Silicon suboxide (SiOx, x ≈ 1) is a substoichiometric silicon oxide with a large refractive index and optical absorption coefficient that oxidizes to silica (SiO2) by annealing in air at ~1000 °C. We demonstrate that nanostructures with a groove period of 200–330 nm can be formed in air on a silicon suboxide film with 800 nm, 100 fs, and 10 Hz laser pulses at a fluence an order of magnitude lower than that needed for glass materials such as fused silica and borosilicate glass. Experimental results show that high-density electrons can be produced with low-fluence femtosecond laser pulses, and plasmonic near-fields are subsequently excited to create nanostructures on the surface because silicon suboxide has a larger optical absorption coefficient than glass. Calculations using a model target reproduce the observed groove periods well and explain the mechanism of the nanostructure formation.

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It has been reported that periodic nanostructures with a period size of 200–330 nm can be formed on silicon suboxide (SiO x , x ≈ 1) with 800-nm, 100-fs laser pulses at a fluence much smaller than that needed for nanostructuring on glasses such as fused silica and borosilicate glass. We demonstrated that a homogeneous SiO2 nanostructure with a period of ∼240 nm can be produced using a two-step ablation process and heat treatment in air at 1000°C for 144 hours. Optical microscopic images of the nanostructured surface illuminated by non-polarized visible light show that the transmittance increases as the reflectivity decreases.

Freeform optics enable improved optical solutions but their fabrication usually requires complicated precision machining processes. We report on an approach for freeform shaping of optical surfaces via a stress-induced viscous deformation of glass plates. We studied the deformation of fused silica substrates covered by specifically laser patterned films of substoichiometric silicon oxide during annealing at about 1100 °C in an oxidizing ambient. The obtained large deformation of the substrates can be understood by a mostly viscous deformation but can be described in analogy to a purely elastic deformation. Our results demonstrate the feasibility of a method for freeform shaping of individual optical substrates that only requires the preparation of a flat surface.

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