Volume photoinscription of glasses: three-dimensional micro- and nanostructuring with ultrashort laser pulses
Ultrafast laser photoinscription of optical materials has seen a strong development in the recent years for a range of applications in integrated photonics. Fueled by its capability to confine energy in micro-domains of arbitrary geometries, it forecasts extensive potential in optical design. The process can locally modify the material structure and the electronic properties, changing in turn the refractive index. It thus lays down a powerful concept for three-dimensional modifications of materials, with the potential to design integrated optical functions. Using fused silica as model glass, this report discusses the physical mechanisms of photoinscription, outlining the possibility of refractive index engineering. We will review basic mechanisms of light propagation, excitation of matter, and energy relaxation concurring to material structural and photophysical modification. A dynamic perspective will be given, indicating relevant times for relaxing different forms of energy (electronic, thermal, etc.). The possibility to structure beyond diffraction limit will be explored, as well as the subsequent optical response of hybrid micro-nanostructures. Different irradiation geometries for photoinscription will be presented, pinpointing their potential to generate optical and photonic systems in three dimensions. Spatiotemporal pulse engineering can optimize the material response toward the achievement of accurate positive and negative index changes. An optimality concept can thus be defined for index design and present optimization concepts will be discussed. A particular potential derives from the utilization of non-diffractive beams with engineered dispersion. Finally, we indicate a range of application domains, from telecom to optofluidics and astrophotonics, outlining the potential of volume micro- and nanoprocessing.