摘要:

Sub‐Doppler resolution of atomic transitions is obtained by spatially resolved laser induced fluorescence in a diverging atomic beam created by laser ablation. Resonances are spatially separated by relativistic effects experienced between the probing radiation and the moving particles thus proving that it is a high‐quality atomic beam. Fast neutral atomic beams of89Y having centroid energies up to 12 eV are interrogated with tuned, single‐frequency laser light. Narrow, spatially separated points of light move along the probe axis and are imaged by detecting only the zero‐velocity components of the fluorescence. Accordingly, the Y 2D3/2→Y 2D03/2transition centered near 16 146 cm−1is resolved into its hyperfine components with line widths on the order of 55 MHz and with an equivalent resolution of ±5 MHz (±0.0002 cm−1). The upper level hyperfine interaction constant derived from level splitting is +89±5 MHz. An equivalent collimation ratio of 9 mrad is estimated for the expanding pulsed atomic beam. We propose that laser ablation of selected targets provides a unique source of neutral or ionic ground‐state atoms and molecules for spectroscopic studies. Details of these experiments are discussed and analyzed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361420

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We discuss the &ggr;‐ray production by Compton backscattering of intracavity storage ring free‐electron laser radiation. We use a semi‐analytical model which provides the buildup of the signal combined with the storage ring damping mechanism and derive simple relations yielding the connection between backscattered photons, brightness and the intercavity laser equilibrium intensity. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360797

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A quantum‐mechanical model is used to predict the energy levels and transition rates of Er in a variety of garnets in order to optimize laser performance. The existence of near resonances among the various Er manifolds as well as the transition rates are used to estimate upconversion and self‐quenching of Er in ten different garnets. Morrison’s quantum‐mechanical point charge program for calculating crystal‐field effects on rare‐earth ions was used to fit experimentally determined Er:YAG energy levels obtained from the literature. Radial factors from this fit were used, along with x‐ray data, to calculate theoretical levels and transition rates for Er3+in other garnets. The calculations suggest that the crystal‐field effect on the upper four levels of the ground4I15/2manifold can dramatically effect resonances and thus affect upconversion and cross‐relaxation processes. Additionally, a method by which they indicate how a large population density can efficiently be built up in the4S3/2manifold is proposed, a necessary condition for upconversion lasing from that manifold. Based on a simple summation of cross‐relaxation resonances that can contribute, LuGG is predicted to enhance this upconversion process. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360798

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have studied the lateral waveguiding properties of gain‐guided and index‐guided II–VI lasers under pulsed conditions and their influence on threshold current densityJthand differential quantum efficiency &eegr;. Thermal index guiding was found to reduce the astigmatism of the gain‐guided devices. The thermally induced lateral field confinement leads to a lowering ofJthand an increase of &eegr; with pulse width with a maximum after a few &mgr;s because of maximum overlap of the near field with the gain profile. For the index‐guided devices the lateral waveguiding is fully determined by the built‐in refractive index profile and no dependencies on pulse width are observed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360799

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Dipole radiation in a two‐dimensional periodic dielectric structure is studied both theoretically and experimentally in this article. For the two‐dimensional square lattice structure with dielectric cylinders, the emission power of an electric dipole in the photonic band structure is computed numerically by a combination of the plane wave method, the dyadic Green’s function, the Poynting theorem, and the triangular integration method. The theoretical results are compared with experimental measurements in a microwave frequency region. The computed radiation spectra in the photonic band structure show good agreement with the experimentally measured results. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360800

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The transfer matrix method has been employed to study the effect of light absorption on optical spectra of GaAs/AlAs Bragg reflectors. Substantial distortion of the Bragg plateau due to the absorption in GaAs layers is found. The saturation value of the reflection coefficient of the infinite Bragg reflector is shown to change nonmonotonically with the absorption coefficient showing a minimum. The particular case of low‐temperature light reflection from the Bragg mirror in the vicinity of the exciton resonance frequency in GaAs layer is considered. The dramatic enhancement (by a factor of 50) of the exciton resonance amplitude in comparison with the reflectivity spectrum of GaAs is found in case of GaAs/AlAs Bragg structure covered by &lgr;/2‐thick GaAs caplayer. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360801

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A theoretical and experimental study about the ability of the photothermal modulated mirage technique to characterize vertical interfaces (thermal resistances) separating two different opaque and thermally thick media with thermal conductivities larger than the medium surrounding the sample and with any value in their thermal diffusivities has been carried out. For the calculation of the mirage deflection, a theoretical model that incorporates the mirage parameters (sizes of probe and pump beams, probe beam height) and which is valid for any value of the thermal resistance characterizing the interface is presented. The model is validated by experimental measurements on a fabricated sample having a controlled interface width that separates two pieces of copper and steel. Fitting the model calculations and the experimental data allows one to retrieve the value of the thermal resistance. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360802

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An expression for the reduced effective thermal conductivity,keff/k1, of a random array of coated or debonded spherical inclusions with pair interactions rigorously taken into account is derived. Pair interactions are evaluated through solution of a boundary value problem involving two coated or debonded spheres with twin spherical expansions. The resultingkeff/k1is ofO(f2) accuracy, wherefis the combined volume fraction of the inclusion and interface. The effect of interfacial characteristics manifested as the reduced thermal conductivity, &sgr;3, and relative thickness, &dgr;/a, of the interfacial layer is thoroughly investigated. It is found thatkeff/k1can be approximately viewed as a function offand the dimensionless dipole polarizability, &thgr;1, over a large parameter domain, despite the existence of higher order polarizabilities in the expression ofkeff/k1. The value of &thgr;1alone determines whether the effective inclusion is enhancing (&thgr;1≳0), neutral (&thgr;1=0), or impairing (&thgr;1<0) to the matrix. Furthermore, the evaluation ofkeff/k1for the present model system can be approximately replaced with that for composites containing inclusions of no interface but possessive of a reduced thermal conductivity of (1+2&thgr;1)/(1−&thgr;1). A contour plot ofkeff/k1on the &thgr;1−fdomain that is useful in estimatingkeff/k1for interfacial properties characterized by an arbitrary combination of &sgr;3and &dgr;/a, is constructed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360803

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The fraction of neonlike ions and the average ionic charge state in laser plasmas formed at the surface of the solid targets of germanium, selenium, and krypton have been obtained at several electron temperatures using the steady‐state collisional radiative ionization model. Dielectronic recombination has been considered. The effect of consideration of dielectronic recombination of ions of one or more charge states on these parameters has been studied. It is observed that the values of electron temperature corresponding to the maximum abundance of neonlike ions in different cases of dielectronic recombination are significantly different from those obtained without consideration of dielectronic recombination. The optimum electron temperature of the plasma is estimated from the maximization of the x‐ray laser gain through temperature‐dependent variables of the gain coefficient and is observed to be the electron temperature at which the abundance of neonlike ions is maximum. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360804

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Physical mechanisms during early phases of pseudospark discharges are still under discussion. Experiments at low rates of current rise (dI/dt<109A/s) were performed to investigate these phenomena, using filtered fast shutter photography of the discharge plasma in the visible wavelength region. Additionally temporally resolved measurements of line emission of the filling gas (hydrogen and helium) and cathode material (copper) were performed. These experiments revealed a discharge phase where the cathode surface involved in the discharge is restricted mainly to the inside of the cathode aperture. The voltage drop during that phase is about 200–400 V and the current density exceeds 1 kA/cm2. A possible explanation of the discharge mechanisms during that discharge phase is self‐sputtering of cathode material from the cathode surface inside the cathode aperture. The described mechanisms might be fundamental for the transition from the transient hollow cathode phase to the high current phase in pseudospark discharges. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362663

出版商:AIP    

年代:1996

数据来源: AIP