Dissecting Exciton-Polariton Transport in Organic Molecular Crystals: Emerging Conductivity Assisted by Intermolecular Vibrational Coupling

TL;DR

The paper investigates exciton-polariton transport in a 1D organic molecular crystal inside a single-mode cavity, focusing on dynamic disorder from intermolecular vibrations modeled by SSH-type electron-phonon coupling. It employs a linear-response Kubo approach, decomposing conductivity into a molecular channel and a cavity-mediated channel, and uses quasi-classical dynamics to compute the spectral function $A(k,\omega)$ and frequency-domain current correlations. Key findings show that polariton formation can suppress bare molecular mobility $\mu_{\text{mol}}$ while enhancing cavity-assisted mobility $\mu_{\text{cav}}$, with the latter amplified by stronger dynamic disorder $\alpha$ via long-range hopping and vibration-assisted scattering; the optical conductivity exhibits characteristic polariton-sideband peaks that shift with the light–matter coupling $g$ and broaden with $\alpha$. Overall, the work demonstrates a tunable transport landscape in organic crystals by engineering light–matter coupling and vibrational dynamics, offering design principles for cavity-modified optoelectronic materials.

Abstract

In this work, we systematically investigate the spectral and transport properties of exciton-polaritons under the explicit influence of intermolecular vibrational coupling, which introduces dynamic disorder. In the context of a one-dimensional molecular chain strongly interacting with a cavity photon, we demonstrate the polaritonic characteristics of the spectral function and its interactions with the electronic band broadened by the coupling disorder. We further dissect the current flux into its bare excitonic contribution and transport via the cavity photon. Our results reveal that the enhancement in the charge carrier mobility and frequency-resolved conductivity stems from the photon-mediated current. More importantly, contrary to the intuition that dynamic disorder hinders transport, intermolecular vibrational coupling can facilitate exciton-polariton transport, offering an additional degree of tunability for material properties.

Figures (7)

• Figure 1: (a) Schematic illustration of a 1D molecular chain inside an optical cavity. $u_n$ denotes the displacement associated with the lattice distortion of site $n$. The exciton transport between the nearest-neighboring molecules is modeled by the bare transfer integral $\tau$ and the Peierls-type electron-phonon coupling $\alpha(u_{m}-u_{n})$. $L$ is the lattice constant. The cavity mode is coupled to the molecular chain with a constant coupling strength $g$, and the decay rate is $\Gamma_c$. (b) The corresponding single excitation matrix in the basis state $|X_n 0\rangle$ for $n=1,\cdots$ and $|G \alpha\rangle$. The orange blocks denote $N$ exciton states with energy $\varepsilon_x$ (diagonal) and the nearest neighbor coupling $-\tau$ (off-diagonal). The cavity photon of frequency $\omega_c$ (red) is coupled to each exciton with a constant coupling strength $g$ (cyan).
• Figure 2: Momentum-resolved spectral function $A(k,\omega)$ with varying electron-phonon coupling strength $\alpha = 498,995,1407$ (in the unit of $\text{cm}^{-1}/$Å), and light-matter coupling strength $g = 30, 90, 150$ (in the unit of $\text{cm}^{-1}$). For all parameters, we observe the upper and lower polariton peaks ($\omega=E_\pm$) at $k=0$ and a broadened version of the electronic band for $k>0$. The dashed black line indicates the bare electronic band. While the electronic band is not affected by the cavity phonon (except at $k=0$), the polariton peaks are lowered by dynamic disorder and broadened in the $k$ dimension, especially when $g$ is small.
• Figure 3: The spectral functions $A(k=0,\omega)$ for (a) $g=30$, (b) $g=90$, and (c) $g=150$ (in the unit of $\text{cm}^{-1}$) exhibit the UP/LP peaks with decreasing heights as $\alpha$ increases. The UP peaks are suppressed more than the LP in the presence of dynamic disorder. In the lower panels, $A(k=0.02\pi/L,\omega)$ for (d) $g=30$, (e) $g=90$, and (f) $g=150$ (in the unit of $\text{cm}^{-1}$) shows three peaks corresponding to the electronic band (middle) and the UP and LP side peaks (right and left). The polariton side peaks appear only when electron-phonon coupling is present, and can be suppressed as $\alpha$ increases.
• Figure 4: Charge carrier mobilities, $\mu_\text{mol}$ (solid line), $\mu_\text{cav}$ (dotted line) and its LP contribution $\mu_\text{cav}(\text{LP})$ (dashed line), are plotted as a function of light-matter coupling strength $g$ for (a) $\alpha = 498$, (b) $\alpha = 995$, and (c) $\alpha = 1407$ (in the unit of $\text{cm}^{-1}/$Å). In general, $\mu_\text{cav}>\mu_\text{mol}$ and $\mu_\text{mol}$ is suppressed as $g$ increases. The difference between $\mu_\text{cav}$ and $\mu_\text{cav}(\text{LP})$ is attributed to the optical dark states, which contribute to the mobility for small $g$. For large $g$, the LP contribution of the mobility is enhanced as $\alpha$ increases, showing vibration-assisted transport.
• Figure 5: Qualitative trend of $C_{JJ}^\text{mol}(\omega)$ and $C_{JJ}^\text{cav}(\omega)$ with increasing $g$ for dynamical disorder $\alpha = 498,\ 995,\ 1407$ (in the unit of $\text{cm}^{-1}/$Å). To show the linear scaling of the polariton side peak position, we shift the data by an offset proportional to $g$ and scale $C_{JJ}^\text{cav}(\omega)$ by $1/g^3$ for clarity. The vertical dotted lines in (a) indicate the conductivity peaks at $\omega/\tau=2,4$. The dashed lines denote the side peaks shifting linearly with $g$.