Correction to Vibro-Polaritonic IR Emission in the Strong Coupling Regime

T method of fitting the microcavity transmission spectrum to derive the cavity absorption spectrum by means of transfer-matrix analysis was flawed. Consequently, in contrast to our initial conclusions, the polaritonic emission appears thermalized and there is no evidence for polariton emission that is blue-shifted with respect to the polariton absorption. For this reason, the interpretation of the blue-shift as a signature of polariton−polariton interactions is obsolete. Two errors have been made: First, the dielectric function of PMMA was only modeled in the range from 1400 to 2100 cm−1 (original Figure 2(b)). However, by virtue of the Kramers−Kronig relations, the relatively strong absorption lines at lower energies (cf. original Figure 2(a)) influence the refractive index in the vicinity of the strongly coupled CO stretching band. Therefore, the simulated free spectral ranges between the cavity and the polariton modes were incorrect. Second, a small aperture was kept inside the Fourier transform infrared (FTIR) spectrometer transmission compartment for both emission and transmission measurements. As a consequence, only a small area of the prepared cavity was sampled. It was not checked which part of the emitting sample was imaged onto the aperture in the transmission compartment. Therefore, it is likely that different spots of the same sample were measured in transmission and emission, respectively. A film thickness inhomogeneity on the order of 10 nm (the polymer film was about 4 μm thick) can cause a spectral shift of the modes on the order of the initially derived polariton blue-shift (17 and 6 cm−1) . Both errors added up in a way that the transmission spectrum fit yielded a match of emission maxima and reflectivity minima at the first and the third cavity mode, as expected from thermalized emission (cf. original Figure 3(b)). The emission maxima appeared however blue-shifted with respect to the reflection minima (cf. original Figure 3(d)). It should be noted that absorption maxima and reflection minima overlap. In an analogous manner to the procedure described in the original paper, we spin-coated an about 1 μm thick PMMA film on a 0.5 mm thick silicon substrate. We measured the sample transmission with an FTIR spectrometer and fitted it by transfer-matrix analysis while optimizing the model of the dielectric function of PMMA. The silicon substrate was treated as an incoherent layer. Its complex refractive index was taken from the literature and was in good agreement with experimental data. To approximate the dielectric function of PMMA, we started from the Lorentz-oscillator model used in the original paper. Next, we adjusted the imaginary part of the PMMA complex refractive index and finally applied Kramers− Kronig relations to obtain the real part of the refractive index. Figure 1 shows the measured transmission spectrum, the fit with the originally used simplified dielectric function, and the fit with the refined dielectric function.