Photochron Streak Camera with GaAs Photocathode

Abstract

The electron-optical streak and framing cameras which have been used for monitoring ultrafast optical processes in the UV-NIR spectral region have incorporated conventional semitransparent, positive electron affinity photocathodes (usually types Sl, S11, S20 or S25). Electron emission from such photocathodes consists of only those “hot electrons” that possess sufficient energy to traverse the potential barrier at the cathode/vacuum interface (Fig. la). This implies that any photoelectrons emitted are likely to have been excited close to the vacuum surface (~ 20nm) and the photoemission process is therefore expected to be very rapid (<10−13s). In contrast, a heavily p-type doped semiconductor can be activated to a condition of negative electron affinity (NEA) such that the conduction band minimum in the bulk lies above the vacuum level. It then follows that electrons excited at comparatively deep sites within the bulk (~ 1µm) of these NEA materials,which subsequently thermalise to the conduction band minimum, can still be photo-emitted, Fig. lb. Although this results in excellent photosensitivities (1500µA/lumen in transmission(l)), theoretical considerations imply that their response times (estimated to be ~ lns (2)) will be substantially greater than their positive electron affinity counterparts but no quantitative experimental results on this topic have been reported. In view of the exploitable features of low dark current, high visible-to-near infrared spectral sensit?ivities and good spatial resolution of NEA cathodes, we decided to make some measurements of their temporal response characteristics in the transmission mode which is most compatible to streak cameras.