Ultra-high spatial resolution diagnostics of femtosecond laser radiation-induced modification morphology in glasses for the fabrication of microfluidic and microphotonic components

Abstract

The light intensity in a focused femtosecond laser pulse can be high enough to initiate non-linear absorption of the radiation in otherwise transparent dielectric media. The highly localized energy deposition into the material results in permanent changes in its chemical and physical properties which can be used in the fabrication of various photonic and microfluidic devices. In this thesis we study the morphology and optical properties of the modification induced by focused femtosecond laser radiation inside fused silica and other inorganic glasses. The characterization of the modified regions is performed using a microreflection refractometry technique and the ultra-high spatial resolution technique of weak selective chemical etching followed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). We demonstrate that the sample irradiation conditions strongly affect the modification morphology. Specifically, for a tight focusing geometry we identify the pulse duration-pulse energy parameter space where three distinct regimes of modification can be achieved. We also address the effects of optical aberrations on the shape, position and the intensity threshold values of the femtosecond laser induced modification. We use the combination of femtosecond laser dielectric modification and selective chemical etching to fabricate high-quality microchannels in fused silica glass. We show a dramatic dependence of the etch rate on the laser polarization and demonstrate that the high differential etch rate inside the modified regions is determined by the presence of polarization-dependent self-ordered periodic nanocracks or disordered nanoporous structures. These nanostructures are much smaller than the wavelength of the femtosecond radiation used for their formation and are the smallest objects ever created by light inside dielectric materials. Exciting potential applications of the nanostructures will be discussed.