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Journal of Experimental and Theoretical Physics Letters

Probe Absorption Spectroscopy As a Tool to Study the Excited States in a Cooperative System of Two Organic Molecules

Ekaterina A. Tarasevich, M. G. Gladush

Published Jul 1, 2024• 1 citation
Publisher

Abstract

The applicability of the method for measuring the probe absorption and gain spectra in a system of quantum emitters excited by a resonant driving field for studying cooperative photoluminescence is considered. The gain and attenuation of a probe signal in the system of two quantum emitters with the dipole–dipole interaction have been calculated for the conditions of observing the cooperative photoluminescence of a pair of impurity molecules described in the literature and close to them. The resulting dependences demonstrate the structure of excited collective molecular states, which can be used to reconstruct the positions of molecules in a matrix and to determine the difference in their photophysical properties. Thus, the probe absorption and gain spectra can be used to analyze the photoluminescence of quantum-entangled particles and to transform light signals.

DOI
10.1134/s0021364024602227

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Abstract We present numerical calculations of the cooperative photoluminescence spectra of two dipole–dipole–coupled dibenzoterrylene molecules embedded in an anthracene crystal at liquid-helium temperature. Using master-equation formalism for non-identical two-level emitters, we reproduce the excitation spectra recently observed in experiment. The excellent agreement between the simulations, analytical model, and measurements confirms the observation of controllable molecular entanglement and validates the theoretical framework. The approach provides an efficient computational tool for predicting and controlling cooperative photoluminescence in solid-state molecular systems. These results establish a basis for future experiments aimed at achieving optical entanglement of organic molecules and developing quantum photonic devices.
We report progress in the theory of photoluminescence and light scattering by two closely spaced particles. This study is based on our original method to derive the master equation for a system of coupled quantum emitters driven by a cw laser. We emphasise on the case when the emitters are two-level systems but notably different in their transition frequencies and transition moments. The master equation is shown to describe the dipole-dipole coupling and the cooperative entanglement between the particles naturally. It is provided by the use of the Bogolyubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchies for reduced density matrices and correlation operators of the material and photonic subsystems. Tackling the hierarchies has also provided us with an elegant way to calculate the photoluminescence excitation spectra subject to the arrangement of the excitation and the position of the detector against the emitters. Our findings may be important for developing novel possibilities for the all-optical nanoscopy and may help construction of entangled systems to be implemented in quantum technologies.
Abstract The current paper demonstrates theoretical analysis of two types of spectral curves for several configurations of system of two-level light emitters, considering the influence of local field and close environment inside a transparent medium. Probe field absorption spectra and resonant fluorescence spectra are calculated under excitation of a strong monochromatic cw laser. The sensitivity of absorption and emission optical spectroscopy method is compared for revealing the effects of the medium on individual emitters and their ensembles. Spectral curves were calculated for model emitters considering local field influence of a transparent dielectric medium and local electron-phonon interactions, which determined the response of the emitters to an external laser field and effective relaxation mechanisms. The calculation formalism is based on a semiclassical approach, while the relaxation processes associated with the phonon contribution are introduced phenomenologically with references to other studies.
The problem of the energy absorption from a weak probe field by a system of atoms driven by a coherent laser field is considered with special reference to the collective behavior of the atomic system. The changes in the nature of the absorption spectra as the intensity of the coherent laser field is changed are examined explicitly for a system of two atoms. Cooperative effects are shown to be important for weak laser fields. For strong fields, it is shown that the absorption spectra under cooperative conditions differ from those without cooperative conditions only by a scale factor.
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.
Dipole-coupled subwavelength quantum emitter arrays respond cooperatively to external light fields as they may host collective delocalized excitations (a form of excitons) with super- or subradiant character. Deeply subwavelength separations typically occur in molecular ensembles, where in addition to photon-electron interactions, electron-vibron couplings and vibrational relaxation processes play an important role. We provide analytical and numerical results on the modification of super- and subradiance in molecular rings of dipoles including excitations of the vibrational degrees of freedom. While vibrations are typically considered detrimental to coherent dynamics, we show that molecular dimers or rings can be operated as platforms for the preparation of long-lived dark superposition states aided by vibrational relaxation. In closed ring configurations, we extend previous predictions for the generation of coherent light from ideal quantum emitters to molecular emitters, quantifying the role of vibronic coupling onto the output intensity and coherence.