Strong Exciton-Photon Coupling in an Organic Single Crystal Microcavity

We demonstrate strong exciton-photon coupling of Frenkel excitons at room temperature in a microcavity composed of a melt grown thin film anthracene single crystal and two distributed Bragg reflectors. Angle-resolved reflectivity and normal incidence photoluminescence under weak excitation are observed. The microcavity spectrum is a function of the anisotropy of the crystalline material and the strong exciton-photon coupling of the excitonic resonances to the cavity photon. The photoluminescence spectrum is found to be completely polarized along the crystal axes.

Figure 1

Right: Inverse transmission (1-T) spectrum of a 120 nm-thick anthracene single crystal channel for electric field polarizations along the $\mathbf{a}$ and $\mathbf{b}$ crystal axes. Because of the large oscillator strength of the first singlet transition in anthracene, there is significant reflection at resonance, and the transmission cannot be taken as a direct measure of the absorption. Left: Photoluminescence spectrum of a 120 nm-thick anthracene single crystal (uncorrected for self-absorption). The vibronic replicas mirror the absorption spectrum, with the highest energy peak distorted by self-absorption. Right inset: Schematic of the fabricated microcavity structure. Left insets: Phase contrast micrographs of the channel area from the completed structure (top) and a thicker area far away from the gold channels where multiple domains can be observed (bottom). The scale bar is 1 mm.

Figure 2

The angle-dependent $p$-polarized reflectivity spectrum of a microcavity containing a 140 nm-thick anthracene crystal. The spectra are taken in 5° increments. The sample is oriented such that (a) the polarization and propagation direction is along $\mathbf{a}$ and (b) the polarization and propagation direction is along $\mathbf{b}$. The lower polariton (LP) and middle polaritons (MP1 and MP2) are indicated by arrows. The gray lines indicate the energies of the bare resonances observed in the absence of a cavity.

Figure 3

Left: Dip positions extracted from $p$- (solid squares) and $s$-polarized (solid diamonds) reflectivity. The position of the DBR side band for $p$ polarization is also indicated (open squares). The dashed lines are a fit to a 4-body coupled harmonic oscillator Hamiltonian. Right: Photonic and excitonic branch content for each polariton branch ($p$ polarization). The data are shown for the sample oriented such that (a) the $p$ polarization is along $\mathbf{a}$ and (b) the $p$ polarization is along $\mathbf{b}$.

Figure 4

Bottom: Normal incidence photoluminescence spectrum taken for three polarizer orientations. The excitation wavelength was $\lambda =325\mathrm{nm}$. Top: 20° reflectivity spectrum with the sample aligned such that $\mathbf{a}\parallel \mathbf{E}$ for $p$ polarization. The reflectivity is shown for 3 different polarizer orientations. In both cases, two LP branch peaks appear off-axis corresponding to both excitonic components.