Patterned structures were created by exposing a SiO₂ sol-gel film containing nano-silicon particles to a Clark MXR CPA-2010 fs laser (387 nm wavelength). The refractive index variation of 0.2 is similar to that of polymer films, however belonging to an entirely superior stability class (structural, chemical, thermal, radiation, etc). The useful optical range of refractive index modulation is beyond 800 nm, respectively near-IR. Of course, the films can be used below 800 nm as absorbers, then the un-cured ones are transparent in visible, while the laser-cured ones opaque. Applications therefore can be holographic plates, optical isolators, etc.

More attractive application is phase matching Mach-Zehnder interferometers in ultra-fast light pulse modulation applications. The active modulating elements may cause static unbalance of the interferometer setup. Balancing this via a bias on the said active elements reduces the available dynamic range of the modulator, hence a precise phase matching element in the optical circuit is very useful – especially when this can be produced without further material deposition, just through irradiation.

The optical transparency of the laser-cured material decreases drastically in visible (up to 600 nm) due to the 1.5 eV bandgap. This onset is correlated with the formation of a cured glassy phase (revealed by Raman spectroscopy)  unaxial optically (determined ellipsometrically). The refractive index value of pristine films is 1.47, while that of laser-cured ones 1.20 - 1.32.  AFM images revealed a homogeneous film with nano-silicon particles under 200 nm.

    The synthesis of laser-modified structures with a strong periodic modulation of refractive indices of the alternating structures was been developed with success. It is interesting to note that the pristine films display a moderately wide bandgap (3.87 eV), contrasting with that of the laser-cured ones – which can be useful in holographic and perhaps photovoltaic applications.

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