Photonic effects in visible-pump infrared-probe vibrational spectroscopies on photovoltaic thin films

Visible-pump infrared-probe vibrational spectroscopy, commonly known as time-resolved infrared (TRIR) spectroscopy, has been widely applied to molecular and hybrid photovoltaic films to probe the interactions between photoinduced carriers and molecular constituents, most notably in lead halide perovskites over the past decade. Several of these studies have reported unexpected observations, including an apparent gigantic enhancement of vibrational transition dipole moments and Fano-like line shapes. Here, we demonstrate that many of these features can be interpreted as signals of purely photonic origin. A spectrally broad carrier response modulates the transmission within the multilayer sample, producing vibrational features in the transient transmission spectrum (-ΔT/T) even in the complete absence of any photoinduced change in the molecular vibrational response. We show that this carrier-induced photonic resonance (CIPR) can mimic excited-state absorption, blue shift, and ground-state bleach of molecular vibrations, depending on the complex dielectric function of the carrier-induced background. We further examine the role of CIPR in recently demonstrated ultrafast nano-FTIR spectroscopy of a perovskite thin film, focusing on how the nature of the tip-sample near-field interaction governs its contribution. The photonic contributions revealed here are essential for the accurate interpretation of past and future TRIR studies of photovoltaic thin films.