Experimental upper bounds for resonance-enhanced entangled two-photon absorption cross section of indocyanine green

Manni He; Bryce P Hickam; Nathan Harper; Scott K Cushing

doi:10.1063/5.0193311

Experimental upper bounds for resonance-enhanced entangled two-photon absorption cross section of indocyanine green

J Chem Phys. 2024 Mar 7;160(9):094305. doi: 10.1063/5.0193311.

Authors

Manni He¹, Bryce P Hickam¹, Nathan Harper¹, Scott K Cushing¹

Affiliation

¹ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.

PMID: 38445732
DOI: 10.1063/5.0193311

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.