Comment on "Isolating Polaritonic 2D-IR Transmission Spectra".

This Viewpoint responds to the analysis of 2D IR spectra of vibration cavity polaritons in the study reported in (Duan et al. , 12, 11406). That report analyzed 2D IR spectra of strongly coupled molecules, such as W(CO) and nitroprusside anion, based on subtracting a background signal generated by polariton filtered free space signals. They assigned the resulting response as being due to excited polaritons. We point out in this Viewpoint that virtually all of the response can be properly reproduced using the physics of transmission through an etalon containing a material modeled with a complex dielectric function describing the ground- and excited-state absorber populations. Furthermore, such a coupled system cannot be described as a scaled sum of the bare molecular and cavity responses.