Abstract Entangled two-photon absorption (ETPA) exhibits a quantum-enhanced two-photon absorption cross-section and a linear scaling with incident power that exceeds classical two-photon absorption (TPA) at low photon flux. Here, we assess the feasibility and applications of ETPA for active plasma spectroscopy. The ETPA-absorbed power for an Ar + species in a helicon plasma scenario is calculated to be as high as 0.6 nW cm −1 along the incident beam path for nondegenerate-entangled photon pairs that are near-resonant with an intermediate level. The ETPA absorption power exceeds classical TPA and Bremsstrahlung emission, a necessary condition for a feasible fluorescence measurement. Applications of ETPA plasma spectroscopy may include trace impurity detection, ground-state excitation, and continuous-wave pumping for high-bandwidth turbulence measurements. Possible benefits of plasma measurements are less harm to optical components with lower incident flux and reduced stray light that can contaminate optical diagnostics.
Abstract Entangled two-photon absorption (ETPA) exhibits a quantum-enhanced two-photon absorption cross-section and a linear scaling with incident power that exceeds classical two-photon absorption (TPA) at low photon flux. Here, we assess the feasibility and applications of ETPA for active plasma spectroscopy. The ETPA-absorbed power for an Ar + species in a helicon plasma scenario is calculated to be as high as 0.6 nW cm −1 along the incident beam path for nondegenerate-entangled photon pairs that are near-resonant with an intermediate level. The ETPA absorption power exceeds classical TPA and Bremsstrahlung emission, a necessary condition for a feasible fluorescence measurement. Applications of ETPA plasma spectroscopy may include trace impurity detection, ground-state excitation, and continuous-wave pumping for high-bandwidth turbulence measurements. Possible benefits of plasma measurements are less harm to optical components with lower incident flux and reduced stray light that can contaminate optical diagnostics.
Entangled two-photon absorption (ETPA) may be a viable technique to continuously drive an excited state population in plasma for high-bandwidth spectroscopy measurements of localized plasma turbulence or impurity density. Classical two-photon absorption commonly requires a high-intensity, pulsed laser, but entangled photons with short entanglement time and high time correlation may allow for ETPA using a lower intensity, continuous-wave laser. Notably, ETPA with non-collinear entangled photon generation allows for cross-beam spatial localization of the absorption or fluorescence signal using a single laser source. Entangled photon generation, the ETPA cross-section, candidate transitions for an Ar-II species, and plans for a proof-of-principle measurement in a helicon plasma are discussed.
Entangled two-photon absorption (ETPA) could form the basis of nonlinear quantum spectroscopy at very low photon fluxes, since, at sufficiently low photon fluxes, ETPA scales linearly with the photon flux. When different pairs start to overlap temporally, accidental coincidences are thought to give rise to a "classical" quadratic scaling that dominates the signal at large photon fluxes and, thus, recovers a supposedly classical regime, where any quantum advantage is thought to be lost. Here, we scrutinize this assumption and demonstrate that quantum-enhanced absorption cross sections can persist even for very large photon numbers. To this end, we use a minimal model for quantum light, which can interpolate continuously between the entangled pair and a high-photon-flux limit, to analytically derive ETPA cross sections and the intensity crossover regime. We investigate the interplay between spectral and spatial degrees of freedom and how linewidth broadening of the sample impacts the experimentally achievable enhancement.
Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.
Optical technologies based on two-photon absorption can provide high resolution and deep tissue penetration but require very high peak power intensities because of the extremely small absorption cross sections associated with this phenomenon. The use of quantum-entangled photon pairs has been reported to increase the two-photon absorption rate compared with that obtainable using classical coherent laser sources. However, the enhancement that can possibly be obtained using entangled two-photon absorption (ETPA) has thus far been realized only at low photon rates, typically ranging from 107 (on the pW level) to 1013 (on the μW level) photons/s. For this reason, ETPA has been regarded as impractical. The present work demonstrates an experimental evaluation of ETPA-excited fluorescence at both low (μW) and relatively high (mW) pump power levels, using a high-gain parametric down-conversion source. Both an increase in the absorption rate and a linear correlation between rate and power were observed within both power ranges. This work also compared quantum and coherent sources. Assessments of the crossover photon flux suggest that the larger spatial-mode diameter of the present quantum source at the focal point (relative to values used in prior research) may have contributed to the rate enhancement seen at high-power levels. The results presented herein are expected to provide a new route to the mitigation of damage often associated with two-photon imaging and photodynamic therapies.
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.
Over the last 50 years entangled photon pairs have received attention for use in lowering the flux in two-photon absorption imaging and spectroscopy. Despite this, evidence for entangled two-photon absorption (ETPA) effects remain highly debated, especially at low-fluxes. Here, we structure the transverse spatial correlations of entangled photon pairs to evidence signs of ETPA at room-temperature in organic and inorganic chromophores, in the low-flux regime. We demonstrate our scheme to be robust to common artifacts that have previously hampered detection of ETPA such as linear absorption and background fluorescence, and show that ETPA scales with transverse correlation area and chromophore two-photon cross-sections. Our results present a step towards verifying ETPA and experimentally exploring entangled light-matter interactions.