Colors of entangled two-photon absorption.

Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.

Quantum Light-Enhanced Two-Photon Imaging of Breast Cancer Cells.

Correct biological interpretation from cell imaging can be achieved only if the observed phenomena proceed with negligible perturbation from the imaging system. Herein, we demonstrate microscopic images of breast cancer cells created by the fluorescence selectively excited in the process of entangled two-photon absorption in a scanning microscope at an excitation intensity orders of magnitude lower than that used for classical two-photon microscopy. Quantum enhanced entangled two-photon microscopy has shown cell imaging capabilities at an unprecedented low excitation intensity of ∼3.6 × 107 photons/s, which is a million times lower than the excitation level for the classical two-photon fluorescence image obtained in the same microscope. The extremely low light probe intensity demonstrated in entangled two-photon microscopy is of critical importance to minimize photobleaching during repetitive imaging and damage to cells in live-cell applications. This technology opens new avenues in cell investigations with light microscopy, such as enhanced selectivity and time-frequency resolution.

Entangled Photon Spectroscopy.

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 107 photons/s, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. New reports have also illustrated an advantage of nonclassical light in Raman imaging as well.From a standpoint of more precise measurements, the use of entangled photons in quantum interferometry may offer new opportunities for chemistry research. Experiments that combine molecular spectroscopy and quantum interferometry, by utilizing the correlations of entangled photons in a Hong-Ou-Mandel (HOM) interferometer, have been carried out. The initial experiment showed that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer. In addition, parameters such as the dephasing time have been obtained with the opportunity for even more advanced phenomenology in the future.

Enhanced Exciton Quantum Coherence in Single CsPbBr3 Perovskite Quantum Dots using Femtosecond Two-Photon Near-Field Scanning Optical Microscopy.

Cesium-halide perovskite quantum dots (QDs) have gained tremendous interest as quantum emitters in quantum information processing applications due to their optical and photophysical properties. However, engineering excitonic states in quantum dots requires a deep knowledge of the coherent dynamics of their excitons at a single-particle level. Here, we use femtosecond time-resolved two-photon near-field scanning optical microscopy (NSOM) to reveal coherences involving a single cesium lead bromide perovskite QD (CsPbBr3) at room temperature. We show that, compared to other nonperovskite nanoparticles, the electronic coherence on a single perovskite QD has a relatively long lifetime of ca. 150 fs, whereas CdSe QDs have exciton coherence times shorter than 75 fs at room temperature. One possible explanation for the longer coherence time observed for the CsPbBr3 perovskite system is related to the exciton fine structure of these perovskite QDs compared to other nanoparticles. These perovskite QDs exhibit interesting optical properties that differ from those of the traditional QDs including bright triplet exciton states. In fact, due to the small amplitude of the energy gap fluctuations of dipole-allowed triplet states in perovskite QDs, the coherent superposition could be preserved for longer times. Furthermore, single-particle excitation approach implemented in this work allows us to remove effects of heterogeneity that are usually present in ensemble averaging experiments at room temperature. The realization of quantum-mechanical phase-coherence of a charge carrier that can operate at room temperature is an issue of great importance for the potential application of coherent electronic phenomena in electronic and optoelectronic devices. These interesting findings provide further evidence of the great potential of these perovskite QDs as candidates for quantum computing and information processing applications.

Investigations of Molecular Optical Properties Using Quantum Light and Hong-Ou-Mandel Interferometry.

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.

Two-Photon Fluorescence Microscopy at Extremely Low Excitation Intensity: The Power of Quantum Correlations.

Quantum entanglement has been shown to imply correlations stronger than those allowed by classical models. The possibility of performing tasks that are classically impossible has made quantum entanglement a powerful resource for the development of novel methods and applications in various fields of research such as quantum computing, quantum cryptography, and quantum metrology. There is a great need for the development of next generation instrumentation and technologies utilizing entangled quantum light. Among the many applications of nonclassical states of light, nonlinear microscopy has the potential to make an impact in broad areas of science from physics to biology. Here, the microscopic image created by the fluorescence selectively excited by the process of the entangled two-photon absorption is reported. Entangled two-photon microscopy offers nonlinear imaging capabilities at an unprecedented low excitation intensity 107, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. The nonmonotonic dependence of the image on the femtosecond delay between the components of the entangled photon pair is demonstrated. This delay dependence is a result of specific quantum interference effects associated with the entanglement and this is not observable with classical excitation light. In combination with novel spectroscopic capabilities provided by a nonclassical light excitation, this is of critical importance for sensing and biological applications.

Two-Photon Excitation of Flavins and Flavoproteins with Classical and Quantum Light.

In this contribution, the entangled two-photon absorption (ETPA) process on naturally occurring flavoproteins was studied. Low temperature responsive protein (LOT6P) and b-type dihydroorotate dehydrogenase (DHOD B), which possess flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) chromophores embedded in the protein environment, were investigated. The ETPA cross-section was measured, and we found that it increases when going from an aqueous solution of the free flavin chromophore to the chromophore embedded in the protein. This enhancement is particularly evident when entangled photons are used as excitation light compared to classical light. Our results prove the potential of ETPA as a sensing technique for fluorescent proteins even for those whose classical TPA cross-section is small compared to well-known fluorescent proteins.

Predicting and Controlling Entangled Two-Photon Absorption in Diatomic Molecules.

The use of nonclassical states of light to probe organic molecules has received great attention due to the possibility of providing new and detailed information regarding molecular excitations. Experimental and theoretical results have been reported which show large enhancements of the nonlinear optical responses in organic materials due to possible virtual-electronic-state interactions with entangled photons. In order to predict molecular excitations with nonclassical light, more detailed investigations of the parameters involved must be carried out. In this report we investigate the details of the state-to-state parameters important in calculating the contribution of particular transitions involved in the entangled two-photon absorption process for diatomic molecules. The theoretical discussion of the entangled two-photon process is described for a set of diatomic molecules. Specifically, we provide detailed quantum chemical calculations which give accurate energies and transition moments for selection-rule allowed intermediate states important in the entangled nonlinear effect for the diatomic molecules. These results are used to estimate in a more accurate manner the nonmonotonic behavior of the entangled two-photon absorption cross-section. We also derive accurate approximations that can be used to predict the period between entanglement-induced transparencies without needing exact values of the transition dipole moments. These results suggest that with the additional parameters allotted by the entangled two-photon absorption (in comparison to the classical case), it may be possible to predict and later control the nonlinear absorption and transparency of a molecule at a constant incident photon frequency.

Entangled Photon Excited Fluorescence in Organic Materials: An Ultrafast Coincidence Detector.

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, TE, 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.

High Yield Ultrafast Intramolecular Singlet Exciton Fission in a Quinoidal Bithiophene.

We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by photoinduced triplet-triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 µs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system.

Two-photon fluorescence spectroscopy and imaging of 4-dimethylaminonaphthalimide peptide and protein conjugates.

We report detailed photophysical studies on the two-photon fluorescence processes of the solvatochromic fluorophore 4-DMN as a conjugate of the calmodulin (CaM) and the associated CaM-binding peptide M13. Strong two-photon fluorescence enhancement has been observed which is associated with calcium binding. It is found that the two-photon absorption cross-section is strongly dependent on the local environment surrounding the 4-DMN fluorophore in the CaM conjugates, providing sensitivity between sites of fluorophore attachment. Utilizing time-resolved measurements, the emission dynamics of 4-DMN under various environmental (solvent) conditions are analyzed. In addition, anisotropy measurements reveal that the 4-DMN-S38C-CaM system has restricted rotation in the calcium-bound calmodulin. To establish the utility for cellular imaging, two-photon fluorescence microscopy studies were also carried out with the 4-DMN-modified M13 peptide in cells. Together, these studies provide strong evidence that 4-DMN is a useful probe in two-photon imaging, with advantageous properties for cellular experiments.

An ultrafast look at Au nanoclusters.

In the past 20 years, researchers studying nanomaterials have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (>3 nm) because of their potential applications, such as in molecular electronics, image markers, and catalysts. In particular, the discovery of metal nanoclusters (<3 nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties for nanomaterials are intriguing, because for metal nanosystems in this size regime both size and shape determine electronic properties. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to probe the metal core directly. Au MPCs have become the "gold" standard in nanocluster science, thanks to the rigorous structural characterization already accomplished. The use of ultrafast laser spectroscopy on MPCs in solution provides the benefit of directly studying the chemical dynamics of metal nanoclusters (core), and their nonlinear optical properties. In this Account, we investigate the optical properties of MPCs in the visible region using ultrafast spectroscopy. Based on fluorescence up-conversion spectroscopy, we propose an emission mechanism for these nanoclusters. These clusters behave differently from nanoparticles in terms of emission lifetimes as well as two-photon cross sections. Through further investigation of the transient (excited state) absorption, we have found many unique phenomena of nanoclusters, such as quantum confinement effects and vibrational breathing modes. In summary, based on the differences in the optical properties, the distinction between nanoclusters and nanoparticles appears at a size near 2.2 nm. This is consistent with simulations from a free-electron model proposed for MPCs. The use of ultrafast techniques on these nanoclusters can answer many of the fundamental questions about the nature of these exciting nanomaterials and their applications.

Bright two-photon emission and ultra-fast relaxation dynamics in a DNA-templated nanocluster investigated by ultra-fast spectroscopy.

Metal nanoclusters have interesting steady state fluorescence emission, two-photon excited emission and ultrafast dynamics. A new subclass of fluorescent silver nanoclusters (Ag NCs) are NanoCluster Beacons. NanoCluster Beacons consist of a weakly emissive Ag NC templated on a single stranded DNA ("Ag NC on ssDNA") that becomes highly fluorescent when a DNA enhancer sequence is brought in proximity to the Ag NC by DNA base pairing ("Ag NC on dsDNA"). Steady state fluorescence was observed at 540 nm for both Ag NC on ssDNA and dsDNA; emission at 650 nm is observed for Ag NC on dsDNA. The emission at 550 nm is eight times weaker than that at 650 nm. Fluorescence up-conversion was used to study the dynamics of the emission. Bi-exponential fluorescence decay was recorded at 550 nm with lifetimes of 1 ps and 17 ps. The emission at 650 nm was not observed at the time scale investigated but has been reported to have a lifetime of 3.48 ns. Two-photon excited fluorescence was detected for Ag NC on dsDNA at 630 nm when excited at 800 nm. The two-photon absorption cross-section was calculated to be ∼3000 GM. Femtosecond transient absorption experiments were performed to investigate the excited state dynamics of DNA-Ag NC. An excited state unique to Ag NC on dsDNA was identified at ∼580 nm as an excited state bleach that related directly to the emission at 650 nm based on the excitation spectrum. Based on the optical results, a simple four level system is used to describe the emission mechanism for Ag NC on dsDNA.

Probing coherence in synthetic cyclic light-harvesting pigments.

A series of π-extended cyclic thiophene oligomers of 12, 18, 24, and 30 repeat units have been studied using methods of ultrafast time-resolved absorption, fluorescence upconversion, and three-pulse photon echo. These measurements were conducted in order to examine the structure-function relationships that may affect the coherence between chromophores within the organic macrocycles. Our results indicate that an initial delocalized state can be seen upon excitation of the cyclic thiophenes. Anisotropy measurements show that this delocalized state decays on an ultrafast time scale and is followed by the presence of incoherent hopping. From the use of a phenomenological model, we conclude that our ultrafast anisotropy decay measurements suggest that the system does not reside in the Förster regime and coherence within the system must be considered. Three-pulse photon echo peak shift experiments reveal a clear dependence of initial peak shift with ring size, indicating a weaker coupling to the bath (and stronger intramolecular interactions) as the ring size is increased. Our results suggest that the initial delocalized state increases with ring size to distances (and number of chromophores) comparable to the natural light-harvesting system.

Optically excited acoustic vibrations in quantum-sized monolayer-protected gold clusters.

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1-4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (< or =2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.

Critical size for the observation of quantum confinement in optically excited gold clusters.

We present a systematic study of optical properties of a series of hexanethiolate-capped Au clusters of varying sizes using femtosecond transient absorption, time-resolved fluorescence, and two-photon absorption cross-sectional measurements. An abrupt change in optical properties and their trends has been found at the 2.2 nm size. Displacively excited vibrations with a period of 450 fs have been detected in the transient absorption signal for smaller clusters < or = 2.2 nm. These results strongly suggest an emerging optical gap between the highest occupied and lowest unoccupied orbitals in the narrow size range at 2.2 nm.

Investigations of energy migration in an organic dendrimer macromolecule for sensory signal amplification.

The issue of macromolecular exciton delocalization length and fluorescence sensing of energetic materials is investigated and modeled from results of nonlinear optical and time-resolved spectroscopy. By using two- and three-photon absorption techniques the fluorescence quenching effects of an organic dendrimer for sensing TNT were carried out. The Stern-Volmer plots for the set of dendrimers were examined and a large quenching constant for the dendrimer G4 was obtained (1400 M(-1)). The quenching constant was found to increase with the dendrimer generation number. The mechanism for the enhanced sensitivity of the dendrimer system was examined by probing the exciton dynamics with femtosecond fluorescence up-conversion. Fluorescence lifetime measurements revealed a multicomponent relaxation that varied with dendrimer generation. Fluorescence anisotropy decay measurements were used to probe the exciton migration length in these dendrimer systems and for the large structure the excitation migration area covers approximately 20 units. All of these results were used in a model that describes the exciton localization length with the fluorescence quenching strength. The use of time-resolved techniques allows for a closer and more detailed description of the mechanism of sensory amplification in organic macromolecules.

Quantum-sized gold clusters as efficient two-photon absorbers.

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.

Strong coupling in macrocyclic thiophene investigated by time-resolved two-photon excited fluorescence.

We report the femtosecond dynamics of fluorescence anisotropy excited through the two-photon absorption (TPA), which provides direct signatures of delocalized electronic excitations for symmetrical macromolecular architectures. Two-photon excited fluorescence anisotropy is strongly correlated with the orientation and value of the transition moment from the excited state to the second and higher lying states. For macromolecular systems it leads to a relatively low initial fluorescence anisotropy and specific femtosecond anisotropy dynamics. We have experimentally demonstrated qualitatively different anisotropy dynamics for two- and one-photon absorption excitations for strongly coupled ring architecture prospective for artificial-light-harvesting applications and possessing an enhanced TPA-absorption cross section.

Enhancement of two-photon absorption cross-section in macrocyclic thiophenes with cavities in the nanometer regime.

The linear and nonlinear optical properties of two thiophene-based cyclic molecules have been investigated. These molecules represent nanometer sized cavities which may be useful for novel photonic devices. By virtue of long-range interactions, these chromophores serve as novel architectures for enhanced two-photon absorption (TPA) properties. Measurements of the different size ring structures showed a 550% increase in the TPA cross-section for the larger macrocycle. Electronic structure calculations have suggested an increase in coupling of the excited states in these systems as the ring size is increased. Measurements of the ultrafast transient absorption and fluorescence were carried out with these systems in order to probe the interaction between the chromophores. The results of the transient decays as well as fluorescence anisotropy decay times gives stronger proof to the suggestion of delocalized states in the cyclic macrocycles. These results provide information regarding the optical properties of these novel systems useful for potential applications in photonics.