摘要:

MgO crystals have been studied by scanning‐electron acoustic microscopy under different experimental conditions. Contrast mechanisms in imaging are discussed and compared. The experimental results obtained by earthing or nonearthing the specimen‐transducer interface suggest the existence of a signal generation mechanism that is related to the ionic nature of these kind of crystals. Electron‐acoustic microscopy appears then to be a useful tool for the characterization of ionic materials.

ISSN:0021-8979    

DOI:10.1063/1.346087

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The transmissive Fabry–Perot interference peaks of a filled twisted nematic (TN) cell as a function of wavelength, from 400 to 600 nm and from 1100 to 2000 nm, have been used for the determination of the cell gap. Theoretical formulations have been developed to calculate the effective indices of refraction including the dispersive effect, as well as the twist geometry of the liquid‐crystal medium for both the ordinary and extraordinary waves. When the light passes through the TN medium an even number of times caused by the interfacial reflections, the equations governing the effective indices of refraction are well behaved. For an odd number of passes, the validity of these equations is restricted to a region adjacent to the Mauguin limit. The cell gaps were derived from the measured effective optical paths and the calculated indices of refraction, which include the dispersive effect with or without the twisted deformation of the liquid‐crystal medium. A general equation has also been obtained to derive the TN cell gap from the effective optical paths, which are measured by using the split‐beam interferometric method.

ISSN:0021-8979    

DOI:10.1063/1.345205

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Noble gas clusters with an average size of several thousand atoms and with energies in the keV range were directed at metal surfaces under small angles of incidence. The scatter products were examined with respect to size, energy, and recoil angle. All observations fit an impact model in which the tangential momentum is preserved during impact, while the normal kinetic energy is converted to cluster heat. The associated temperature rise results in the evaporation of cluster atoms. Small clusters evaporate completely; large clusters are reduced in size. The evaporated atoms continue in a forward direction (together with the residual cluster fragments) and form a well‐collimated and energetic atom beam.

ISSN:0021-8979    

DOI:10.1063/1.345289

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

It is shown that the concept of electromagnetic momentum can be used to calculate the force acting on the projectile in a railgun, thereby refuting some recent papers on the subject.

ISSN:0021-8979    

DOI:10.1063/1.346056

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The characteristics of the surface polariton existing on the surface of a free electron metal in the proximity of a higher permittivity prism dielectric are investigated including the effects of the finite length of the prism dielectric and the slow spatial variation of the amplitude of the surface polariton. The radiative decay characteristics of the surface polariton and the characteristics of the antiresonance in the angular response of the reflectivity of light incident on the metal surface via the air gap from the prism dielectric are treated, and the effect of a small loss in the metal on these characteristics is also presented.

ISSN:0021-8979    

DOI:10.1063/1.345285

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We consider in this paper an arbitrary stack of dielectric plates with given voltage distributions at both lower and upper free surfaces of the stack. We address the problem of finding the potential distribution at any interface within the stack. This problem is of fundamental importance regarding a recently proposed electro‐optical technique for testing printed‐circuit boards. We have developed a simple and powerful matrix formalism relying on Fourier transformations of voltage distributions. The theory is verified against a particular kind of stack that is of practical importance in the already mentioned application. This stack is composed of three successive layers: an isotropic dielectric, a one‐dimensional elastomeric conductor, and an electro‐optic crystal (Bi4Ge3O12). The last surface of the stack is grounded using a conductive (and transparent) electrode, whereas a given potential distribution is applied to the first surface. The potential distribution on the ungrounded face of the electro‐optic plate (that side in contact with the elastomer) is measured using an optical reflective polarimetric technique. Our results demonstrate good agreement between theory and experiment.

ISSN:0021-8979    

DOI:10.1063/1.345257

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A simulated thin‐film transistor (TFT) circuit has been built to drive the twisted nematic (TN) cell for the measurements of charge retention and the transmission versus peak voltage applied to the drain electrode of the simulated TFT using the gate pulse width as a parameter. The established rule that the transmission of the TN cell depends only on the rms voltage applied to the cell has been confirmed by calculating the rms voltage of the charge retention curves in correlation with the measured transmissions. The deviation of the decaying charge retention curves from the exponential behavior has been observed and can be qualitatively explained by a combination of the dielectric and transport properties of nematic liquid‐crystal medium.

ISSN:0021-8979    

DOI:10.1063/1.345263

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have modeled the response of a nonlinear periodic structure by means of the Abele´s 2×2 matrix method. Our structure differs from the usual rejection‐band filter designs, in that we have chosen the filter elements to be index matched in the absence of radiation, providing a rejection band that both grows and shifts as a function of incident intensity. The intensity output function of the model not only directly demonstrates optical bistability, but also limiting, switching, self‐pulsing, and chaos.

ISSN:0021-8979    

DOI:10.1063/1.345271

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The output characteristics of an optically pumped titanium vapor laser on various buffer gas species and pressures have been measured, and the role of buffer gas on the laser output is accordingly discussed. He, Ne, Ar, and Kr are used as buffer gases. The optimum condition for the maximum output energy is 4 Torr of He with 10 &mgr;s delay time.

ISSN:0021-8979    

DOI:10.1063/1.345226

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We introduce a new technique to modify the coherence properties and the far‐field intensity distributions of optical fields. Use is made of a combination of Bragg diffraction and Raman–Nath diffraction in an acousto‐optic deflector. The high‐frequency Bragg grating in the deflector is phase modulated by a synthetic low‐frequency Raman–Nath regime grating that has a structure of an optimized phase‐only computer‐generated hologram. We demonstrate, e.g., electronic control of the state of spatial coherence of light, Gaussian to flat‐top conversion, and the generation of arrays of equally intense but mutually incoherent light spots.

ISSN:0021-8979    

DOI:10.1063/1.345232

出版商:AIP    

年代:1990

数据来源: AIP