Volume 100, Issue 11, 01 December 2006
- 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
Space charge field and dynamics of the grating formation in a photorefractive polymer-dispersed liquid crystal based on a photoconducting polysiloxane100(2006); http://dx.doi.org/10.1063/1.2372434View Description Hide Description
A photorefractive system composed of liquid crystaldroplets dispersed in a photoconducting polymer is characterized by means of two-beam coupling. The amplitudes and phases of the spatial variations of the refractive index and the absorption coefficient are measured using the moving grating technique. Dynamic measurements indicate that the phase of the refractive index modulation is nearly constant while the amplitude increases gradually. The maximum value of the internal space charge field can be estimated from the amplitude of the phase grating and is found to be of the order of one-hundredth of the external bias field.
Highly sensitive and ultrafast light modulation with a vanadyloxophthalocyanine-doped composite polymer guided wave mode device100(2006); http://dx.doi.org/10.1063/1.2397291View Description Hide Description
We propose a guided wave mode (GWM) device with prism coupling, which is composed of a low-refractive-index polymer layer and a vanadyloxophthalocyanine-doped polymer layer. Reflectance dips due to the GWM were observed at specific incident angles depending on the wavelength and the thickness of each layer. The reflectance at the GWM can be controlled by the real part and/or the imaginary part of the complex refractive index of the waveguide layer. The dependence of reflectance showing a minimum of almost zero at a certain value defined as was confirmed in the visible to near infrared region. The value varies linearly with the intensity of the evanescent wave. Highly sensitive and ultrafast control of reflection in the GWM geometry was demonstrated by the photoinduced complex refractive-index change upon femtosecond laser excitation of vanadyloxophthalocyanine.
100(2006); http://dx.doi.org/10.1063/1.2372409View Description Hide Description
In this work we present a study of energy transfer processes in -doped phosphate glasses with different contents and concentrations. Fluorescence and thermal lens (TL) techniques were used in order to analyze the and -other impurity interactions. The role of groups is also presented and discussed. The results show that in the low concentration region up to , the interaction between and OH radicals is the dominant process reducing the lifetime, increasing the TL dioptric power, and then endangering the performance of the system. For the high concentration limit, the -other impurity interaction, which is favored by energy migration, is comparable to the energy transfer, even for high concentration. The nonradiative decay rate due to hydroxyl groups follows Förster-Dexter theory [Ann. Phys.2, 55 (1948);J. Chem. Phys.21, 836 (1953)] except at low concentration, being more active at low levels, where quenching rate is probably due to several impurities.
100(2006); http://dx.doi.org/10.1063/1.2397293View Description Hide Description
The electrical control of the lasing wavelength in two-section quantum dot lasers is investigated. By changing the optical loss in the absorber section, the control of the ground-state(GS) and excited-state(ES) lasing thresholds and output powers is achieved. Additionally, a complex self-pulsation dynamics with simultaneous oscillations of the GS and ES intensities is observed. The experimental results are well explained in the framework of a rate equationmodel.
Electroluminescence efficiency of blue quantum-well diodes with and without an electron reservoir layer100(2006); http://dx.doi.org/10.1063/1.2398690View Description Hide Description
The temperature dependence of the electroluminescence(EL) spectral intensity has been investigated in detail between and at various injection current levels for a set of two blue multiple-quantum-well(MQW)light-emitting diodes(LEDs) with and without an additional -doped electron reservoir layer (ERL). The radiative recombination efficiency of the main blue emission band is found to be significantly improved at all temperature regions and current levels when the additional ERL is introduced. For high injection currents, i.e., large forward bias voltages , a quenching of the EL intensity is observed for for both LED structures, accompanying appearance of short-wavelength satellite emissions around . Furthermore, the low-temperature intensity reduction of the main EL band is stronger for the LED without the ERL than with the ERL. For low , i.e., small , however, no quenching of the EL intensity is observed for both LEDs even below and the short-wavelength satellite emissions are significantly reduced. These results of the main blue emission and the short-wavelength satellite bands imply that the unusual evolution of the EL intensity with temperature and current is caused by variations of the actual potential field distribution due to both internal and external fields. They significantly influence the carrier capture efficiency by radiative recombination centers within the active MQW layer and the carrier escape out of the active regions into high-energy recombination centers responsible for the short-wavelength satellite emissions.
100(2006); http://dx.doi.org/10.1063/1.2400090View Description Hide Description
Amorphousmultilayers were prepared by successive evaporations of and powders onto substrates maintained at . The evolution of the structure was investigated by x-ray reflectometry, transmission electron microscopy, infrared-absorption spectrometry and Raman spectrometry for annealing temperatures less than . These experiments allowed us to follow the phase decomposition of the alloy and to observe the appearance of amorphous and crystallized germanium aggregates. The evolution of the photoluminescence in the range of was correlated to the structure of the films.
Highly transparent cathodes comprised of rare earth and Au stacked layers for top-emission organic light emitting diodes100(2006); http://dx.doi.org/10.1063/1.2388875View Description Hide Description
We report the transparent cathodes comprised of a low work functionrare earth element(RE) and a Au capping layer for top-emission organic light emitting diodes (TOLEDs). The , , , and cathodes possess a transmittance up to when the RE/Au thickness is . The light extraction efficiencies for the TOLEDs with a siliconanode and a RE/Au cathode are computed to be 9%–13%, markedly higher than that for a similar device but with an cathode. The surface plasmon polaritons at the cathode are found to be much weaker than those at the cathode.
Analysis of the linewidth enhancement factor (-factor) in compressively strained InGaAsP quantum wire lasers100(2006); http://dx.doi.org/10.1063/1.2388850View Description Hide Description
The linewidth enhancement factor (-factor) of compressively strained (CS) InGaAsP/InP quantum wire (QWire) lasers is theoretically studied using an eight-band formalism. It is found that tensile strained (TS) barriers lower the value of the -factor by suppressing elastic strain relaxation. When multiple QWire layers are stacked vertically, the -factor decreases owing to the increase in the modal gain. However, the decrease is lower than expected, because enhanced strain relaxation in multiple-layer stack tends to increase the -factor. Comparison of the -factors of QWires and QWells at the lasing frequency shows that the peak value of the -factor decreases monotonically with increasing confinement, but an improvement in the -factor of QWires at threshold condition relative to the -factor of QWell lasers is not achieved unless the QWire width is reduced below a critical value. These results provide useful guidelines for designing CS QWire lasers with reduced values of the -factor.
Light-polarization characteristics of electroluminescence from light-emitting diodes prepared on -plane GaN100(2006); http://dx.doi.org/10.1063/1.2382667View Description Hide Description
Light polarization and emission spectra from quantum-well light-emitting diodes(LEDs) were investigated. The LEDs were prepared on the plane of wurtzite GaN.Polarization and spectrum measurement was performed at different observation angles with respect to the LED surface. Partially polarized electroluminescence was confirmed at any angle of observation, where the emission intensity tended to be greater when a polarizer was aligned along the axis of the LEDstructure. The results clearly indicated the inclination of the axis relative to the LED surface. As a result, two light polarizations were identified and they were assigned to two different electronic transitions in relation to emission peak energies. Possible alteration of the valence-band structure was suggested due to the induced strain.
100(2006); http://dx.doi.org/10.1063/1.2396671View Description Hide Description
An infrared, two-wavelength pumping scheme for generating UV in -doped fibers is investigated and proposed as an alternative because the pump wavelengths are accessible from laser diodes. Spectral characterizations of fiber samples with different concentrations revealed that moderate concentrations are best suitable to produce UV emission when single—or double-line pumping with 1117 and . Detailed spectroscopic measurements realized to the fiber with the best performance, the , allowed to obtain the copumping wavelengths (in the region) that enhanced the UV emission. For example, when applying tens of milliwatts at , which represented a 28% increase of total pump power, the UV emission increased in an avalanchelike fashion up to three orders of magnitude. Then, a high-power source that currently exists in the market and a moderate power source under development are possible to be used as pumps for this scheme.
Enhanced Zn–Cd interdiffusion and biexciton formation in self-assembled CdZnSe quantum dots in thermally annealed small mesas100(2006); http://dx.doi.org/10.1063/1.2390547View Description Hide Description
By controlled annealing of small ZnSe mesas with embedded CdZnSe quantum dots(QDs) at considerably low temperatures, significant changes of the QD luminescence have been observed. To investigate the spectral evolution of single exciton lines several thermal annealing steps were performed successively, and large energy shifts of single exciton (X) lines due to annealing were traced. In annealedQDs the biexciton (XX) emission is drastically enhanced. Biexciton binding energies less than were recorded for the thermally annealed CdZnSe QDs, indicating a considerable change in the QD confinement. The pronounced energy shifts of the QD luminescence is attributed to the Cd–Zn interdiffusion between the CdZnSe QDs and the surrounding ZnSe matrix. In small mesas interdiffusionactivation energy of less than was determined. This value of is half of that recorded for the Cd–Zn interdiffusion in large QD ensembles, indicating that the sidewalls of the etched mesas play an important role in the observed diffusion process.
100(2006); http://dx.doi.org/10.1063/1.2393009View Description Hide Description
This work reports on the synthesis and the optical-structural investigation of light-emitting microstructures obtained from amorphous aluminum nitride(AlN) thin films doped with chromium. The samples were prepared by radio frequency sputtering an Al target, with approximately of chromium, in an atmosphere of pure nitrogen. After deposition, the samples were thermally annealed up to under a flow of dry oxygen. The experimental investigation included photoluminescence, cathodoluminescence, energy dispersive x-ray spectroscopy, and scanning and transmission electron microscopy techniques. According to the experimental data, filmsannealed at for exhibit relatively strong luminescence at . Further thermal annealing at this temperature induces the development of microscopic features onto the surface of the films that emit red light at approximately . In view of their distinctive luminescence characteristics and spatial dimensions, the observed features have been designated by ruby microstructures (RbMS’s). For the present AlN films, the occurrence of these RbMS’s is associated with ions in oxygen-rich environments (-alumina) that arise in consequence of the thermal treatments and of the presence of microimperfections (pores, voids, etc.) in the samples.
100(2006); http://dx.doi.org/10.1063/1.2397284View Description Hide Description
Luminescence and phosphorescence properties are investigated in two series of strontium aluminates, and , codoped with rare earth (lanthanoid, Ln) ions. Persistent phosphorescence is observed for the phosphors codoped with Pr, Nd, Dy, Ho, and Er. In the phosphorescence process of these phosphors, the excited electron in the ion is transferred between the Eu and the codopant Ln ions through the states of these ions. When the trivalent codopant ion captures the excited electron in the shell, the ion is reduced to the divalent state and, therefore, the energy necessary to thermally relieve the electron from the ion is determined by the energy difference between the and the configurations of the divalent Ln ion. The trapping depths (activation energies) of the Ln ions, estimated theoretically on the basis of this mechanism, coincide with the observed depths for the persistently phosphorescent phosphors, and those of the nonpersistent Ln ions are larger or smaller by than the former. It is also elucidated that the retrapping of the electron released from a trap plays an important role in the persistent phosphorescence process.
100(2006); http://dx.doi.org/10.1063/1.2372445View Description Hide Description
In this work the origin of the phase transition induced gain depression in diode pumped doped lasers has been systematically investigated. The mechanisms capable of intracavity beam distortions are discussed and their relative contribution to the gain depression is investigated. We also provide evidence of two effects: a phase transition induced polarization switching and an enhancement in the laser performance when phase transition is overcome.
100(2006); http://dx.doi.org/10.1063/1.2398556View Description Hide Description
/metals thin film bilayered structures were examined for write-once-type optical recording materials. Decreases in the reflectance of and structures at a certain elevated temperature were observed. The phenomena were attributed to the formation of metal sulfides with high optical absorption coefficients by the interface reaction between transparent and highly reflective metals. In contrast to or , an increase in the reflectance was observed in , which was presumably due to the change of optical interference conditions caused by the decrease in the film thickness and the improvements of the crystallinity of Au caused by the decrease of the lattice defects along the grain boundaries of Au. Examination of the /metals bilayered structures as optical disc memories revealed that the reactions occurred within submicrosecond during the recording laser irradiations. The recording sensitivities of or which corresponded to the reaction temperatures was able to be controlled by Al doping to Ag or Cu. The reflectance of the /metals samples except the kept enough value to be optically readable in the wider range from than the recording layer containing organic dye of the conventional compact disc recordable. The spectra after the heat treatments showed the smaller wavelength dependence than that before the heat treatment. These features indicate the promising properties of these structures in application to optical recording media that can be used in the wide wavelength range.
100(2006); http://dx.doi.org/10.1063/1.2395599View Description Hide Description
A number of methods for improvement of the efficiency of laser cooling in semiconductors are suggested and analyzed theoretically. It is shown that by combining band gap engineering in the electronic domain and photonic band gap engineering in the optical domain, one can attain an enhancement in refrigeration efficiency provided the nonradiative efficiency is not drastically affected.
100(2006); http://dx.doi.org/10.1063/1.2399887View Description Hide Description
A -doped single crystal has been grown by the Czochralski technique along the (0 0 1) orientation. The refractive indices were measured precisely as a function of wavelength. A Raman spectrum, polarized absorption spectra,fluorescence spectra, and fluorescence decay curve have been recorded at room temperature. The standard and modified Judd-Ofelt theories have been performed to analyze the absorption spectra to determine the spectroscopic parameters, including the Judd-Ofelt intensity parameters , radiative transition probabilities, radiative lifetimes, and branching ratios. The stimulated emission cross sections and fluorescence lifetime of the promising laser level were obtained.
100(2006); http://dx.doi.org/10.1063/1.2396670View Description Hide Description
An advanced dielectric function has been designed to compute the photonic band structures of non-close-packed inverse opalsfabricated using conformal infiltration and by a recently described sacrificial-layer technique. A model is proposed to correctly simulate complex dielectric structures resulting from conformal backfilled infiltrations. While large photonic band gaps(PBGs) and a reduced refractive index requirement (RIR) are predicted to occur in these inverse structures, the results also indicate a high degree of sensitivity to the dielectric/air network topology enabling fine PBG tailoring. Optimized structurally modified non-close-packed inverse opals with lower refractive indices offer enhanced optical properties compared to narrow PBGs observed in conventional inverse shell opals using high index materials such as silicon or germanium. Three-dimensional finite-difference time-domain computations predict that many experimentally achievable non-close-packed inverse structures exhibit significantly enhanced PBG properties: a RIR as low as 2.65 and PBG width of . Additionally, a PBG width of is predicted for an optimized high index non-close-packed inverse structure in which the interstitial air void is smaller than in structures fabricated by conformal means. The robustness and simplicity of this technique combined with predicted adjustable PBG properties is therefore demonstrated to provide alternative fabrication routes to the synthesis of photonic crystal devices operating in the visible with lower refractive indices.
Wavelength dependence of electro-optic effect in tetragonal lead magnesium niobate lead titanate single crystals100(2006); http://dx.doi.org/10.1063/1.2384813View Description Hide Description
The refractive indices,birefringence, and linear electro-optic (EO) effect of single crystals have been characterized as a function of wavelength. The dispersions of refractive indices for both ordinary and extraordinary rays were accurately described by a two-term Sellmeier dispersion equation, in which the parameters connected to the energy band structure were determined. A strong dispersion was found for birefringence and EO coefficient . With the increasing wavelengths, both of them decrease fast. The two-term Sellmeier dispersion model, in which one oscillator frequency is polarization dependent, was shown to represent closely all the results. The polarization potential, which specifies the magnitude of the oscillator frequency shift, was .
100(2006); http://dx.doi.org/10.1063/1.2399896View Description Hide Description
An analytic model describing the distribution of the electron temperature created by absorption of an optical beam with a cylindrical symmetry in a layered structure was developed. Main attention was paid to the contribution of the lattice heating in the stationary and nonstationary regimes. It was shown that both the spatial distribution of the incident stationary beam and the temporal distribution of the incident pulses can be retrieved from the spatial and temporal electron temperature dependences near the illuminated surface. Electron temperature distributions can be measured using the thermoelectric effect. Experimental results of the spatial and temporal measurements of the thermoelectric voltage were compared with the theoretical calculations and a satisfactory agreement between experimental and theoretical results was found near the incident beam center for the quasistationary regime. The experimentally derived Seebeck detector’s responsivity equals .