Ultrafast thermal modification of strong coupling in an organic microcavity

Abstract

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.

Untargeted effects in organic exciton–polariton transient spectroscopy: A cautionary tale

“…For instance, within the twotemperature model of Liu et al, the <10 ps dynamics in microcavity transient reflectivity could be predominantly described by carrier thermalization. 63 Similar dynamics are reported for hot carrier cooling in other photonic structures. 78,79 Ground-state bleaching should roughly follow the dynamics of the bare film, resulting in persistent signatures on the order of nanoseconds.…”

“…78,79 Here, the dominant effect is likely pumpinduced changes to the refractive index of the metallic mirrors, similar to suggestions using TIPSpentacene microcavities. 63 Similar effects can be obtained within organic films, as reported for instance from a PMMA waveguide mode not coupled to any electronic excitations. 80 We further highlight the pronounced growth kinetics observed in the PIA at 775 nm.…”

“…80 We further highlight the pronounced growth kinetics observed in the PIA at 775 nm. These may potentially be related to the coherent phonon oscillations within the Ag mirror, 63 but we additionally note that photoexcitation has been reported to create localized strain in thin films, resulting in propagating strain waves that manifest as time-domain oscillations in the transmission signal. [81][82][83][84] These effects can be difficult to detect when superimposed with bulk electronic signatures.…”

“…Instead, we can describe the critical characteristics of these features using a simple model that only considers polariton contraction, thermal expansion, and bulk refractive index changes. Moving beyond structures based on metallic mirrors, 62,63 we find that these optical effects do not require direct photoinduced carrier heating within the mirror and can be similarly prominent in dielectric structures. Moreover, the richer spectrum of dielectric microcavities provides numerous additional signatures of non-specific photoinduced effects.…”

“…While clearly identified in vibro-polariton studies and implicated in the blueshifts of organic polariton condensates, 61 it has been considered and explicitly discarded in the transient spectroscopy of exciton-polaritons. 52,62 Recent work by Liu et al on exciton-polaritons suggests that other effects-the modulation of molecular excited-state absorption spectra by the cavity structure, 62 and carrier heating/thermalization dynamics within metal mirrors 63 -should likewise be taken into account.…”