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Physical Review A

Two-photon absorption by spectrally shaped entangled photons

Hisaki Oka

Published Mar 12, 2018• 40 citations
Publisher

Abstract

We theoretically investigate two-photon excitation by spectrally shaped entangled photons with energy anticorrelation in terms of how the real excitation of an intermediate state affects two-photon absorption by entangled photons. Spectral holes are introduced in the entangled photons around the energy levels of an intermediate state so that two-step excitation via the real excitation of the intermediated state can be suppressed. Using a three-level atomic system as an example, we show that the spectral holes well suppress the real excitation of the intermediate state and recover two-photon absorption via a virtual state. Furthermore, for a short pulse close to a monocycle, we show that the excitation efficiency by the spectrally shaped entangled photons can be enhanced a thousand times as large as that by uncorrelated photons.

DOI
10.1103/physreva.97.033814

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We theoretically investigate two-photon excitation by spectrally shaped entangled photons with energy anticorrelation in terms of how the real excitation of an intermediate state affects two-photon absorption by entangled photons. Spectral holes are introduced in the entangled photons around the energy levels of an intermediate state so that two-step excitation via the real excitation of the intermediated state can be suppressed. Using a three-level atomic system as an example, we show that the spectral holes well suppress the real excitation of the intermediate state and recover two-photon absorption via a virtual state. Furthermore, for a short pulse close to a monocycle, we show that the excitation efficiency by the spectrally shaped entangled photons can be enhanced a thousand times as large as that by uncorrelated photons.
We theoretically investigate the dynamics of two-photon excitation by correlated photons with energy anticorrelation in terms of how the excitation efficiency depends on incident pulse width. A three-level atomic system having an intermediate state is used to evaluate the efficiency of two-photon excitation. It is shown that for shorter pulses closer to a monocycle pulse the excitation efficiency by correlated photons is enhanced to become 100100100 times as large as that by uncorrelated photons.
The true role of entanglement in two-photon virtual-state spectroscopy (Saleh et al 1998 Phys. Rev. Lett. 80 3483), a two-photon absorption spectroscopic technique that can retrieve information about the energy level structure of an atom or a molecule, is controversial. The consideration of closely related techniques, such as multidimensional pump–probe spectroscopy (Roslyak et al 2009 Phys. Rev. A 79 , 063409), suggests that spectroscopic information might also be retrieved by using uncorrelated pairs of photons. Here we show that this is not the case. In the two-photon absorption process, the ability to obtain information about the energy level structure of a medium depends on the spectral shape of existing temporal (frequency) correlations between the absorbed photons. In fact, it is a combination of both the presence of frequency correlations (entanglement) and their specific spectral shape that makes the realization of two-photon virtual-state spectroscopy possible. This result helps in selecting the type of two-photon source that needs to be used in order to experimentally perform the two-photon virtual-state spectroscopy technique.
It is well known that energy-time entanglement can enhance two-photon absorption (TPA) by simultaneously optimizing the two-photon resonance and the coincidence rate of photons at the absorber. However, the precise relation between entanglement and the TPA rate depends on the coherences of intermediate states involved in the transition, making it a rather challenging task to identify universal features of TPA processes. In the present paper, we show that the theory can be simplified greatly by separating the two-photon resonance from the temporal dynamics of the intermediate levels. The result is a description of the role of entanglement in the TPA process by a one-dimensional coherence in the Hilbert space defined by the arrival-time difference of the two photons. Transformation into the frequency-difference basis results in Kramers-Kronig relations for the TPA process, separating off-resonant contributions of virtual levels from resonant contributions. In particular, it can be shown that off-resonant contributions are insensitive to the frequencies of the associated virtual states, indicating that virtual-state spectroscopy of levels above the final two-photon excited state is not possible.
We theoretically investigate two-photon excitation by correlated photons with energy anticorrelation. A three-level atomic system is used to evaluate selectivity and efficiency of two-photon excitation. It is shown that tailored squeezing of two-photon probability distribution concurrently enhances excitation efficiency and high-contrast selective excitation, without phase manipulation or pulse-shaping techniques. We also discuss candidates to generate such squeezed photons in real material systems.
Abstract We theoretically analyze the transient dynamics in two-photon absorption (TPA) driven by a rectangular pulse consisting of entangled photons with time delay in terms of verifying the quantum virtual-state spectroscopy (VSS) method. We consider two types of three-level atomic systems: one is the atomic system having an intermediate state between ground and excited states, considered in common TPA, and another is the atomic system having an intermediate state above the excited state, assumed in the original VSS theory. We show that for common TPA configuration VSS does not occur at all, whereas for the original VSS configuration VSS-like signals can be obtained regardless of whether the incident photons are entangled or not, where photon entanglement simply contributes to the enhancement of the signal. In addition, by introducing frequency cutoff to the spectrum of incident photons, we also show that the VSS-like signal disappears even for the original VSS configuration when the intermediate state is completely virtually excited. Thus, the obtained results suggest that both the broad spectrum of incident photons causing the real excitation of intermediate state and energy configuration of intermediate state above the excited state are essential for VSS-like signals and that VSS in itself does not occur in a strict sense at least. The effect of unphysical parameters appearing in the original VSS scheme, recently theoretically predicted, is also discussed.
We theoretically investigate strongly enhanced vibrational-mode-selective two-step excitation by ultrabroadband frequency-entangled photons. A diatomic molecule having three sets of vibronic states is used to evaluate the excitation efficiency of the two-step excitation. We show that the photon entanglement has little influence on the populations of intermediate vibrational modes of intermediate states but selectively increases the population of a single vibrational mode of excited states, resonant with the total energy of the entangled photons. For an ultrashort pulse close to a monocycle with high degree of entanglement, the population of the vibrational mode is enhanced 2500 times more than by uncorrelated photons, and the selectivity nearly reaches unity. Our results indicate that the two-step excitation with ultrabroadband frequency-entangled photons can achieve highly selective and strongly enhanced excitation of a single vibrational mode.