Experimental upper bounds for resonance-enhanced entangled two-photon absorption cross section of indocyanine green
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1.
California Institute of Technology
Abstract
Resonant intermediate states have been proposed to increase the efficiency of entangled two-photon absorption (ETPA). Although resonance-enhanced ETPA (r-ETPA) has been demonstrated in atomic systems using bright squeezed vacuum, it has not been studied in organic molecules. We investigate for the first time r-ETPA in an organic molecular dye, indocyanine green (ICG), when excited by broadband entangled photons in near-IR. Similar to many reported virtual state mediated ETPA (v-ETPA) measurements, no r-ETPA signals are measured, with an experimental upper bound for the cross section placed at 6(±2) × 10−23 cm2. In addition, the classical resonance-enhanced two-photon absorption (r-TPA) cross section of ICG at 800 nm is measured for the first time to be 20(±13) GM, where 1 GM equals 10−50 cm4 s, suggesting that having a resonant intermediate state does not significantly enhance two-photon processes in ICG. The spectrotemporally resolved emission signatures of ICG excited by entangled photons are also presented to support this conclusion.
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Acknowledgement
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant No. DE-SC0020151 (S.K.C.). The authors also thank the Caltech Beckman Institute Biological Imaging Facility for assisting with the classical two-photon absorption measurements.
Contributions
Manni He: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Resources (equal); Software (equal); Writing – original draft (equal); Writing – review & editing (equal). Bryce P. Hickam: Conceptualization (supporting); Data curation (supporting); Formal analysis (supporting); Methodology (equal); Resources (supporting); Software (equal). Nathan Harper: Formal analysis (supporting); Methodology (supporting); Resources (supporting); Writing – review & editing (equal). Scott K. Cushing: Conceptualization (equal); Funding acquisition (lead); Methodology (supporting); Writing – original draft (equal); Writing – review & editing (equal).
Data Availability
The data that support the findings of this study are available within the article and its supplementary material. Additional data are available from the corresponding author upon reasonable request.
Conflict of Interest
The authors have no conflicts to disclose.
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- ISSN
- 1089-7690
- United States Department of Energy
- DE-SC0020151