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.
Analysis of translucent and opaque photocathodes.
By an analysis of the photodetection process, the response of photodetectors to wide band, noncoherent light and guidelines for its improvement are determined. In this paper, the phenomenon of multiple reflections within the emitter of a reflecting-translucent and a reflecting-opaque photocathode is analyzed. Geometrical and optical configurations and solid state parameters are evaluated in terms of their effect on the photodetection process. The quantum yield, the percent of incident light absorbed, and the collection efficiency are determined as functions of the thickness of the emitting layer. These results are then employed to suggest areas of improvement in the use of state-of-the-art photocathodes.
Interferenz-Photokathoden erhöhter Ausbeute mit frei wählbarem spektralem Maximum
ZusammenfassungDie teildurchlässige Metallschicht eines Interferenzspiegels wird als photoelektrisch empfindliche Schicht ausgebildet. Dann ist für die Strahlung einer bestimmten, frei wählbaren Wellenlänge das Reflexionsvermögen der Interferenz-Photokathode Null bzw. sehr klein. Der größte Teil der eingestrahlten Lichtenergie wird in der Photoschicht absorbiert. Gegenüber der bisher üblichen photoelektrischen Einfachschicht wurde die Absorption um den Faktor 3, die Elektronenemission dagegen bis um den Faktor 9 gesteigert. Diese überraschende Erhöhung der Ausbeute wird gedeutet: Bei der Interferenz-Photokathode gelingt es, die Lichtenergie nahezu vollständig in den oberflächennahen Schichtlamellen zu absorbieren. Wegen der kurzen Reichweite der Photoelektronen im Metall haben nur die in den oberflächennahen Schichtlamellen angeregten Elektronen eine hohe Austrittswahrscheinlichkeit. Die Steigerung der Ausbeute gegenüber der üblichen Photoschicht beruht also darauf, daß die eingestrahlte Lichtenergie fast vollständig absorbiert wird und die Schichten infolge des günstigen Absorptionsverlaufes eine hohe effektive Quantenausbeute haben.