Modern methods of laser-based nanocomposite fabrication and treatment rely on a deep understanding of the interplay between a set of mechanisms involved, such as nanoparticle growth and decay, material phase transformation, degradation, and damage. In this work, scanning multipulse femtosecond laser irradiation is used for Au:TiO 2 nanocomposite formation. Depending on laser scan speed, two different regimes are observed revealing different gold nanoparticle growth rates. The regime of the remarkably fast laser-induced growth of Au nanoparticles is found to be accompanied by the cavity formation in titania film around the particles. The transition between two formation regimes is found to be abrupt, confirming the catastrophic mechanism of Au nanoparticle growth. The obtained results are analyzed based on the developed numerical model including effects such as nanoparticle absorption, local field enhancement, photoinduced free carrier generation, plasmon-assisted electron emission, and thermal heat transfer from nanoparticles toward titania matrix. Our modeling reveals the crucial role of collective thermoplasmonic effects caused namely by bimodal nanoparticle size distribution. The performed analysis also suggests that spall in the solid state is responsible for final matrix degradation if nanoparticles become large enough. The considered laser-based formation of optical nanocomposites is crucial not only for the better understanding of ultrashort laser interactions with glass− metallic nanocomposite materials but also for numerous applications in photocatalysts, solar cells, and chemosensors.