Mechanisms of blueshifts in organic polariton condensates

Organic polaritonics, utilizing Frenkel excitons in organic microcavities, has shown promise for room-temperature polariton condensates and applications like polariton transistors. However, the mechanisms behind polariton nonlinearities, particularly the blueshift observed at condensation thresholds across diverse organic materials, remain unclear. Unlike inorganic systems with Wannier-Mott excitons and strong Coulomb exchange interactions, the localized nature of Frenkel excitons significantly weakens these interactions, necessitating a different explanation for the blueshift phenomenon. This study aims to investigate the underlying mechanisms responsible for this energy blueshift in organic polariton condensates, bridging the gap in understanding the nonlinear dynamics of these promising materials.

Previous research on inorganic semiconductor microcavities has established that the blueshift of polariton modes is a key indicator of polariton interactions, primarily attributed to repulsive Coulomb exchange interactions between Wannier-Mott excitons. However, the highly localized nature of Frenkel excitons in organic semiconductors drastically reduces the significance of these Coulomb interactions. While room-temperature polariton condensation and lasing have been demonstrated in organic microcavities using various molecular materials, the origin of the ubiquitous step-like energy shift observed at condensation thresholds remains an open question. Existing studies have hinted at potential contributions from various nonlinear optical phenomena, but a comprehensive understanding is still lacking.

The researchers utilized organic microcavities consisting of a BODIPY-G1 dye dispersed in a polystyrene matrix sandwiched between two distributed Bragg reflectors (DBRs). Angle-resolved reflectivity measurements were performed to characterize the strong coupling regime and determine parameters like vacuum Rabi splitting and exciton-photon detuning. Non-resonant optical excitation using 2 ps pulses at 400 nm was employed to investigate polariton condensation. Time-integrated polariton photoluminescence was analyzed as a function of excitation density to observe the blueshift and linewidth narrowing at the condensation threshold. To investigate potential mechanisms, the authors performed open- and closed-aperture Z-scan techniques to measure the optical nonlinearities of the bare intracavity medium. Amplified spontaneous emission (ASE) measurements were also conducted to characterize the spectral distribution of optical gain. Finally, a theoretical model incorporating saturation of molecular optical transitions, intermolecular energy transfer, and coupled rate equations was developed to explain the observed blueshifts and step-like behavior.

The study found that the blueshift in organic polariton condensates is not primarily due to intracavity optical Kerr effect or gain-induced frequency pulling. Measurements showed that the observed blueshift exhibits sub-linear dependence on the square of the exciton fraction, ruling out dominant contributions from pair-polariton or polariton-exciton scattering. The researchers demonstrated that the blueshift originates from the interplay of two effects stemming from the saturation of molecular optical transitions: (1) quenching of the Rabi splitting due to Pauli blocking in strongly coupled molecules; and (2) renormalization of the cavity mode energy due to changes in the effective refractive index caused by the saturation of weakly coupled molecules. A crucial observation was the step-like increase in the blueshift at the condensation threshold. This step-like behavior, along with the corresponding increase in the degree of linear polarization, was explained by a model incorporating intermolecular energy transfer. Below the threshold, energy transfer leads to depolarization. Above the threshold, stimulated relaxation to the ground polariton state becomes dominant, leading to a higher degree of linear polarization and a step-like blueshift. A coupled rate equation model quantitatively captures the observed dependence of emission intensity, energy shift, and polarization on excitation density. Further experiments with varying dye concentrations confirmed the model's predictions, highlighting the importance of both strongly and weakly coupled molecules in the blueshift phenomenon. Specifically, a sample with lower dye concentration showed a larger blueshift due to stronger saturation effects. The ratio between the contributions of the cavity mode energy renormalization to the overall polariton blueshift, and the Rabi quenching, remains virtually constant across the whole range of exciton-photon detuning, being close to unity.

This study provides a comprehensive explanation for the blueshifts observed in organic polariton condensates, highlighting the significance of saturation of molecular optical transitions and intermolecular energy transfer. The findings demonstrate that the mechanisms underlying these blueshifts differ significantly from those in inorganic systems. The model presented successfully accounts for both the magnitude and the step-like behavior of the blueshift at the condensation threshold, unifying seemingly disparate observations. The results contribute significantly to the understanding of nonlinear dynamics in organic polariton systems and have implications for the design and optimization of organic-based polaritonic devices.

The research definitively shows that blueshifts in organic polariton condensates originate from the interplay of saturation of molecular optical transitions (leading to Rabi splitting quenching and cavity mode energy renormalization) and intermolecular energy transfer. The step-like increase in blueshift and linear polarization at the condensation threshold is a direct consequence of the competition between stimulated relaxation and energy transfer. This comprehensive model advances our understanding of organic polaritonics and offers crucial insights for future device development. Further investigations could explore the influence of molecular structure and intermolecular interactions on the energy transfer dynamics and blueshift magnitude.

The model relies on certain approximations, such as the small saturation parameter assumption. The precise values of some parameters, such as the intermolecular energy transfer rates, may be subject to some uncertainty. The study focused on a specific type of organic microcavity and dye; further research is needed to determine the generality of the findings across different organic materials and cavity designs. The experimental observation of blueshifts at the onset of nonlinear emission does not provide a sufficient condition for distinguishing between polariton condensation and lasing in the weak-coupling regime.