Volume 99, Issue 9, 01 May 2006
- 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
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
99(2006); http://dx.doi.org/10.1063/1.2193648View Description Hide Description
Ultrashort laser pulses have been investigated as a means for the generation of deep narrow holes in metal samples. The drilling is observed to occur in two stages: the rapid formation of a narrow channel and the subsequent broadening of this channel to its final width. An in situ depth-gauging technique shows that this broadening develops regularly down through the hole. The coupling of light energy to the sample during this second stage of drilling and its polarization dependence are discussed.
99(2006); http://dx.doi.org/10.1063/1.2194119View Description Hide Description
Two-dimensional (2D) honeycomb photonic crystals with cylinders and connecting walls have the potential to have a large full band gap. In experiments, 2D photonic crystals do not have an infinite height, and therefore, we investigate the effects of the thickness of the walls, the height of the slabs, and the type of the substrates on the photonic bands and gap maps of 2D honeycomb photonic crystal slabs. The band structures are calculated by the plane wave expansion method and the supercell approach. We find that the slab thickness is a key parameter affecting the band gap size, while on the other hand the wall thickness hardly affects the gap size. For symmetric photonic crystal slabs with lower dielectriccladdings, the height of the slabs needs to be sufficiently large to maintain a band gap. For asymmetric claddings, the projected band diagrams are similar to that of symmetric slabs as long as the dielectric constants of the claddings do not differ greatly.
99(2006); http://dx.doi.org/10.1063/1.2194124View Description Hide Description
The linear and nonlinear changes in the refractive index of a Pöschl-Teller quantum well are studied. The Pöschl-Teller potential can easily become asymmetric by a correct choice of its parameter set. We use this feature to investigate the intensity-dependent refractive index of a GaAsquantum well within the density matrix formalism. Numerical results reveal large values of linear and nonlinear changes in the refractive index.
99(2006); http://dx.doi.org/10.1063/1.2196237View Description Hide Description
We investigate the physical mechanisms responsible for waveguide formation in silicaglass induced by intense femtosecond laser pulses from a Ti-sapphire laser at as well as from a femtosecond optical parametric amplifier at . It is demonstrated that the densification taking place at the irradiated region is the principal cause for refractive index change in the waveguides written with both 0.8 and pulses. The birefringence induced by the stress arising from such densification and its behavior against thermal annealing are also studied.
99(2006); http://dx.doi.org/10.1063/1.2192267View Description Hide Description
The interactions between ions OH groups in Er-doped -based glasses were investigated for samples with various Er OH concentrations. From the analyses of the total decay rate of the level of it was confirmed that the OH groups are dominant quenching centers of excited and a cause of concentration quenching of band emission. The attempts to remove the OH groups in the melting process were then performed. Highly dehydrated glass was obtained by keeping the dew point of the melting atmosphere at the order of . The solution of the OH groups in reheating processes for fiber fabrication was also investigated. It was revealed that they diffused from the glass surface to the inside with an order of millimeters when the glasses were annealed above softening temperature. Finally, we fabricated highly Er-doped, length compact -band fiber amplifiers with various OH concentrations. By removing the OH groups in the core glasses, the gain characteristics drastically improved and a gain at was achieved with pumping at .
99(2006); http://dx.doi.org/10.1063/1.2193067View Description Hide Description
Amplification of terahertz radiation on intersubband transitions has been analyzed by numerical Monte Carlo simulation for -type delta-doped Ge films with in-plane transport configuration of applied electric and magnetic fields. A significant increase of the gain is found, compared to existing bulk -Ge lasers, due to spatial separation of light and heavy hole streams, which reduces scattering of light holes on ionized impurities and heavy holes. The considered device has potential as a widely tunable laser with high duty cycle and operating temperatures up to .
99(2006); http://dx.doi.org/10.1063/1.2195422View Description Hide Description
Excess polarization dependent loss (PDL) was investigated for GaNwaveguide devices grown by molecular beam epitaxy(MBE). The loss for transverse magnetic polarization strongly depended on the edge dislocation density in the crystal, because the dislocations capture electrons and act like a wire-grid polarizer. By means of MBE regrowth on GaNgrown with metal-organic chemical vapor deposition (MOCVD), the PDL was reduced to with an edge dislocation density of , whereas it was approximately for an all-MBE-grown sample. An ultrafast all-optical switch utilizing the intersubband transition was fabricated with a multiple quantum well structure that was regrown with MBE on MOCVD-grown GaN. An extinction ratio of as high as was achieved with a control pulse energy of , which is attributable to the reduction of the excess PDL.
99(2006); http://dx.doi.org/10.1063/1.2195883View Description Hide Description
High-efficiency visible light emission in N-and-B-doped epilayers was observed in photoluminescencemeasurements at room temperature. The orange-yellow light emission due to the recombination of donor-acceptor pairs (DAPs) has a broad spectrum with a peak wavelength of and a full width at half maximum of at . The high B concentration of more than improves the emission efficiency of the DAP recombination at a high temperature. Compared with the photoluminescence spectrum of GaN at , a high quantum efficiency of 95% was estimated for the highly B-doped sample. From time-resolvedphotoluminescencemeasurements, a DAP recombination time of was obtained, which is in good agreement with the calculated value by the rate equation with the assumption of a 95% internal quantum efficiency. This is quite promising as a light-emitting medium by optical pumping, as well as monolithic light sources combined with nitride-based light-emitting diodes grown on the DA-doped SiC epilayer.
99(2006); http://dx.doi.org/10.1063/1.2199973View Description Hide Description
An examination into a chiralliquid crystal active waveplate device using a one-dimensional model, giving numerical results, is presented. The model calculates the director and flow configuration by minimizing the free energy of the chiral nematic modeled with fixed boundary conditions. The static case of varying the flexoclectric coefficients, electric field magnitude, and dielectricanisotropy is examined and it is shown that both flexoelectricity and dielectric effects alter the birefringence of the device in the presence of an electric field. The dynamic reaction to a rotating electric field is then examined.
99(2006); http://dx.doi.org/10.1063/1.2195891View Description Hide Description
Terahertz time-domain spectroscopy has been employed to measure the optical constants of ytterbium-doped yttriumaluminumgarnet (Yb:YAG) crystals for nominal values of 0.0, 0.1, 0.2, 0.5, 0.8, and 1.0 in the frequency range from . The real refractive indices for Yb:YAG crystals increase linearly with the concentration of . The experimental results can be fitted by the Sellmeier equation. The results imply that the phonon modes of the Yb:YAG crystals shift to lower frequencies with substitution of Yb for Y. The extinction coefficients of the Yb:YAG crystals are smaller than 0.05.
99(2006); http://dx.doi.org/10.1063/1.2195903View Description Hide Description
This paper reports on electrically controlled holographic optical elements (HOEs) in the form of parabolic mirrors that have the ability to collect light. These mirrors were written in a material known as holographicpolymer dispersed liquid crystal, where the liquid crystal embedded within the device allows for electronic control of the optical properties of the mirrors. Essentially, we have replicated the function of a focusing mirror in a lightweight, thin film, electro-optic device. In this work, we have experimentally characterized the optical and electro-optical properties of the HOEs and provided a qualitative explanation of the operation of these structures. This and further investigations of the properties of electro-optic HOEs will open avenues in bulk HOEs and integrated photonic device applications.
99(2006); http://dx.doi.org/10.1063/1.2196148View Description Hide Description
We report on the observation of ultraviolet lasing in optically pumped ZnO nanonails synthesized by thermal chemical vapor deposition method. The lasing threshold was found to be . Very sharp emission peaks (full width at half maximum of ) were observed in the emission spectrum, indicating a high factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonail.
99(2006); http://dx.doi.org/10.1063/1.2196220View Description Hide Description
We have demonstrated the use of microspectroscopy for measuring the and fluorescence spectra in double-clad Cr:YAG crystal fiber. The emission spectra of and are detected from core and inner cladding. The spectrum in the inner cladding shows a broad-band emission from , while the emission of occurs in the range of with a peak around . The characteristic of Cr ion at high-field sites shows a narrow-band emission ( for ; for ), whereas that at low-field sites shows a broad-band emission ( for ; for ). The emission intensity ratio of high-field sites to low-field sites in the inner cladding with different compositions has been investigated. It varies from 20% to 29% for and from 7.1% to 11.3% for when the concentration of increases from .
- PLASMAS AND ELECTRICAL DISCHARGES
99(2006); http://dx.doi.org/10.1063/1.2193170View Description Hide Description
Atmospheric and near-atmospheric pressureglow discharges generated in both pure helium and helium-air mixtures have been studied using a plasma chemistry code originally developed for simulations of electron-beam-produced air plasmas. Comparisons are made with experimental data obtained from high-pressureglow discharges in helium-air mixtures developed by applying sinusoidal voltage wave forms between two parallel planar metallic electrodes covered by glass plates, with frequencies ranging from and electric field strengths up to . The code simulates the plasma chemistry following periodic pulsations of ionization in prescribed environments. Many of the rate constants depend on gas temperature, electron temperature, and . In helium plasmas with small amounts of air added, rapid conversion of atomic helium ions to molecular helium ions dominate the positive ion kinetics and these species are strongly modulated while the radical species are not. The charged and neutral species concentrations at atmospheric pressure with air impurity levels up to are predicted. The negative ion densities are very small but increase as the air impurity level is raised, which indicates that in helium-based systems operated in open air the concentration of negative ions would be significant. If water vapor at typical humidity levels is present as one of the impurities, hydrated cluster ions eventually comprise a significant fraction of the charged species.
99(2006); http://dx.doi.org/10.1063/1.2191477View Description Hide Description
An examination of the influence of target composition and viewing angle on the extreme ultravioletspectra of laser produced plasmas formed from tin and tin doped planar targets is reported. Spectra have been recorded in the region from plasmas created by a , full width at half maximum intensity, Nd:YAG laser pulse using an absolutely calibrated grazing incidence vacuum spectrograph. The influence of absorption by tin ions in the plasma is clearly seen in the shape of the peak feature at , while the density of tin ions in the target is also seen to influence the level of radiation in the region.
Charge exchange collisions and ion velocity distribution at the electrode of low pressure capacitive rf discharges99(2006); http://dx.doi.org/10.1063/1.2191622View Description Hide Description
An analytic theory of the time-averaged ion distribution function (IDF) in the self-consistent sheath field of a low pressure capacitively coupled rf discharge is developed. A reliable simple quantitative description of the ion energy spectra formation is obtained and test calculations of charge exchange cross section reconstructions from IDFs are presented.
99(2006); http://dx.doi.org/10.1063/1.2197032View Description Hide Description
The early stages of the expansion of fully ionized cathode spot cathodic arc plasmas from a localized initial volume are simulated by an all particle molecular dynamics model. The particles are given an initial temperature and interact by Coulomb forces. Although there are limits on the number of particles and the starting density, the model is relevant in describing processes occurring in cells that are the smallest structures of a cathodic arc. Our model predicts that a cell undergoes an explosive expansion leading to a halo of electrons surrounding an ion-rich core. The ion kinetic energies are lower than experimental observation but may be consistent with them after extrapolation to higher initial densities. For mixed charge state plasmas, the ion kinetic energy increases with ion charge state.
99(2006); http://dx.doi.org/10.1063/1.2193647View Description Hide Description
A capacitively coupled, atmospheric pressure plasma has been developed that produces a high concentration of reactive species at a gas temperature below . The concentration of ground-state oxygen atoms produced by the discharge was measured by NO titration, and found to equal , or , using in argon at . The ozone concentration determined at the same conditions was . A model of the gas phase reactions was developed and yielded O atom and concentrations in agreement with experiment. This plasma sourceetched Kapton® at at and an electrode-to-sample spacing of . This fast etch rate is attributed to the high O atom flux generated by the plasma source.
99(2006); http://dx.doi.org/10.1063/1.2192947View Description Hide Description
Charged particle beams normally contain a complicated pulse shape structure when created. This structure is created by particular equipment and techniques such as high bandwidth laser systems driving photocathodes, and may drive effects that degrade beam quality or produce coherent electromagnetic radiation. While often encountered, such structure is generally poorly diagnosed and difficult to control. To study the effects of pulse shape structure in intense beams, we have developed a system using combined thermionic emission and photoemission to produce carefully tailored pulse shapes in an electron beam. In this paper, we discuss the performance of this system and derive limiting curves to explain the range of electron beam pulse shapes measured with it. Suggestions for improved design of future photomodulation systems are also made.
99(2006); http://dx.doi.org/10.1063/1.2197036View Description Hide Description
The observation of magnetooptical Faraday effects in water in experiments with electrical breakdown is presented. After high-voltage breakdown, the ionized channel with current was generated. The magnetic field from the current channel induces a circular birefringence which results in rotation of the polarization plane of a probing laser (, ). In spite of fast opposite radius drop of the magnetic field in radial direction, the Faraday rotationeffect drops very slowly. The rotation of the polarization plane was . The optical measurements are in good agreement within with the electrical measurements of the current.