Dynamical and statistical behaviors of Frenkel excitons in a matter system consisting of N two-level-atoms and interacting with an external field are studied with the molecular field and random phase approximations. Existence of a phase transition is suggested both in the exciton system and exciton-photon coupled system. This phase transition is the second order transition whose order parameter is the polarizability. A new type of condensation of the photon-exciton coupled wave, in which a macroscopic number of excitons and photons participate, has been found as a result of appreciable contribution of photons to the phase transition. When the incident light is coherent, the condition of phase transition is modified by photon induced exciton-exciton interaction. The energy dispersion of elementary excitations is derived.
Dynamical and statistical behaviors of Frenkel excitons in a matter system consisting of N two-level-atoms and interacting with an external field are studied with the molecular field and random phase approximations. Existence of a phase transition is suggested both in the exciton system and exciton-photon coupled system. This phase transition is the second order transition whose order parameter is the polarizability. A new type of condensation of the photon-exciton coupled wave, in which a macroscopic number of excitons and photons participate, has been found as a result of appreciable contribution of photons to the phase transition. When the incident light is coherent, the condition of phase transition is modified by photon induced exciton-exciton interaction. The energy dispersion of elementary excitations is derived.
A formal, regorous theory of Frenkel excitons interacting with an external electromagnetic field is doveloped within the framework of a two-level-atom model in which the dipole approximation is employed in describing the atom-field (taken as a c-number) interaction. The model exciton Hamiltonian is shown to be identical in form with the Heisenberg model Hamiltonian for S=1/2 spins with anisotropic long-range exchange interactions. Instead of treating excitons as a non-ideal Bose gas, atomic polarization operators and level population defference are taken up as two fundamental quantities to describe Frenkel excitons. A pair of equations satisfied by these two quantities are derived. It is shown that Frenkel excitons are generally expressed as quantum-mechanical nonlinear polarization waves, the nonlinearity being characterized by the level population. A discussion is given on the interrelationship between such polarization waves and those derivable from the classical Drude-Lorentz model. With the aid of several approximation procedures similar to those employed in nonlinear optics, a set of equations describing nonlinear exciton-photon coupled modes are derived. Brief discussion is given on the eigenfrequencies of nonlinear Frenkel excitons and nonlinear polaritons. The existence of nonlinear exciton-photon coupled modes whose properties are different from those of polariton modes is suggested.
Interactions of nonlinear Frenkel excitons with an intense external radiation field (rf) are studied by taking the atomic polarization, the atomic population difference and the electric field as a set of variables to study the coherence and nonlinearity properties of the exciton-rf system. To equations for these quantities, terms are added phenomenologicaly to take care of pumping and dissipative effects. Two kinds of solutions are obtained under certain conditions, one representing the existence of the stationary coherent polarization and the macroscopic number of photons with wave number equal to that of the excitons and another envelope-kink-like, steady exciton-rf coupled modes around the stationary coherent states
In this work, we study the exciton-polariton condensate phase transition in a microcavity matter-light system in which electron-hole Coulomb interaction and matter-light coupling effects are treated on an equal footing. In the framework of the unrestricted Hartree-Fock approximation applying the two-dimensional exciton-polariton model, we derive the self-consistent equations determining simultaneously the excitonic and the photonic condenstate order parameters. In the thermal-equilibrium limit, we find a condensed state of the exciton-polariton systems and phase diagrams are then constructed. At a given low temperature, the condensate by its nature shows a crossover from an excitonic to a polaritonic and finally photonic condensed state as the excitation density increases at large detuning. Without the detuning, the excitonic condensed state disappears whereas the polaritonic or photonic phases dominate. The crossover is also found by lowering the Coulomb interaction at a finite matter-light coupling. Lowering the Coulomb interaction or increasing the temperature, the excitonic Mott transition occurs, at which the exciton-polariton condensates dissociate to free electron-hole/photon. Depending on temperature and excitation density, the phase transition of the exciton-polariton condensates is also addressed in signatures of photoluminescence mapping to the photonic momentum distribution.
The atom - electromagnetic field interaction is studied in the Dicke model, wherein a single field mode is interacting with a collection of two level atoms at thermal equilibrium. It is found that in the superradiant phase of the system, wherein the Bose-Einstein condensation of photons takes place, the notion of photon as an elementary electromagnetic excitation ceases to exist. The phase and intensity excitations of the condensate are found to be the true excitations of electromagnetic field. It is found that in this phase, the atom interacts with these excitations in a distinct coherent transition process, apart from the known stimulated emission/absorption and spontaneous emission processes. In the coherent transition it is found that while the atomic state changes in course of the transition process, the state of electromagnetic field remains unaffected. It is found that the transition probability of such coherent transition process is macroscopically large compared to other stimulated emission/absorption and spontaneous emission processes.
A quantum theory of nonlinear excitations in a matter-photon system is developed by generalizing the conventional two-level-atom model of a matter in several directions, on the assumption that the overlapping of atomic wave functions on different atomic sites is negligible.The model matter Hamiltonian is shown to be expressible in a generalized form' of the Heisenberg model Hamiltonian with anisotropic long-range exchange interactions with spin operators replaced by quasi-Pauli operators.From approxirpate formal solutions of a set of nonlinear equations satisfied by the quasi-Pauli operators ~nd photon operators, brief discussion is given on nonlinear Frenkel excitons and nonlinear exciton-photon coupled modes.An attempt is made to formulate a theory of higher-order nonlinear physical properties of the matter-photon system by including interactions of magnetic as •well as electric dipoles of atoms with photons.A brief discussion is also given on the coherence properties. of the matter-photon system which are characterized by the simultaneo~s existence of •a photon coherent state and an atomic-polarization coherent state.*> Throughout this paper, we will often use a simplified .notation l, rather than l 1, whenever appropriate, to avoid the use of complicated symbols.Also, the complex conjugate of a quantity• A is denoted by A*.
Bose-Einstein condensates of exciton-polaritons in inorganic semiconductor microcavities are known to possess strong interparticle interactions attributed to their excitonic component. The interactions play a crucial role in the nonlinear dynamics of such systems and can be witnessed as the high energy shift of polariton states. However, the localised nature of Frenkel excitons in strongly coupled organic microcavities precludes interparticle Coulomb exchange-interactions that affect nonlinear dynamics and change mechanisms of polariton energy shifts accordingly. We have scrutinized the origins of energy shifts in connection with nonlinear dynamics in Frenkel exciton-polariton condensates and examined the possible contributions: intracavity optical Kerr-effect, gain-induced frequency pulling, polariton interactions and effects related to saturation of optical transitions for "dark"- and "bright" molecules [1]. Unlike the conventional strongly coupled semiconductor microcavities, we have shown that nonlinear interactions within the condensate do not rely on polariton interactions but instead originated from Pauli-blocking principle forbidding double excitation of the same molecular states. We have developed a theoretical model explaining the omnipresent energy shift of the condensate wavefunction together with its spectral and polarization features at the BEC transition in a consistent way. The crucial role of intermolecular energy transfer and “dark” exciton reservoir has been demonstrated for the first time. We believe the principles explored in this work are relevant to other systems exhibiting strong coupling of Frenkel excitons with a cavity mode, regardless of a cavity type, whether one dealing with Fabry-Perot cavities or plasmonic nanocavities etc and; therefore, provide general insight on nonlinear phenomena in composite light-matter condensates. [1] T. Yagafarov, D. Sannikov, A. Zasedatelev, K. Georgiou, A. Baranikov, O. Kyriienko, I. Shelykh, L. Gai, Z. Shen, D. G. Lidzey, P. Lagoudakis, Mechanisms of blueshifts in organic polariton condensates, Commun Phys 3, 18 (2020). https://doi.org/10.1038/s42005-019-0278-6