Dark Vibronic Polaritons and the Spectroscopy of Organic Microcavities

Organic microcavities are photonic nanostructures that strongly confine the electromagnetic field, allowing exotic quantum regimes of light-matter interaction with disordered organic semiconductors. The unambiguous interpretation of the spectra of organic microcavities has been a long-standing challenge due to several competing effects involving electrons, vibrations, and cavity photons. Here we present a theoretical framework that is able to describe the main spectroscopic features of organic microcavities consistently. We introduce a class of light-matter excitations called dark vibronic polaritons, which strongly emit but only weakly absorb light in the same frequency region of the bare electronic transition. A successful comparison with experimental data demonstrates the applicability of our theory. The proposed microscopic understanding of organic microcavities paves the way for the development of optoelectronic devices enhanced by quantum optics.

Figure 1

Spectral signals. (a) Reflection $R$, transmission $T$, and absorption $A=1-R-T$ of an external pump $I_p(\omega )$. Absorption is due to spontaneous emission into bound modes of the microcavity. (b) Leakage photoluminescence $S_{\mathrm{LPL}}(\omega )$ following weak laser pumping at frequency ${\omega }^{\prime }>\omega$. Bound photoluminescence [27] is shown as horizontal wavy arrows.

Figure 2

Model microcavity spectra for $N=20$ emitters. (a) Absorption spectra $A=1-R-T$ (dashed line) and bound mode absorption (solid line), indicating the lower (LP) and upper polariton (UP) peaks. Vertical bars indicate the peak position and dipole oscillator strength. (b) LPL spectra including transitions that project the material onto the absolute ground state (dashed line) or onto states in the ground manifold with up to one quantum of vibration (solid line). The red arrow indicates the highest energy polariton considered. (c) Experimental LPL spectra for an ensemble of cyanine dye J-aggregates in a microcavity, adapted from data provided by Bill Barnes for Fig. 3 in Ref. [14]. The red arrow indicates the laser pump energy. (d) Relative emission intensity at the lower polariton frequency $I_{\mathrm{LP}}$, for a Gaussian polariton population centered at ${\omega }_p$ with standard deviation ${\sigma }_p/{\omega }_v=0.5$. Transitions to the ground state with up to ${\nu }_{\max }$ vibrational quanta are included. We use ${\lambda }^2=1$, $\sqrt{N}\mathrm{\Omega }/{\omega }_v=2.4$, $\kappa /{\omega }_v=0.9$, and $N{\gamma }_e/{\omega }_v=3.0$, where ${\omega }_v$ is the vibrational frequency.