Volume 107, Issue 9, 01 May 2010
Index of content:
- Lasers, Optics, and Optoelectronics
107(2010); http://dx.doi.org/10.1063/1.3406152View Description Hide Description
We theoretically study the transmission and Faraday rotationeffect in the heterostructure composed of an all-dielectric photonic crystal and a magnetic metal. Under the tunneling mechanism, the electromagnetic fields can totally enter the structure and localize at the interface between photonic crystal and magnetic metal. Because of the localized electromagnetic fields, the transmission and Faraday rotation in the heterostructure are simultaneously boosted.
107(2010); http://dx.doi.org/10.1063/1.3407518View Description Hide Description
Two-dimensional (2D) subwavelength imaging using a near-field antenna array probe is demonstrated experimentally at a probe-to-object separation distance of . Field perturbations caused by the presence of small objects are detected by monitoring the input reflection coefficient as the probe is scanned. The probe is designed to produce a subwavelength focal spot with a full-width half-maximum beam width. Propagating components are suppressed through destructive interference, enhancing the evanescent field enough to resolve objects spaced by . The poor resolving capability of a conventional single-element probe at this working distance is also addressed; it is shown that even objects spaced over a wavelength apart are inaccurately detected. A full 2D imaging experiment is carried out using eight scattered objects. The single element probe is unable to resolve any of the objects, while all but the two most closely spaced pairs are resolved by the array probe.
107(2010); http://dx.doi.org/10.1063/1.3408315View Description Hide Description
Judd–Ofelt (JO) analysis of ions spectral properties in doped and codoped glasses are performed based on the optical absorptionspectra at room temperature. Three phenomenological parameters , , and are determined. The effects of on the JO parameters are discussed in terms of the variations in bonding nature and ligand fields around sites.
107(2010); http://dx.doi.org/10.1063/1.3357291View Description Hide Description
We demonstrate theoretically and numerically that tunable slow light can be realized in planar semiconductormetamaterials with the unit cell composed of two different elements in a broad terahertz regime. In the unit cell, one element is a semiconductor split ring resonator and another one is a semiconductor cut wire. The interaction between the two elements of the unit cell, induced directly or indirectly by the incident electromagnetic wave, leads to a transparent window, resembling the classical analog of electromagnetically induced transparency. This transparent window, caused by the coupling of bright-bright modes or dark-bright modes, can be continuously tuned in a broad frequency regime. The strong normal phase dispersion in the vicinity of this transparent window results in the slow light effect. This scheme provides an alternative way to achieve tunable slow light in a broad frequency band and can find important applications in active and reversibly tunable slow light devices.
107(2010); http://dx.doi.org/10.1063/1.3415533View Description Hide Description
Tunable second harmonic generation of pulsed laser radiation in a GaSe crystal was demonstrated. The nonlinear optical properties of GaSe and phase-matching conditions were investigated, and the best fitted dispersion relationships for this crystal were also determined. External conversion efficiency of 0.1% with unfocused pumped radiation had been achieved and maximum single pulse energy of 0.19 mJ was generated at .
107(2010); http://dx.doi.org/10.1063/1.3419716View Description Hide Description
We have recently proposed a solid state heat pump based on photon mediated heat transfer between two large-area light emitting diodes coupled by the electromagnetic field and enclosed in a semiconductor structure with a nearly homogeneous refractive index. Ideally the thermophotonic heat pump (THP) allows heat transfer at Carnot efficiency but in reality there are several factors that limit the efficiency. The efficient operation of the THP is based on the following construction factors and operational characteristics: (1) broad area semiconductor diodes to enable operation at optimal carrier density and high efficiency, (2) recycling of the energy of the emitted photons, (3) elimination of photon extraction losses by integrating the emitting and the absorbing diodes within a single semiconductor structure, and (4) eliminating the reverse thermal conduction by a nanometer scale vacuum layer between the diodes. In this paper we develop a theoretical model for the THP and study the fundamental physical limitations and potential of the concept. The results show that even when the most important losses of the THPs are accounted for, the THP has potential to outperform the thermoelectric coolers especially for heat transfer across large temperature differences and possibly even to compete with conventional small scale compressor based heat pumps.
Many-body design of highly strained GaInNAs electroabsorption modulators on GaInAs ternary substrates107(2010); http://dx.doi.org/10.1063/1.3360937View Description Hide Description
Electroabsorption in highly strained GaInAs and GaInNAs quantum wells(QWs) grown on GaInAs or quasi-GaInAs substrates is investigated by using microscopic many-body theory. The effects of various parameters, such as strain, barrier height, substrate composition, and temperature are thoroughly examined. It is shown that the value of the absorption coefficient strongly depends on the depth of the QWs under large bias electric field due to the small overlap integral of wave functions between the conduction and valence bands. The use of GaInNAs QWs makes the strain in the well layer very small. Further, the effective quantum-well depth is increased in GaInNAs QWs due to the anticrossing interaction between the conduction and N-resonant bands, making it possible to obtain larger absorption coefficient under large bias electric fields without using wide-band gap materials for barriers.
107(2010); http://dx.doi.org/10.1063/1.3383045View Description Hide Description
We find that the phase shifts of reflected light within band gap of two-dimensional photonic crystal(PC) are as follows: with frequency altering from the lower edge to the upper edge of first stop band, the reflection phase shift varies from to 0 for the PC’s unit cell with the high-index material near the center, while it varies from 0 to for that with low-index material near the center. For the higher-order stop band, there exists a certain value of filling fraction, which makes the phase shifts in higher-order stop bands almost the same as that in the first stop band. When the filling fraction is far from that value, the phase shifts are significantly different. The further study on the Bloch modes demonstrates that their distribution of electric field and magnetic field determines the phase shifts. Moreover, we have found that, in the overlap area of transverse magnetic and transverse electric stop band, the phase difference between two polarizations of reflected light can remain invariant in a broad frequency region. Based on this property, the broadband and angle-insensitive phase retarders are designed. These interesting phase characteristics will bring about many potential applications.
Impurity-related photoluminescence line shape asymmetry in GaAs/AlAs multiple quantum wells: Fractional-dimensional space approach107(2010); http://dx.doi.org/10.1063/1.3342673View Description Hide Description
The optical transitions in 20 nm wide silicon and beryllium -doped GaAs/AlAs multiple quantum wells with various doping levels were investigated at different excitation intensities. A fractional dimensionality model was used to describe the free hole-donor and free electron-acceptor transitions in the quantum wells. The measured photoluminescencespectra from samples of different doping level related to donor-impurity or acceptor-impurity induced effects in the photoluminescence lineshape, were compared within the framework of these model calculations. Both experimentally and theoretically it was shown that acceptor and donor related optical transitions and photoluminescence line shapes were related to the difference in the effective masses of holes and electrons. This effect also leads to a difference in the photoluminescencespectra in which the luminescence band for the donor related spectrum is narrower in comparison to the acceptor related spectrum.
- Plasmas and Electrical Discharges
High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes107(2010); http://dx.doi.org/10.1063/1.3399650View Description Hide Description
High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathodeoscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphitecathode. In a coaxial vircator, velvet cathode is superior to the graphitecathode due to its shorter turn on time and better electron beam uniformity.
107(2010); http://dx.doi.org/10.1063/1.3406153View Description Hide Description
A fully electromagneticplasma model for an asymmetric capacitively coupled plasma discharge is used to understand the interaction between the external radio-frequency (rf) distributed circuit and the plasma. The plasma is excited using a 150 MHz rfsource connected to the top electrode, the bottom electrode is connected to a shorted transmission line, and the electrodes are separated from the chamber walls through dielectric rings. Under typical conditions, the electron density peaks in the center of the plasma chamber due to the standing electromagnetic wave and the rf current from the top electrode primarily returns through the bottom electrode. When the electrical length of the bottom transmission line is adjusted such that it presents a large (open-circuit) impedance at the plasma chamber interface, the rf return current shifts from the bottom electrode to the chamber wall. As a consequence, the peak in electron density also moves from the center of the chamber toward its outer periphery.
Transitions between corona, glow, and spark regimes of nanosecond repetitively pulsed discharges in air at atmospheric pressure107(2010); http://dx.doi.org/10.1063/1.3309758View Description Hide Description
In atmospheric pressure air preheated from 300 to 1000 K, the nanosecond repetitively pulsed (NRP) method has been used to generate corona,glow, and sparkdischarges. Experiments have been performed to determine the parameter space (applied voltage, pulse repetition frequency, ambient gas temperature, and interelectrode gap distance) of each discharge regime. In particular, the experimental conditions necessary for the glow regime of NRP discharges have been determined, with the notable result that there exists a minimum and maximum gap distance for its existence at a given ambient gas temperature. The minimum gap distance increases with decreasing gas temperature, whereas the maximum does not vary appreciably. To explain the experimental results, an analytical model is developed to explain the corona-to-glow (C-G) and glow-to-spark (G-S) transitions. The C-G transition is analyzed in terms of the avalanche-to-streamer transition and the breakdown field during the conduction phase following the establishment of a conducting channel across the discharge gap. The G-S transition is determined by the thermal ionization instability, and we show analytically that this transition occurs at a certain reduced electric field for the NRP discharges studied here. This model shows that the electrode geometry plays an important role in the existence of the NRP glow regime at a given gas temperature. We derive a criterion for the existence of the NRP glow regime as a function of the ambient gas temperature, pulse repetition frequency, electrode radius of curvature, and interelectrode gap distance.
107(2010); http://dx.doi.org/10.1063/1.3374711View Description Hide Description
Computational simulations of air glowdischarge phenomena in the pressure range typical of plasma actuator applications for high speed flow control are presented. The model is based on a self-consistent, multispecies, and multitemperature continuum description of the plasma. A reduced air plasma model suitable for multidimensional simulations with 11 species and 21 gas phase chemical reactions is validated against experimental results in the literature. The discharge model predicts experimentally observed glow mode discharge operation, the current-voltage characteristics of the discharge, and spatial profiles of the electron temperature and positive ion number densities. For pressures of order 1 Torr, and are the dominant positive ion species in the discharge, and the concentration of negative ion is comparable to electron concentration. The two-dimensional structure of the discharge is predicted by the model is found to be in agreement with qualitative observations from the experiments.
H-atom interaction with amorphous hydrocarbon films: Effect of surface temperature, H flux and exposure time107(2010); http://dx.doi.org/10.1063/1.3369286View Description Hide Description
In the present paper, we study the interaction between atomic hydrogen generated in a microwave afterglow with amorphous hydrogenated carbonfilms. A simple surface model is described and compared with the experimental results. Erosion rate is time dependent and exhibits a transient regime before reaching a constant value. Estimate of the modified film thickness by ellipsometry shows that thickness increases with time and becomes constant and equal to 1.4 nm when reaching the permanent regime. In addition, this limit is independent on the conditions, e.g., on hydrogen flux and temperature. Erosion rate depends linearly on hydrogen flux arriving at the surface and shows an exponential increase with surface temperature. A simple model proposed in the paper is in good agreement with the experimental data and allows giving an estimate of the erosionactivation energy. This value is in agreement with the energy involved in the reaction between hydrogen atom and carbon atom in hybridization.
Ion energy distribution near a plasma meniscus with beam extraction for multi element focused ion beams107(2010); http://dx.doi.org/10.1063/1.3369287View Description Hide Description
An earlier study of the axial ion energy distribution in the extraction region (plasma meniscus) of a compact microwaveplasmaion source showed that the axial ion energy spread near the meniscus is small and comparable to that of a liquid metal ion source, making it a promising candidate for focused ion beam(FIB) applications [J. V. Mathew and S. Bhattacharjee, J. Appl. Phys.105, 96101 (2009)]. In the present work we have investigated the radial ion energy distribution (IED) under the influence of beam extraction. Initially a single Einzel lens system has been used for beam extraction with potentials up to 6 kV for obtaining parallel beams.In situ measurements of IED with extraction voltages upto −5 kV indicates that beam extraction has a weak influence on the energy spread (±0.5 eV) which is of significance from the point of view of FIB applications. It is found that by reducing the geometrical acceptance angle at the ion energy analyzer probe, close to unidirectional distribution can be obtained with a spread that is smaller by at least 1 eV.
107(2010); http://dx.doi.org/10.1063/1.3371688View Description Hide Description
A multiple beamelectrode system (MBES) is used to provide focused ion beamlets of elements from a compact microwaveplasma. In this study, a honeycomb patterned plasmaelectrode with micron size apertures for extracting ion beamlets is investigated. The performance of the MBES is evaluated with the help of two widely adopted and commercially available beam simulation tools, AXCEL-INP and SIMION, where the input parameters are obtained from our experiments. A simple theoretical model based upon electrostatic ray optics is employed to compare the results of the simulations. It is found that the results for the beam focal length agree reasonably well. Different geometries are used to optimize the beam spot size and a beam spot is obtained. The multiple ion beamlets will be used to produce microfunctional surfaces on soft matter like polymers. Additionally, the experimental set-up and plans are presented in the light of above applications.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Growth of Bi doped cadmium zinc telluride single crystals by Bridgman oscillation method and its structural, optical, and electrical analyses107(2010); http://dx.doi.org/10.1063/1.3275054View Description Hide Description
The II-VI compound semiconductorcadmiumzinc telluride (CZT) is very useful for room temperature radiation detection applications. In the present research, we have successfully grown Bi doped CZT single crystals with two different zinc concentrations (8 and ) by the Bridgman oscillation method, in which one experiment has been carried out with a platinum (Pt) tube as the ampoule support. Pt also acts as a cold finger and reduces the growth velocity and enhances crystalline perfection. The grownsingle crystals have been studied with different analysis methods. The stoichiometry was confirmed by energy dispersive by x-ray and inductively coupled plasma mass spectroscopy analyses and it was found there is no incorporation of impurities in the grown crystal. The presence of Cd and Te vacancies was determined by cathodoluminescence studies. Electrical properties were assessed by I-V analysis and indicated higher resistive value for the crystal grown with higher zinc concentration (with Cd excess) compare to the other .
107(2010); http://dx.doi.org/10.1063/1.3399770View Description Hide Description
The photocurrent (PC) variation in photoconductive (CGS) layers had been investigated as a function of temperature. Three peaks A, B, and C of the PC spectra were associated with the band-to-band transitions. Thus, the parameters of the crystal-field splitting and spin-orbit splitting were directly acquired through the PC measurement. The and were 0.0903 eV and 0.2130 eV at 10 K, respectively. From the relations of peak position and temperature, the temperature dependence of the band-gapenergy is well described by . Also, the is estimated to be 1.7952, 1.8855, and 2.0985 eV at the valence-band states of , , and , respectively. However, the behavior of the PC was different from that generally observed in other semiconductors, the PC intensities decreased with decreasing temperature. From the relation of versus , where is the PC density, two dominant levels were observed, one at high temperature and the other at low temperature. By comparing the results of the PC and photoluminescence, we confirmed that the activation energy of 82.7 meV at high temperatures is related to the dissociation energy of donor level due to Se vacancies. Consequently, we suggest that in photoconductive CGS layers, the trapping center due to native defects and impurities limits the PC signal with decreasing temperature.
107(2010); http://dx.doi.org/10.1063/1.3400130View Description Hide Description
We investigate size dependent strengthening mechanisms in sputtered Fe/W multilayers with individual layer thickness, h, varying from 1 to 200 nm. Microstructure analyses reveal that Fe/W has incoherent bcc/bcc interface when h is greater than 5 nm. When h decreases to 1–2.5 nm, the interface becomes semicoherent, and Fe and W show significant lattice distortions comparing to their bulk counterpart due to interface constraint. The layer thickness dependent drastic variations in x-ray diffraction profiles are simulated well by using an analytical model. Film hardness increases with decreasing h, and approaches a maximum value of 12.5 GPa when h is 1 nm. The layer thickness dependent film hardnesses are compared with analytical models. Koehler’s image force plays a major role in determining the maximum strength of composites at smaller h.
107(2010); http://dx.doi.org/10.1063/1.3402458View Description Hide Description
The time-resolved luminescence spectra of the doped precursor glass reveal that ions locate in both the oxygen-coordination environment and the fluorine-coordination one. After crystallization induced by heat treatment, the orthorhombic nanocrystals with mean size of 22 nm embedded homogeneously in the glassy matrix. The Stark splitting emission, the low electric dipole transition, the disappearance of the charge transfer band and the reduction in value indicate the partition of into the lattice. Moreover, the nearly single-exponential luminescence decay curves of the and levels for the 0.1% doped glass ceramic evidence that ions mainly occupy the sites.