Ultrastrong coupling of vibrationally dressed organic Frenkel excitons with Bloch surface waves in a one-sided all-dielectric structure

We demonstrate a transition from weak, to strong, to ultrastrong coupling of Frenkel molecular excitons and Bloch surface wave photons at room temperature using a one-sided, all-dielectric optical structure. The all-dielectric structure comprises an organic semiconductor thin film on the surface of a distributed Bragg reflector. We investigate the evolution of multiple vibronic polariton branches and their dominant absorption peaks as a function of coupling and in-plane momentum, which is absent in previous ultrastrong coupling systems. Measurements are interpreted using both the transfer matrix method and a coupled-oscillator model without the rotating wave approximation. The dependence of Rabi splitting on the number of excitons and electrical field amplitude is also modeled showing a transition to ultrastrong coupling at film thicknesses $\ge 50$ nm. This low-loss polaritonic structure enables us to study phenomena such as organic exciton-polariton dynamics, ultralong range polariton propagation, and high efficiency energy transport in the ultrastrong coupling regime at room temperature.

Figure 1

(a) Absorption spectrum of DBP. Absorption peaks of the 0-0, 0–1, and 0–2 vibronic transitions are indicated by vertical dashed lines. Inset: DBP molecular structure. (b) Schematic of the one-sided optical cavity capped by DBP. The right-angle prism is used for optical coupling to the DBR capped by a $d$-nm-thick DBP organic layer. The measurement angle is θ. (c). Simulated transverse electric (TE) optical field intensity profile of the Bloch surface wave (BSW) with a 30 nm DBP layer. (d) Calculated dispersion relation of the BSW mode without ($d=0$ nm) and with a 50-nm-thick DBP layer. Open circles show the measured dispersion of the bare cavity.

Figure 2

Simulated TE-polarized reflectivity spectra of a (a) $d=30$-nm-thick DBP layer on a DBR at incidence angles of $41.7^{\circ }$, $44.3^{\circ }$, $45.7^{\circ }$, $47.7^{\circ }$, $50.3^{\circ }$, $51.6^{\circ }$, and (b) $d=50$-nm-thick DBP layer on a DBR at incidence angles of $44.3^{\circ }$, $45.7^{\circ }$, ${47}^{\circ }$, $47.6^{\circ }$, $48.3^{\circ }$, ${49}^{\circ }$, $49.6^{\circ }$, $50.9^{\circ }$ (curves from bottom to top). Measurements corresponding to (a) and (b) are shown in (c) and (d), respectively. Red dashed lines provide guides for the polariton branches; vertical black dashed lines indicate the uncoupled exciton energies. Four polariton branches are observed from low to high energy: the lower (LP), first middle (MP1), second middle (MP2), and upper polariton branch (UP).

Figure 3

Dispersion relations of BSW-exciton polaritons in the ultrastrong-coupling regime ($d=50$ nm). (a) Least-squares fit (solid lines) to data (circles) of the angle-dependent reflectivity minima. The fits assume three coupled oscillators with coupling strengths of $\hslash g_1=205$ meV, $\hslash g_2=177$ meV, and $\hslash g_3=136$ meV. Colored horizontal dashed lines are guides for the uncoupled 0-0, 0–1, 0–2 exciton energies; black dashed line: calculated dispersion of the BSW photon (Ph). (b) Hopfield coefficients of LP: photon (black line) and three excitons (colored lines). (c) Virtual photon (black) and exciton (colored) content of the polariton ground state (GS).

Figure 4

(a) Vacuum Rabi splitting energy vs DBP thickness. Dots are the extracted vacuum Rabi splitting energies ($\hslash {\mathrm{\Omega }}_1$). The dashed line is a fit to the constant field approximation. The solid line accounts for field attenuation, with fitting parameters $A^{\prime }=4.45\pm 0.50\mathrm{eV}$, $l=53\pm 8\mathrm{nm}$, and $d_0=2.9\pm 0.8\mathrm{nm}$. (b) Simulated reflectivity of samples with $d=2$, 5, 10, 15, 25, 30, 40, and 50 nm (curves from bottom to top) at angles corresponding to $|{\omega }_{cav}-{\omega }_{ex1}|=0$. The blue dashed line is a guide for the evolution of low-energy absorption peak, and red dashed lines are guides for the high-energy absorption peaks as coupling strength increases.