*4.3. Substrates*

We use mostly used glass substrates for high-throughput coating campaigns and switched to silicon substrates for measuring the optical properties of the coatings on a high-index substrate (BT Electronics, one-sided polished 1 mm thick and two-sided polished 0.345 mm thick).

## *4.4. Optical Methods*

Optical performance was measured as follows: Spectroscopic ellipsometry permitted us to extract the refractive index of the coatings; reflectance in the visible-to-near IR range leads to the reflection coefficient evidencing anti-reflection in some cases; mid-IR provided us with the transmittance of the samples.

The variable angles spectroscopic ellipsometer we used was a HORIBA Jobin Yvon UVISEL working here in the 250–1300 nm range and we analysed eight angles in the range 55–75◦. We worked only on the thinnest grade of nanoparticles (4 nm) as the material made out of the largest ones scatters too much light.

The reflection measurements were performed at 'normal' incidence with basic bench-top equipment from Avantes. The sample was illuminated via the same co-axial fibre, from which the light emerging from the sample is also collected (Avantes reference FCR-7UVIR400-2-BX/ME). Neither the illumination nor the collection are actually at perfect normal incidence: The illumination fibre is not collimated (numerical aperture 0.22). The lamp shines at the spectral range of 200–2500 nm, but due to the two detectors we use, there is an unfortunate detection gap around 800–1000 nm.

Transmission in the mid-IR range was measured on a specific system, namely fast infrared imaging spectroscopy (FIIST [33,34]), which permits us to image the optical transmission in the range of 2–6 μm.

#### **5. Appendix: Optical Performances**
