Harold Wieder

Optical properties of amorphous SixGe1−x(H) alloys prepared by R.F. Glow discharge

Amorphous alloys of Si and Ge have been prepared over the entire composition range by r.f. plasma decomposition of mixed gases of SiH4 and GeH4. The optical absorption edge is found to have a linear dependence on alloy composition given by: Eg = 0.95 + 0.70x (eV).

A Study of Thermomagnetic Remanence Writing on EuO

An improved method is described of measuring the temperature in a thermomagnetically written bit. It uses the temperature shift of absorption in the storage material to deduce the temperature from transmission changes measured during the application of a thermal pulse. The transmission kinetics which were measured in a film of EuO for several energy pulses supplied by a GaAs laser are presented. By fitting these curves to those generated by a computer program simulation of the experiment, details of the temperature profile within the bit are revealed. For moderate power levels, about half of the power absorbed by the bit is retained by it in the first 400 nsec, while radial losses do not produce significant thermal spreading in the same time period.

Fast photoconductor coupled liquid‐crystal light valve

Short low‐energy pulses of laser light have been used to switch spots on a liquid crystal (LC) through a coupled photoconductive film of glow‐discharge‐produced amorphous Si (GD/a‐Si). The a‐Si responds rapidly to a light pulse, yet retains sufficient photoconductivity in the slow portion of the decay to match the slower response time of the LC. Spots have been written with laser pulses 10 μs long, about 10−3 the response time of the LC, and with a pulse energy of 1–10 μJ/cm2, less than 10−4 the energy required for thermal writing.

Reactive Optical Information Processing. II: Factors Affecting the Applicability and Efficiency of the Method

The efficiency with which information may be recovered from a phase object is shown to be greater when the object is inserted into a laser cavity than when it is illuminated passively by the same laser. The parameters of both the laser and object which affect this efficiency are discussed. Examples of information processed in coherent, as well as in partially coherent, light are presented.

``Self''‐demagnetizing effects in thermomagnetic writing on MnAlGe films

During thermomagnetic writing on a film of MnAlGe, sequential cooling from the exterior to the interior of the laser‐heated spot allows the periphery of the spot to achieve reverse magnetization before the interior of the spot has cooled significantly below the Curie point. This, in turn, produces a component to the demagnetizing field at the interior of the spot which opposes the normal demagnetizing field and which strongly affects the shape of the magnetization profile measured in this material.

Reactive Optical Information Processing. I: Theory of Information Recovery and Resonator Mode Structure

A method of processing optical information is described in which the use of absorptive spatial filters is replaced by suitable mirrors with spatially varying reflectances. When these mirrors are made the terminal planes of a laser resonator, the light which is lost in conventional spatial filtering systems, is stored in the laser cavity. It is shown that shallowly modulated phase objects may be imaged outside the resonator without any appreciable effect on the modes of the resonator.

Novel Method for Measuring Transient Surface Temperatures with High Spatial and Temporal Resolution

Rapid temperature fluctuations occurring at the surfaces of solids over areas as small as a few wavelengths in diameter can be measured by a new technique. The sample is coated with a semiconductor film which exhibits a suitable shift of its absorption edge with temperature. It is shown that changes in the transmission of an incident monochromatic beam can be used to measure the thermal transients of the underlying surface with a spatial resolution as high as ∼1λ and a temporal resolution as high as 1 nsec.

SEQUENTIAL EXCITATION OF LONGITUDINAL MODES IN GaAs LASERS

Experimental evidence is presented which demonstrates that, in the vicinity of threshold, GaAs lasers exhibit longitudinal mode hopping, in agreement with the theory of Popov et al.

Influence of Mode Number and Mode Degeneracy on the Output of a Ruby Laser

The conjugate‐concentric ruby resonator is a system in which a ruby lens images each of the external mirrors on the other. By merely perturbing the geometry of the resonator, it is possible to obtain three distinct types of output: quasicontinuous, regular spiking, and random spiking. From the results of various experiments which have been performed on the system, it was concluded that two parameters are sufficient to determine which of these outputs will be obtained from a multimode resonator. Regular spiking is favored over random spiking if the mode number exceeds some critical value; quasicontinuous behavior is favored over regular spiking if, in addition, the modes exhibit degeneracy. The experimental results are compared with some existing theories.

Effect of internal demagnetizing fields on the thermomagnetic writing process in MnAIGe films

Considerable structure has been found in the magnetization of spots written thermomagnetically on a film of MnAlGe by an impinging GaAs laser beam. Regions of the spot only 1 μm apart can exhibit strong, but opposing, remanence. We have investigated the effect of an external bias field on the behavior of different regions of the spot. From these experiments we show that the structure cannot be attributed either to local variations in film coercivity or to the fact that the magnetization reverses sign at a temperature well below the Curie point. These results support our previous conclusion that the effect is due to a demagnetizing field originating from within the spot during the writing process.