Photoemission and absorption under coherent and entangled-photon-pair illumination

The phenomena of subthreshold photoemission and absorption under coherent and entangled-photon-pair illumination are reviewed, and the generation and properties of entangled-photon pairs are surveyed. Three prominent forms of subthreshold photoemission are examined: one-photon Fermi-tail photoemission (FTP), two-photon photoemission (TPP), and entangled-two-photon photoemission (ETPP). Experimental methods for measuring subthreshold photocurrents and photoelectron count rates are discussed, along with strategies for enhancing selected contributions. Experimental observations of FTP from a CsK₂Sb photocathode in a photomultiplier tube (PMT), under both coherent and entangled-photon-pair illumination, are reviewed, and the role of FTP as a noise source in two-photon measurements is elucidated. TPP from Na and CsK₂Sb photocathodes in a PMT under classical-light illumination is considered, as are TPP and ETPP from a CsK₂Sb photocathode in a channel photomultiplier (CPM) under coherent and entangled-photon-pair illumination. The observation of ETPP is facilitated by the use of a CPM, which suppresses FTP, and by low-intensity illumination, which minimizes TPP. Quantum models of TPP and ETPP accord well with experiment. Entangled-two-photon absorption (ETPA) is analyzed, as are its applications in entangled-two-photon fluorescence microscopy (ETPFM) and entangled-two-photon spectroscopy (ETPS). The three principal forms of subthreshold absorption parallel those of subthreshold photoemission: singleton-induced Boltzmann-tail absorption; cousin-induced/singleton-pair-induced two-photon absorption; and twin-induced ETPA. Heuristic particle and fully quantum models of these processes are compared, and experimental studies of ETPA and ETPFM, together with methods for enhancing their observability, are summarized.