Optical factors in the photoemission of thin films.
Expressions are derived for the dependence of the photoemission from thin films deposited on transparent substrates on the direction of illumination, as determined by optical factors alone. For normal incidence, the photoemission of very thin-film transparent photocathodes may be expected to be about 1.4 times as great for illumination from the glass side as for illumination from the vacuum side. Furthermore, if the index of refraction of the film is large compared with that of the substrate, the ratio, for natural light, is also almost independent of the angle of incidence for a wide angular range. For thicker films, both the optical constants of the film and the mean depth of origin of the photoelectrons play an important role. However, for strongly absorbing as well as nearly transparent photoemissive films, substantially higher photocurrent should be obtainable from a photoemissive film of optimum thickness illuminated through the substrate than for a thick film with similar optical constants illuminated from the vacuum side. An assumed simple proportionality between optical stimulus, here given by the square of the field intensity within the photosensitive film, and photoexcitation implies that the photoeffect studied is a volume effect and not dependent on the direction of the electric vector within the film.
Photomultiplier sensitivity variation with angle of incidence on the photocathode.
A description is given of measurements of the variation of sensitivity and reflection coefficient of the photocathode of an EMI 9558B photomultiplier with angle of the incident beam. They were made for three wavelengths, 0.69 microm, 0.56 microm, and 0.42 microm, and both p and s polarization states. Theoretical computations for each of the experimental measurements are given and compared with the experimental data.
IMPROVED QUANTUM EFFICIENCY LASER DETECTORS.
Abstract : This report covers the theoretical and experimental work completed during the first 4 months of the contract. An equation was derived relating photoelectric response to the optical and solid-state properties of photoemitters. The analysis includes the influence of multiple reflections of light from the photocathode boundaries and shows interference enhanced photoresponse for certain values of wavelength of the incident radiation and photocathode thickness. Photoelectric response for light entering the photocathode from the vacuum interface is compared to the response for light entering the photocathode from the glass interface for S-1 and S-20 photocathodes. The yield is generally found to be higher for light entering the photocathode from the glass substrate. It is believed that the relative yields of the two cases is partially determined by multiple reflections within the photocathode. The measurements of photoelectric yield versus photocathode thickness and experimental problems relating to the measurements are discussed. Experiments to be conducted during the remaining month of the contract are outlined. (Author)