Collective Rayleigh Scattering from Molecular Ensembles under Strong Coupling

Abstract

Rayleigh scattering is usually considered to be the elastic scattering of photons from subwavelength physical objects, such as small particles or molecules. Here, we present the spectroscopic study of the scattering properties of molecules embedded in an optical cavity under strong coupling conditions, where the collective interaction between the molecules and the cavity gives rise to composite light-matter excitations known as cavity polaritons. We show that the polaritonic states exhibit strong resonant Rayleigh scattering, reaching ∼ 25% efficiency. Since the polaritonic wavefunctions in such systems are delocalized, our observations correspond to the collective scattering of each photon from a large ensemble of molecules.When quantum emitters are placed inside an optical cavity, their interaction with the quantized electromagnetic mode of the cavity may become large enough to overcome the incoherent processes taking place in the system [1]. This regime, which is known as strong light-matter coupling, has been extensively studied in hybrid molecular-photonic systems over the past decade, as it offers exciting possibilities for controlling the photophysical and chemical properties of molecules [2,3]. The coupling between an ensemble of molecules and an optical resonator is quantified by the vacuum Rabi frequency, which is roughly given byHere d is the transition dipole element of the molecules, is Planck's constant, ω c is the cavity resonance frequency, is the background dielectric constant inside the cavity, V c the cavity mode volume and N is the number of molecules inside the cavity. The √ N dependence of the coupling strength indicates that, under strong coupling conditions, the interaction between the molecules and the optical mode is collective. As a result, the eigenstates of the coupled system, known as cavity polaritons, represent a coherent superposition of a photon and a material excitation which is delocalized across a macroscopically large ensemble of molecules [1,4]. The collective nature of strong coupling in molecular systems is currently attracting considerable attention [5][6][7][8][9][10], as it gives rise to fascinating effects. These include, for instance, long-range spatial coherence [11][12][13], enhanced transport [14-18] and energy transfer [19][20][21], and even collective molecular reactivity [22][23][24][25].The progress in this evolving field is intimately linked to the extensive spectroscopic study of organic strongly coupled systems, which has gradually revealed the properties of the polari- * Corresponding author talschwartz@tau.ac.il

Infrared Open Cavities for Strong Vibrational Coupling

“…Among various promising applications of open cavities are those that exploit their strong interaction with molecular degrees of freedom. Unlike the Fabry–Perot-style photonic cavities, which are transparent only to the light at their corresponding resonance frequencies, SLPR arrays allow for convenient spectroscopic access to molecules deposited on the surface. To date, open cavity applications have mostly been in the visible and near-infrared spectral regions .…”

Virtual Issue on Polaritons in Physical Chemistry

“…7−10 Studies for molecules are usually performed by optical transmission or reflection measurements, although it is also possible to observe coupling between molecular transitions and cavity modes through scattering. 11 Scattering is usually not a strong effect for molecules. However, the polariton states involve a large number of molecules and thus can have large light-scattering cross sections.…”

“…However, the polariton states involve a large number of molecules and thus can have large light-scattering cross sections. 11 Recent research on these systems has focused on experiments that provide information about the dynamics of the polariton states. This is a complex issue, with different types of measurements providing different information.…”

Strong Coupling in a Self-Coupled Terahertz Photonic Crystal

“…We note that here, in contrast to Figure b, we do not detect the free induction decay from the upper polariton. This may reflect some real physical mechanism involved in the generation of the free induction decay signal in such strongly coupled systems, which might also be related to asymmetries observed for spontaneous emission , and elastic scattering in organic strongly coupled systems or simply be a result of the limited signal-to-noise ratio of the measurements.…”