Volume 97, Issue 10, 15 May 2005
- 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
- interdisciplinary and general physics
- proceedings of the 49th annual conference on magnetism and magnetic materials
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
97(2005); http://dx.doi.org/10.1063/1.1896436View Description Hide Description
A tunable focal lens using flat electro-optical liquid-crystal cell with uniform pixel-free electrodes is developed. The lenslike gradient refractive index profile is created in the cell via the spatially distributed polymer network obtained by photopolymerization using a spatially nonuniform laser beam. The conditions of the polymer network generation are optimized to improve the optical quality of the lens and its focusing properties. Low optical loss (scattering) is achieved for a focal length smoothly tunable from infinity to . Obtained results can be applied to develop lenses that have no moving parts and allow the electro-optical zooming.
Chromophore orientational mobility and index grating rise time in azo-dye-doped photorefractive polymer composites97(2005); http://dx.doi.org/10.1063/1.1887833View Description Hide Description
The influence of chromophore solubility enhancements on photorefractive grating rise time and device lifetime is investigated. Three azo chromophores differing primarily in compatibility with a polyvinylcarbazole host polymer were synthesized. Aromatic substitutions to the chromophore increased the device lifetime from several days to years although electric-field-induced poling experiments indicated that chromophore orientational mobility is severely hindered, resulting in photorefractive grating rise times approaching several hours. The incorporation of a flexible butyl chain to the aromatic substituted chromophores significantly enhanced the orientational mobility. These chromophores could be loaded as high as with no degradation in transparency for one year following fabrication.
Terahertz wave detection performance of photoconductive antennas: Role of antenna structure and gate pulse intensity97(2005); http://dx.doi.org/10.1063/1.1905792View Description Hide Description
We studied the receiver performance of two photoconductive antennas (bow tie and dipole antennas) fabricated on the same low-temperature-grown GaAs substrate to clarify the effect of the antenna structure and gate pulse intensity on terahertz wave detection. We observed the gate pulse intensity dependence of the temporal profiles of the terahertz waves or terahertz spectra. For both antennas, the sensitivity in the low-frequency regime was enhanced compared to that in the high-frequency regime for large gate pulse intensities. This is because the carrier trap time increased due to the saturation of the GaAsdefect levels. We also observed that the peak-to-peak amplitude of the terahertz wave detected by one antenna was not always larger than that detected by the other antenna, and the peak-to-peak amplitude of the bow tie antenna was larger (smaller) than that of the dipole antenna when the gate pulse intensity was high (low). This was explained by the gate pulse intensity dependence of the frequency-dependent detection sensitivity and also by the resonance frequency of the antenna structure.
97(2005); http://dx.doi.org/10.1063/1.1899224View Description Hide Description
We employ femtosecond laser pulses (80 fs, 1.59 eV, and 80 MHz) to study the optical second-harmonic (SH) response of ternary alloys ( about 0.2) grown by the vertical Bridgman method. The alloy segregates into a Pb-rich and a Cd-rich phase, the latter dominating the SH response of the ternary alloy by at least two orders of magnitude. Several sample regions show a regular layer-by-layer accommodation of the Pb-rich and Cd-rich phases as seen by a periodic alternation of the alloy’s SH response on a length scale. Furthermore, we employ polarization-resolved SH imaging as well as SH imaging at different azimuthal angles to obtain spatially resolved mappings of the sample, which are sensitive to the composition as well as the growth orientation of the material system. We observe an azimuthal phase shift of approximately 30° between coherent macroscopic regions (several ) in the Cd-rich phase of the ternary alloy. We interpret these regions as large area crystalline grains of (111) and (411) crystal orientations and approximately equal composition. Hence, SH imaging is shown to spatially resolve regions of different growth directions within the sample.
Investigation of the spectra of phosphorescent organic light-emitting devices in relation to emission zone97(2005); http://dx.doi.org/10.1063/1.1900283View Description Hide Description
The dependence of the electroluminescent spectra on the emission zone of blue electrophosphorscent light emitting diodes (PHOLEDs) was investigated. The light emission of a PHOLED was tuned from blue to greenish blue by adjusting the position of the emission zone in the PHOLED. Experimental results agreed well with the numerical simulation based on the effect of the wide-angle optical interference by the metalcathode. The comparison of the numerical results and the electroluminescent spectra of the PHOLED was then extended to serve as the basis of another method to determine the location of the emission zones of PHOLEDs.
Optically pumped terahertz laser based on intersubband transitions in a GaN∕AlGaN double quantum well97(2005); http://dx.doi.org/10.1063/1.1900929View Description Hide Description
A design for a GaN∕AlGaN optically pumped terahertz laser emitting at 34 μm is presented. This laser uses a simple three-level scheme where the depopulation of the lower laser level is achieved via resonant longitudinal-optical-phonon emission. The quasibound energies and associated wave functions are calculated with the intrinsic electric field induced by the piezoelectric and the spontaneous polarizations. The structures based on a double quantum well were simulated and the output characteristics extracted using a fully self-consistent rate equationmodel with all relevant scattering processes included. Both electron-longitudinal-optical phonon and electron-acoustic-phonon interactions were taken into account. The carrier distribution in subbands was assumed to be Fermi–Dirac-like, with electron temperature equal to the lattice temperature, but with different Fermi levels for each subband. A population inversion of 12% for a pumping flux at room temperature was calculated for the optimized structure. By comparing the calculated modal gain and estimated waveguide and mirror losses the feasibility of laser action up to room temperature is predicted.
Magneto-optic Kerr effect: Incorporating the nonlinearities of the analyzer into static photometric ellipsometry analysis97(2005); http://dx.doi.org/10.1063/1.1904722View Description Hide Description
A static photometric ellipsometer allows the measurement of the magneto-optic Kerr effect at a very high signal-to-noise ratio. To gain information about the state of magnetization a mathematical description of the optical components is required. This involves the use of trigonometric functions, e.g., within the Müller calculus. As these are transcendental functions the inversion of the formulae defining the final state of polarization is impossible. The commonly applied linearization is only valid for thin samples with small Kerr angles. For large Kerr angles the nonlinearity of the analyzer setup becomes important. The influence of these nonlinearities on static ellipsometry is studied analytically and numerically. A method is developed to reveal the magnetic properties with high accuracy despite of the nonlinearities due to the optical setup. The applicability of the method is demonstrated for the experimental data obtained from an ion-irradiated ironfilm on silicon, revealing the magnetic behavior of the investigated sample. The encountered effects are of fundamental importance for the full vectorial magnetization analysis when large Kerr signals are obtained in experiments.
Generation of continuous-wave terahertz radiation using a two-mode titanium sapphire laser containing an intracavity Fabry–Perot etalon97(2005); http://dx.doi.org/10.1063/1.1904724View Description Hide Description
Continuous-wave terahertz (THz) radiation was generated by photomixing two modes of a titanium sapphire laser. The laser was induced to oscillate on two modes by placing a Fabry–Perot etalon in the laser resonator. The frequency of terahertz radiation, which was equal to the difference frequency of the two modes, was varied by adjusting the free spectral range (FSR) of the etalon. Photomixing was performed by logarithmic spiral antennas fabricated on low-temperature-grown GaAs; and the emitted THz radiation was characterized. The THz power, measured by a Golay cell, was at and at . The THz frequency, as determined by a Fourier transform interferometer, was seen to correspond to the etalon FSR. The current-voltage characteristics of photomixers were also determined, and photocurrent modulation was observed by the autocorrelation of the laser beam.
97(2005); http://dx.doi.org/10.1063/1.1904706View Description Hide Description
We present a model for calculating the optical gain in a midinfrared quantum cascade laser in a magnetic field, based on solving the set of rate equations that describe the carrier density in each level, accounting for the optical- and acoustic-phonon scattering processes. The confinement caused by the magnetic field strongly modifies the lifetimes of electrons in the excited state and results in pronounced oscillations of the optical gain as a function of the field. Numerical results are presented for the structure designed to emit at , with the magnetic field varying in the range of . The effects of band nonparabolicity are also included.
Real-time photodisplacement imaging using parallel excitation and parallel heterodyne interferometry97(2005); http://dx.doi.org/10.1063/1.1905793View Description Hide Description
A parallel photodisplacement technique that achieves real-time imaging of subsurface structures is presented. In this technique, a linear region of photothermal displacement is excited by a line-focused intensity-modulated laser beam and detected with a parallel heterodyneinterferometer using a charge-coupled device linear image sensor as a detector. Because of integration and sampling effects of the sensor, the interference light is spatiotemporally multiplexed. To extract the spatially resolved photodisplacement component from the sensor signal, a scheme of phase-shifting light integration combined with a Fourier analysis technique is developed for parallel interferometry. The frequencies of several control signals, including the heterodyne beat signal, modulation signal, and sensor gate signal, are optimized so as to eliminate undesirable components, allowing only the displacement component to be extracted. Two-dimensional subsurface lattice defects in silicon are clearly imaged at a remarkable speed of only for an area of . Thus, the proposed technique allows for real-time imaging more than 10 000 times faster than conventional photoacoustic microscopy.
97(2005); http://dx.doi.org/10.1063/1.1903107View Description Hide Description
The Si/ one-dimensional photonic crystals of heterostructural multilayers with two periods, and , have great potential for multiple-wavelength-transmission filters. These structures were prepared by inserting pairs of (as the defect region) in the middle of two sets of two pairs of , so that the structure becomes . means the number of pairs in the defect region. The complex refractive indices of Si and are assumed to be and in the transfer matrix calculation. The number of transmission channels or defect branches is given by , that is, . For large , the photonic band gap exists in a normalized frequency range of 0.0846–0.3838, which corresponds to the wavelength range of 0.84–6.67 μm. The defect branches are placed on a branch band between two symmetric flat bands. For a filling factor , a matching condition of optical length in two alternating layers, the branches at the center of the branch band are divided into a uniform frequency interval. In particular, we claim that the transmission-defect branches can be precisely tuned by controlling the incident angles without external applied bias.
Investigation of dye-doped red emitting organic electroluminescent devices with metal-mirror microcavity structure97(2005); http://dx.doi.org/10.1063/1.1913794View Description Hide Description
Organic electroluminescent (EL) devices with planar microcavity structure, indium-tin-oxide/-diphenyl--bis(3-methylphenyl)--biphenyl--diamine/tris(8-hydroxyquinoline)-aluminum (AlQ):4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran/, were fabricated. The Ag and Al layers acted as not only hole-injection layer and cathode, respectively, but reflective mirrors, resulting in strong microcavity effects, such as spectral narrowing and directional emission. The effects of device parameters on the EL performance were studied in detail and were discussed in terms of conventional microcavity theory. On-axis light magnification with a coefficient (EL enhancement ratio between cavity and noncavity devices) of was observed, which was consistent with the theoretical calculation. At the same time, optimized microcavity device with bright pure red emission showed maximum luminance of , peak at , Commission International de I’Eclairage coordinates of and , and high EL efficiency of were obtained.
97(2005); http://dx.doi.org/10.1063/1.1899223View Description Hide Description
A multiguide directional coupler with a gratinglike indium-tin-oxide electrode on a planar nematic liquid-crystal film is studied. The linearly polarized beam can be coupled into adjacent channels with the periodic modulation of the refractive index by applying a periodic electric field. The intensity distribution of coupling in the transverse direction varies with the distance of beam propagation. The coupling effects, which depend on the polarization of the incident beam and the temperature of the liquid crystals, are also discussed.
97(2005); http://dx.doi.org/10.1063/1.1900933View Description Hide Description
The potential of optical traps for the study of weak interactions in colloidal systems is well recognized. In this paper, a theoretical model is developed for two oscillating interacting hard spheres in a fluid in the low Reynolds number limit. The amplitude of oscillation of the particle and the phase lag between the motion of the particles and the optical traps are a function of both the hydrodynamic coupling between the particles and the interaction potential. The effect of the particle-particle interaction is most pronounced for particles oscillated asynchronously along their line of centers. The experimental realization of this scheme is demonstrated for the model system of polystyrene latices in water and the theoretical predictions are compared with the experimental results. The extension of this idea to oil-in-water emulsions is discussed.
- PLASMAS AND ELECTRICAL DISCHARGES
Absolute and relative density measurements in a dielectric barrier discharge by diode-laser absorption spectroscopy and resolved plasma emission97(2005); http://dx.doi.org/10.1063/1.1894585View Description Hide Description
The population density of metastable Ar in a 70% , short-pulsed ( full width at half maximum) dielectric barrier discharge was determined from the time-resolved diode-laser absorption as a function of pulse repetition rate for a gas pressure of . The relative population density change of and was also determined from first negative and second positive plasma emission for the same pulse repetition rates and pressure. The net power deposited per pulse was obtained from measurements of the voltage and current wave forms. The fractional energy dissipated in metastable Ar production was estimated from the power and absorptionnumber densitymeasurements to be roughly 20% for pulse repetition rates , decreasing to at for applied voltage and .
97(2005); http://dx.doi.org/10.1063/1.1891276View Description Hide Description
Optical and electrical measurements are performed in methane/diluent mixtures in a 250-ns pulsed-dc discharge using a fast-rise-time pulser. In particular, emission and laser-induced-fluorescence studies of CH produced by direct-electron-impact dissociation reveal the postdissociation kinetics of CH as well as a means of estimating the CH density. In a 20-Torr mixture of argon/methane/nitrogen, the instantaneous CH density is determined to be at the end of the steady-state voltage and current condition. Kinetics analysis reveals that neutral chemistry with methane is the largest loss process for the CH fragments. Analysis of rotational temperatures indicates that less than 30% of the input electrical energy is deposited as heat in the gas, with the balance going toward dissociation, excitation of internal molecular states, and ionization.
97(2005); http://dx.doi.org/10.1063/1.1904719View Description Hide Description
A critical issue in high-intensity laser-solid interactions is the effect of the laser prepulse on the target, but the experimental details of these lower-intensity interactions are often difficult to measure due to the subsequent high-intensity pulse. We have performed target experiments using a duration, wavelength laser pulse, specifically designed to mimic the typical amplified spontaneous emission(ASE) prepulse from a high-power Ti:Sapphire laser. Using this “artificial” ASE prepulse, we find that the threshold for relevant changes to typical solid targets occurs at a fluence of , or , well below the plasma formation threshold. Notably, the results are not consistent with simple surface vaporization, and suggest that the ASE prepulse causes multiatom clusters to be ejected from the target surface. In a full high-intensity experiment, this ablated material would then strongly interact with the subsequent primary laser pulse.
97(2005); http://dx.doi.org/10.1063/1.1905798View Description Hide Description
A dual frequency rf sheath is analyzed using a simple rf sheath model to study the interaction between the two driving rf currents and their effect on sheath parameters. A symmetric rf discharge with defined electron density and dc sheath potential is modeled using a sharp boundary sheath approximation. Three results of this study are reported: (1) reproduction of trends in ion energy distribution functions predicted and measured in previous studies, (2) a frequency-mixing-dependent relationship between the dc sheath potential and applied rf potential, and (3) an additional asymmetry in the ion energy distribution functiongenerated by the intermodulation components resulting from the nonlinear sheath.
Atomic hydrogen densities in capacitively coupled very high-frequency plasmas in : Effect of excitation frequency97(2005); http://dx.doi.org/10.1063/1.1900290View Description Hide Description
Absolute hydrogen atom densities in pure hydrogen capacitive discharges were measured as a function of excitation frequency (13.56, 27.12, and 40.68 MHz), nominal electrical power, and gas pressure (between 0.1 and 1 Torr). Quantitative measurements were made using two-photon absorption laser-induced fluorescence (TALIF), put on an absolute scale by comparison with the TALIF signal from a known density of krypton gas, as proposed by Niemi, Schultz von Gathen, and Döbele [J. Phys. D34, 2330 (2001)]. The H atom density increases with gas pressure and electrical power, and at a given power and pressure it increases significantly with excitation frequency. The latter can be attributed in part to increased electron density. However, time-resolved TALIF measurements in the afterglow showed that the H atom surface loss probabilities are not constant, becoming somewhat smaller when the sheath voltage is lowered, as is the case when the excitation frequency is increased, contributing to the increase in H density.
Charge-state-resolved ion energy distributions of aluminum vacuum arcs in the absence and presence of a magnetic field97(2005); http://dx.doi.org/10.1063/1.1906291View Description Hide Description
The charge-state-resolved ion energy distributions (IEDs) of aluminum vacuum arc plasma species were measured and analyzed for different geometric and magnetic field configurations. The IEDs were fitted by shifted Maxwellian distributions. Plasma expansion in the absence of a magnetic field showed higher ion energies for higher charge states. The introduction of a magnetic field (independent of geometric configuration) resulted in a broader distribution and increased average ion energies. The energy gain was approximately proportional to the charge state, which may be due to the presence of electric fields in the magnetized plasma. The evolution of ion energy distributions is relevant to thin-film growth, and it is shown that the IEDs can be modified by suitable magnetic field configurations.