摘要:

The compressed monolayers of a rodlike polymer at the air–water interface were transferred onto solid substrates by a horizontal lifting method and studied by the liquid crystal alignment technique. It has been found that the rodlike polymer monolayers have a well ordered orientation on a local region while on a larger scale they consist of disordered domains within which the polymers are uniformly oriented. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360454

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Image blurring as a result of stochastic particle–particle interactions has been investigated for projection electron‐ and ion‐beam lithography systems. A comparative analysis of the currently available analytical theories is presented. The results from these theories are also compared with Monte Carlo simulation results and experimental data. Large variations in results and serious disagreements between the different theoretical approaches are found. We have formulated a new theory on the basis of a simple, analytical approach that overcomes most of the difficulties experienced by earlier theories with two key concepts: consideration of nearest‐neighbor interactions only, and a randomization length, over which the interactions are correlated. Our model displays satisfactory functional and numerical agreement with Monte Carlo simulation results over a large range of beam currents, as well as with the only available experimental data. The physical basis of our model also enables us to understand the origins of the discrepancies arising from earlier theories. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360455

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A series of photorefractive composite materials has been developed in which an inert polymer has been doped with second‐order nonlinear optical chromophores, charge transporting agents, and photocharge generation sensitizers. The composites show high photoconductivity, large photorefractive responses, and optical transparency at many wavelengths. Photorefractivity has been demonstrated at wavelengths of 633, 514.5, and 488 nm, with net two‐beam coupling gains. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360456

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A detailed study of stimulated emission mechanisms as well as laser emission capability has been carried out on Hg1−xCdxTe (0.44<x<0.5) separate‐confinement heterostructures grown by molecular beam epitaxy. At low temperature, spontaneous photoluminescence (PL) occurs on extrinsic levels below the gap whereas optical gain exhibits a maximum of stimulated emission shifted towards higher energy, close to the gap. As temperature increases, spontaneous PL is shifted from the extrinsic states to the band‐to‐band transition by a thermally activated detrapping of the carriers. Above 100 K, spontaneous and stimulated emission vary in a similar way with temperature. Laser emission has been observed up to room temperature for all the heterostructures. The use of quantum wells in the active layer and graded index in the barriers has allowed a significant reduction of the excitation density threshold, as compared to a single separate‐confinement heterostructure (SCH) of same composition. However, the high‐temperature characteristic temperatureT0is found to be similar in the two structures. A SCH with a higher energy gap exhibits a more favorable behavior with temperature. These experimental results have been compared to theoretical models. The experimentally observedT0can be well simulated by taking into account the Auger effect. From the experimental data, the Auger constant has been determined for each heterostructure. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360457

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In Cd(S,Se)‐doped glasses the optical properties are strongly dependent on the size of the nanocrystals, but can be also largely modified by changes in the crystal stoichiometry; however, the information on both stoichiometry and size is difficult to obtain in crystals smaller than 10 nm. The intensity scattered at small angles is classically used to get information about nanoparticles sizes. Moreover the variation of amplitude of this intensity with the energy of the x ray—‘‘the anomalous effect’’—near the selenium edge is related to stoichiometry. Anomalous small‐angle x‐ray scattering has been used as a tentative method to get information about stoichiometry in nanocrystals with size lower than 10 nm. Experiments have been performed on samples treated for 2 days at temperatures in the range 540–650 °C. The samples treated at temperatures above 580 °C contain crystals with size larger than 4 nm. For all these samples the anomalous effect has nearly the same amplitude, and we found the stoichiometryx=0.4 for the CdSxSe1−xnanocrystals. This agrees with the previous results obtained by scanning electron microscopy and Raman spectroscopy. The results are also confirmed by measurements of the position of the optical absorption edge and by wide‐angle x‐ray scattering experiments. For the sample treated at 560 °C, the nanocrystal size is 3 nm and the stoichiometryx=0.6 is deduced from the anomalous effect. For samples treated at lower temperatures the anomalous effect is not observable, indicating an even lower selenium content in the nanocrystals (x≳0.7). We observed differences in the Se content of nanocrystals for different heat treatments of the same initial glass. These results may be very helpful to interpret the change in the optical properties when the temperature of the treatments decreases in the range 560–590 °C. In this temperature range, compositional effects seem to be of the same order of magnitude as the effects of the quantum confinement. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360458

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Spatial resolution of optically addressed spatial light modulators (OASLMs) is degraded by several different transfer processes in these devices. We have developed a general transient charge transport model to calculate and simulate the resolution limits of OASLMs due to the following charge spreading mechanisms during the transfer process in which the input image is converted into a particular charge distribution in the photosensor layer. (i) The effect of charge drift in the photosensor bulk on resolution increases with the thickness of the photosensor and the light‐modulating layers. It also increases with the total amount of photogenerated charge collected at the interface. (ii) The effect of charge diffusion in the photosensor bulk on resolution is largely independent of the carrier mobility in the semiconductor photosensor. In most cases the corresponding spatial frequencyf50%is proportional to &sqrt;Vsc/dsc, wheredscis the photosensor thickness andVscis the voltage drop in that layer. To have high‐sensitivity OASLMs the transit time of charge carriers from the photosensor bulk to the interface must be much shorter than the recombination lifetime. (iii) The effects that charge drift, diffusion, and trapping at the photosensor‐light‐modulating layer interface have on resolution depend strongly on the interface properties. Decreasing the mobility or the trapping time of charge carriers at the interface can dramatically improve the resolution of OASLMs. The resolution ranges from 3 to 875 line pairs/mm for respective diffusion lengths of 10 to 0.1 &mgr;m at the interface. The combined effect on resolution from each of the charge spreading and other resolution‐degrading mechanisms is also discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360459

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Physical mechanisms causing the refractive index increase &Dgr;nin uniaxial oxidized ferroelectric crystals subjected to double doping are discussed. The analysis shows that the index changes of the two different dopants, &Dgr;nAand &Dgr;nB, are additive only if the corresponding changes of the spontaneous polarization are small. Ti indiffusion and proton exchange (TIPE) waveguides in LiNbO3are considered as an example of the case when this condition is not fulfilled. The hydrogen–lithium substitution in such waveguides leads to a strong reduction of the spontaneous polarization of the virgin crystals and the final index change &Dgr;nis not a sum of &Dgr;nAand &Dgr;nB. A physical model is developed to explain light waveguiding in double‐doped ferroelectric structures. Experimental data obtained for TIPE waveguides confirm the model. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360460

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A fast approximate method is described to calculate the acoustic scattering from a one‐dimensional Dirichlet band‐limited fractal surface. The formulation is based on a perturbation of the Green’s function allowing an approximation of the propagator in the kernel of the Helmholtz integral equation, which reduces the integral equation to a convolution equation. This allows us to find a solution using Fourier transforms rather than the usual matrix inversion that is required. We have shown that in the limit of smallk&sgr;, wherekis the incident wave number and &sgr; is the rms height, it is possible to find accurate closed form expressions for the reflection coefficientsRn, the spectral components of the normal gradient of the field &psgr;n, the scattered fieldpsca, and the angular scattering coefficientIscarepresenting the scattering from a band‐limited fractal surface. For small values ofk&sgr;≪1, we have used the generalized Rayleigh method [D. L. Jaggard and X. Sun, J. Appl. Phys.68, 5456 (1990)] to determine the theoretical linear relationship which exists between the slope of the absolute value of the reflection coefficients in dB versus the reflection mode and the fractal dimensionD. This theoretical relationship has been verified by using the Green’s function perturbation method. This relationship and an analogous relationship between the scattering coefficient and the scattering angle allows the determination of the fractal dimensionDand the rms height &sgr; from the scattering pattern whenk&sgr;≤0.2. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360461

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An inverse analysis utilizing the conjugate gradient method of minimization and adjoint equation is used successfully to solve the inverse problem in estimating the temperature‐dependent thermal conductivity of a composite material. No prior information is available on the functional form of the unknown thermal conductivity in the present study, thus, it is classified as the function estimation in inverse calculation. The accuracy of the inverse analysis is examined by using the simulated exact and inexact measurements that were obtained within the medium. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360462

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Laser‐induced‐fluorescence (LIF) spectroscopy was used to study, with spatial and temporal resolution, the processes by which diatomic sulfur S2is formed and lost in SF6plasmas. We present results concerning the relative S2number density in steady‐state or pulsed discharges in a reactive ion etching (RIE) reactor operated at different SF6gas pressures and RF powers, in the presence or absence of a silicon wafer. It is found that S2is formed mainly on surfaces under conditions when the F‐atom density is high, but that volume formation can also occur when the F‐atom population is depleted (namely, when Si is present). It is also shown that loss of S2is mainly due to diffusion out of the inter‐electrode space to the main reactor volume, excluding electron‐impact dissociation processes. It is apparent that, in a RIE reactor, the only electron process pertinent to the balance of S2density is the formation of its precursors (probably S atoms and SF molecules) by fragmentation of the SF6gas. The remaining reactions controlling the density of S2are neutral‐neutral interactions in the volume and on surfaces. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360463

出版商:AIP    

年代:1995

数据来源: AIP