Investigations of Organic Molecules Using Entangled Photons as a Novel Spectroscopic Tool

Abstract

Entangled states have been utilized in a wide range of fields and their quantum properties have been taken advantage of across quantum information science to achieve outcomes that are not available through classical means. Despite being used in quantum computing, cryptography and communication, the use of entangled states of light in spectroscopy is a new area where a lot is still needed to be understood. The goal of this thesis is to expand on the understanding of quantum entanglement as it applies to spectroscopy of different types. Firstly, in entangled two-photon absorption (ETPA) spectroscopy measured via transmission (absorption) and fluorescence methods. Secondly, to experimentally realize the utilization of quantum entangled photons in the novel application of virtual state spectroscopy and finally to prove the viability of entangled photons in a Hong-Ou-Mandel interferometer for spectroscopic use. In this thesis, the optical properties of a series of organic chromophores with differing structure-function properties were studied with classical and entangled light. It was observed that when used to excite some of these chromophores, entangled light causes enhancements in their nonlinear responses, which aids in our understanding of the structure-function relation to entangled light excitation, and the sensitivity of entangled light in resolving different absorption pathways. The experimental and molecular parameters necessary for the observation of entangled two-photon excited fluorescence signals were investigated, and it was found that in addition to high dipole moments and good quantum yields, there is a strong dependence on the detuning energies between the molecule’s intermediate states and the entangled photons; small detuning energies lead to a resonance effect and coupling between the intermediate states and the photons leading to higher fluorescence signals. This makes the transition pathway preferred for fluorescence measurements and enables enhancements in the fields of imaging and microscopy. The idea of using quantum entangled light as a source to measure the intermediate or virtual energy levels of molecular transitions has been theorized but this work will be an expanded experimental achievement. ETPA signals are measured at different interbeam delay times for a pair of organic trimers that differ in their planarity. By using this data to obtain an energy spectrograph and comparing the information to results obtained via classical nonlinear spectroscopic methods, a broader picture of the compounds’ excitation processes is attained. This is a step towards greater employment of this specialized virtual state spectroscopy technique. Additionally, Hong-Ou-Mandel (HOM) interferometry has been used primarily for metrology, but in this research the quantum interference properties of entangled light interacting with an organic sample’s electronic states is used to determine information about the coherent excitation of organic molecules. This is the first time an HOM interferometer is being used to excite an organic sample and obtain information about optical molecular properties in this way. The dephasing time of the sample was extracted, and this proof-of concept experiment can now be expanded for further time-resolved and nonlinear spectroscopy with entangled photons.