Volume 82, Issue 20, 19 May 2003
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- 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
82(2003); http://dx.doi.org/10.1063/1.1575927View Description Hide Description
The bleached state transmittance of a charge-unbalanced, complementary electrochromic (EC) device may show residual coloration due to the presence of residual charges. In this study, EC devices were fabricated with configurations and solidpolymer electrolytes, and tin oxide). These devices, referred to as EC1 and EC2, were initially fabricated from fully bleached EC layers and from fully colored EC layers, respectively. The change in electrochromic properties as a function of charge capacity ratio for each device was then compared. In comparison to EC2 devices, EC1 devices provided better bleached-state transmittances and higher coloration efficiencies over a wider range of and were less sensitive to changes in value. This may arise because the absorbance caused by the residual charges in the colored state is greater and more sensitive to the charge capacity ratio than that in the bleached state.
82(2003); http://dx.doi.org/10.1063/1.1575509View Description Hide Description
Simulations of InAlAs/AlGaAs self-assembled quantum-dot arrays containing as many as 30 individual dots are used to identify a mechanism for lateral excitation transfer through partially delocalized heavy-hole states. Individual hole states exhibit wave-function splitting between several dots in the array, as well as partial confinement in the wetting layer, and have strong overlap with multiple conduction-band electron states in different quantum dots. Electron–hole pair energies involving these partially delocalized hole states correspond well with narrow resonances seen in the experimental photoluminescence excitation spectra taken for similar quantum-dot arrays using low-temperature near-field scanning optical microscopy.
82(2003); http://dx.doi.org/10.1063/1.1575938View Description Hide Description
Single component photorefractive materials based on small molecules have been synthesized. The sidechains located in the electron-withdrawing group of these molecules were systematically changed in order to investigate their influence. It was demonstrated that the photorefractive performance of these materials could be fine-tuned by changing the length of the sidechain. Excellent optical quality and photorefractive properties were obtained. A large net optical gain of at a low external field (38.3 V/μm) and a diffraction efficiency of 82% were observed at 780 nm. The results indicate that an optimum sidechain length exists for this type of molecules.
Efficient frequency doubling of a femtosecond pulse with simultaneous group-velocity matching and quasi phase matching in periodically poled, MgO-doped lithium niobate82(2003); http://dx.doi.org/10.1063/1.1576296View Description Hide Description
We demonstrate efficient frequency doubling of 95-fs pulses with a small temporal broadening in a 10-mm-long, periodically poled, 5-mol % MgO-doped lithium niobate crystal. Simultaneous group-velocity matching and quasi phase matching were achieved by the off-diagonal nonlinear optical coefficient at telecommunications range.
82(2003); http://dx.doi.org/10.1063/1.1576504View Description Hide Description
Photoinduced anisotropic and optical switchingproperties have been studied in two kinds of azobenzene sidechain polymers, a disazobenzene sidechain polymer (DAP–PGMA) and a monoazobenzene sidechain polymer P(MMA–co–MAZ). Picosecond optical switchingdynamics have been observed in both DAP–PGMA and P(MMA–co–MAZ) films. The rise time of the optical switching in DAP–PGMA film is about 40 ps, which is much faster than that of P(MMA–co–MAZ) film (∼100 ps). DAP–PGMA shows enhanced photoanisotropy and larger birefringence than that of P(MMA–co–MAZ) The influence of the azobenzene sidechain groups on the photoanisotropic dynamics is discussed.
82(2003); http://dx.doi.org/10.1063/1.1574404View Description Hide Description
We report a very high average output power at room temperature for quantum-cascade lasers emitting at For high-reflectivity-coated 2-mm-long cavities, a low threshold current density of was obtained at room temperature. From 300 to 400 K, the characteristic temperature was 198 K. A maximum average output power of 0.67 W was achieved. In addition, 0.56 W average output power was observed at a duty cycle of 56%.
82(2003); http://dx.doi.org/10.1063/1.1576511View Description Hide Description
The effects of nitrogen fraction on the temperature dependence of quantum-well emission was investigated using steady-state photoluminescence between 2 and 300 K. At low temperatures, a characteristic S-shape behavior indicative of carrier localization was observed for each of the samples. This is believed to result from the large miscibility gap induced by the nitrogen, which results in structural/compositional fluctuations in the well. In the high temperature regime where the emission has a linear dependence, a strong reduction in emission temperature dependence was observed with increasing nitrogen. The temperature dependence was modeled using the band anticrossing approach, with the interaction matrix element parameter and the nitrogen level parameter γ used as fitting parameters.
Improved temperature characteristics of laser diodes with nonidentical multiple quantum wells due to temperature-induced carrier redistribution82(2003); http://dx.doi.org/10.1063/1.1577384View Description Hide Description
Laser diodes with nonidentical multiple quantum wells could have the lasing wavelength very insensitive to temperature variation. For temperature varying from 33 to 260 K, the lasing energy changes less than 5 meV, while the band gapenergy changes more than 50 meV. The origin is due to the strongly temperature-dependent Fermi–Dirac distribution, which favors carriers in high-energy states at large temperature. The temperature-induced carrier redistribution could even cause negative characteristic temperature for a certain temperature range because the low-energy quantum wells behave like reservoirs to overcome the detrimental influence of temperature.
Analysis of thermal stress in wafer bonding of dissimilar materials for the introduction of an InP-based light emitter into a GaAs-based three-dimensional photonic crystal82(2003); http://dx.doi.org/10.1063/1.1576911View Description Hide Description
Thermal stresses generated by differences in the thermal expansion coefficients of InP and GaAs are analyzed in an attempt to introduce an InP-based light emitter into GaAs-based three-dimensional photonic crystal. Observations of the GaAs/InGaAsP bonding interface by scanning acoustic microscopy reveal that debonding occurs at approximately 300 °C due to thermal stress. Calculations of thermal stress by a two-dimensional finite element method suggested that thermal stress could be reduced by thinning the substrate, which was confirmed experimentally. Using these results, a three-dimensional photonic crystal with light emitter was successfully fabricated.
82(2003); http://dx.doi.org/10.1063/1.1576908View Description Hide Description
We report the shortest-wavelength (λ∼8.5 μm) room-temperature laser operation so far achieved for GaAs-based quantum-cascade (QC) lasers. By depositing InAsmonolayers in the device active regions during growth, we are able to both reduce the emission wavelength and minimize thermally activated carrier leakage into quasicontinuum states. This approach results in laser operation up to 305 K, with a peak optical power of ∼10 mW at room temperature. A reduced temperature sensitivity of the threshold current compared with similar QC lasers is also observed.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
82(2003); http://dx.doi.org/10.1063/1.1575507View Description Hide Description
Reversible and bistable terahertz (THz) radiation by sweeping the temperature has been observed in a dipole-type photoswitching device fabricated on magnetoresistive charge-ordered manganite thin film. Based on simultaneous measurements of the photocurrent, we show that this THz functionality of arises from the instability of metallic patches embedded with a charge-ordered insulating phase, which is created by the illumination of visible light under an electric field at constant temperature, and that it can also be controlled by a subsequent change of temperature.
82(2003); http://dx.doi.org/10.1063/1.1575931View Description Hide Description
We present a midinfrared absorption study of an structure containing two planes of strongly coupled self-assembledquantum dots. We find that the dominant absorption occurs for light polarized in the growth plane contrasting with uncoupled dots which exhibit stronger absorption for light polarized in the growth direction in the same energy range. Results from the coupled dot sample indicate that the confinement length in the growth direction is increased, lowering the energy of the -like ground state and hybridizing -like excited states and wetting layer states. A significantly increased normal incidence photocurrent signal is measured for the sample containing coupled dots, relative to samples containing up to 30 layers of uncoupled dots, confirming the enhancement of the normal incidence absorption.
82(2003); http://dx.doi.org/10.1063/1.1573368View Description Hide Description
Intersubband optical transitions in strain-compensated superlatticesgrown using gas-source molecular-beam epitaxy on (001)GaAs are investigated by means of midinfrared absorption and low-temperature photoluminescence. Strong absorption corresponding to the transition from the first to second electronic subband is measured at wavelengths between 5.6 and 10.5 μm. The data indicate that the conduction band offset between the strained and the strained is 370 meV and the electron effective mass in the strained well is This material system is an interesting GaAs-based candidate for applications in midinfrared intersubband emitters and detectors.
82(2003); http://dx.doi.org/10.1063/1.1575508View Description Hide Description
Analyzer-basedimaging is a powerful phase-sensitive technique that generates improved contrast compared to standard absorptionradiography. Combining numerically two images taken on either side at of the full width at half-maximum (FWHM) of the rocking curve provides images of “pure refraction” and of “apparent absorption.” In this study, a similar approach is made by combining symmetrical images with respect to the peak of the analyzer rocking curve but at general positions, These two approaches do not consider the ultrasmall angle scattering produced by the object independently, which can lead to inconsistent results. An accurate way to separately retrieve the quantitative information intrinsic to the object is proposed. It is based on a statistical analysis of the local rocking curve, and allows one to overcome the problems encountered using the previous approaches.
82(2003); http://dx.doi.org/10.1063/1.1576309View Description Hide Description
We report on the epitaxialgrowth of wide-band-gap cubic-phase thin films on Si(100) by pulsed-laser deposition and fabrication of oxide-semiconductor-based ultraviolet photodetectors. The challenges of large lattice and thermal expansion mismatch between Si and have been overcome by using a thin buffer layer. The heteroepitaxy of cubic-phase on Si was established with epitaxial relationship of and The minimum yield of the Rutherford backscattering ion channeling in layer was only 4%, indicating good crystalline quality of the film. Smooth surface morphology with rms roughness of 0.6 nm was observed using atomic force microscopy. Photodetectors fabricated on show peak photoresponse at 225 nm, which is in the deep UV region.
82(2003); http://dx.doi.org/10.1063/1.1575503View Description Hide Description
We investigate oxygen vacancy ordering in epitaxialthin filmsgrown by sputter deposition on (001) and (001) After annealing at under oxygen partial pressures greater than those used during deposition,films transform to a long-range oxygen vacancy ordered structure with orthorhombic symmetry. Observed orientation variants of the oxygen vacancy ordered structures are different for the two substrates. We discuss the relationship between film stress due to lattice and thermal mismatch with the substrate, and vacancy ordering.
82(2003); http://dx.doi.org/10.1063/1.1575932View Description Hide Description
We report on the fabrication of terminating-layer-controlled (LSMO) thin films and in situ characterization of the films using photoemission spectroscopy. The terminating layer of the LSMO films was changed from B to A site by inserting one atomic layer of SrO between the LSMO film and a -terminated substrate. The successful control of the terminating layer was confirmed by measuring the angular dependence of core-level spectra. Detailed analysis of the core levels revealed considerable differences in chemical bonding states of Sr atoms in the surface regions of films with different terminating layers.
82(2003); http://dx.doi.org/10.1063/1.1569414View Description Hide Description
Positron annihilation measurements show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than to below as the distance from the interface region increases from 1 to 300 μm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of suggesting that the Ga vacancies are complexed with the oxygen impurities.
photoluminescence from Er-doped amorphous films prepared by pulsed laser deposition at room temperature: The effects of oxygen concentration82(2003); http://dx.doi.org/10.1063/1.1573335View Description Hide Description
We have fabricated Er-doped amorphous films by laser ablation of a target in He atmosphere. The photoluminescence intensity at 1.54 μm was highly dependent on the oxygen content in the film, which turned out to be changed significantly by the ambient He pressure. Also, we have adopted time-of-flightquadrupolemass spectroscopy to obtain kinetic energies of ionic species in a plume produced by laser ablation. Si and Er ions do not overlap spatially as they expand toward the Si substrate and Er ions impinge on the preformed layer.