Volume 90, Issue 12, 19 March 2007
- 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
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
Sixfold symmetry heptapodlike and threefold symmetry tetrapodlike ZnS structures have been fabricated by thermal evaporation of a ZnS and SiO mixture source in at . Both the heptapods and tetrapods have been formed by the self-assembly of ZnSnanowires with the preferred (0001) orientations. Structures and morphologies of the obtained heptapods and tetrapods are thoroughly studied using x-ray diffraction,scanning electron microscopy, and high-resolution transmission electron microscopy. Room-temperature photoluminescence spectra taken from the nanostructures show a strong green emission centered at .
- LASERS, OPTICS, AND OPTOELECTRONICS
90(2007); http://dx.doi.org/10.1063/1.2714099View Description Hide Description
A solid-state laser material of composite all-ceramic is applied as a source of a high-power, diode edge-pumped microchip laser. quasi-continuous-wave and continuous-wave (cw) output powers were obtained from the -diameter, Yb dopedceramic core with a thickness. The cw output power densities of and in the core area and volume, respectively, are the highest for an active-mirror solid-state laser. The maximum thermal stress in the ceramic core is estimated to be at the noncooled surface and is twice the tensile strength of single-crystal.
90(2007); http://dx.doi.org/10.1063/1.2714193View Description Hide Description
The authors demonstrate that thin film organic photovoltaic cells are efficient detectors of surface plasmonpolaritons (SPPs). For radiation in a Kretschmann configuration, the external quantum efficiency in fullerene–copper phthalocyanine photovoltaic cells is doubled at resonance to 12%. In thin heterojunction organic photovoltaics, SPP detection relies on a substantial increase in absorption when the incoming radiation is coupled to guided SPPs rather than unguided photons. SPP scattering and nonradiative losses are negligible; however, optical modeling shows that cathode metal penetration into the neighboring organic semiconductor is a major source of loss for SPP or photonic excitation.
90(2007); http://dx.doi.org/10.1063/1.2714292View Description Hide Description
The authors demonstrate that photoinduced birefringence in azo-dye-doped polymers is strongly enhanced by hydrogen bonding between the guest molecules and the polymer host. The primary mechanism behind the enhancement is the possibility to use high dye doping levels compared to conventional guest-host systems because dye aggregation is restrained by hydrogen bonding. Moreover, hydrogen bonding reduces the mobility of the guest molecules in the polymer host leading to a larger fraction of the induced birefringence to be preserved after the excitation light has been turned off.
Polarized optical feedback from an extremely short external cavity for controlling and stabilizing the polarization of vertical cavity surface emitting lasers90(2007); http://dx.doi.org/10.1063/1.2714301View Description Hide Description
The authors present experimental results showing that polarization selective optical feedback from an extremely short external cavity induces switching between the two fundamental transverse modes with orthogonal linear polarization of the otherwise (without optical feedback)polarization stable vertical cavity surface emitting laser. Moreover, when properly selecting the external cavity length within a subwavelength range the polarization state of the emitted light can be chosen in either of the two fundamental transverse modes and stabilized against injection current variations. The numerical results reported are in good agreement with experiments.
90(2007); http://dx.doi.org/10.1063/1.2714330View Description Hide Description
The coupling of electrical, optical, and thermal effects in broad-area semiconductor lasers has been investigated using a multilateral mode mathematical model.Numerical solutions for the active layer temperature rise are input into the thermal source terms of elasticityequations, leading to the prediction of the thermoelastic stresses which occur in regions of high defect concentration. The magnitude of this prediction is compared with the size of other stresses reported elsewhere in experimental observations of the degraded facet of broad-area devices.
90(2007); http://dx.doi.org/10.1063/1.2715001View Description Hide Description
The authors report the observation of room-temperature intersubband luminescence at from quantum wells under optical pumping at . The quantum wells are designed to exhibit three bound states in the conduction band. The emission arises from the intersubband transition. Photoluminescence excitation spectroscopy shows that the emission is only observed for -polarized excitation at wavelengths corresponding to the intersubband transition. The measured external quantum efficiency is .
90(2007); http://dx.doi.org/10.1063/1.2715035View Description Hide Description
The authors observed the superluminal propagation at negative group velocity in toluene solution firstly. The theoretical analysis was made based on density matrix model of superluminal propagation. The simulation result was well consistent with experimental measurement. The largest time advancement of was got at of sample concentration; the corresponding group velocity is .
Method to improve the quality of acousto-optic imaging using an ultrashort and focused ultrasound pulse90(2007); http://dx.doi.org/10.1063/1.2713126View Description Hide Description
The letter shows that the optical scattering and the length of ultrasound-wave train are key factors affecting the quality of acousto-opticimaging. A method using an ultrashort and focused ultrasound pulse is provided. The method has the potential to improve the quality of acousto-opticimaging.
90(2007); http://dx.doi.org/10.1063/1.2717018View Description Hide Description
The authors extracted the thermal resistance and the electrical power dependence of the electronic temperature of quantum-cascade lasers (QCLs) operating at , in the lattice temperature range of . The low electron-lattice coupling constant can be related to the beneficial effect of the high conduction band offset, peculiar to the material system, on the electron leakage. The authors found an active region cross-plane thermal conductivity value , which is approximately three times larger than that measured in QCLs with heterostructures.
90(2007); http://dx.doi.org/10.1063/1.2714293View Description Hide Description
The authors present experimental and numerical results relative to the polarization-resolved spectral reflectivity of one-dimensional periodic microstructures, evaluated in the near-infrared region at non-normal incidence. The tested hybrid quarter-wavelength microstructures, fabricated by electrochemical deep etching of silicon, consist of arrays of silicon walls and air gaps, with 3 and periods and aspect ratio of up to 100. A theoretical Monte Carlo analysis taking into account the presence of a Gaussian statistical distribution for the structure porosity has been carried out and the calculated wavelength dependence of the reflectivity at non-normal incidence has been confirmed by experimental data.
90(2007); http://dx.doi.org/10.1063/1.2714995View Description Hide Description
Two-photon absorption has been measured in a nano-optical material (quantum dots) based on the nonconjugated conductive polymer, iodine-doped poly(-pinene). The measurement has been made using open-aperture scan at with pulses. Exceptionally large two-photon absorption coefficient has been observed with a peak of magnitude that appeared at which is close to half of the optical gap corresponding to the charge-transfer transition involving the isolated double bond. Saturation of absorption was observed at lower intensities and shorter wavelengths. The results have been attributed to the quantum dots (subnanometer size) formed upon doping.
Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses90(2007); http://dx.doi.org/10.1063/1.2714999View Description Hide Description
The authors introduce a method for cross correlating vacuum-ultraviolet with near-infrared femtosecond light pulses in a perpendicular geometry. Photoelectrons generated in an atomic gas by laser-assisted photoionization are used to create a two-dimensional image of the cross-correlation volume, thereby mapping time onto a space coordinate. Thus, information about pulse duration and relative timing between the pulses can be obtained without the need to scan an optical delay line. First tests using vacuum-ultraviolet pulses from the free-electron laser at the Deutsches Elektronen Synchrotron set an upper limit for their temporal jitter with respect to external optical laser pulses.
90(2007); http://dx.doi.org/10.1063/1.2715023View Description Hide Description
The authors study the regime of self-pulsations in a two-state quantum dot laser with saturable absorber. Experiments demonstrate and theory explains the appearance of self-pulsations at low relaxation oscillation frequency. The system exhibits a period doubling sequence of bifurcations leading to chaos.
90(2007); http://dx.doi.org/10.1063/1.2715043View Description Hide Description
The authors report on electroluminescence measurements combined with photoluminescence excitation spectroscopy on a single InGaAsquantum well placed in the intrinsic region of a photodiode. They show that at low current density, the spectra are dominated by the spectrally narrow excitonic emission. Moreover when increasing carrier injection, they observe the progressive transition from excitons into free electron-hole pairs. This structure meets all criteria to be integrated in a semiconductor microcavity and the present demonstration of excitonelectroluminescence is the first step toward the achievement of the strong coupling regime under electrical injection.
90(2007); http://dx.doi.org/10.1063/1.2715096View Description Hide Description
The authors present a theoretical study of the ground state optical transition matrix element in quantum dots(QDs) with a dilute amount of nitrogen. They have investigated the interplay between the nitrogen to the conduction band mixing and piezoelectric field on the optical matrix element. With a reduced amount of indium and an increased amount of nitrogen in the QD, the optical matrix element becomes on the average larger and less sensitive to the variation of both the QD shape and size than is the case of an InNAs QD. The optical characteristics at room temperature and wavelength are discussed.
Identification of modes and single mode operation of sapphire-bonded photonic crystal lasers under continuous-wave room temperature operation90(2007); http://dx.doi.org/10.1063/1.2715107View Description Hide Description
Sapphire-bonded photonic crystal lasercavities were fabricated and characterized under room temperature continuous-wave operation, and the single mode lasing with a side-mode-suppression ratio of was observed. The lasing modes of the photonic crystalcavities were characterized and compared to calculated spectra from the three-dimensional finite-difference time-domain method.
90(2007); http://dx.doi.org/10.1063/1.2715113View Description Hide Description
The energy distribution of negative ions has been measured during the reactive magnetron sputtering of 13 different target materials by the means of energy resolved mass spectrometry. For the same series of target materials the ion-induced secondary electron emission coefficient was determined in an earlier published research. A correlation between this ion-induced secondary electron emission coefficient and the emission of the high energetic negative ions was observed. This correlation should be taken into consideration in the selection of oxides for their high electron emission coefficients as these materials will emit at the same time negative ions.
Fabrication and device characteristics of Schottky-type bulk GaN-based “visible-blind” ultraviolet photodetectors90(2007); http://dx.doi.org/10.1063/1.2715114View Description Hide Description
The authors present the fabrication and characterization of vertical-geometry Schottky-type ultraviolet (UV) photodetectors based on a bulk substrate. By using low temperature rapid thermal annealing of the semitransparent Schottky contacts (nickel with 7% vanadium), they obtained an ultralow dark current of at reverse bias. A responsivity of at zero bias was measured for wavelength shorter than the absorption edge of GaN, and it was found to be independent of the incident power in the range measured . The devices are visible blind, with an UV/visible contrast of over six orders of magnitude. An open-circuit voltage of was also obtained under a broadband UV illumination.
90(2007); http://dx.doi.org/10.1063/1.2715126View Description Hide Description
The role of extended defects in determining the atomic scale surface morphology of nonpolar -plane gallium nitride has been elucidated. The heteroepitaxially grownfilms are commonly reported to yield striated surface morphologies (slate morphology) correlated with their high densities of basal plane stacking faults. Here, the growth window was explored to allow nonslate morphologies for hydride vapor phase epitaxy. Lateral epitaxial overgrowth was then utilized to produce films with three regimes of different extended defect contents. Elimination of stacking faults from the yielded step-flow features with an average step height of 4–7 ML even for slate morphology growth conditions.
Study of self-diffraction process in photoconducting polymer-nematic liquid crystal hybrid structure90(2007); http://dx.doi.org/10.1063/1.2715127View Description Hide Description
The authors present results of the measurements of self-diffraction process inside the nematic liquid crystal cell with a photoconducting polymeric layer poly(-vinylcarbazole) doped with trinitrofluorenone molecules. The dynamics of diffraction grating fading process versus applied voltage and different laser power has been studied. Three-step process of grating’s fading was observed, connected with the charge carriergeneration and their mobility in the photoconducting polymer. Theoretical calculations assuming proposed mechanism responsible for the grating decay are in a very good agreement with experimental results.