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

TL;DR

This paper reviews phenomena of subthreshold photoemission and absorption under coherent and entangled-photon illumination, discussing experimental methods, observations, quantum models, and applications in microscopy and spectroscopy.

Contribution

It provides a comprehensive survey of subthreshold photoemission and absorption processes, highlighting experimental techniques, observations, and quantum models for entangled-photon interactions.

Findings

FTP observed from CsK₂Sb photocathodes under various illumination conditions

ETPP observed using channel photomultiplier with entangled photons

Quantum models align well with experimental results

Abstract

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 CsK$_2$Sb 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 CsK$_2$Sb photocathodes in a PMT under classical-light illumination is considered, as are TPP and ETPP from a CsK$_2$Sb 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.