摘要:

Recent advances in nonsilica fiber technology have prompted the development of suitable materials for devices operating beyond 1.55&mgr;m. The III–V ternaries and quaternaries (AlGaIn)(AsSb) lattice matched to GaSb seem to be the obvious choice and have turned out to be promising candidates for high speed electronic and long wavelength photonic devices. Consequently, there has been tremendous upthrust in research activities of GaSb-based systems. As a matter of fact, this compound has proved to be an interesting material for both basic and applied research. At present, GaSb technology is in its infancy and considerable research has to be carried out before it can be employed for large scale device fabrication. This article presents an up to date comprehensive account of research carried out hitherto. It explores in detail the material aspects of GaSb starting from crystal growth in bulk and epitaxial form, post growth material processing to device feasibility. An overview of the lattice, electronic, transport, optical and device related properties is presented. Some of the current areas of research and development have been critically reviewed and their significance for both understanding the basic physics as well as for device applications are addressed. These include the role of defects and impurities on the structural, optical and electrical properties of the material, various techniques employed for surface and bulk defect passivation and their effect on the device characteristics, development of novel device structures, etc. Several avenues where further work is required in order to upgrade this III–V compound for optoelectronic devices are listed. It is concluded that the present day knowledge in this material system is sufficient to understand the basic properties and what should be more vigorously pursued is their implementation for device fabrication. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365356

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The influence of aperture diameter on image contrast and resolution in scanning near-field optical microscopy has been investigated. Evanescent standing wave fields at glass–air interfaces were probed with well-characterized aluminum-coated fiber tips. A strong decrease in image contrast was found when the aperture diameter exceeds a critical value. Numerical simulations with the concept of an intensity transfer function using the axial component of the Poynting vector inside the probe demonstrate that this phenomenon is the result of the increasing influence of higher-order waveguide modes on power transmission through the probe. It has been shown that standing wave patterns are convenient and reliable reference devices for individual probe characterization and quantification of their resolving power. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364373

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The novel possibilities of phase feature detection in radiography at a third generation synchrotron radiation source are used to image, both in projection and in computed tomography, a cracked silicon single crystal and metal matrix composites strained in tension. Through an instrumentally very simple technique, based on Fresnel diffraction, phase jumps related to the interface between the matrix and the reinforcing phases of the composites are detected even when these phases show very similar x-ray attenuation. Strain-induced cracks with openings below the micrometer range are also visible through the phase modulation they introduce, illustrating the potential of the technique for assessing damage in materials with improved resolution and sensitivity. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364374

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The image quality of radiographic images from image plate detectors is limited by two factors: first, the quality of the image plate itself and, second, the scanning procedure of the individual scanning device. In this study the impact of the latter on the image quality is investigated. A method to calculate the spatial resolution in the form of the line-spread function and the photostimulated luminescence intensity for various scanning laser beam intensities is presented. Theoretical results are compared to experimental ones obtained from a commercially available image plate. It is shown experimentally that the spatial resolution is reduced with increasing laser intensity of the readout beam, that the detected information shifts spatially and that the response of the photostimulated luminescence does not vary linearly with the intensity of the laser light. The results are explained by light scattering in the phosphor layer of the image plate and information bleaching during the readout procedure. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364375

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Time-of-flight measurements were made of neutral helium atom beams extracted from a repetitive, pulsed, positive-point corona discharge. Two strong neutral peaks, one fast and one slow, were observed, accompanied by a prompt photon peak and a fast ion peak. All peaks were correlated with the pulsing of the discharge. The two types of atoms appear to be formed by different mechanisms at different stages of the corona discharge. The fast atoms had energies of 190 eV and were formed at the onset of the pulsing, approximately 0.7 &mgr;s before the maximum of the photon peak. The slow peak, composed of electronically metastable He atoms, originated 30–50 &mgr;s after the photon pulse, and possessed a nearly thermal velocity distribution. The velocity distribution was typical of an undisturbed supersonic expansion with a stagnation temperature of 131 K and a speed ratio of 3.6. Peak intensities and velocities were measured as a function of source voltage, stagnation pressure, and skimmer voltage. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364376

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The picosecond optical limiting characteristics of the optical limiting dyes hexamethylindotricarbocyanine iodide (HITCI) and chloroaluminum phthalocyanine (CAP) are compared and contrasted at 532 nm. From single pulse transmittance experiments, HITCI is shown to possess a serious limitation in its optical limiting behavior, where the regime of induced absorption becomes one of induced transmission for high irradiances. CAP, on the other hand, although possessing a smaller ratio of the excited-state to ground-state absorption cross-section, continues to exhibit optical limiting over a much broader dynamic range. From cross-polarized time-resolved excitation-probe measurements, HITCI exhibits a sharp negative spike around zero delay which originates from orientational coherent grating effects, while CAP does not exhibit any spike. These nonlinearities are explained theoretically as a complex interplay between a series of excited manifolds for the dyes and the radiation properties of the interacting laser pulse. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364377

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A quantitative model is presented to describe the formation of volume holograms in a polymeric medium containing photopolymerizable acrylate monomers that undergo spatially modulated gelation as a result of exposure to a visible “write” beam. The model refines the simple diffusion model of Zhao and Mouroulis [J. Mod. Opt.41, 1929 (1994)], by including cure dependence of both the photoreaction kinetics and the monomer diffusivity. These dependences are determined by experimental measurements, using near infrared spectroscopy to quantify the degree of cure and the time dependence of the hologram formation to infer the cure-dependent diffusivity. The cure-dependent diffusion coefficient can be fit by an expression from a free-volume theory, and the cure-dependent reaction rate coefficient is found to be proportional to the diffusivity, showing the reaction rate to be diffusion limited. With the model parameters determined experimentally, predictions are then made of the first, second, and third harmonics of the grating profile, and these are found to be in good agreement with the measured values. The results show the validity of the model and its usefulness in predicting the optimal exposure conditions and performance of a given holographic material. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364378

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Reflective twisted nematic displays where the output polarizer is eliminated are analyzed and demonstrated. The absence of the rear output polarizer has many advantages, including much higher brightness and higher resolution. In this article, we examine the solution space for the design of such reflective displays. Both the cases of a simple reflective display and the use of a retardation film within the display to compensate for color dispersion are explored. It is shown that excellent black and white displays can be obtained using film compensation. Experimentally, over 90&percent; reflectance from a compensated reflective display has been obtained with low color dispersion. The optical response of this reflective display was also found to be quite fast and suitable for video rate applications. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364379

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Two-dimensional simulations were performed to optimize the waveguiding region’s aluminum composition of an AlGaN/GaN separate confinement heterostructure (SCH) laser with large band discontinuities. Results demonstrate that the active region’s thickness, waveguiding regions’ thickness, and the material composition of the waveguiding region should be optimized, based on losses of the materials in the structure. Results also demonstrate that the threshold current of a SCH laser may be larger than that of a double heterostructure laser. The increase is caused by a competition between the active region and the waveguiding region which has a parasitic effect on the laser’s modal gain because of the waveguiding region’s slightly larger band gap, so that below the lasing threshold, photon emission and population inversion can occur in both regions. At the minimum threshold current the structure is optimized to strongly confine both the guided optical mode and the charge carriers responsible for the gain. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364380

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A powerful and a stable numerical method is developed to solve the Boltzmann equation of electrons moving under the action of an electric field in weakly ionized gases involving space and time gradients. It is based on the classical two term development of the distribution function and on a strongly implicit procedure following position and energy axis and an explicit approach along the time axis. This numerical algorithm is successfully applied to determine the spatiotemporal variation of the electron distribution function and the associated swarm parameters (mean energy, drift velocity, ionization and attachment coefficients, etc.) in the case of nonthermal electrical discharges in different gases (He, Ar andO2)under different applied electric fields and initial and boundary conditions. The transient phase, the following steady state phase and also the electrode effects are clearly emphasized and analyzed for each gas discharge studied. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364381

出版商:AIP    

年代:1997

数据来源: AIP