Volume 91, Issue 3, 01 February 2002
- lasers, optics, and optoelectronics (pacs 42)
- plasmas and electrical discharges (pacs 51-52)
- structural, mechanical thermodynamic, and optical properties of condensed matter (pacs 61-68, 78)
- electronic structure and transport (pacs 71-73)
- magnetism and superconductivity (pacs 74-76)
- dielectrics and ferroelectricity (pacs 77)
- nanoscale science and design
- device physics (pacs 85)
- applied biophysics (pacs 87)
- interdisciplinary and general physics (pacs 1-41, 43-47, 79, 81-84, 89-99)
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
91(2002); http://dx.doi.org/10.1063/1.1427431View Description Hide Description
The photorefractive response of several dye-doped nematic liquid crystal samples is studied experimentally. Particular attention is given to the observation and modeling of the multiexponential dynamics of the photorefractive grating, and of the photocurrent. Both involve at least two different mechanisms with largely different time constants. These phenomena occur on time scales on the order of seconds, for applied voltages about 1 V, and light intensities as low as milliwatts/cm2.
91(2002); http://dx.doi.org/10.1063/1.1428101View Description Hide Description
Inter-subband laser performance can be critically dependent on the nature of the electron distributions in each subband. In these first Monte Carlodevice simulations of optically pumped inter-subband THz lasers, we can see that there are two main causes of electron heating: intersubband decay processes, and inter-subband energy transfer from the “hot” nonequilibrium tails of lower subbands. These processes mean that devices relying on low electron temperatures are disrupted by electron heating, to the extent that slightly populated subbands can have average energies far in excess of the that of either the lattice or other subbands. However, although these heating effects invalidate designs relying on low temperature electron distributions, we see that population inversion is still possible in the high-THz range at 77 K in both stepped and triple-well structures, and that our 11.7 THz triple-well structure even promises inversion at 300 K.
91(2002); http://dx.doi.org/10.1063/1.1427433View Description Hide Description
Spectroscopic and laser properties have been characterized for incorporated into crystal belonging to the apatitestructure family. The standard Judd–Ofelt analysis has been applied to the measuredoptical absorption intensities to determine the radiative decay rates, branching ratios, and emission cross sections of principal intermanifold transitions of from the and states to the lower-lying manifolds in the visible region. The measured room temperaturefluorescence lifetimes of the (594 nm) and (625 nm) transition are 325 and 105 μs, respectively, while the Judd–Ofelt analysis predicts the radiative lifetimes for the and states to be 822 and 116 μs, respectively. The room temperature emission cross sections of the and intermanifold transitions have been also determined to be and respectively.
91(2002); http://dx.doi.org/10.1063/1.1426240View Description Hide Description
We report a morphology-dependent resonance (MDR) laser emission of rhodamine-doped ellipsoidal microcavity of the poly(methylmethacrylate) particles. Elliptical particles showed different lasing properties: the lasing threshold, intramode spectral separation, mode intensity, and spectral position, when an excitation is pointed to the different locations on the particle. This differs from the well-understood MDR lasing in spherical microparticles.
Fabrication and characterization of different types of two-dimensional AlGaAs photonic crystal slabs91(2002); http://dx.doi.org/10.1063/1.1429795View Description Hide Description
Photonic crystals having two-dimensional periodicity on a length scale of 320–450 nm are fabricated by electron beamlithography in combination with high-aspect-ratio dry etching. To achieve three-dimensional control of the optical properties, three kinds of dielectric waveguide structures based on AlGaAs heterostructures, that is, semiconductor-clad, air-bridge, and oxide-clad structures, are investigated. The ability to guide light through such photonic crystal devices is demonstrated by optical transmission measurements. Clear photonic band gap effects resulting in 30 dB attenuation of the transmitted light can be observed in the stop-band regions. The measured results are in good agreement with theoretical band-structure calculations and with numerically computed transmission spectra obtained by the finite-difference time-domain method.
91(2002); http://dx.doi.org/10.1063/1.1427405View Description Hide Description
Annealed proton-exchanged wave guides in X-cut were characterized by infrared reflection and Raman scatteringspectroscopy. The measured spectra were related to the multiple crystalline phases of as identified by direct wave guide index profiling. Unique relationships were found to exist between the crystalline phases and corresponding spectra, which thus can be used to identify phase contents in a given wave guide. Vibrational bands of the studied spectra were correlated with the electro-opticproperties of phases. Phase-dependent anisotropy of protonic conductivity was predicted based on the observed polarization dependence of OH libration bands. Finally, the electro-optic coefficient was theoretically estimated for all phases and compared with previously reported experimental data.
Reflection of infrared radiation from lamellar gratings on a silicon wafer: Spectroscopy of nonspecular orders91(2002); http://dx.doi.org/10.1063/1.1430538View Description Hide Description
Spectral efficiencies in the wavelength range from 1.5 to 14 μm of nonspecular orders reflected by grating-on-silicon structures with periods comparable to the wavelengths have been measured. The measurements were carried out at varying incident and scattered angles while keeping their sum fixed. The spectra consist of broadened peaks at wavelengths close to those predicted by the grating equation. A mechanism of the broadening due to finite incident beam width has been discussed. Simulations using rigorous diffraction theory and wavelet analysis have been performed. The results obtained agree well with the experimental data.
91(2002); http://dx.doi.org/10.1063/1.1430887View Description Hide Description
An infrared photodetector that contains a superlattice structure and a blocking barrier was investigated. The photodetector shows advantages including a low operating voltage (<0.7 V), wide detection bandwidth, flexible miniband engineering, and in particular, voltage-tunable spectral responsivity. The blocking barrier not only prohibits the dark current but also acts as a high-pass energy filter to tune the spectral responsivity. The zero background peak detectivity of our detector is (9.7 μm) at −0.5 V and (6.7 μm) at −0.1 V. Since the spectral responsivity at different biases is independent, temperature sensing is feasible by taking the ratio of the measured photocurrent at different biases. A demonstration of temperature sensing by using our superlattice infrared photodetector with a blocking barrier is also shown in this article. Those results show the superlattice is a simple and efficient structure for the design of a multicolor infrared photodetector.
91(2002); http://dx.doi.org/10.1063/1.1432116View Description Hide Description
We have characterized in detail the performance of an optically pumped semiconductor laserdevice which emits radiation on the third order laser mode. The sample was grown by molecular beam epitaxy and consists of an quantum well sandwiched between an double heterojunction. The structure was specially designed to emit laser radiation at 775 nm on the third order mode and used as a pump source towards generating twin photons at 1.55 μm on the fundamental mode via parametric fluorescence by utilizing the principle of modal phase matching between fundamental and third order laser modes in III–V semiconductors. The spectral and pumping power dependencies of the single pass optical gain of the structure were measured by varying the length of the excited region on the sample and recording the corresponding amplified spontaneous emissions. A maximum gain of 120 cm−1 was measured. The threshold for laser emission of barlike samples with surface waveguide ridges was measured systematically vs cavity length and sample temperature. The optimum cavity length for obtaining the lowest threshold was greater than 1.2 mm. Threshold equal to 42 kW/cm2 was measured at room temperature for a 1.44 mm long cavity, and the characteristic temperature which expresses the threshold in the exponential form was equal to 117 K. The transverse angular dependence of the third order mode laser radiation was measured for different pumping powers for processed as well as unprocessed samples.
91(2002); http://dx.doi.org/10.1063/1.1428805View Description Hide Description
Up-conversion luminescenceproperties and energy transfer processes in and co-doped -based fluoride glasses have been studied under 800 nm light excitation. Blue up-converted emission around 478 nm which can be assigned to the transition, was strongly observed. Up-conversion luminescence intensity exhibited an -concentration dependence. Among and both and have ground state absorption bands due to the and transitions, respectively, which can be directly pumped by 800 nm light. However, no emissions were observed in singly doped and doubly dopedglasses under 800 nm excitation. Therefore, a possible up-conversion mechanism may be proposed as follows: Energy transfer firstly occurs from to when is excited by 800 nm light, then the energy is transferred from to which is on the excited state and, finally, blue up-conversion emission of is observed through the transition. It was also demonstrated that the energy on the level was back-transferred to in the high-concentration region, quenching the up-conversion luminescence.
91(2002); http://dx.doi.org/10.1063/1.1426247View Description Hide Description
We have derived the global band gaps for general two-dimensional (2D) photonic band gap(PBG)structures formed by square or triangular arrays of metal posts. Such PBGstructures have many promising applications in active and passive devices at microwave,millimeter wave, and higher frequencies. A coordinate-space, finite-difference code, called the photonic band gapstructure simulator (PBGSS), was developed to calculate complete dispersion curves for lattices for a series of values of the ratio of the post radius (r) to the post spacing (a). The fundamental and higher frequency global photonic band gaps were determined numerically. These universal curves should prove useful in PBG cavity design. In addition, for very long wavelengths, where the numerical methods of the PBGSS code are difficult, dispersion curves were derived for the transverse-magnetic (TM) mode by an approximate, quasi-static approach. Results of this approach agree well with the PBGSS code for The present results are compared with experimental data for transverse-electric (TE) and TM mode PBGresonators built at Massachusetts Institute of Technology (MIT) and the agreement is found to be very good.
Absolute concentrations of the radical in the state measured by cavity ring down spectroscopy in an atmospheric oxyacetylene flame91(2002); http://dx.doi.org/10.1063/1.1427402View Description Hide Description
Measurements of absolute concentrations of in an atmospheric oxyacetylene flame are presented. Cavity ring down spectroscopy (CRDS) was applied to measure lateral column density profiles at a number of vertical positions. By means of Abel inversion processing and estimated radial temperature profiles, the column densities were converted to absolute concentration profiles, which have central maxima with peak concentrations between and Comparison of the measuredA state concentration profiles with calculated ground state profiles supports an earlier suggestion that the gas-phase mechanism used in the calculations needs improvement. This work demonstrates the applicability of CRDS for measuring absolute concentrations of electronically excited species in atmospheric flames, a result which can be of importance to combustion research.
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
91(2002); http://dx.doi.org/10.1063/1.1425422View Description Hide Description
Experimental results of high-current electron beamgeneration in a planar diode with a ferroelectric plasmacathode and a control grid are presented. The plasma is formed as a result of noncomplete discharge on the surface of a sample when a driving pulse is applied to its rear electrode. This noncomplete surface discharge leads also to the appearance of fast plasma flows which fill the anode–cathode gap prior to the application of the accelerating electric field. The measuredplasma densities and flow velocities allow us to estimate the bias potential of the control grid which is needed for the suppression of the plasma flows. The dependence of the plasma density on the grid potential is presented. Using the control grid, operation of the diode with the ferroelectric plasmacathode is achieved with negligible plasma prefilling of the anode–cathode gap.
91(2002); http://dx.doi.org/10.1063/1.1426246View Description Hide Description
Charge exchange between xenon ions and xenon atoms is the source of a detrimental low energy plasma in the vicinity of electrostaticspacecraft thrusters. Proper modeling of charge-exchange induced spacecraft interactions requires knowledge of the respective charge-exchange cross sections. Guided-ion beammeasurements and semiclassical calculations are presented for xenon atom charge-exchangecollisions with and at energies per ion charge ranging from 1 to 300 eV. The present measurements for the symmetric exchange system are in good agreement with several earlier experimental studies and semiclassical calculations based on the most recently computed interaction potentials. The cross sections are ∼30% higher than predictions by the Rapp and Francis model [D. Rapp and W. E. Francis, J. Chem. Phys. 37, 2631 (1962)]. The present symmetric charge exchangemeasurements are the first to cover the ion energy range from 40 to 600 eV. The cross sections are in good agreement with low-energy drift tube measurements and are significantly lower than previous higher energy measurements. A simple model for symmetric two-electron transfer is proposed that is in good agreement with the present measurements. The onset for the asymmetric charge-exchange process, is observed to be at 10 eV. For this process, a cross section of is measured for a energy of 600 eV.
91(2002); http://dx.doi.org/10.1063/1.1425425View Description Hide Description
The space and time variations of the light emission in a macroscopic plasma display panel (PDP) discharge cell have been studied with an image-intensified charge-coupled device. The discharge cell is similar to a real PDP cell with a coplanar electrode configuration. The cell dimensions are on the order of 100 times larger than those of a real PDP cell and the operating pressure is about 100 times smaller. Different xenon–neon gas mixtures have been investigated. Optical filters have been used to measure infrared emission from xenon excited states 823.1 and 828.0 nm, and visible emission from neon at 640.2 nm. The measurements show that the neon visible emission occurs only above the cathode while xenon infrared emission occurs above both cathode and anode. Standing striations can be observed above the anode. The strong xenon emission above anode indicates that this region is more efficient than the cathode region in terms of ultraviolet production. The measurements are in excellent qualitative agreement with similar measurements performed in real PDP cells. However the velocity of plasma spreading above the cathode in the macrocell is significantly larger than in a real PDP cell. The interpretation of this discrepancy is given in a companion paper (part II) [J. Appl. Phys. 91, 1000 (2002)] where the experimental results are compared with results from a fluid model of the plasma.
Physical phenomena in a coplanar macroscopic plasma display cell. II. Comparisons between experiments and models91(2002); http://dx.doi.org/10.1063/1.1425426View Description Hide Description
Measurements of infrared and visible emission in a macroscopic discharge cell similar to a plasma display panel (PDP) cell are analyzed using a two-dimensional fluid model of the discharge. The comparisons between experiments and models show a good qualitative agreement but the plasma spreading velocity above the cathodesurface is much faster in the experiments. We find that including photoemission (in a simplified way) in the model considerably increases the agreement between experiments and models. With a well-chosen photoemission coefficient, the model reproduces the trends observed in the experiments when the gas mixture (between 2% and 10% of xenon in neon) or the applied voltage is changed. The influence of photoemission on the current rise time and on the velocity of plasma spreading above the dielectricsurfaces is more important in the macrocell than in a similar (same dimension × pressure) PDP cell because resonant photon transport does not follow the similarity laws.
91(2002); http://dx.doi.org/10.1063/1.1425078View Description Hide Description
Plasma columns sustained at high enough gas pressures undergo radial contraction as manifested by their glow not entirely filling the radial cross-section of the discharge tube. This phenomenon has been reported with direct current, radio frequency, and microwavedischarges. However, its modeling is still incomplete, in particular for rf and microwavedischarges, a situation attributed to a lack of experimental data. To fill this gap, we took advantage of the extreme flexibility in terms of field frequency, tube diameter and gas nature of surface-wave sustained discharges to achieve a parametric study of this phenomenon. Special attention was paid to filamentation, specific to rf and microwavedischarges, which is the breaking of a single channel of plasma into two or more smaller filaments as a result of the skin effect. We used emission spectroscopy as the main diagnostic means. Electron density was obtained from Stark broadening of the line, while molecular-band spectra emitted by the OH radical and the molecule were employed to determine the discharge gas temperature, leading to its radial distribution upon performing Abel inversion. For a given tube radius, contraction is shown to increase with decreasing thermal conductivity of the discharge. As a result, He and discharges are the least contracted, while contraction increases with increasing atomic mass of noble gases. Of all these discharges, the discharge appears to be the closest to local thermodynamic equilibrium.
91(2002); http://dx.doi.org/10.1063/1.1425429View Description Hide Description
To elucidate the dynamics of static charge elimination, we measured charge decay and residual potential (balance or offset voltage) in gases with various ion mobilities. It was observed that surface charge decay, especially for positive charges, occurs much faster in nitrogen than in air. The residual potential on the probe is negative in pure nitrogen and increases toward positive values with the injection of small quantities of air in front of the ionizer. The fluctuations in the residual potential are generally less than 3 V peak-to-peak. For ionizer operations in nitrogen environments, the charge decay rate increases with superficial gas-flow rate. The results are consistent with a theory of the interaction between bipolar ions and a charged object.
Ion and neutral energy flux distributions to the cathode in glow discharges in Ar/Ne and Xe/Ne mixtures91(2002); http://dx.doi.org/10.1063/1.1430891View Description Hide Description
The work presented in the article is a study of the heavy particle (ion and neutral) energy flux distributions to the cathode in conditions typical of discharges used for luminous signs for advertising (“neon” signs). The purpose of this work is to evaluate the effect of the gas mixture on the sputtering of the cathode. We have combined two models for this study: a hybrid model of the electrical properties of the cathode region of a glow discharge and a Monte Carlo simulation of the heavy particle trajectories. Using known sputtering yields for Ne, Ar, and Xe on ironcathodes, we estimate the sputtered atom flux for mixtures of Ar/Ne and Xe/Ne as a function of the percent neon in the mixture.
91(2002); http://dx.doi.org/10.1063/1.1429773View Description Hide Description
A theoretical model of the plasmas generated by a sheath-helix antenna is developed for axisymmetric perturbations. The system configuration consists of a cylindrical plasma column inside a dielectric tube of radius The eigenvalue equation is obtained and the eigenfunction is identified as the Bessel function of the first kind of order zero. The radial wave numbers ξ and η for the eigenfunction are described in terms of the rf frequency and plasma density. A full dispersion relation is analytically obtained, including influence of the finite plasma size, antenna effects, the electron inertia, and ion mass contributions. It is shown from the dispersion relation that the radial mode number ξ approaches infinity at the lower-hybrid frequency, exhibiting a resonance condition. Meanwhile, the radial wave number η approaches at the lower-hybrid frequency. A cross-sectional view of the light emission in experiment indicates that the helicon-plasma density at the lower-hybrid frequency has a hollow profile. The azimuthal component of the perturbed electric field observed experimentally is very similar to the theoretical model of at the lower-hybrid frequency. The emission peak coincides with the radial location of the strongest electric-field intensity.