摘要:

The feasibility of superconducting strips as position‐sensitive detectors for &agr; particles was investigated. For this purpose films have been prepared of different materials and dimensions using standard evaporation and laser ablation in an UHV system. Films were patterned into strips and current‐biased strips were irradiated with 5.5 MeV &agr; particles. The energy deposited by the particle in the strip creates a normal‐conducting region, which in turn causes a voltage drop. The time evolution of the voltage drop across the strip as a function of bath temperature and bias current has been investigated. Information about the site of the strip hit by the &agr; particle was obtained from monitoring the propagating normal‐conducting zones. A lateral position sensitivity of ±30 &mgr;m for a tantalum strip detector has been achieved. The propagation of normal‐state zones in the strip cannot provide information on the energy of the particle. A model describing the time evolution of a normal‐conducting zone in a current‐biased superconducting strip was used to test and interpret the experimental results and allows one to estimate the thermal conductivity of the strip material and the heat transfer from the strip into the substrate. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358810

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The successful development of the Plasma Opening Switch (POS) for inductive storage applications has been largely confined to negative polarity operation. Some models of POS behavior suggest that this is because in a positive polarity coaxial configuration, the weaker magnetic field at the cathode position retards the switch opening process. This article describes experiments in which both conductor radii in the POS region were significantly reduced. Anode‐ and cathode‐side current monitors indicate that voltages greater than open‐circuit are generated at the POS position, but there is a significant amount of electron flow out of the POS, depending upon load impedance. Flow impedance analysis indicates that a relatively small gap appears in the POS plasma after switch opening. Switch performance is also compared between flashboard and carbon gun plasma sources, with the latter operated both in positive and negative polarity. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358811

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The variation of the modal propagation loss of planar SiGe/Si heterojunction waveguides with Ge concentrations ranging from 1.3% to 10% has been determined for both TE and TM polarizations at wavelengths of 1.15 and 1.523 &mgr;m. The results show that at 1.15 &mgr;m wavelength the propagation loss increases with increasing Ge concentration due to the band‐edge absorption, which dominates the waveguide loss characteristics, while at the wavelength of 1.523 &mgr;m it decreases with increasing Ge concentration. The polarization sensitivity is only found at the longer wavelength and is thought to be due to the interaction of the evanescent field at the SiGe/Si interface with the Si substrate. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359573

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Nonlinear optical films were vapor deposited onto Si(100) and indium‐tin‐oxide‐coated glass. These films are guest‐host‐type polymer and showed electrooptical (EO) properties after poling. The host polymer is Teflon AF 1600 (AF) and the guest is dimethylaminonitrostilbene (DANS). Deposition is done by coevaporation. EO effects were found in films containing 5–25 vol % DANS. The EO coefficientr33, is found to be a function of composition, poling temperature, and cooling rate during poling. The highest EO coefficient obtained is 2.4 pm/V from a film with 10 vol % DANS and poled at a temperature of 130 °C. Very little or no EO effects were found for films with ≳25 vol % DANS. This is found to be a result of phase separation and subsequent crystallization of DANS. A decrease of EO effect at higher poling temperature is possibly a result of thermal disorder which was ‘‘frozen’’ during cooling. DANS was also found to react with Teflon AF 1600 at a higher DANS concentration. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358812

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Vertical‐cavity surface‐emitting lasers (VCSELs) with thermally stable electrical characteristics, including slowly changing operating voltages and series resistance, have been achieved over a wide temperature range between 100 and 380 K. A stable and low threshold voltage (1.9 V at 300 K to 3.0 V at 100 K) results from the temperature insensitive carrier transport across the continuously graded heterojunction interfaces of a distributed Bragg mirror, and from the very low series resistance that has been achieved by reducing the spreading resistance of the VCSEL. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358813

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Potassium fluoroboratoberyllate KBe2BO3F2(KBBF) has been revealed theoretically and experimentally as a novel ultraviolet nonlinear optical crystal, but it is found to be very difficult to grow in a large size, because of the weak binding interaction between the (Be2BO3)∞units, which leads to an apparent layer habit in the growth. By using a molecular engineering approach, oxygen bridges when brought in to strengthen the binding between the infinite units are found to be useful to overcome the above shortcoming of KBBF, and in the light of it another new ultraviolet nonlinear optical crystal—strontium boratoberyllate Sr2Be2B2O7(SBBO) has been discovered. The linear optical properties of SBBO are similar to KBBF’s, but its nonlinear optical properties are better than that of the latter.d22(SBBO)&bartil;d22(&bgr;‐BaB2O4), which is two times higher thand11of KBBF. SBBO has very good mechanical properties, and it is also not deliquescent. So SBBO is expected to have great potential for the application in ultraviolet nonlinear optical devices. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358814

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Calculations of the electric field in the excitation pulse of a 8‐cm‐diam copper‐vapor laser are presented. The electric field evolution is governed by a diffusion equation. Both radial and axial diffusions are considered. The boundary conditions are determined by the geometry of the tube, which is coaxially shielded by a metal sleeve, considering the tube inductance and parasitic capacitance. The diffusion coefficients are determined by the electrical conductivity of the plasma that is calculated based on measured plasma parameters. These calculations are essential both for modeling the excitation process and for estimation of the overall tube impedance in such lasers. Inadequate penetration of the electric field into the plasma because of high pre‐pulse conductivity turns out to be a main cause of power and pulse repetition rate limitations of this laser. The optimization of the tube geometry in order to minimize the field penetration problem is discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358815

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experimental data are presented along with calculations which describe the near‐threshold transverse modes in AlAs/GaAs/InGaAs vertical‐cavity surface‐emitting lasers with two different Bragg reflector systems. The Bragg reflector systems are composed of either AlAs/GaAs quarter‐wave stacks or a combination of AlAs/GaAs and ZnSe/CaF. The temperature dependence of the lasing mode is studied for three different structures, and it is shown that higher‐order lasing modes are favored with strong cavity detuning when the cavity resonance exists at a wavelength greater than the bulk emission peak. Important aspects of the lasing behavior are described by calculating the spontaneous mode which occurs at the lasing wavelength. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358816

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

X‐ray laser gain in laser‐produced plasmas for collisionally pumped 3p‐3stransitions in neon (Ne)‐like krypton ions has been estimated. The dependence of the gain on the temperature‐dependent average ionic charge stateZ¯(Te) and the fraction &dgr;(Te) of Ne‐like ions evaluated by using the plasma ionization model has been studied. It is observed that the temperature dependence of these parameters has a profound effect on the gain estimate, showing the importance of including the ionization model precisely for accurate modeling of x‐ray laser gain. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358817

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this article, we describe a method based on mirage effect in rear configuration, i.e., a modulated pump beam illuminates one side of the sample and the temperature gradient is probed on the opposite side. We show that a detailed investigation of the amplitude and phase of the photothermal deflection angle, versus the modulation frequency, connected with a numerical analysis, provides a determination of the thermal conductivity coefficient in the normal direction to the sample surface. The method, suitable for thin slabs of low thermal diffusivity materials, is calibrated with a series of known samples and then applied to a polydiacetylene single crystal. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358818

出版商:AIP    

年代:1995

数据来源: AIP