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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Applications of correlated photon statistics with a biphoton source in an organic material

Robert E. French, Theodore G. Goodson III

Published Feb 3, 2004• 2 citations
Publisher

Abstract

The process of generating entangled photons by a biphoton source is examined in the context of the detection of correlated two-photon absorption (TPA) induced fluorescence in an organic material. This effect may have possible applications in spectroscopy as well as chemical and biological sensing. The use of certain organic branched molecules which possess large cross sections for random TPA is presented in relation to the magnitude of the correlated two-photon absorption rate. The results of the angular dependence of the biphoton statistics are presented. Estimates of the expected correlated two-photon-absorption rate and subsequent number of emitted photons from the organic material are discussed.

DOI
10.1117/12.538094

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The process of generating entangled photons by a biphoton source is examined in the context of the detection of correlated two-photon absorption (TPA) induced fluorescence in an organic material. This effect may have possible applications in spectroscopy as well as chemical and biological sensing. The use of certain organic branched molecules which possess large cross sections for random TPA is presented in relation to the magnitude of the correlated two-photon absorption rate. The results of the angular dependence of the biphoton statistics are presented. Estimates of the expected correlated two-photon-absorption rate and subsequent number of emitted photons from the organic material are discussed.
We report the fluorescence emission from organic systems selectively excited by entangled pairs of photons. We have demonstrated a linear dependence of this two-photon excited fluorescence on the excitation intensity which is a unique nonclassical feature of two-photon interactions induced by entangled photons. The entangled photon (ETPA) excited fluorescence has been detected in several organic molecules possessing a high entangled photon absorption cross section. The ETPA fluorescence showed a nonmonotonic dependence on the delay between signal and idler beams. The fluorescence signal was detectable within the signal–idler relative delay time interval of ∼100 fs. This time is comparable with the estimated entanglement time, T E, making the ETPA-excited fluorescence in organic materials an ideal ultrafast coincidence detector. These results have widespread impact in applications ranging from spectroscopy to chemical and biological sensing, imaging, and microscopy.
By utilizing entanglement property of photons the entangled two-photon absorption (ETPA) effect of an organic material, porphyrin dendrimer, is demonstrated through the comparison of the property of entangled photons to the property of quantum-correlated photons. The ETPA showed a cross-section 31 orders of magnitude higher than the cross-section of the classical two-photon absorption (TPA). This high cross-section is comparable to the cross-section of the resonant single-photon absorption. The entangled absorption effect is compared to the correlated TPA effect to determine the degree of correlation between the entangled and quantum correlated photon pairs. The experimental data describe the different degree of correlation of the non-entangled and entangled photon pairs by demonstrating linear and nonlinear relationship of the absorbed photon flux to the input photon flux. The linearity of the ETPA is an interesting quantum effect because the two-photon absorption is an inherently nonlinear process. Virtual state spectroscopy is also demonstrated as a novel spectroscopic method to investigate the properties of the virtual state from non-monotonic behavior of the cross-section which is represented by controlling temporal property of the entanglement. These results from the quantum spectroscopy methods show a unique quantum property which is not feasible to detect using classical methods.
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.
Conspectus The enhanced interest in quantum-related phenomena has provided new opportunities for chemists to push the limits of detection and analysis of chemical processes. As some have called this the second quantum revolution, a time has come to apply the rules learned from previous research in quantum phenomena toward new methods and technologies important to chemists. While there has been great interest recently in quantum information science (QIS), the quest to understand how nonclassical states of light interact with matter has been ongoing for more than two decades. Our entry into this field started around this time with the use of materials to produce nonclassical states of light. Here, the process of multiphoton absorption led to photon-number squeezed states of light, where the photon statistics are sub-Poissonian. In addition to the great interest in generating squeezed states of light, there was also interest in the formation of entangled states of light. While much of the effort is still in foundational physics, there are numerous new avenues as to how quantum entanglement can be applied to spectroscopy, imaging, and sensing. These opportunities could have a large impact on the chemical community for a broad spectrum of applications. In this Account, we discuss the use of entangled (or quantum) light for spectroscopy as well as applications in microscopy and interferometry. The potential benefits of the use of quantum light are discussed in detail. From the first experiments in porphyrin dendrimer systems by Dr. Dong-Ik Lee in our group to the measurements of the entangled two photon absorption cross sections of biological systems such as flavoproteins, the usefulness of entangled light for spectroscopy has been illustrated. These early measurements led the way to more advanced measurements of the unique characteristics of both entangled light and the entangled photon absorption cross-section, which provides new control knobs for manipulating excited states in molecules. The first reports of fluorescence-induced entangled processes were in organic chromophores where the entangled photon cross-section was measured. These results would later have widespread impact in applications such as entangled two-photon microscopy. From our design, construction and implementation of a quantum entangled photon excited microscope, important imaging capabilities were achieved at an unprecedented low excitation intensity of 10 7 photons/s, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. New reports have also illustrated an advantage of nonclassical light in Raman imaging as well. From a standpoint of more precise measurements, the use of entangled photons in quantum interferometry may offer new opportunities for chemistry research. Experiments that combine molecular spectroscopy and quantum interferometry, by utilizing the correlations of entangled photons in a Hong–Ou–Mandel (HOM) interferometer, have been carried out. The initial experiment showed that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer. In addition, parameters such as the dephasing time have been obtained with the opportunity for even more advanced phenomenology in the future.
We utilize quantum entangled photons to carry out nonlinear optical spectroscopy in organic molecules with an extremely small number of photons. For the first time, fluorescence is reported as a result of entangled photon absorption in organic nonlinear optical molecules. Selectivity of the entangled photon absorption process is also observed and a theoretical model of this process is provided. Through these experiments and theoretical modeling it is found that while some molecules may not have strong classical nonlinear optical properties due to their excitation pathways; these same excitation pathways may enhance the entangled photon processes. It is found that the opposite is also true. Some materials with weak classical nonlinear optical effects may exhibit strong non-classical nonlinear optical effects. Our entangled photon fluorescence results provide the first steps in realizing and demonstrating the viability of entangled two-photon microscopy, remote sensing, and optical communications.
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.