Volume 103, Issue 5, 01 March 2008
- 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
103(2008); http://dx.doi.org/10.1063/1.2887995View Description Hide Description
Concave laser forming and convex forming are all required for the complicated curved surface plate forming. Concave laser forming can be readily obtained by temperature gradient mechanism, while convex forming may be achieved based on buckling mechanism. To achieve precise control of bending direction of plate, buckling critical condition is analyzed when the buckling mechanism plays a dominant role, and a judgment criterion of working conditions of buckling mechanism, value, is derived. To verify the validity of the value, the experiments and numerical simulations are carried out. The results suggest that the bending direction of the plate can be exactly judged according to the value. In addition, the effects of the heating location and starting point on the bending direction are discussed, which provide further insight into the convex forming process and is helpful for the parameter selection of future process planning.
103(2008); http://dx.doi.org/10.1063/1.2841471View Description Hide Description
Polarization rotation or selection appears in materials with optical activity, or those with Faraday effect, or in liquid crystals. In this letter we present a structure, with an analogous response, using stacked extraordinary transmission subwavelength hole arrays modified to be nearly self-complementary. This produces a polarization selector because of the negative index of refraction for one of its linearly polarized eigenwaves. Simulation results and experiments at millimeter wavelengths confirm these features. Applications in miniaturized devices are envisioned as well as the possibility to scale to optical wavelengths.
103(2008); http://dx.doi.org/10.1063/1.2841418View Description Hide Description
From three dimensional whispering gallery cavities of GaAs photonic quantum ring fabricated in hyperboloid drum shape by chemically assisted ion beametching with the central active region diameter of , we have observed single mode lasing near with a record low injection threshold of in continuous wave operation at room temperature. This indicates that the quantum ring lasing phenomena associated with the three dimensional whispering gallery modes continue to persist, even at the submicron range overcoming the conventional two dimensional whispering gallery mode limit.
High-resolution observations of an amorphous layer and subsurface damage formed by femtosecond laser irradiation of silicon103(2008); http://dx.doi.org/10.1063/1.2885111View Description Hide Description
Using transmission electron microscopy(TEM), we observed the micro- and nanostructures of silicon after irradiation by duration pulses centered at wavelength. Specimens irradiated with a single pulse of fluence and with five pulses, each with a fluence of , exhibited various structures which included amorphous phases. The amorphous phases were pure silicon, as was revealed by high-resolution TEM imaging, nanobeam diffraction patterns, high-angle annular dark-field images, conventional diffraction images, and energy-dispersive x-ray spectra. Irradiation with a single pulse of produced neither amorphousmaterial nor lattice defects. Single-pulse irradiation at a fluence of and irradiation by four pulses at led to substantial subsurface damage around the center of the laser spot. It is concluded that multiple-pulse irradiation produces crystallographic damage more readily than a single pulse.
103(2008); http://dx.doi.org/10.1063/1.2844552View Description Hide Description
We report a circular metallic aperture with a subwavelength circular slit in the microwave regime, in which we experimentally demonstrate that this aperture can excite and focus surface plasmons. Under normal illumination, there is no focusing of the surface plasmons. However, by changing the incident angle, it is possible to focus surface plasmons. We showed that under a 20° illumination angle surface plasmons focus at away from the center on the surface of the aperture.
103(2008); http://dx.doi.org/10.1063/1.2841488View Description Hide Description
Highly photoconducting thin films with conducting grain boundaries were obtained, using “chemical spray pyrolysis” technique. By varying the atomic ratio of the precursor solution used for spray pyrolysis, the photoconductivity of these films could be tailored. Conducting grain boundaries were found only for samples with a specific stoichiometry and these films exhibited photoresponse to intrinsic and extrinsic excitation wavelengths in the range of . Postdeposition vacuum annealing of these films enhanced the grain boundary conductivity, caused the films to exhibit persistent photoconductivity for both intrinsic and extrinsic excitations and extended the extrinsic photoresponse to wavelengths beyond . Photoresponse to excitation wavelength of was observed in films with and without conducting grain boundaries which proved that the extrinsic photoresponse to this wavelength was an effect associated with the defect chemistry of the .
103(2008); http://dx.doi.org/10.1063/1.2890156View Description Hide Description
An in-line one-fiber approach to realize simultaneous measurement of salinity and temperature is proposed. The sensor system, which consists of multiplexed polymer-coated fiber Bragg gratings, showed that the polyimide-coated grating responds to variations of both temperature and salinity, while the acrylate-coated grating is only sensitive to the environmental temperature. The experimental results indicated that the temperature sensitivity of the acrylate-coated grating in water was for redshifted Bragg wavelength with increasing temperature, and the temperature and the salinity sensitivities of the polyimide-coated grating were (redshifted) and (blueshifted), respectively, which are in excellent agreement with the theoretical analysis.
103(2008); http://dx.doi.org/10.1063/1.2890751View Description Hide Description
We present results from a numerical study on heating in a thick layer of induced by long laser pulses at photonenergies close to the band gap of the material. A number of highly nonlinear mechanisms contribute to the heating, their relative importance being dependent on laser wavelength, instantaneous irradiance, and materialtemperature. Mechanisms studied include one- and two-photon absorptions across the band gap, intervalence band absorption between light- and heavy hole bands, electron-hole recombination, free-carrier absorption, excess carrier temperatures, and refractive index changes. The increase in band gap with temperature eventually terminates one-photon absorption from the valence to the conduction band, and further heating is driven by much weaker absorption processes. The varying band gap also introduces changes in electron- and light hole masses and thereby in the separation between the light- and heavy hole bands, thus strongly affecting intervalence band absorption. At the shortest laser wavelength of , the simulations indicate that surface melting will occur at fluence levels in the range of , while more than will be required for melting at wavelengths beyond .
103(2008); http://dx.doi.org/10.1063/1.2885729View Description Hide Description
It has been established that deviations from stoichiometry during the growth of ZnSe crystals result in point defects, which influence its electronic properties. We report on detailed photoluminescence results and their systematic analysis for ZnSenanowires. We studied photoluminescence from vapor-phase grown undoped ZnSenanowiresgrown under excess Zn conditions, and in particular the dependence on excitation intensity. Luminescencespectra were characterized by strong near-band-edge luminescence with negligible deep-level emission. We observed excitonic emission at 2.794 eV related to the neutral donor at . The binding energy of the exciton was found to be 7 meV, and that of the donor was 35 meV. Two donor-acceptor pair transitions at 2.714 and 2.686 eV were also observed, which can be related to the defect complexes of native defects with other native defects or with common unintentional shallow donors and acceptors. The ionization energies of both donors were 27 meV, whereas those of the acceptors were 102 and 139 meV, respectively.
103(2008); http://dx.doi.org/10.1063/1.2890390View Description Hide Description
In this work, we experimentally explored the negative-refraction superprism effect in photonic crystals by investigating the frequency-dependent negative refractive index of photonic crystals at microwave frequencies from . By detecting the spatially distributed intensity of a transmitted microwave whose incident angle is well controlled, we determined the propagating path, the refractive angle, and the refractive index of the transmitted microwave. The highly dispersive relationship between the negative refractive index and the frequency of microwave observed indicates potential applicability of this method for being used in a microwave wavelength demultiplexer by utilizing the negative-refraction superprism effect in photonic crystals.
- PLASMAS AND ELECTRICAL DISCHARGES
103(2008); http://dx.doi.org/10.1063/1.2842402View Description Hide Description
A low energy NB source, which consisted of a surface wave plasma (SWP) source and two large diameter carbonelectrodes, was developed for damageless etching of ultralarge-scale integrated devices. Ion beams were extracted from the SWP using two carbonelectrodes, accelerated and injected to the process chamber, and then neutralized without energy loss by a charge exchangereaction. The energydistribution functions of an Ar ion beam and an Ar atom beam was observed using a quadrupole mass spectroscope equipped with an energy analyzer. The energy of the Ar ion beam and the Ar atom beam was controlled by the acceleration voltage. ion and N ion beams were also extracted from a nitrogen plasma source. The intensity ratio of the N ion beam to the ion beam was 5:9, indicating that N ions were efficiently generated in the nitrogen SWP. The ion and N ion beams were changed to molecule and N atom beams, respectively, through a charge exchangereaction without energy loss. The energy of these beams was controlled by the acceleration voltage and was in the region less of than . When the acceleration voltage is higher than , not only the primary peaks due to the ion beam or N ion beam were observed but also a low energy second peak was observed in the energy distribution. The energy of the low energy second peak was controlled by the acceleration voltage. It was concluded that the low energy second peak corresponds to the molecule ion beam and the N ion beam, which is extracted from the second plasma generated in the space between the two carbonelectrodes.
Numerical analysis of the production profile of atoms and subsequent ions in large negative ion sources103(2008); http://dx.doi.org/10.1063/1.2887996View Description Hide Description
The production and transport processes of atoms are numerically simulated using a three-dimensional Monte Carlo transport code. The code is applied to the large JAEA negative ion source under the Cs-seeded condition to obtain a spatial distribution of surface-produced ions. In this analysis, the amount of atoms produced through dissociation processes of molecules is calculated from the electron temperature and density obtained by Langmuir probe measurements. The high-energy tail of electrons, which greatly affects atom production, is taken into account by fitting a single-probe characteristic as a two-temperature Maxwellian distribution. In the atom transport process, the energy relaxation of the atoms, which affects the surface ion production rate, is taken into account. The result indicates that the surface ion production is enhanced near the high-electron-temperature region where atom production is localized.
103(2008); http://dx.doi.org/10.1063/1.2838227View Description Hide Description
Single and double probes are simple and common tools for plasma measurements. In the case of nonstationary plasmas, the values of the plasma density obtained with these tools may differ significantly from the correct values measured, e.g., by microwave methods. The reason for such discrepancy could be the Bohm criterion failure during the plasma transition to the steady state. Indeed, the Bohm criterion, which is commonly used as a boundary condition at the plasma-sheath edge, directly determines the ion saturation current to the probe surface. The transition-time duration is studied and explained quantitatively for various plasmas produced by a version of a ferroinductor-coupled plasma source, which has its magnetic core fully immersed in the plasma. Corresponding conversion factors for probe measurements have been evaluated. Also, the influence of a certain amount of “hot” non-Maxwellian electrons on probe characteristics has been investigated.
103(2008); http://dx.doi.org/10.1063/1.2841450View Description Hide Description
Glow discharge at atmospheric pressure using a dielectric barrier discharge can induce fluid flow and operate as an actuator for flow control. In this paper, we simulate the physics of a two-dimensional asymmetric actuator operating in helium gas using a high-fidelity first-principles-based numerical modeling approach to help improve our understanding of the physical mechanisms associated with such actuators. Fundamentally, there are two processes in the two half-cycles of the actuator operation, largely due to the difference in mobility between faster electrons and slower ions, and the geometric configurations of the actuator (insulator and electrodes). The first half-cycle is characterized by the deposition of the slower ion species on the insulator surface while the second half-cycle by the deposition of the electrons at a faster rate. A power-law dependence on the voltage for the resulting force is observed, which indicates that larger force can be generated by increasing the amplitude. Furthermore, one can enhance the effectiveness of the actuator by either increasing the peak value of the periodic force generation or by increasing the asymmetry between the voltage half-cycles or both. Overall, the increase in the lower electrode size, applied voltage, and dielectric constant tends to contribute to the first factor, and the decrease in frequency of applied voltage tends to contribute to the second factor. However, the complex interplay between the above factors determines the actuator performance.
103(2008); http://dx.doi.org/10.1063/1.2837890View Description Hide Description
A detailed physical model for asymmetric dielectric barrier discharge (DBD) in air at low voltages (1.5–2 kV) is developed. Modeling of DBD with an applied sinusoidal voltage is carried out in two dimensions. The leading role of charging the dielectricsurface by electrons in the cathode phase is shown to be critical, acting as a harpoon that pulls positive ions forward and accelerates the gas in the anode phase. The positive ion motion back toward the exposed electrode is shown to be a major source of inefficiency in the sinusoidal or near-sinusoidal voltage cases. Based on understanding of the DBD physics, an optimal voltage waveform is proposed, consisting of high repetition rate, short (a few nanoseconds in duration), negative pulses combined with a positive dc bias applied to the exposed electrode.
103(2008); http://dx.doi.org/10.1063/1.2844478View Description Hide Description
We discuss the physics of a photoluminescence-based pulse-energy detector that will be used to characterize hard x-ray free-electron-laser pulses at the Linac Coherent Light Source. We tested the detector in a quasi-steady-state mode of operation at the conventional synchrotron light source Stanford positron electron assymetric ring 3 at the Stanford Synchrotron Radiation Laboratory. We also developed a Monte-Carlo model for the x-ray interaction with the nitrogen gas and the resulting ultraviolet signal and found good agreement with the experimental data. Remaining discrepancies can, to a large extent, be attributed to the varying luminescence behavior of different chamber wall materials.
103(2008); http://dx.doi.org/10.1063/1.2841770View Description Hide Description
By focusing a pulsed single mode Nd:YAG laser, we created low temperature plasmas at various pressures with various target gases and collected spectral light emissions to investigate the possibility of turbulent behavior in these types of plasmas.Characteristicfluctuation frequencies, chaotic dimensions, spectral indices, and turbulent fluctuationenergies are determined from fluctuations in these spectral light emissions. Values calculated for the spectral index and the chaotic index for each plasma event are found to be within the known values for other turbulentplasma systems. Thus, turbulent fluctuations on a nanosecond time scale are confirmed in the time evolutions of various singly ionized and neutral spectral lines of various gases.
Investigation of plasma immersion ion implantation of nickel-titanium rod by multiple-grid particle-in-cell simulation103(2008); http://dx.doi.org/10.1063/1.2890157View Description Hide Description
A multiple-grid-particle-in-cell numerical method has been developed. This method uses grids of different cell sizes and details are needed in only one part of the simulation region and not others. Hence, there are fewer nodes in the simulation thereby reduced computational time without sacrificing details. In the multiple-grid system, a phenomenon is identified to arise at the interface between two grids and a half-cell weighting method is utilized to solve the weighting issue at the boundary. It is shown that the expression of the change of momentum has no weighting function. This method is employed to numerically simulate the plasma immersion ion implantation process into a nickel titanium rod measuring long and in diameter used in orthopaedic surgery. To conduct more uniform implantation, the NiTi rod is elevated on the sample stage by a metal rod. The nitrogen implantation fluences and depth profiles are simulated and compared to experimental values determined by x-ray photoelectron spectroscopy.
103(2008); http://dx.doi.org/10.1063/1.2844495View Description Hide Description
In this paper plasma dynamics and ionization of propellant gas are modeled within the anode holes used for gas injection of a Hall thruster. Under conditions of anode coating with dielectric material, the discharge current should close within these holes, which results in ionization and formation of plasma jets emanating from the openings. The model shows that gas ionization inside the anode holes is very significant. For instance, the electron density increases by two orders of magnitude under certain conditions. The potential drop in the anode region which includes the electrostatic sheath inside the hole and potential drop along the hole might be positive or negative, depending on the anode hole radius.
103(2008); http://dx.doi.org/10.1063/1.2887987View Description Hide Description
Low-damage, high-rate, and highly selective low-etching can be simultaneously satisfied using a plasma with an environmentally harmonized gas chemistry . Such a plasma can drastically reduce the irradiation damage by ultraviolet (UV) photons during low-etching, because the intensity of UV in plasma is much lower than that in conventional plasma. The etching selectivity of SiOCH to a photoresist can be drastically improved by using plasma because of reducing F radical generation. In addition, pulse-time-modulated plasma causes a drastic increase in the etching rate because a large amount of negative ions can be generated. These results show that plasma is a very promising candidate for low-damage and highly selective low-etching.