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Entangled photon pairs have been used for molecular spectroscopy in the form of entangled two-photon absorption and in quantum interferometry for precise measurements of light source properties and time delays. We present an experiment that combines molecular spectroscopy and quantum interferometry by utilizing the correlations of entangled photons in a Hong-Ou-Mandel (HOM) interferometer to study molecular properties. We find that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer, and the resulting signal contains information pertaining to the light-matter interaction. We can extract the dephasing time of the coherent response induced by the excitation on a femtosecond time scale. A dephasing time of 102 fs is obtained, which is relatively short compared to times found with similar methods and considering line width broadening and the instrument entanglement time As the measurement is done with coincidence counts as opposed to simply intensity, it is unaffected by even-order dispersion effects, and because interactions with the molecular state affect the photon correlation, the observed measurement contains only these effects and no other classical losses. The experiments are accompanied by theory that predicts the observed temporal shift and captures the entangled photon joint spectral amplitude and the molecule's transmission in the coincidence counting rate. Thus, we present a proof-of-concept experimental method based of entangled photon interferometry that can be used to characterize optical properties in organic molecules and can in the future be expanded on for more complex spectroscopic studies of nonlinear optical properties.

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

New donor–acceptor light harvesting polymers with either a 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene or a 4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithienothiophene donor subunit and either a 2-(nonadecan-9-yl)-5-(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione, (E)-1,1′-bis(2-octyldodecyl)-[3,3′-biindolinylidene]-2,2′-dione, or 3-ethyl-1-(thieno[3,4-b]thiophen-2-yl)heptan-1-one acceptor were synthesized. In this study, the effects of donor conjugation length and donor-acceptor combination on the optical properties of the synthesized polymers were investigated using steady state, up-conversion, quantum chemical simulations and ultrafast and transient spectroscopic techniques. At high energies, it was observed that polymers with extended donor conjugation had significantly enhanced absorption, whereas those with stronger acceptors showed higher extinction coefficients at lower energies. Fluorescence up-convers...

A new series of donor–acceptor ladder-type molecules were synthesized via Scholl reaction. These molecules contain up to 25-fused rings but still show good air stability and good solubility. The ring-fusing reaction is found to be sensitive to the nature of the side-chain in donor units. The molecular conformations were investigated by 2D NMR and DFT calculations. The photophysical properties were investigated and an intense intramolecular charge-transfer was observed. All of the molecules exhibit two-photon absorption (TPA) activity, and their TPA cross-section shows a linear relationship with increasing conjugation length of the thienoacene-PDI derivatives.

Acceptor doped perovskites show promise as fuel cell proton conducting ceramic membranes and continue to be rapidly developed. Proton conduction in barium zirconate with 6.25% ordered yttrium doping at the zirconium site is explored. The dopant increases octahedral distortions in the lattice similar to our earlier studied 12.5% dopant system though the angle distribution is slightly broader. Normal modes promoting vibrations about a non-transient octahedral tilt are found for both concentrations at frequencies thermally accessible at typical proton conduction temperatures. The energy of proton binding sites and transition states roughly increases with distance away from the dopant though there are many oscillations in the trend due to local proton induced distortions. Protons can both locally enhance and decrease the octahedral distortions induced by the dopant. Transition state energies roughly increase with proton-dopant distance. Rotational and intraoctahedral transfer transition state energies vary through a similar magnitude range though the intraoctahedral transition states are often higher in energy. Interoctahedral transfer transition states tend to have the highest energies. The diversity of minima and transition states suggests that proton conduction in 6.25% yttrium doped barium zirconate is more complex than conduction in 12.5% doped barium zirconate.

Investigations of the optical effects in thienoacene chromophores with different central atoms were performed. These chromophores provide a basis for the comparison of the linear, two-photon, and entangled two-photon properties in organic molecules with varying degrees of dipolar or quadrupolar character. Linear absorption and emission as well as time-dependent density functional theory calculations were performed for the chromophores investigated. Measurements of the classical two-photon absorption (TPA), entangled two-photon absorption (ETPA), as well as entangled two-photon fluorescence were experimentally performed for the four chromophores. Electronic structure calculations were utilized to provide estimates of the classical two-photon absorption coefficients. The results of the measured entangled two-photon cross sections were compared with theoretical estimates for the molecules investigated. It is found that the dipole (transition or permanent) pathway can have an effect on the trends in the entangled photon absorption process in dipolar organic chromophores. This study helps predict the properties of the entangled two-photon effect in chromophores with different dipolar and quadrupolar character.