Waveguide-like structures written in transparent polycrystalline ceramics with an ultra-low fluence femtosecond laser

We present a laser processing study of optically transparent ytrria stabilized zirconia (YSZ) ceramics (ZrO2-8 mol. % Y2O3) using unamplified femtosecond (fs) laser pulses of a few nJ and high repetition rate (70 MHz). The ceramics were fabricated using current activated pressure assisted densification (CAPAD) and have fine grain size and minimal porosity, producing a transparent material. Irradiation using fs laser pulses caused permanent changes in the optical properties of the irradiated zone. These laser written structures were found to confine He-Ne laser light (632 nm) in effect functioning as waveguide like structures and were written into the YSZ ceramics using a remarkably low per-pulse energy (5nJ). The number of passes with the laser i.e total incident pulses per unit area was found to significantly affect the waveguide writing. We believe that waveguides are regions were the concentration of oxygen vacancies and/or their associated free electrons have been altered by laser irradiation. We are not aware of previous reports of low fluence fs laser pulses being used to influence vacancy related defects to produce waveguides in ceramics. This new mechanism opens the door for writing strictures in optical ceramics with lower power than previously thought feasible.