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arXiv:1901.02605 · Physical Review Letters

Temperature-Controlled Entangled-Photon Absorption Spectroscopy

Roberto de J. León‐Montiel, Jiŕí Svozilík, Juan P. Torres, Alfred B. U’Ren

Published Jul 9, 2019• 53 citations
arXivHTMLPDF

Abstract

Entangled two-photon absorption spectroscopy (TPA) has been widely recognized as a powerful tool for revealing relevant information about the structure of complex molecular systems. However, to date, the experimental implementation of this technique has remained elusive, mainly because of two major difficulties: first, the need to perform multiple experiments with two-photon states bearing different temporal correlations, which translates into the necessity to have at the experimenter's disposal tens, if not hundreds, of sources of entangled photons; second, the need to have a priori knowledge of the absorbing medium's lowest-lying intermediate energy level. In this work, we put forward a simple experimental scheme that successfully overcomes these two limitations. By making use of a temperature-controlled entangled-photon source, which allows the tuning of the central frequencies of the absorbed photons, we show that the TPA signal, measured as a function of the temperature of the nonlinear crystal that generates the paired photons, and a controllable delay between them, carries all information about the electronic level structure of the absorbing medium, which can be revealed by a simple Fourier transformation.

DOI
10.1103/physrevlett.123.023601

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Entangled two-photon absorption spectroscopy (TPA) has been widely recognized as a powerful tool for revealing relevant information about the structure of complex molecular systems. However, to date, the experimental implementation of this technique has remained elusive, mainly because of two major difficulties: first, the need to perform multiple experiments with two-photon states bearing different temporal correlations, which translates into the necessity to have at the experimenter's disposal tens, if not hundreds, of sources of entangled photons; second, the need to have a priori knowledge of the absorbing medium's lowest-lying intermediate energy level. In this work, we put forward a simple experimental scheme that successfully overcomes these two limitations. By making use of a temperature-controlled entangled-photon source, which allows the tuning of the central frequencies of the absorbed photons, we show that the TPA signal, measured as a function of the temperature of the nonlinear crystal that generates the paired photons, and a controllable delay between them, carries all information about the electronic level structure of the absorbing medium, which can be revealed by a simple Fourier transformation.
Abstract Entangled two-photon absorption spectroscopy has been widely recognized as a promising tool for revealing important information about the structure and dynamics of complex molecular systems. In this contribution, we present the basics of the theory behind this technique and describe a novel experimental scheme known as temperature-controlled entangled-photon absorption spectroscopy, which constitutes the first, realistic proposal for the experimental implementation of nonlinear spectroscopy with quantum light.
In entangled two-photon absorption (eTPA) spectroscopy, information about the energy-level structure of an arbitrary sample is retrieved by Fourier transforming sets of measured two-photon absorption probabilities of entangled photon pairs where the degree of entanglement and the delay time between the photons are varied. This works well for simple systems but quickly becomes rather difficult when many intermediate states are involved. We propose and discuss an extension of eTPA spectroscopy that solves this problem by means of repeated measurements at different pump wavelengths, thus reducing the experimental complexity of eTPA spectroscopy by removing the need to control the correlations of the entangled photons. Specifically, we demonstrate that our extension works well for a variety of realistic experimental setups.
The Two-photon absorption processes (TPA) is of great interest due to its variety of technical applications. Two-photon transitions can be excited using light with different statistical properties, such as laser light (coherent light), pseudo-thermal light and entangled photons-pairs (ETPA). For a TPA process driven with entangled photons (ETPA), a linear dependence of the signal with the photon flux has been theoretically predicted and experimentally demonstrated by various groups. The main motivation for studying ETPA relies on the capability of inducing TPA with a low photon flux. This is advantageous to analyze, among other things, biological samples with a reduced probability of destroying them in the process. However, ETPA experiments developed to date raise many questions regarding the difficulty of isolating a signal in organic molecules. One of the main problems in these experiments is the difficulty to study independently all the systematic effects due to the complex structure of organic molecules. An alternative to clarify the origin of the signal is to study the ETPA process in atomic systems. In this case, the possibility to excite narrow band transitions with tunable single frequency lasers can provide a clear signature of the ETPA process. To achieve this objective, the first step consists of understanding TPA with coherent light, or classical TPA, and measuring the corresponding cross-section in atomic systems. Once the classical regime is characterized, the ETPA process can be studied considering decoherence effects and coherence between intermediate states. For this, the ETPA cross section must be calculated and measured. In this work we present a theoretical calculation of the ETPA transition probability and cross-section with the density operator formalism for the transition 62S1/2 →82S1/2. The theoretical results will be complemented with an experiment with atomic Cesium based in a coincidence detection scheme.
Entangled two-photon absorption (eTPA) has been recognized as a potentially powerful tool for the implementation of ultra-sensitive spectroscopy. Unfortunately, there exists a general agreement in the quantum optics community that experimental eTPA signals, particularly those obtained from molecular solutions, are extremely weak. Consequently, obtaining spectroscopic information about an arbitrary sample via conventional methods rapidly becomes an unrealistic endeavor. To address this problem, the authors introduce an experimental scheme that reduces the amount of data needed to identify and classify unknown samples via their electronic structure. Their proposed method makes use of machine learning to extract information about the number of intermediate levels that participate in the two-photon excitation of the absorbing medium. This is achieved by training artificial neural networks (ANNs) with various eTPA signals where the delay between the absorbed photons is externally controlled. Inspired by multiple experimental studies of eTPA, the authors consider model systems comprising one to four intermediate levels, whose energies are randomly chosen from four different intermediate-level bandgaps, namely, Δλ=10, 20, 30, and 40 nm. Within these bandgaps, and with the goal of testing the efficiency of their artificial intelligence algorithms, the authors make use of three different wavelength spacing 1, 0.5, and 0.1 nm. The authors find that for a proper entanglement time between the absorbed photons, classification average efficiencies exceed 99% for all configurations. Their results demonstrate the potential of ANNs for facilitating the experimental implementation of eTPA spectroscopy.
Two-photon absorption (TPA) is a nonlinear optical process with wide applications in fluorescence microscopy and spectroscopy of organic molecules and compounds, as it enables atoms or molecules to transition into states that may otherwise be inaccessible by a single photon, either because of selection rules or energy conservation. Despite its common use, TPA is a rather inefficient process, often requiring very large photon numbers to produce significant results. It has been demonstrated and analytically described [1,2], that using frequency entangled pairs of photons can in some cases enhance the efficiency of TPA. For this to occur, the joint spectral amplitude (JSA) of the photon-pair state, which describes the spectral entanglement between the pair, must be engineered according to the spectral properties of the TPA response-function of the specific system in question. In this work, we consider different cases for the spectral functionality of TPA and propose that the control of the group indices of the modes involved in the pair-generation process is a key property in the design of photon-pair sources for optimized TPA.
Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics and biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead to tens of orders of magnitude lower incident fluxes than in conventional two-photon absorption schemes. However, the role of different entangled degrees of freedom in ETPA was unclear following recent experimental studies, when compared to earlier theoretical works. Here, we first demonstrate a linear dependence of the ETPA rate with the photon-pair rate, a clear signature of ETPA, and estimate the values for the concentration-dependent ETPA cross section for Rhodamine 6G. We then investigate the signature of energy-time entanglement and polarization dependence in the ETPA fluorescence rate and demonstrate a strong dependence of the signal on the interphoton delay that reflects the coherence time of the entangled two-photon wave packet.