A versatile tunable microcavity for investigation of light-matter interaction

Piezoelectric and microfluidic tuning of an infrared cavity for vibrational polariton studies

We developed a microfluidic system for vibrational polariton studies, which consists of two microfluidic chips: one for solution mixing and another for tuning an infrared cavity made of a pair of gold mirrors and a PDMS (polydimethylsiloxane) spacer. We show that the cavity of the system can be accurately tuned with either piezoelectric actuators or microflow-induced pressure to result in resonant coupling between a cavity mode and a variational mode of the solution molecules. Acrylonitrile solutions were chosen to prove the concept of vabriational strong coupling (VSC) of a CN stretching mode with light inside the cavity. We also show that the Rabi splitting energy is linearly proportional to the square root of molecular concentration, thereby proving the relevance and reliability of the system for VSC studies.

Polariton-assisted manipulation of energy relaxation pathways: donor-acceptor role reversal in a tuneable microcavity

Resonant interaction between excitonic transitions of molecules and localized electromagnetic field allows the formation of hybrid light-matter polaritonic states. This hybridization of the light and the matter states has been shown to significantly alter the intrinsic properties of molecular ensembles placed inside the optical cavity. Here, we have observed strong coupling of excitonic transition in a pair of closely located organic dye molecules demonstrating an efficient donor-to-acceptor resonance energy transfer with the mode of a tuneable open-access cavity. Analysing the dependence of the relaxation pathways between energy states in this system on the cavity detuning, we have demonstrated that predominant strong coupling of the cavity photon to the exciton transition in the donor dye molecule can lead not only to an increase in the donor-acceptor energy transfer, but also to an energy shift large enough to cause inversion between the energy states of the acceptor and the mainly donor lower polariton energy state. Furthermore, we have shown that the polariton-assisted donor-acceptor chromophores' role reversal or "carnival effect" not only changes the relative energy levels of the donor-acceptor pair, but also makes it possible to manipulate the energy flow in the systems with resonant dipole-dipole interaction and direct energy transfer from the acceptor to the mainly donor lower polariton state. Our experimental data are the first confirmation of the theoretically predicted possibility of polariton-assisted energy transfer reversal in FRET systems, thus paving the way to new avenues in FRET-imaging, remote-controlled chemistry, and all-optical switching.

Polariton-assisted splitting of broadband emission spectra of strongly coupled organic dye excitons in tunable optical microcavity

Resonance interaction between a localized electromagnetic field and excited states in molecules paves the way to control fundamental properties of a matter. In this study, we encapsulated organic molecules with relatively low unoriented dipole moments in the polymer matrix, placed them in tunable optical microcavity and realized, for the first time, controllable modification of the broad photoluminescence (PL) emission of these molecules in strong coupling regime at room temperature. Notably, while in most previous studies it was reported that the single mode dominates in the PL signal (radiation of the so-called branch of the lower polariton), here we report on the observation of two distinct PL peaks, evolution of which has been followed as the microcavity mode is detuned from the excitonic resonance. A significant Rabi splitting estimated from the modified PL spectra was as large as 225 meV. The developed approach can be used both in fundamental research of resonant light-mater coupling and its practical applications in sensing and development of coherent spontaneous emission sources using a combination of carefully designed microcavity with a wide variety of organic molecules.