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arXiv:2205.07485

Aspects of Two-photon Absorption of Squeezed Light: the CW limit

Christian Drago, John E. Sipe

Published May 16, 2022• 0 citations
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Abstract

We present a theoretical analysis of two-photon absorption of classical and squeezed light valid when one-photon absorption to an intermediate state is either resonant or far-detuned from resonance, and in both the low and high intensity regimes. In this paper we concentrate on continuous-wave excitation, although the approach we develop is more general. We calculate the energy removed from an incident field for typical experimental parameters and consider the limiting cases when the photon pairs are narrowband or broadband compared to the molecular linewidths. We find an enhancement of the two-photon absorption due to resonant contributions from the large squeezed light bandwidth and due to photon bunching in the low intensity regime. However, in both cases, for the parameters we choose, the one-photon absorption is the dominant process in the region of parameter space where a large enhancement of the two-photon absorption is possible.

DOI
10.48550/arxiv.2205.07485

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An unconventional perturbation treatment of the theory of two-photon absorption in a four-level atomic system close to two-photon resonance is developed on the basis of the optical Bloch equations. The results for the longtime populations of the intermediate and upper states are valid in low- and medium-intensity regimes. In both regimes the upper state population shows a lorentzian resonance centred on zero two-photon detuning with a width equal to the natural linewidth of the upper state. The intermediate state population shows a similar behaviour in the intermediate regime only, with no significant dependence on two-photon detuning in the low-intensity regime. However, in both regimes the population of the virtual intermediate state is at least comparable to that of the resonant upper state. The perturbation theory results have been checked by direct numerical calculations based on the Bloch equations. Numerical calculations confirm the presence of a high-intensity regime characterized by power shifts, broadening and saturation effects.
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In this thesis we study three aspects of squeezed light. First, we investigate the generation of pulsed, near‑pure, degenerate, single‑temporal‑mode squeezed light produced by dual‑pump spontaneous four wave mixing in integrated silicon‑nitride coupled‑ring resonators. We show that parasitic spontaneous four wave mixing processes inherent to the dual‑pump configuration can substantially degrade the amount of generated squeezing and state purity. To mitigate these effects we explore pump detuning, engineered dispersion, and an auxiliary‑ring structure as suppression strategies. We then optimize the incident pump amplitude and phase, demonstrating that longer pump pulses can yield highly squeezed, temporally separable states and that an appropriate pre‑chirp tracks the effects of self‑ and cross‑phase modulation, increasing intracavity pump amplitudes and the amount of generated squeezing. Next, we analyze the spectral–temporal structure and photon statistics of strongly correlated squeezed light. We develop a general formalism based on the Whittaker–Shannon interpolation of the joint‑temporal amplitude that is particularly useful in the long‑pulse and continuous‑wave limits, where the instantaneous photon density is low but the total photon number may be large. This packet‑based description provides a local, physically transparent account of the state and its photon statistics that complements the Schmidt decomposition, and extends to nonclassical states for which a Schmidt decomposition is not available. Finally, we examine whether or not squeezed illumination can enhance two‑photon excitation relative to classical light, and whether any enhancement is experimentally detectable. We construct a model including radiative and nonradiative broadening to compute scattered and absorbed energy, and apply it to fluorescence from cesium atoms in a magneto‑optical trap driven by classical and squeezed light in both continuous‑wave and pulsed regimes. Under ideal conditions squeezed light can produce a measurable enhancement in both limits, but the gains are moderate and smaller than some recent molecular reports; additional broadening or loss would further reduce detectability.