Fig. 3: Structural and optical characterisation.

a Tilted cross-sectional Scanning Electron Microscopy images showing a tunable increase of the thickness of the modified volume with the laser power, enabling three-dimensional nanopatterning capabilities. The YIG surface appears smooth, while the sidewall roughness in the modified volume suggests strain relaxation at the edge. Scale bar: 200 nm. b Experimental data measured from images in panel (a) and simulated values (see Supplementary Fig. 2) of the modified depth as a function of the laser power. The plot reveals a nearly linear relation which allows for continuous depth-control. c Optical images of the irradiated areas, showing no sizeable optical contrast within the patterns at all powers. Scale bar: 25 μm d Spectral reflectance as a function of the irradiation power. In the YIG transparency window above λ = 450 nm, no sizable difference is observed between the irradiated and pristine regions. The oscillatory behaviour due to thin film interference is consistent with a 1 μm-thick film. Below 450 nanometres, in the YIG absorption band, the regions irradiated above P_th display lower reflectivity, indicating an enhanced optical absorption upon irradiation, suggesting the creation of oxygen vacancies. The 405 nm UV writing laser (pink) is efficiently absorbed by YIG. e–g Raman spectra of the YIG film as a function of the irradiation power, and corresponding zoomed in view of the A_1g (f, g) peak regions. The characteristic modes of single crystal YIG are present at all powers, with a minor left-side broadening and red-shift especially of the A_1g peaks, above the threshold power, indicating a strained system.