Investigation of output parameters of a 4.3-μm longitudinal-discharge CO2 laser

A simple hot-cell-free 4.3-m CO₂ laser with longitudinal d.c. discharge is described and the results of its parametrization are presented. It is shown that for effective operation of the 4.3-m (10° 1-10°0 band) CO₂ laser, use of active mixtures containing 4 to 5% of carbon dioxide with 20% nitrogen is needed. The laser is mechanically Q-switched producing peak powers in excess of 60 W with a pulse duration of 300 ns FWHM. The 4.3-m laser is easily tunable and operates on the P and R branches of the 10°1-10°0 band of CO₂. The peculiarities of spectral performances are discussed.     

Photorefractive measurements on electron-irradiated semi-insulating GaAs

We present photorefractive measurements at 1.06 m and 1.3 m performed in electron-irradiated GaAs. Irradiation with electrons of kinetic energies 1 MeV introduces intrinsic electrically active defects which modify the Fermi-level position and allow to modify the electron-hole competition mechanism of the photorefractive effect. Furthermore, it is shown that the optical absorption in the 1.3–1.5 m spectral range can be increased, which might allow to enlarge the useful spectral range of GaAs towards optical telecommunication windows. The native and irradiation-induced defects are assessed by electron paramagnetic resonance and optical absorption spectroscopy conducted at T=300 K and 77 K. The direct influence of an irradiation-induced mid-gap defect on the photorefractive effect is experimentally and theoretically demonstrated.     

Nonrelativistic Lamb shift for muonic molecules

A recently developed formula [R. Bukowski and B. Jeziorski, Phys. Rev. A46 (1992) 5437]. has been applied to estimate the soft-photon Lamb shift contribution to the energies of the muonic molecules pp, dd, tt, pd, pt and dt. The corresponding corrections to the dissociation energies for the excited P states of dd and dt have been found to be almost identical and equal to 0.048 meV. The magnitude of this stabilizing effect is too small to affect seriously the formation rates predictions.     

A new doping superlattice photodetector

A new GaAs photodetector with high sensitivity in the whole 0.8–1.4m wavelength range has been fabricated from totally depleted GaAs doping superlattices grown by molecular beam epitaxy. Photoexcited electrons and holes are separated in real space by the space-charge field of the doping superlattice immediately after excitation, yielding a high quantum efficiency of this device. Because of the complete depletion, the doping superlattice behaves like a highly resistive material, which allows application of high electric field along the layers via selectiven ⁺ — andp ⁺ -electrodes. The sensitivity of this device at 1.3 m reaches more than 90% of the original band edge response at 0.85 m, and the external quantum efficiency amounts to 65% at 0.85 m. This excellent photoresponse at longer wavelengths arises from an extremely high electric field composed of the intrinsic space charge field and applied external field, and from the existence of pronounced tail states in the forbidden gap region of the superlattice.     

Positivity of entropy production in the presence of a random thermostat

We study nonequilibrium statistical mechanics in the presence of a thermostat acting by random forces, and propose a formula for the rate of entropy productione() in a state . When is a natural nonequilibrium steady state we show thate()0, and sometimes we can provee()>0.     

En">Characters and Composition Factor Multiplicities for the Lie Superalgebra $${\mathfrak{g}}{\mathfrak{l}}$$ ( m / n )

The multiplicities a of simple modules L in the composition series of Kac modules V lambda for the Lie superalgebra (m/n ) were described by Serganova, leading to her solution of the character problem for (m/n ). In Serganova's algorithm all with nonzero a are determined for a given this algorithm, turns out to be rather complicated. In this Letter, a simple rule is conjectured to find all nonzero a for any given weight . In particular, we claim that for an r-fold atypical weight there are 2r distinct weights such that a = 1, and a = 0 for all other weights . Some related properties on the multiplicities a are proved, and arguments in favour of our main conjecture are given. Finally, an extension of the conjecture describing the inverse of the matrix of Kazhdan–Lusztig polynomials is discussed.     

X-ray coherence and ultra small angle resolution at grazing incidence and exit angles

We measure the nonspecular x-ray scattering at grazing incidence and exit angles from a GaAs surface, modulated by an one-dimensional lateral grating of effectively variable periodicity 1 md60 m. Due to the projection onto the sample surface, the lateral coherence length is enlarged by a factor of more than 100. The line broadening of the diffraction maxima is used to deduce the number of periods that add up coherently for a given periodd and given angle of incidence. We demonstrate that the scattering geometry used allows for large coherently illuminated lengths on the sample and ultra small angle resolution of one- or two-dimensional objects. Thus structures of m size and larger will be accessible by coherent x-ray scattering.     

On the Haar measure of the quantumSU(N) group

We prove that the Haar state associated to the compact matrix quantum groupSU (N) is faithful for ]–1,1[,0, and anyN2.     

Towards slow μ− production via muon catalyzed fusion

Slow ^– production via dd-CF using a two-layer arrangement is investigated. To determine its feasibility, experimental measurements are now in progress using the muonic X-ray detection method. The following experimental steps are being considered: (1) measurement of the number of ^– stopped inside a solid H₂/D₂ layer by detecting p K X-rays, (2) hot d emission detection by placing a secondary target at a distance of 10–30 mm from the layer and by detecting specific delayed X-rays, (3) measurement of the disappearance of d emission as the added D₂ layer is increased, (4) dd-CF measurement by detecting fusion protons, and (5) slow ^– emission detection. Results of the initial test experiment are presented.     

An Approach to Linear Scalable DH-PHEMT Model for Millimeterwave Application

This paper describes a scalable small-signal equivalent circuit for 0.25 m gatelength Double Heterojunction delta-doped PHEMTs. The scaling rules for all elements except the pad capacitances and bondwire inductances have been determined. Good agreement is obtained between simulation results and measured results for 2 times 20 m , 2 times 40 m, 2 times 60 m, 2 times 100 m gate width (number of gate fingers times unit gate width) DH PHEMT.     

Analysis of the effects of doping and barrier design on the small-signal modulation characteristics of long-wavelength multiple quantum well lasers

This theoretical work has modelled the small signal response of InGaAsP and InGaAlAs multiple quantum well (MQW) lasers based on an ambipolar carrier transport model. The MQW parameters such as barrier bandgap, barrier width and the number of quantum wells have been optimized for high-speed modulation. The effect of the p-type doping and the strain of the InGaAs well have also been investigated.For the InGaAsP-based system, the optimization for maximum 3 dB bandwitdth shows that the optimum width is about 5 nm for 1.1 m barriers and 7 nm for 1.2 m barriers. The optimum barrier bandgap wavelength is about 1.1 m for the barrier width of 6 nm, about 1.15 m for 8 nm and 10 nm barriers. The p-doped MQW exhibits a higher modulation bandwidth because of its high differential gain and improved carrier distribution among the MQWs. The compressively strained InGaAs quantum well system has the potential for a higher modulation bandwidth. For the InGaAlAs-based system, the optimization for maximum 3 dB bandwidth shows that the optimum width is about 4 nm for a barrier wavelength of 1.10 m, and 6 nm for 1.2 m. The optimum barrier bandgap wavelength is about 1.1 m for a barrier width of 4 nm, and about 1.2 m for 6, 8 and 10 nm.     

Positivity of entropy production in nonequilibrium statistical mechanics

We analyze different mechanisms of entropy production in statistical mechanics, and propose formulas for the entropy production ratee() in a state . When is steady state describing the long term behavior of a system we show thate()0, and sometimes we can provee()>0.     

A microwave μSR spectrometer

A microwave-SR (WSR) spectrometer has been developed which operates in the frequency region of 1–2 GHz. The spectrometer geometry is that of longitudinal field (LF)SR and is typically used in time integral measurements. The high frequency of this instrument distinguishes it from the currently used RFSR spectrometers and allows high field measurements to be made for normal Mu type systems. This allows one to avoid low field broadening mechanisms that may be present, either from electronic spin-lattice relaxation or nuclear hyperfine interactions.Technical details of the tunable microwave cavity and the RF coupling scheme are presented. Experimental spectra of Mu lines in SiO₂ and GaAs are displayed.     

Light absorption in laser-heated cavities

The absorption of laser light in 0.25–1 mm diameter gold cavities, irradiated for the purpose of generating high-temperature blackbody radiation with intense laser radiation of either =0.44 m or =1.3 m wavelength, was investigated. For =0.44 m radiation the absorption exceeded 0.9 for all conditions, but dropped to only 0.3 for the smallest cavities irradiated at =1.3 m. Entrance hole and cavity filling with plasma seems important for the understanding of the observations.     

Slow and monoenergetic (3Heμ−)+ beam production and novel applications

Based upon the recent discovery at UT MSL/KEK, a new idea is proposed for producing a slow and monoenergetic (3.2 keV) (³He^–)⁺ ion beam by using particle decay of the (d³He) muon molecule formed during the (d) to³He transfer reaction. The proposed intense (³He) beam as well as the less intense (⁴He) beam will open up way to various new types of important CF experiments.     

Determination of in-depth thermal strain distribution in Molecular Beam Epitaxy GaAs on Si

In-depth stress distribution GaAs layers grown by Molecular Beam Epitaxy (MBE) on Si (001) has been studied by X-ray diffraction, photoluminescence and Raman spectroscopy. In order to determine the stress state at different distances to the interface GaAs/Si, layers of different thickness were prepared by chemical etching of the grown samples. We observe a non-uniform residual strain distribution through the GaAs on Si epilayer. Residual strain of thermal origin is larger in the highly defective region ( 0.4 m) near the GaAs/Si interface where we have found a non-elastic relation between measured in-plane (a ) and in growth direction (a ) lattice parameters. However, thermal strain is partially relaxed by formation of 10⁷ cm^–2 dislocations in the region of better crystalline quality near the external surface.     

Tunable electronic Raman laser at 16 μm

6s–9s electronic Raman scattering in caesium vapour is used to generate tunable 16m radiation. Output energies in excess of 8 J were obtained, with peak powers in the kilowatt range.     

Feasibility of an experiment to determine the branching ratio for the emission of a heavy neutrino after muon capture in3He

The triton energy of the muon capture reaction ³He t+v, where ³ He is the ground state of muonic³He, has been measured in order to investigate a possible heavy v admixture into the flavour with high sensitivity. ³ He has been formed via the pd fusion reaction by stopping ^– in an ionization chamber (IC) filled with an H/D gas mixture of 3% D concentration at a pressure of 161 bar. In a first short experiment 650 triton events were observed yielding an upper limit for the -heavy v mixing strength of 2.3×10^–3 atE _0v=60 MeV.     

Sealing of bore-holes in Si crystals by epitaxial overgrowth below 560° C

Liquid phase epitaxy (LPE) is applied to seal small bore-holes (diameter 250–400 m) in Si crystals by lateral overgrowth. The overgrowth layers have a typical thickness of 20 m, are mechanically stable and gas-tight. Using Ga as solvent for the LPE process the overgrowth can be performed at temperatures as low as 560°C.     

High efficiency cw far infrared lasers at 119 μm and 127 μm

The optically pumped FIR laser lines at 119 m from CH₃OH and at 127 m from¹³CD₃OH are known to be the most powerful in the far infrared spectral region. We report on efficiency measurements for our waveguide laser system. The effect of various parameters was investigated, resulting in the highest efficiency ever reported for the 119 m line. The Stark effect and others parameters of the 127 m were measured, and a new¹³CD₃OH laser line at 175 m discovered, with the same pump transition. These measurements are helpful for completing the assignment already proposed for the 127 m line.