Hybridization of molecules via a common photonic mode

Abstract

Atoms and molecules usually hybridize and form bonds when they come in very close proximity of each other. In this work, we show that molecules can hybridize even through far-field electromagnetic interactions mediated by the shared mode of an optical microcavity. We discuss a collective enhancement of the vacuum Rabi splitting and study super- and subradiant states that arise from the cavity-mediated coupling both in the resonant and dispersive regimes. Moreover, we demonstrate a two-photon transition that emerges between the ground and excited states of the new optical compound. Our experimental data are in excellent agreement with the predictions of the Tavis-Cummings Hamiltonian and open the door to the realization of hybrid light-matter materials.

Keywords: cavity quantum electrodynamics; hybridization; polaritonic states; strong coupling.

Fig. 1.

Experimental scheme. (A) A continuous-wave (CW) spectroscopy laser beam is coupled into the cryogenic microcavity. The transmitted laser light is detected with a photon-counting avalanche photodiode (APD). On the reflection side, we also have the possibility of detecting the red-shifted fluorescence photons with a second APD. During the measurement, a CW locking laser beam at λ = 700 nm is used to monitor cavity length changes. Asph: aspheric lens, DM: dichroic mirror, BP: bandpass filter, BS: nonpolarizing beam splitter, PBS: polarizing beam splitter, PM: planar mirror, QWP: quarter-wave plate, HWP: half-wave plate, and BPD: balanced photodiode. (B) Close-up view of the microcavity arrangement: an anthracene (AC) crystal doped with dibenzoterrylene (DBT) is placed between a flat and a curved mirror at the end of a pedestal. The curved micromirror (μM) has a radius of curvature of 10 μm. Inset Left: front view of a pedestal with six microfabricated curved mirrors; one is chosen for experimentation. Inset Right: the molecular structures of DBT and AC. (C) Schematics of the energy levels of a DBT molecule, indicating the vibrational manifolds of the ground and excited states.

Fig. 2.

Cavity transmission when coupled to two molecules. Columns and rows group the measurements according to the frequency detuning between the two molecules and the detuning of the cavity resonance, respectively; see legends on Top and on the Right-hand side. Blue symbols show experimental data while the green curves represent the results of theoretical fits. The pink symbols and the red curve illustrate the measured spectrum of the bare cavity and a Lorentzian fit with a linewidth of 3.46 GHz, respectively. Coupling strengths were $g_1/2\pi =0.82\pm 0.01$ GHz and $g_2/2\pi =0.6\pm 0.02$ GHz. Three distinct detuning values between the molecules are presented: (A) ${\delta }_{12}/2\pi =0.91\pm 0.01$ GHz, (B) $0.24\pm 0.01$ GHz, and (C) $0.05\pm 0.04$ GHz. (D–I), show the cavity tuned to the red and blue sides, respectively. In each case, the cavity resonance is set at the origin of the horizontal axis. Red dashed curves display the extrapolated empty cavity transmission spectrum.

Fig. 3.

Two-photon transition. (A) Off-resonance transmission spectrum of the cavity in the presence of two molecules that are dispersively coupled to it. The vertical axis is expressed in the units of mega counts per second (Mcps). The x-axis is shared with (B). The origin marks the cavity frequency. (B) A series of red-shifted fluorescence spectra recorded from the cavity at increasing laser powers. At higher laser powers, a two-photon peak appears in the Middle of the sub- and superradiant states. The excitation powers correspond to 0.1, 0.9, 4, 80, 190, 381 photons per cavity life time, in the increasing order, respectively. Inset shows the energy diagram of the coupled system and the two-photon transition (red arrows). Here, the parameters were cavity linewidth $\kappa /2\pi =1.58$ GHz, coupling strengths $g_1/2\pi =0.7$ GHz and $g_2/2\pi =0.72$ GHz.

Fig. 4.

Coupling many molecules. (A) Two molecular pairs coupled to the cavity mode. (B) Same as (A) but with smaller frequency differences within each pair. The following parameters apply: coupling strengths $g_1/2\pi =0.5$ GHz, $g_2/2\pi =0.73$ GHz, $g_3/2\pi =0.82$ GHz, $g_4/2\pi =0.6$ GHz, and cavity linewidth $\kappa /2\pi =3.46$ GHz. (C–H) Snap shots of the coupled system in (B) for different cavity frequency detunings. (I) Evolution of the five resonance frequencies in (C–H). Red and gray data points represent subradiant states. Inset shows a close-up of the region around the origin, set to the resonance of the cavity with the blue-detuned pair. (J) Same as (I) but for resonance linewidths. Inset shows a close-up of the region around −3 GHz. (K) Symbols show the cavity transmission spectrum resulting from the coupling of eight molecules. The dashed curve shows the bare cavity resonance for comparison, placed at the origin with a linewidth of $\kappa /2\pi =1.4$ GHz. The solid curve presents the theoretical fit, which allows us to extract $g_i/2\pi =0.5,0.25,0.25,0.1,0.3$ GHz for five of the molecules. (L) Calculated excited-state populations of the eight molecules.