Volume 101, Issue 2, 15 January 2007
- 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
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
101(2007); http://dx.doi.org/10.1063/1.2409618View Description Hide Description
We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. The bimolecular upconversion process in these systems is based on the presence of a metastable triplet excited state of the macrocycles giving rise to dramatically different photophysical characteristics relative to the other known methods for photon upconversion such as two-photon absorption, parametric processes, second harmonic generation, and sequential multiphoton absorption. The chosen blue emitter molecules have suitably positioned triplet levels: in the case of an oligofluorine—essentially higher and in the case of diphenylanthracene—lower than the sensitizer porphyrinplatinum triplet level and thus two excitation pathways for photon upconversion were observed and investigated.
101(2007); http://dx.doi.org/10.1063/1.2422711View Description Hide Description
The phase stability in the capture and acceleration scenario (CAS) is studied and compared with that of conventional linear electron accelerators (CLEAs). For the CAS case, it has been found that a slow phase slippage occurs due to the difference between the electron velocity and the phase velocity of the longitudinal acceleratingelectric field. Thus, CAS electrons cannot remain in a fixed small phase region of the accelerating field to obtain a quasimonoenergy gain in contrast to the stability of phase oscillation in CLEAs. Also, the energy spread of the output electron beam for the CAS case cannot be kept as small as the CLEA because there is no good phase bunching phenomenon generated by phase oscillation.
Imaging concentric modulations in transverse modes of a vertical-cavity surface emitting laser using a scanning near-field optical microscope101(2007); http://dx.doi.org/10.1063/1.2423138View Description Hide Description
Concentric standing waves have been spatially imaged in the near-field regime within the optical aperture of a vertical-cavity surface emitting laser (VCSEL) using a scanning near-field optical microscope. Using the microscope’s near-field collection mode and subsequent detection via a spectrometer it has been possible to design an experiment to determine the spatial location of multiple lasing modes in addition to concentric standing waves. At low injection current above threshold the standing waves influence and modulate the optical emission from multiple transverse modes. These results are discussed in relation to cavity and aperture effects, and pattern formation in VCSELs. Surface defects arising in the aperture are also seen to affect the optical output and are briefly discussed.
101(2007); http://dx.doi.org/10.1063/1.2404327View Description Hide Description
Integrated waveguideamplifiers based on high concentration doped crystals are numerically studied by combining a full-vectorial finite element based modal analysis and propagation-rate equations. Using realistic input data, such as the absorption/emission cross sections and Er level lifetimes measured on grown crystal samples, we investigate the amplifier performance by optimizing the total Er concentration. We predict optimum gain coefficient up to and broad amplification bandwidth exceeding with pumping.
101(2007); http://dx.doi.org/10.1063/1.2409765View Description Hide Description
Noncontact, nonintrusive photocarrier (PCR) and photothermal radiometry (PTR) as well as Raman spectroscopy were used to characterize GaSb and Te-doped GaSb wafers used to grow GaSb-based heterostructures for infrared applications. The results indicated excellent sensitivity of PTR and PCR to the Te distribution on the wafer surface. The results from the three methodologies were consistent and indicated that the Te was segregated toward the edge of the GaSb wafer. The PTR and PCR laser-based techniques show great potential advantages to characterize the homogeneity of the Te doping in the GaSb wafers and for other steps in the fabrication of semiconductorheterostructures.
101(2007); http://dx.doi.org/10.1063/1.2409490View Description Hide Description
The coupled dipole method is a volume integral equation method which allows computation of the scattered field from an arbitrary object (shape and relative permittivity). This method has been extended to the computation of optical forces. In this article we further extend the coupled dipole method to the computation of optical torque. First, we establish the equation to obtain the optical torque using the coupled dipole method, stressing the importance of the radiative reaction term. Second, we compare our theory to existing models for validation. Third, we apply our method to the computation of optical torque, from a plane wave circularly polarized on a micropropeller. The influence of geometry and relative permittivity on the optical torque is studied.
101(2007); http://dx.doi.org/10.1063/1.2425003View Description Hide Description
We examine the impact of various annihilation processes on the laser threshold current density of a multilayer organic laser diode by numerical simulation. Our self-consistent numerical model treats the dynamics of electrons, holes, and singlet as well as triplet excitons in the framework of a drift-diffusion model. The resulting particle distributions enter into an optical model. In our approach, a three layer waveguide structure is taken into account and the resulting laser rate equations are solved. Various annihilation processes are included as reactions between the different particle species in the device employing typical annihilation rates and material properties of organic semiconductors. By systematically varying the device dimensions and the annihilation rate coefficients, the dominating quenching processes are identified. The threshold current density is found to depend sensitively on the thickness of the emission layer. The influence of annihilation processes on the threshold current density is quantified as a function of the emission layer thickness and various annihilation rate coefficients. Using typical annihilation rate coefficients singlet-polaron annihilation is found to be the dominating quenching process. Maximum annihilation rate coefficients are calculated allowing a threshold current density below . Singlet-triplet annihilation is recognized as another main loss process for singlet excitons. In our model the singlet exciton density is increased by triplet-triplet annihilation whereas it is diminished by singlet-triplet annihilation. The ratio of the rate coefficients for singlet-triplet and triplet-triplet annihilations is identified to be critical for the total number of singlet excitons being quenched by triplet excitons.
101(2007); http://dx.doi.org/10.1063/1.2408344View Description Hide Description
Absorption spectra of the , , , and multiplet manifolds of in sites in cubic garnet (TbGaG) are investigated at sample temperatures between and room temperature. Absorption measurements extend from . From analyses of temperature-dependent (hot-band) absorption spectra, many of the crystal-field split energy (Stark) levels of the multiplet manifolds of are identified and confirmed from analyses of the fluorescence spectra observed between 485 and , representing transitions from the to the manifolds. Each manifold is split by the crystal field into Stark levels. Some of these manifolds, including the ground-statemanifold, consist of Stark levels that are accidentally degenerate, or nearly so, making transitions to or from these levels appear as unresolved spectra, even at the lowest temperature investigated . To resolve these spectra, we have investigated the Zeeman and magneto-opticalspectra for representative manifolds, , and at temperatures of 78 and and magnetic fields up to . The data are interpreted using the Stark levels and wave functions from a crystal-field splitting calculation that involved 80 individual Stark levels identified from the optical spectra of the and quintet states reported in this study. Good agreement is obtained between the calculated and the experimental Stark levels. The calculated energy and symmetry label for each Stark level in the , , and manifolds suitably interpret the spectral properties observed in the magneto-opticalspectra, including the experimental assignment reported in the literature for the ground state as a quasidoublet .
Improved uniformity of target illumination by combining a lens array and the technique of spectral dispersion101(2007); http://dx.doi.org/10.1063/1.2424533View Description Hide Description
A scheme using a lens array and the technique of spectral dispersion is presented to improve target illumination uniformity in laser produced plasmas. Detailed two-dimensional simulation shows that a quasi-near-field target pattern, of steeper edges and without side lobes, is achieved with a lens array, while interference stripes inside the pattern are smoothed out by the use of the spectral dispersion technique. Moving the target slightly from the exact focal plane of the principal focusing lens can eliminate middle-scale-length intensity fluctuation further. Numerical results indicate that a well-irradiated laser spot with small nonuniformity and great energy efficiency can be obtained in this scheme.
Gain properties of an complex in a poly(methylmethacrylate) matrix for broadband optical amplification101(2007); http://dx.doi.org/10.1063/1.2409607View Description Hide Description
Emission properties of an erbium organic complex embedded in a polymer matrix are investigated for laser and optical amplification applications in the telecommunication range. The evaluation of stimulated emission cross sections is inferred from the Judd-Ofelt model [B. R. Judd et al., Phys. Rev.169, 130 (1968);G. S. Ofelt, J. Chem. Phys.37, 511 (1962)] based on experimental spectroscopic data, resulting in the determination of the three , , and Judd-Ofelt coefficients. These parameters are then used to calculate the oscillator strengths between states and allow us to evaluate spontaneous emission probabilities, radiative lifetimes, branching ratios, and quantum efficiencies of levels. Stimulated emission cross-sections are determined using the McCumber method [D. E. Mc Cumber, Phys. Rev.134, A299 (1964)]. Net gain cross sections are calculated for different values of the inversion population in ions. Results show that organic polymer hosts doped with erbium complexes are very attractive materials for amplification in the telecommunicationspectral range, as they display high gain cross section as compared to other erbium-containing materials, coupled with a wide and relatively flat amplification spectrum (up to ) which covers the whole spectral range currently used for wavelength division multiplexing operation.
101(2007); http://dx.doi.org/10.1063/1.2427098View Description Hide Description
Carrier recombination through radiative and nonradiative processes in lattice-matched multi-quantum-wells (MQWs) was investigated by temperature-dependent time-resolvedphotoluminescence(PL)spectroscopy. The MQW samples with different well widths were grown on InP substrates by molecular beam epitaxy. The PL decay times and the PL intensities were measured as functions of temperature. For a doping level of , the dominant mechanism of the radiative process was found to be free carrier recombination while excitonic recombination was absent due to the effect of strong carrier screening. The nonradiative mechanism was determined to be hole capture through multiphonon emission (MPE). The expressions of the nonradiative MPE recombination lifetime, the PL decay time, and the PL intensity were deduced as functions of temperature and were used to fit the measured temperature dependence of the PL decay times and the PL intensities. The MPE activation energies and relative defect densities for the samples with different well widths were obtained. A simple method is suggested to investigate the interfacial defects of quantum wells.
101(2007); http://dx.doi.org/10.1063/1.2430911View Description Hide Description
We report the self-formation of quasiperiodic voidstructure with the length of several hundred micrometers inside the crystal. The quasiperiodical voids along the propagation direction of the laser beam were formed spontaneously after the irradiation of a single femtosecond laser beam which was focused at a fixed point inside the crystal sample. The length of the void array varied with the focal depth beneath the sample surface. The possible mechanism of the self-formed voidstructure was discussed.
Thermal lens spectroscopy through phase transition in neodymium doped strontium barium niobate laser crystals101(2007); http://dx.doi.org/10.1063/1.2430938View Description Hide Description
In this work the anisotropic thermal lens (TL) effect was studied in doped crystals with two different stoichiometries ( and 0.61). TL measurements were carried out as a function of temperature from 30 up to so that the effect of phase transition was investigated. A strong enhancement in the TL effect during the ferro-to-paraelectric phase transition has been observed. Additionally, we have found an inversion of the TL signal with the crystal stoichiometry. This fact is explained in terms of a different thermal expansion along the axis for both crystals, as measured by x-ray diffraction experiments. The obtained results open the possibility to tailor a crystal stoichiometry that minimizes the TL effects.
101(2007); http://dx.doi.org/10.1063/1.2431080View Description Hide Description
A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and energy. The positions of the shock waves and the contact surface (boundary that separates the compressed ambient gas and the vapor plume) were obtained when the incident laser energy that is transferred to the vapor plume and to the background gas, , and the vaporized sample mass, , are specified. The values for and were obtained from a comparison of the calculated trajectories of the external shock wave and the contact surface with experimental results for a copper sample under different laser fluences. Thus and , which are the two dominant parameters for laser ablation and which cannot be measured directly, can be determined. In addition, the internal shock wave propagation within the vapor plume was determined; the interaction of the internal shock wave with the sample may be one of the mechanisms inducing liquid sample ejection during laser ablation.
101(2007); http://dx.doi.org/10.1063/1.2431085View Description Hide Description
Laser ablation of copper with a laser pulse at was studied with a series of synchronized shadowgraph ( laser pulses at ) and emission images (spectral line at ). Data were obtained at two laser pulse energies (10 and ) and in three background gases (He, Ne, and Ar) at atmospheric pressure. The laser energy conversion ratio and the amount of sample vaporized for ablation in each condition were obtained by the theoretical analysis reported in paper I from trajectories of the external shock wave, internal shock wave, and contact surface between the Cu vapor and the background gas. All three quantities were measured from shadowgraph and emission images. The results showed that , the amount of energy that is absorbed by the copper vapor, decreases as the atomic mass of the background gas increases; and , the mass of the sample converted into vapor, is almost independent of the background gas [Horn et al., Appl. Surf. Sci.182, 91 (2001)]. A physical interpretation is given based on the phenomena observed in shadowgraph and emission images during the first tens of nanoseconds after the beginning of the laser pulse for ablation in different background gases. In addition, an internal shock wave was observed in the emission images during the first tens of nanoseconds after the laser pulse, which strikes the surface and should be one of the mechanisms inducing the liquid sample ejection. Also, a significant vortex ring near the target was observed in emission images at longer times after the laser pulse which distorts the otherwise hemispherical expansion of the vapor plume.
101(2007); http://dx.doi.org/10.1063/1.2422712View Description Hide Description
A guest-host polymer system with potential use in electro-optic devices is discussed. The polymer host is a high , fully aromatic polyimide and the guest chromophore is disperse red 19. Relaxation mechanisms of polar order after electric field alignment procedure have been investigated by measuring the isothermal decay of the macroscopic nonlinear optical coefficient at different temperatures below glass transition temperature , upon the removal of the poling electric field. All the decay curves can be fitted by a double exponential function. Below , the slower relaxation time shows an Arrhenius temperature dependence. An extrapolation to room temperature allows to predict the time stability which results to be longer than . In addition, absorption spectra measurements of the films were performed before and immediately after poling procedure to estimate independently the polar order of the dipoles through the decrease of the absorption coefficients.
101(2007); http://dx.doi.org/10.1063/1.2426376View Description Hide Description
A study of the frequency response of solid-state impact ionization multipliers (SIMs) is presented that emphasizes the role of resistive and capacitive elements of the device to establish response limitations. SIMs are designed to amplify input currents from an external source through the impact ionization mechanism. An equivalent circuit model for the SIM is developed based on its current versus voltage characteristics, which is used to derive a frequency response model. Theoretical frequency response matches very closely to measured responses for first generation SIM devices constructed on -type silicon epitaxial layers with nickel silicide Schottky contact injection points. Devices were measured using a photodiode as a current source under light intensities between and . These SIMs were shown to have a low frequency response that follows a relationship. Using an external photodiode with an effective capacitance of , frequency response for a input current was limited to . A large effective barrier resistance due to the Schottky contact and space chargeresistance dependent on device geometry dominate the response. Future SIM designs with higher frequency response will have to significantly lower both the input barrier resistance and space chargeresistance.
Effect of injection current on the repeatability of laser diode junction voltage-temperature measurements101(2007); http://dx.doi.org/10.1063/1.2427097View Description Hide Description
The junction-voltage temperature relationship of a laser diode is used to determine the temperature of the device in the range to . We consider changes that occur to this relationship when the diode is driven at its nominal operating current and above. It is found that estimates of the parameters that appear in a function fitted to the voltage-temperature data are sensitive to driving currents at and above the nominal operating current. Changes in parameter estimates are attributed to degradation due to surface recombination at facets occurring when the laser diode is overdriven. An indication of this degradation is the change in the ideality factor that occurs as the current to the device is raised. This work indicates that the temperature of the junction can be determined over the range to with a standard uncertainty of .
- PLASMAS AND ELECTRICAL DISCHARGES
Control of plasma space potentials and chemical vapor deposition of nanocrystalline diamond films in surface-wave excited low-pressure plasmas101(2007); http://dx.doi.org/10.1063/1.2409761View Description Hide Description
A dc biasing method, developed in this work, has been investigated for the control of plasma space potentials and the chemical vapor deposition of nanocrystalline diamond (NCD) films in a planar surface-wave excited plasma at gas pressures below . A negative dc voltage was applied to a specially shaped thin metal plate attached below the upper dielectric window with respect to the grounded substrate and discharge chamber, instead of the conventional positive substrate dc biasing method. Plasma parameters were measured using a single-probe and depositedfilms were evaluated by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. The application of the dc bias voltage enabled the net dc bias current to be varied and plasma space potentials to be decreased over a wide range in the bulk region, resulting in the control of the bombarding ion energy on the grounded substrate. The vertical plasma parameter profiles showed the spatial difference in electron temperature between the local surface-wave region near the upper dielectric window and the bulk region (below ). It was found that the spatial difference in electron temperature permits the control of net currents and plasma space potentials in the dc biasing method. NCD films were deposited with smooth surfaces , a deposition rate of about , and a continuous surface coverage on Si substrates maintained at a temperature of about for hydrogen-based plasmas by biasing with to the metal plate.
101(2007); http://dx.doi.org/10.1063/1.2425196View Description Hide Description
The dynamics of barrier microdischarges operating in a Townsend regime is studied analytically and via kinetic particle-in-cell/Monte Carlo simulations. It is shown that statistical fluctuations of the number of charged particles in the discharge gap strongly influence the dynamics of natural oscillations of the dischargecurrent and may even lead to a disruption of the discharge. Analysis of the statistical effects based on a simple model is suggested. The role of external sources in stabilizing microdischarges is clarified.