In Figure , we compare the optical transmittance of IHO-films on glass used here as working electrodes to commercially available FTO substrates. It is apparent that the IHO substrates are superior to FTO for all wavelengths above 1000 nm, particularly above 1500 nm. At 2000 nm, the IHO transmittance is still > 80 %, while the FTO transmittance has decreased to 5 %. In the visible range of the spectrum, we observe fluctuations in the transmittance of the IHO substrates, which we attribute to interference effects. This is supported by the finding that the frequency of the fluctuations increases with the IHO-film thickness. While such fluctuations can be problematic for techniques that measure ΔA spectra (such as EMAS), the thickness dependence may be used to fine-tune the fluctuations into less important spectral regions (for details see Figures S1+S2). Figure and 1d display the oxidative and reductive EMAS spectra of an exemplary IHO substrate (see SI for the EMAS results of other IHO substrates). Noticeably, the oxidative spectra are mirrored images of the reductive spectra. Removing electrons from the material leads to less energetic plasmonic oscillations, indicated by the increasing bleach towards higher wavelengths. Overall, we note a weak differential absorbance (ΔA) below 1 mOD for the spectral range from 800 -1900 nm and electrochemical potentials from -1.5 V to + 1.0 V. Similar results are obtained in TBAHFP/MeCN-d 3 (Figure ). In comparison, the FTO substrates exhibit roughly an order of magnitude larger ΔA in the same range, including practically immeasurable conditions above 1800 nm (see Figures S4+S5).