Spatio-Temporal Ultrafast Laser Tailoring for Bulk Functionalization of Transparent Materials

In the past decade, ultrashort laser sources have had a decisive impact on material processing for photonic applications. Since the first demonstration of bulk photowriting of light guides, the control of the irradiation outcome and the machining of more complex devices have been ongoing challenges. However, the technique is usually restricted to the elemental association of an ultrashort source with a focusing lens. It is thus limited in the processing speed and in the achievable bulk modifications. This requires higher flexibility and adaptivity to the material properties. Accompanying studies of material modifications in space and time, we propose here that automated spatio-temporal tailoring of the laser pulses is an efficient manner to overcome these limitations. More precisely, we demonstrate the generation of multiple processing foci for synchronous photomachining of multiple devices in the bulk. Thus, we report on the parallel photowriting of waveguides, light couplers, light dividers in 2D/3D and wavelength division demultiplexers in fused silica glass. The method relies on laser wavefront dynamic modulation. As another significant breakthrough, we show that the domain of photowriting can be extended to deep focusing despite of the associated optical distortions. The technique is based on preserving the energy density at arbitrary depths. We indicate that this can be achieved by wavefront shaping or temporal profile tailoring as efficient correction approaches for propagation. The design of the spatial phase mask and of the temporal profile is conducted by an evolutionary optimization loop based on the evaluation of the irradiation outcome in fused silica and borosilicate Crown glass BK7. Additionally, we take advantage of a particular photoinscription regime to machine embedded waveguides with well-defined polarization function in fused silica. We also have unveiled a singular interaction regime where regular structuring takes place before the focal region. We developed a Fresnel propagation code that explains this particular regime. In addition, we evidence a single pulse processing window where a purely spherical void is generated. For the first time, the dynamics of the energy coupling to the glassy matrix is evaluated for various temporal pulse profiles. Enhanced energy confinement in the case of picosecond pulses is confirmed by characterization of the transient electronic gas and of the subsequent pressure wave in various glasses. We also show that if distinct temporal pulse profiles lead to similar permanent modifications, the transient excitation scenarios can be drastically different. The dynamics of energy deposition in multi-pulse regimes are also investigated. These pump-probe studies were carried out with a self-build time-resolved microscopy system with temporally shaped pump irradiation. We also developed a new method based on the Drude model to differentiate the electronic and matrix contributions to the contrast of the microscopy images allowing to map the free carrier density and lifetime variations and the refractive index changes in the interaction zone. te l-0 04 88 64 7, v er si on 2 21 F eb 2 01 2