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arXiv:1506.08974

Self-consistent molecular adaptation induced by strong coupling

A Justyna, Peter Kirton, Simone De Liberato, Jonathan Keeling

Published Jun 30, 2015• 6 citations
arXivHTMLPDF

Abstract

Recent observations of temperature and concentration dependence of optical spectra in strongly-coupled organic microcavities [A. Canaguier-Durand et al. Angew. Chem. Int. Ed. 52, 10533 (2013)] have been explained as showing that the fraction of molecules coupled to light vs those uncoupled increases as the Rabi splitting increases, driven by thermodynamics. In this paper, starting from a microscopic model, we investigate the physics of how molecules might self-consistently adapt their coupling to light, considering both rotational and vibrational degrees of freedom. In both cases we do find that the matter-light coupling can lead to a self-consistent change of the molecular states, with consequent temperature-dependent signatures in the absorption spectrum. However, for typical parameters, these effects are much too weak to explain the measurements. We propose instead another mechanism, of vibrational dressing, that has the right magnitude to be at the origin of the observed data.

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Recent observations of temperature and concentration dependence of optical spectra in strongly-coupled organic microcavities [A. Canaguier-Durand et al. Angew. Chem. Int. Ed. 52, 10533 (2013)] have been explained as showing that the fraction of molecules coupled to light vs those uncoupled increases as the Rabi splitting increases, driven by thermodynamics. In this paper, starting from a microscopic model, we investigate the physics of how molecules might self-consistently adapt their coupling to light, considering both rotational and vibrational degrees of freedom. In both cases we do find that the matter-light coupling can lead to a self-consistent change of the molecular states, with consequent temperature-dependent signatures in the absorption spectrum. However, for typical parameters, these effects are much too weak to explain the measurements. We propose instead another mechanism, of vibrational dressing, that has the right magnitude to be at the origin of the observed data.
This dissertation studies the effects of strong matter-light coupling on properties of organic molecules and inorganic semiconductors. The interplay of complex intramolecular dynamics and strong coupling of a photon to molecular transitions results in new physics having no counterparts in other systems. In contrast, low-energy optically active excitations of semiconductors (excitons) usually do not feature such complexity. However, the combination of strong electronic correlations and strong matter-light coupling leads to new physics. Firstly, the effect of strong coupling between molecular vibrations and infrared photons on Raman scattering (RS) is considered. This is motivated by the experiment of Ref. [1] showing up to 10³ enhancement of RS signal under strong coupling. While the exact analytical results of this dissertation predict around 100% enhancement of total RS signal, they cannot explain orders of magnitude enhancement, leaving the question open for further studies. Next, the effects of strong coupling of an optical photon and a molecular electronic transition on molecular lasing properties are discussed. Starting from a microscopic description of a driven-dissipative system, an exact (in the thermodynamic limit) mean-field solution is developed. It allows to uncover the mechanism of molecular lasing in the weak and strong coupling regime and to obtain a non-equilibrium lasing phase diagram. Finally, a semiconductor with different densities of electrons and holes, strongly coupled to a microcavity photon, is studied. While finite electron-hole density imbalance is detrimental for excitonic condensation, it may still lead to a condensed state of excitons with finite centre of mass momentum coexisting with unpaired electrons. On the other hand, due to its low mass, a photon favours zero center of mass momentum condensation. The variational mean-field calculations reveal that the interplay of these effects leads to a variety of novel states with coexisting polariton condensate and unpaired electrons.
Room temperature cavity quantum electrodynamics with molecular materials in optical cavities offers exciting prospects for controlling electronic, nuclear and photonic degrees of freedom for applications in physics, chemistry and materials science. However, achieving strong coupling with molecular ensembles typically requires high molecular densities and substantial electromagnetic field confinement. These conditions usually involve a significant degree of molecular disorder and a highly structured photonic density of states. It remains unclear to what extent these additional complexities modify the usual physical picture of strong coupling developed for atoms and inorganic semiconductors. Using a microscopic quantum description of molecular ensembles in realistic multimode optical resonators, we show the that the emergence of a vacuum Rabi splitting in linear spectroscopy is a necessary but not sufficient metric of coherent admixing between light and matter. In low finesse multi-mode situations we find that molecular dipoles can be partially hybridised with photonic dissipation channels associated with off-resonant cavity modes. These vacuum-induced dissipative processes ultimately limit the extent of light-matter coherence that the system can sustain.
Emission and absorption spectra of molecular films are impacted by low-frequency molecular vibrations. These vibrations define the linewidths of the absorption and emission spectral peaks, as well as the Stokes shift. In cavities that use a molecular film as an active medium, low-frequency molecular vibrations facilitate the thermalization of light, enabling the formation of Bose-Einstein condensation. In this work, I employ perturbation theory for Lindblad superoperators and derive the contribution of the low-frequency molecular vibrations to the thermalization rate of light in a weak coupling regime between light and matter. The derived thermalization rate applies for any cavity design but depends on the local microscopic properties of low-frequency molecular vibrations. I provide an estimation for the thermalization rate, which requires only knowledge of the macroscopic parameters of the system: light-matter interaction strength, resonant frequencies of the cavity and excitons, number of molecules in the illuminated area, and the linewidth temperature dependence of the 0-0 peak in the emission spectra of standalone molecular film.
Room-temperature cavity quantum electrodynamics with molecular materials in optical cavities offers exciting prospects for controlling electronic, nuclear and photonic degrees of freedom for applications in physics, chemistry and materials science. However, achieving strong coupling with molecular ensembles typically requires high molecular densities and substantial electromagnetic-field confinement. These conditions usually involve a significant degree of molecular disorder and a highly structured photonic density of states. It remains unclear to what extent these additional complexities modify the usual physical picture of strong coupling developed for atoms and inorganic semiconductors. Using a microscopic quantum description of molecular ensembles in realistic multimode optical resonators, we show that the emergence of vacuum Rabi splitting in linear spectroscopy is a necessary but not sufficient metric of coherent admixing between light and matter. In low-finesse multi-mode situations, we find that molecular dipoles can be partially hybridized with photonic dissipation channels associated with off-resonant cavity modes. These vacuum-induced dissipative processes ultimately limit the extent of light-matter coherence that the system can sustain.This article is part of the theme issue 'The quantum theory of light'.
Polaritons, mixed light-matter quasiparticles, undergo a transition to a condensed, macroscopically coherent state at low temperatures or high densities. Recent experiments show that coupling light to organic molecules inside a microcavity allows condensation at room temperature. The molecules act as saturable absorbers with transitions dressed by molecular vibrational modes. Motivated by this, we calculate the phase diagram and spectrum of a modified Tavis-Cummings model, describing vibrationally dressed two-level systems, coupled to a cavity mode. Coupling to vibrational modes can induce re-entrance, i.e. a normal-condensed-normal sequence with decreasing temperature and can drive the transition first-order.
The coupling of an isolated quantum state to a continuum is typically associated with decoherence and decreased lifetime. For coupling rates larger than the bandwidth of the associated continuum, decoherence can be mitigated, and new stable eigenstates emerge. Here, we laser-couple diatomic molecules of highly excited Rydberg atoms, so-called Rydberg macrodimers, to a continuum of free motional states. Enabled by their small vibrational eigenfrequencies, we achieve the regime of strong continuum couplings and observe the appearance of new resonances. We explain the observed spectroscopic features as molecular states emerging in the presence of the light field using a Fano model. For atoms arranged on a lattice, we predict the strong continuum coupling to even stabilize triatomic molecules and find the first signatures of these by observing three-atom loss correlations using quantum gas microscopy. Our results present a mechanism to control decoherence and bind polyatomic molecules using strong light-matter interactions. Published by the American Physical Society 2024