases with the fourth power of the amplitude of rf potential. Thus, the induced fluid velocity also increases. The induced velocity shows momentum injection very close to the actuator surface. There is, however, a very small increase in the induced velocity with the forcing frequency. For the specific range of operational parameters considered, the proposed force relation may help speed up the plasma actuator design process.