Femtosecond laser direct-writing of waveguide Bragg gratings in a quasi cumulative heating regime

Femtosecond laser induced step-like structures inside transparent hydrogel due to laser induced threshold reduction

In the area of laser material processing, versatile applications for cutting glasses and transparent polymers exist. However, parasitic effects such as the creation of step-like structures appear when laser cutting inside a transparent material. To date, these structures were only described empirically. This work establishes the physical and chemical mechanisms behind the observed effects and describes the influence of process and material parameters onto the creation of step-like structures in hydrogel, Dihydroxyethylmethacrylat (HEMA). By focusing laser pulses in HEMA, reduced pulse separation distance below 50 nm and rise in pulse energy enhances the creation of unintended step-like structures. Spatial resolved Raman-spectroscopy was used to measure the laser induced chemical modification, which results into a reduced breakdown threshold. The reduction in threshold influences the position of optical breakdown for the succeeding laser pulses and consequently leads to the step-like structures. Additionally, the experimental findings were supplemented with numerical simulations of the influence of reduced damage threshold onto the position of optical breakdown.

In summary, chemical material change was defined as cause of the step-like structures. Furthermore, the parameters to avoid these structures were identified.

Formation of periodic disruptions induced by heat accumulation of femtosecond laser pulses

The absorption and heat accumulation of successive ultrashort laser pulses in fused silica leads to melting of the material. We analyze the structure and formation of disruptions that occur within the trace of the molten material. We employed focused ion beam (FIB) milling to reveal the inner structure of these disruptions. The disruptions consist of several small voids which form a large cavity with a diameter of several tens of micrometer. Based on the observations, we suggest a model explaining the formation of these disruptions as a results of a fast quenching process of the molten material after the laser irradiation has stopped. In addition, we analyzed the periodic and non-periodic formation of disruptions. The processing parameters strongly influence the formation of disruptions.

Fabrication of fully integrated antiresonant reflecting optical waveguides using the femtosecond laser direct-write technique

Utilizing a recently developed two-step fabrication process, we have experimentally demonstrated what is to the best of our knowledge the first ever integrated antiresonant reflecting optical waveguide (ARROW), directly inscribed into bulk optical glass. The waveguide consists of an array of high-index contrast rods that surround an unmodified core. The transmission spectrum reveals the formation of discrete bandgaps that can be tuned by varying the diameter of the individual rods.