摘要:

We present an analysis of the effect of digitization at a moderate resolution on the determination of various morphological quantities for a model three‐dimensional digitized medium. Specifically, we study systems of digitized overlapping spheres which have many of the features found in man‐made and geologic materials. The goal of this paper is to demonstrate which quantities are most sensitive to the process of digitization and resolution. In addition, the question of whether three‐dimensional data sets are necessary to obtain reliable results concerning material structure is answered in the affirmative. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359134

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The near‐surface regions of high‐purity fused silica disks (Corning 7940) were modified by implantation with Bi2+ions at 160 keV and at room temperature. The glasses implanted with a nominal dose of 6×1016Bi2+ions/cm2at ∼5 &mgr;A/cm2were subsequently annealed in oxidizing and inert atmospheres. The annealing temperature ranged from 500 to 1100 °C. The doses retained before and after the anneals were determined from Rutherford backscattering. A broad optical‐absorption band at ∼5 eV (248 nm) was resolved and attributed to surface plasmon resonance from colloidal bismuth metal particles precipitated after implantation and observed by transmission electron microscopy. After annealing in oxygen for 60 min at temperatures ≥1000 °C the bismuth in the sample decreased by ∼70%. Annealing in an oxidizing atmosphere oxidized the metallic precipitates, and oxidation changed the optical absorption of the samples. For samples annealed in argon, a relatively small decrease in retained dose by ∼12% was observed for temperatures ≥1000 °C. Based on the difference in the effects of annealing in oxygen and in argon, the bismuth atoms in the samples annealed in argon are not oxidized and remain in the particles. The small changes in the distribution of bismuth after annealing in argon are due to diffusion of bismuth atoms from colloidal particles to larger particles, i.e., Ostwald ripening, as well as evaporation from the surface of the fused silica. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359135

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A simplified energy‐level scheme is proposed for the photochemical cycle of the bacteriorhodopsin molecule. Rate equations are solved for the detailed light‐induced processes based on this model and the intensity‐induced population densities in various states of the molecule at steady state are computed which are used to obtain an analytic expression for the absorption coefficient of the modulation beam. Modulation of the probe laser‐beam transmission by the modulation‐laser‐beam intensity‐induced population changes is analyzed. It is predicted that for a probe beam at 412 nm up to 82% modulation can be achieved using a laser beam intensity of 3.2 W/cm2at 570 nm. For temperatures ∼77 K, the transmission at 610 nm can be switched from zero to 81% for modulating laser intensity of 11 W/cm2. Construction of a spatial light modulator based on bacteriorhodopsin molecules is proposed and some of its features are discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359136

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Results on annealing experiments of He+ion‐implanted KNbO3planar and channel waveguides are reported. Annealing at 150 °C for several hours leads to a reduction of the waveguide attenuation constant by more than 5 dB cm−1at a wavelength of 457 nm without significant change of the profile of the mode confining barrier. A minimum waveguide attenuation constant of 1.3 dB cm−1in planar and 2.2 dB cm−1in channel waveguides at 515 nm was achieved. Second‐harmonic generation measurements in the waveguides showed that the conversion efficiency can be improved by more than a factor of 2 by the annealing process. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359137

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Planar waveguides inz‐cut LiTaO3have been fabricated by ion exchange in pyrophosphoric acid. The dependence of the refractive index change and waveguide depth on the ion‐exchange time and temperature, and the anneal time and temperature has been investigated. In addition, waveguide transmission and inversion depths have been measured. The ion exchange temperature and duration were varied between 230 and 293 °C and 15 and 90 min, respectively. The anneal temperature and duration were varied between 300 and 594 °C, and 16 and 840 s, respectively. Under certain conditions the depth of the waveguide was shown to have a simple dependence on the processing conditions. The minimum waveguide coupling and propagation loss was measured to be 0.4 dB. Inversion was observed for anneals of 180 s and temperatures of 525–594 °C. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359549

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In computer simulations the mechanisms that lead to room‐temperature continuous‐wave green upconversion lasing in Er3+:LiYF4are investigated. The rate‐equation system considers the full erbium level scheme up to2H9/2, ground‐state depletion, excited‐state absorption on the pump and laser wavelengths, three interionic processes, stimulated emission, and the crystal and resonator data of the experiments. Experimental results performed at the University of Hamburg, Germany, are reproduced in the simulation. The influence of different parameters as pump wavelength, absorption cross sections, interionic parameters, dopant concentration, and temperature is investigated. An avalanche effect which exploits the strong cross relaxation from the upper laser level and the upconversion from4I13/2leads to an efficient population of the upper laser level. At higher dopant concentrations the cross relaxation becomes detrimental to stimulated emission due to the depletion of the upper laser level. This concentration dependence can be considered as a general behavior of rare‐earth‐doped avalanche lasers. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359138

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

New experimental and theoretical results on TE/TM bistability in 1.3 &mgr;m ridge‐waveguide InGaAsP/InP bulk lasers at room temperature are presented. Measured polarization resolved light power–current (P–I) characteristics as well as lateral near‐ and far‐field patterns are compared with results from a theoretical model based on the paraxial wave equations for TE‐ and TM‐polarized modes and the diffusion equation for the carrier distribution. The model was numerically evaluated by use of the beam propagation method. The observed TE/TM bistability is explained by the interplay of three different effects: (i) Tensile stress of about 109dyn/cm2promotes the TM gain strongly enough to compete with the TE mode. (ii) Improved TM waveguiding due to an enhancement of the effective refractive index near the beam axis caused by carrier depletion with increasing current leads to the onset of TM lasing and TE/TM switching. (iii) The TE/TM transition is accompanied by an abrupt increase of spatial hole burning in the lateral carrier distribution. Because of this nonlinear effect, a lower current is needed to switch the laser back to TE, giving rise to a hysteresis loop in theP–Icharacteristics and to TE/TM polarization bistability. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359139

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Thin Cr‐based films were made by sputtering with oblique incidence of the deposition flux onto a substrate. The films had an inclined columnar microstructure, as verified by atomic force microscopy and scanning electron microscopy. The optical transmittance showed pronounced angular selectivity as well as spectral selectivity. Scaling relations were found for the thickness‐dependent transmittance. The optical data could be reconciled with a model based on the Bruggeman effective medium theory. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359140

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The influence of absorption upon behavior of the reflection coefficient in water for an anisotropic periodically multilayered medium is studied. From observing a trough of the experimental reflection coefficient of a carbon/epoxy composite immersed in water, the propagation modes of the Floquet waves in an infinite anisotropic periodically multilayered medium in vacuum have been studied. This was explained by a mode that we called themultilayeredRayleighmode. This mode results from a combination of the Floquet waves which propagate in a periodically multilayered medium. This occurs when all the Floquet waves are inhomogeneous. Analyzing the results obtained in the isotropic case permits the more complicated cases to be explained. By analogy with the isotropic case, themultilayeredRayleighwaveis related to the poles and the zeros of the reflection coefficient. The existence of a critical attenuation becomes evident when the reflection coefficient in water for the multilayered medium reaches zero. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359141

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The free electron laser gain formulae for low‐, intermediate‐, and high‐gain domains are parametrized by means of a simple expression reproducing linear and exponential regimes. The proposed reformulation provides a useful tool of analysis because of its remarkable simplicity and because it offers the possibility of deriving comparably simple gain formulae that include inhomogeneous broadening effects. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359142

出版商:AIP    

年代:1995

数据来源: AIP