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Fluorescence Lifetime Measurements Using Entangled-Photon Coincidences

Audrey V. Eshun, Xiyu Yi, Ashleigh Wilson, Sam Jeppson, Shervin Kiannejad, Mike Rushford, Tiziana Bond, Ted A. Laurence

Published Jan 1, 2023• 0 citations
Publisher

Abstract

We have utilized entangled photon pair coincidences to measure the fluorescence lifetime of the organic dye Rhodamine 6G, allowing lifetime measurements to be conducted at low intensities with a CW laser and without ultrafast pulsed lasers.
DOI
10.1364/cleo_at.2023.jth2a.34

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We have used photon pair correlations generated via spontaneous parametric downconversion (SPDC) to measure the fluorescence lifetime of the organic dye rhodamine 6 G, demonstrating that fluorescence lifetime measurements can be achieved using a continuous wave (CW) laser, without pulsed or modulated lasers. Our entangled photon method, quantum fluorescence lifetime (Q-FL) measurements, uses one photon to excite fluorescence and the resulting fluorescence photon is timed and referenced to the arrival time of the other entangled photon. Thus, we can exploit the short timescale of photon pair correlations to conduct experiments that are typically carried out with pulsed lasers and we show that the inherent timing of the photons is fast enough to resolve the nanosecond scale fluorescence lifetime of the sample. This measurement paves the way towards using the time correlations of entangled photons for fluorescence imaging; capitalizing on the presence of fast, sub-100 ps correlations that have not been demonstrated classically.
Quantum light sources, such as entangled photons, offers unique advantages for bio-sensing of light sensitive samples, providing background free and high SNR measurements at ultra-low photon flux. We built a modular set-up based on SPDC entangled photon pairs generation by using a pulsed laser and several single-photon detectors with different time resolution and quantum efficiency to investigate fluorescent complexes. This toolkit paves the way for minimally invasive fluorescence lifetime sensing towards next generation in vivo studies.
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.
Fluorescence lifetime experiments are a standard approach for measuring excited-state dynamics and local environmental effects. Here, we show that entangled photon pairs produced from a continuous-wave (CW) laser diode can replicate pulsed laser experiments without phase modulation. As a proof of principle, picosecond fluorescence lifetimes of indocyanine green are measured in multiple environments. The use of entangled photons has three unique advantages. First, low-power CW laser diodes and entangled photon source design lead to straightforward on-chip integration for a direct path to distributable fluorescence lifetime measurements. Second, the entangled pair's wavelength is easily tuned by adjusting the temperature or electric field, allowing a single source to cover octave bandwidths. Third, femtosecond temporal resolutions can be reached without requiring major advances in source technology or external phase modulation. Entangled photons could therefore provide increased accessibility to time-resolved fluorescence while also opening new scientific avenues in photosensitive and inherently quantum systems.
The authors present an experimental study of entangled two-photon absorption in solvated rhodamine 6G, providing strong evidence that the orders-of-magnitude increases in two-photon-absorption efficiency by using entangled light reported in previous studies cannot be explained by the community's current understanding of the process. Thus, the sought-after advantages of using two-photon absorption of time-frequency-entangled photon pairs as a practical tool for enhancing molecular spectroscopy and biological imaging remains elusive.