Two-Photon Absorption in Molecules by time-frequency-entangled photon pairs: the roles of photon-number correlations and spectral correlations

Abstract

While two-photon absorption (TPA) and other forms of nonlinear interactions of molecules with isolated time-frequency-entangled photon pairs (EPP) have been predicted to display a variety of fascinating effects, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This paper presents a detailed theoretical study of quantum-enhanced TPA by both photon-number correlations and spectral correlations, including an account of the deleterious effects of dispersion. While such correlations in EPP created by spontaneous parametric down conversion can increase the TPA rate significantly in the regime of extremely low optical flux, we find that for typical molecules in solution this regime corresponds to such low TPA event rates as to be unobservable in practice. Our results support the usefulness of EPP spectroscopy in atomic or other narrow-linewidth systems, while questioning the efficacy of such approaches for broadband systems including molecules in solution.

Spatial properties of entangled two-photon absorption

“…A linear dependence in the case of laser beam attenuation, and quadratic in the case of SPDC beam attenuation, confirmed that the measured signal was due to ETPA and not caused by direct detection of scattered pump, down-converted photons or single-photon absorption of any type [21]. It also constitutes one of the most robust demonstrations of genuine ETPA [23][24][25].…”

Hot-Band Absorption Can Mimic Entangled Two-Photon Absorption

“…It has been predicted that if entangled photons generated via spontaneous parametric down conversion (SPDC) are used for excitation, then the resulting entangled two-photon absorption (E2PA) rate should scale linearly with the excitation flux, and the process efficiency can be boosted relative to C2PA at low photon flux. , Entangled photon excitation might therefore enable ultralow-power two-photon excited fluorescence imaging, which would be particularly advantageous for limiting perturbation and damage of fragile biological samples. The favorable scaling behavior stems from the linear dependence of the 2PA rate on the second-order correlation function, g (2) . , In addition to this absorption efficiency enhancement from the photon statistics, further enhancement is possible from the spectral shape and bandwidth of the frequency anticorrelated photon pairs. , Whether these mechanisms can provide a practical advantage for E2PA in molecules is still unclear.…”