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.     

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.     

μ + SR studies of UM2Al3, M=Ni,Pd

Positive-muon spin rotation ( ⁺ SR) measurements have been carried out in the new heavy-fermion superconductors UM₂Al₃, M=Ni and Pd. In UNi₂Al₃ the observed ⁺ frequencies in zero applied field indicate commensurate antiferromagnetic (AF) ordering of U moments 0.1 _B /U atom. These moments coexist with superconductivity and have the highest values observed in an AF heavy-fermion superconductor. The absence of well-defined frequencies in zero-field ⁺ SR in the AF state of UPd₂Al₃ suggests symmetric ⁺ stopping sites. In this system ⁺ SR linewidths belowT _c yield a preliminary value of 8000 Å for the London penetration depth.     

Adhered Supported Carbon Nanotubes

Carbon nanotubes (NTs) in excess of 200 m long are grown by catalytic pyrolysis of hydrocarbon vapors. The nanotubes grow continuously without the typical extinction due to catalyst encapsulation. A woven metal mesh supports the nanotubes creating a metal supported nanotube (MSNT) structure. The 140 m wide mesh openings are completely filled by 70 nm diameter multiwalled nanotubes (MWNTs). The MWNTs are straight, uniform and highly crystalline. Their wall thickness is about 10 nm (30 graphite layers). The adherent NTs are not removed from the support in a Scotch tape pull test. A 12.5 cm² capacitor made from two MSNT structures immersed in 1 M KCl has a capacitance of 0.35 F and an equivalent series resistance of 0.18 . Water flows through the MSNT at a flow velocity of 1 cm/min with a pressure drop of 15 inches of water. With the support removed, the MWNTs naturally form a carbon nanocomposite (CNC) paper with a specific area of 80 m²/gm, a bulk density of 0.21 g/cm³, an open pore fraction of 0.81, and a resistivity of 0.16 -cm.     

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.     

Infrared absorption of glassy silicon dioxide

The infrared reflectivity of glassy silicon dioxide has been measured in the region of 2–35 m at room temperature. The reflectivity curve has been analyzed by means of Kramers-Kronig analysis and the optical constants have been determined. In addition to the three known peaks at 9 m, 12·5 and 22·5 m, the absorption curve exhibits further peaks at 17·5 and 11 m. A correlation with the transmissivity measurements has shown that in this region even the transmissivity curve exhibits a decrease. An attempt has been made at an interpretation of the spectrum according to Matossi's model of vibrations of the free tetrahedron SiO₄.The authors thanks Dr. A. Vako for enabling us to perform the measurement with the adapter for absolute reflectance and Dr. V. iek for his permission to use the computer programme and for stimulating discussions.     

Induced geometry and confinement

A classical, Poincaré invariant dynamical system is developed which contains, besides the natural metric _v , an induced metricg _v that is generated by a real scalar dynamical field. It is shown that scalar fields whose dynamics are governed by the induced metric can be consistently introduced. Also, point particles which follow timelike quasi-geodesic trajectories can be introduced. The reaction forces acting ong _v due to the presence of these fields and particles are computed. A discussion of causality and geometrical confinement is given.     

A Strategy for a Vanishing Cosmological Constant in the Presence of Scale Invariance Breaking

Recent work has shown that complex quantum field theory emerges as a statistical mechanical approximation to an underlying noncommutative operator dynamics based on a total trace action. In this dynamics, scale invariance of the trace action becomes the statement 0 = Re Tr T , with T the operator stress energy tensor, and with Tr the trace over the underlying Hilbert space. We show that this condition implies the vanishing of the cosmological constant and vacuum energy in the emergent quantum field theory. However, since the scale invariance condition does not require the operator T to vanish, the spontaneous breakdown of scale invariance is still permitted.     

High-resolution spatial characterization of laser produced plasmas at soft x-ray wavelengths

In this article, we describe the setup and application of a system for the spatial characterization of laser-produced plasma x-ray sources. While pinhole cameras are normally used for this purpose, we employed a zone plate to act as the x-ray lens. Together with an x-ray CCD camera as the detector, a spatial resolution of up to 2 m was achieved. Due to the wavelength-dependent focal length of a zone plate, the monochromaticity of the image was better than /=150, and the large aperture of the zone plate allowed single-laser-shot images to be collected. Methanol and ethanol were used as liquid-jet target systems. Two different Nd:YAG lasers with pulse durations of 3 ns and 10 ns produced the plasmas. Our measurements concentrated on the line emission of carbon in the soft x-ray spectral range, namely, the hydrogen-like -line at 3.37 nm and the helium-like -line at 4.03 nm. We investigated the influence of different nozzle sizes, laser energies, and pulse durations on the source size of the plasma. Depending on the experimental conditions, plasma diameters of 17–60 m were measured. PACS 41.50.+h; 52.38.-r; 52.70.La     

Self-consistent calculation of electron density and muon hyperfine field in transition metals and their compounds

The use of finite cluster models to represent the electronic structure of solids in the framework of self-consistent local density theory is reviewed. The embedding problem is discussed, and practical variational approaches to treating crystalline potentials and wavefunction boundary conditions are presented. Hyperfine fields at ⁺ sites in Fe, Ni, and Co are found to result from a delicate balance between negative contributions of deep-lying (bound, paired) states and positive contributions due to polarization of band levels near the Fermi energy. The nature of muon screening, and the potential for ⁺ in Cu and Al are briefly considered. Finally, we explore the likely binding sites and bonding mechanisms for ⁺ in the defect compound VO _x .     

Experimental survey of the sticking problem in muon catalyzed dt fusion

The sticking process dt + n, which constitutes the most severe limit to the number of fusions which a muon can catalyze, is reviewed. Many attempts were made to determine by calculations and measurements the probability for initial sticking _s ⁰ (immediately after dt fusion) and for final sticking _s (after the came to rest). Previous results based on neutron disappearance rates and on the observation of -X-rays were controversial and also in some disagreement with theory. New data are reported from PSI on direct observation of final sticking, using a setup with the St. Petersburg ionization chamber. These data mark a significant improvement in reliability and may clarify questions concerning previous discrepancies. The new results is _s(0.56±0.04)%, lower than the theory prediction _s=(0.65±0.03)%, at medium density.     

Micropropulsion using laser ablation

The micro-laser plasma thruster (LPT) is a new micropropulsion device that uses laser ablation to create very small thrusts (0.1–100 N) for pointing and positioning micro- and nano-satellites. In this paper, we discuss the expected performance of the LPT. For a ms-pulse device, target materials are restricted to those of low thermal conductivity, e.g. polymers. Volume ablation theory adequately describes their behavior. In a ns-pulse version, exhaust velocity can be an order of magnitude higher with correspondingly lower thrust-to-power ratio. The theory for surface absorbers describes the observed behavior. PACS 45.55.Px; 42.70.Jk; 52.50.Jm; 42.62.Cf; 42.70.Km; 78.66.Qr     

A matrix method for estimating the Liapunov exponent of one-dimensional systems

Let : [0, 1][0, 1] be a piecewise monotonie expanding map. Then admits an absolutely continuous invariant measure. A result of Kosyakin and Sandler shows that can be approximated by a sequence of absolutely continuous measures _n invariant under piecewise linear Markov maps _itn. Each _itn is constructed on the inverse images of the turning points of . The easily computable measures _n are used to estimate the Liapunov exponent of . The idea of using Markov maps for estimating the Liapunov exponent is applied to both expanding and nonexpanding maps.     

Application of μ SR to radiation damage in metals

Proton-irradiated Al, Cu and Ni foils have been investigated by the SR technique. No trapping of ⁺ has been observed in the temperature range investigated (<300 K). In Ni a temperature-independent increase of the depolarisation rate by a factor of three is found for the irradiated target. This change is attributed to field inhomogeneities produced by defects.     

Low temperature nuclear orientation of149, 151Gd in gadolinium

^{149, 151}Gd fused in gadolinium metal have been oriented at low temperature. The main results are: ¦ (¹⁴⁹Gd,7/2^–¦=0.97(6)_N, ¦(¹⁵¹Gd,7/2^–¦= 0.77(6)_N and I(¹⁴⁹Eu,939 keV)=7/2. Some M1/E2 and E2/M3 mixing ratios in Eu daughter nuclei are also established.     

Quantization of particle system with anomalous moments

A procedure is outlined for Hamiltonization of a particle system with anomalous magnetic moments and presumed electrical dipole moments, in the zero-plane formalism. Hamiltonization is undertaken in the covariant Lorentz gauge A = 0.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 78–81, September, 1987.     

Mössbauer evidence of reentrant spin-glass phase in the Fe0.45Mn0.25Al0.30 disordered alloy

Zero and longitudinal field SR have been used to study the evolution of spin correlations and magnetic order resulting from 10at% substitution of Zr for Nb in the weak itinerant antiferromagnet NbFe₂. Rapidly fluctuating atomic fields (=0.02s^–1) are found to persist almost unchanged to low temperatures. These coexist with a narrow distribution of static fields (=0.2s^–1) which evolves from a gaussian to lorentzian form between 90K and the Curie temperature of 43K. It is suggested that these effects may be a consequence of topologically frustrated exchange between Fe sites in the C14 structure.     

μ+ Knight shift in Be

A first principles calculation of the Knight shift of a ⁺ in beryllium was carried out using a cellular multiple scattering technique (Keller 1972, 1977). The main conclusion is that the ⁺ induces the segregation of one state (of each spin) from the conduction band to lower energies, a molecular orbital of the ⁺ and its nearest neighbours. This state is occupied (with two antiparallel spin electrons) but it has less than 35% of ⁺ character. The screening is then completed with the conduction electrons properly. The observed Knight shift is then the sum of a diamagnetic shielding and a typical enhanced spin susceptibility term of opposite sign and smaller value.     

Interaction between the nucleon and muon currents in elastic neutrino-nucleon interactions

The spin relations of neutrino-nucleon interaction with the formation of a muonv(V) N N are investigated within the framework of the two-component and four-component theories of two neutrinos under the assumption of a point neutrino. The difference between the predictions of the two theories is identified. It is shown that the four-component theory of two neutrinos, unlike the two-component theory, accommodates the V + A variant of interaction between the muon current (v) and the barion current.The authors are grateful to Prof. A. A. Sokolov and to R. Sh. Yakh'yaev for evaluating the results.     

The Spectrum of Ground-State Reversed Transition of CH3OH Far-Infrared Laser

Using the CH₃OH molecular energy levels data base management program, we deduced that when a CH₃OH laser pumped by CO₂-9P(16) line, there should be a FIR laser line of 918m wavelength, which corresponding to a transition in ground-state reversed three-level energy system. For the first time, the spectra of ground-state reversed three-level system transitions were calculated by solving the density matrix equations, and the spectrum characteristics of 918m line were studied. Experimentally, the CH₃OH FIR laser line of 918m wavelength was obtained by pumping with a TEA-CO₂ laser. The experimental results were in good agreement with the theoretical calculations.