The concept of modifying molecular dynamics in strongly coupled exciton-polariton systems is an emerging topic in photonics due to its potential to produce customized chemical systems with tailored photophysical properties. However, before such systems can be realized, it is essential to address the open questions concerning the nature and strength of electronic interactions between exciton-polaritons and localized excited states in chemical system as well as the proper way to measure such interactions. Here, we use transient optical spectroscopy to investigate dynamical interactions between exciton-polaritons, singlet excitons, and triplet excitons in a molecular singlet fission system that is strongly coupled to an optical microcavity.We identify some of the major limitations to modify molecular dynamics in the strong coupling regime. Simultaneous excitation of cavity polaritons and "reservoir" states, defined as dark polaritons and dark excitons (e.g. triplets) from coupled molecules and excitons from uncoupled molecules, always occurs. In addition, slow conversion from reservoir states to cavity polaritons results in minimal changes to the overall population dynamics. Furthermore, we demonstrate how in addition to the usual population dynamics, transient optical measurements on microcavities reveal information pertaining to modification of the exciton-polariton transition energies due to changes in the population of molecular excited states and the exciton-photon coupling conditions.As a consequence of weak interactions between reservoir states and cavity polaritons, judicious design considerations are required to achieve modified chemical dynamics, necessitating the use 2 of molecular systems with long excited-state lifetimes or strong coupling approaches that require a small number of molecules.

exchange of energy between excitons and photons is faster than any loss process. The momentum-energy dispersions of cavity polaritons are characterized by anticrossing of the exciton and cavity dispersions consisting of an upper-polariton (UP) branch (UPB) and a lower-polariton (LP) branch (LPB), with the minimal energy separation between UPB and LPB at the anticrossing point referred as the vacuum Rabi splitting energy (ℏΩ R ). [1] Due to the large oscillator strength and binding energy of organic materials, organic cavity polaritons exhibit large ℏΩ R even at room temperature. [2][3][4] When ℏΩ R is comparable (20%) to the uncoupled exciton energy (E Ex ), the system is characterized by the ultrastrong coupling (USC) regime, where the rotating wave approximation (RWA) is no longer applicable, and the antiresonant exciton-photon coupling terms can significantly modify cavity-polariton properties. [5][6][7][8] The hybrid light-matter characteristics of both single and multiple cavity polaritons are of significant theoretical and practical importance, such as organic light-emitting diodes (LEDs), [9,10] roomtemperature polariton condensation and lasing, [11][12][13] polaritonmediated energy transfer, [14][15][16] conductivity enhancement, [17,18] and superfluidity. [19] However, nonlinear optical processes, such as the optical Kerr effect and optical harmonic generation, arising from cavity polaritons in organic materials are now beginning to receive attention. Importantly, optical harmonic spectral dispersion measurements, which are widely used to study the excited states of organic molecular materials, [20,21] can be utilized to study the intrinsic nonlinearity of the hybrid light-matter system. Recently, the enhancement of resonant second-harmonic generation (SHG) of the lower polariton is reported from strongly coupled organic crystalline nanofiber microcavities, where the SHG wavelength is resonant with the lower polariton. [22] Also, the enhanced third-harmonic generation (THG) was observed from cavity polaritons in the ultrastrong coupling regime when the pump wavelength is resonant with the lower polariton. [23] Here, we experimentally and theoretically demonstrate the dispersion of the upper-polariton-enhanced THG using anglevarying reflection configuration in the ultrastrong coupling regime, and observe the THG intensity is largest when the THG wavelength is resonant with the upper polariton, in contrast to Organic cavity polaritons are bosonic quasiparticles arising from the strong interaction between organic molecular excitons and photons within microcavities. The spectral dispersion of third-harmonic generation near resonance with cavity polaritons is studied experimentally via angle-resolved reflected third-harmonic generation measurements with several pump wavelengths. Moreover, a three-step nonlinear optical transfer matrix model is used to simulate the third-harmonic generation using the sum-overstates dispersive nonlinear coefficients, which include polariton states. The angle-dependent experimen...

We demonstrate brightening of dark molecular isomeric states via strong coupling in optical microcavities.

There is growing interest in using strongly coupled organic microcavities to tune molecular dynamics, including the electronic and vibrational properties of molecules. However, very little attention has been paid to the utility of cavity polaritons as sensors for out-of-equilibrium phenomena, including thermal excitations. Here, we demonstrate that non-resonant infrared excitation of an organic microcavity system induces a transient response in the visible spectral range near the cavity polariton resonances. We show how these optical response can be understood in terms of ultrafast heating of electrons in the metal cavity mirror, which modifies the effective refractive index and subsequently the strong coupling conditions. The temporal dynamics of the microcavity are strictly determined by carriers in the metal, including the cooling of electrons via electron-phonon coupling and excitation of propagating coherent acoustic modes in the lattice. We rule out multiphoton excitation processes and verify that no real polariton population exists despite their strong transient features. These results suggest the promise of cavity polaritons as sensitive probes of non-equilibrium phenomena.