## 4 Discussion
In this study, the EDS results for _fs_-laser surface-modified zirconia, LD glass ceramics, and VITA _En_ materials demonstrated that _fs_-laser treatment facilitated the formation of carbon, and the reduction of oxygen. [PERSON] et al. [55] hypothesized that the formation of carbon monoxide (CO) and CO\({}_{2}\) occurs in the irradiated area at temperatures at or above 900 \({}^{\circ}\)C. The formation of zirconium carbide in zirconia, along with residual oxygen content, has been observed in several recent studies [56; 57; 58]. The underlying mechanism is attributed to destabilized carbon and zirconium, resulting in the formation of zirconia and subsequent oxygen reduction from zirconia, which eventually leads to CO formation [59]. As the temperature increases during the carbothermal reduction of zirconia, carbon atoms substitute for oxygen atoms in the structure of zirconia [60]. According to [PERSON] et al. [59], the destabilization process of zirconia proceeds until the zirconium is consumed. Heterogeneous nucleation and growth of zirconium carbon monoxide (ZrCO) occur within the carbon black agglomerates, possibly originating from the co-condensation of zirconia and CO moieties, followed by solid-state diffusion to zirconium carbon dioxide (ZrCO\({}_{2}\)). In the subsequent step, ZrCO\({}_{2}\) is further reduced with CO until a stable carbide is ultimately formed [58]. The first step of the mechanism is explained by the observation that the ZrO\({}_{2}\) phase is depleted during the formation of the ZrC phase [61]. The reduction in oxygen and subsequent increase in carbon in all of the _fs_-laser irradiated samples in this study, across all tested _fs_-laser parameters, can be explained by the reduction of an oxycarbide (O\({}_{\rm x}\)C\({}_{y}\)) outer layer through both carbon and oxygen being cleaved off and subsequent CO\({}_{2}\) formation [58; 61; 62]. Our work is believed to be the first study reporting on chemical changes in zirconia, LD glass ceramics, and VITA _En_ under 30-_fs_ laser irradiation. Our findings align with earlier findings by [PERSON] et al. [63], who used 256 _fs_ pulses, a 1028 nm wavelength, and a 100 kHz pulse frequency. They reported minor changes for zirconium, yttrium, and oxygen fractions prior to and after _fs_-laser irradiation. Notably, a key factor determining the efficiency of laser ablation of dielectric materials is the laser pulse duration [22; 64; 65]. Ultra-short _fs_-laser irradiation offers superior precision, minimal thermal effects, and enhanced processing capabilities, making it a highly effective tool for processing dielectrics, particularly when high-quality and intricate work is required [21; 22; 23; 65]. The high peak power of _fs_-pulses, especially those with a pulse width of less than 50 _fs_ in duration, induces non-linear absorption processes, such as multiphoton absorption, allowing the laser light to interact with materials that are otherwise transparent at the laser wavelength. In longer pulse processes, such as in _ns_- or _ps_-laser processes, plasma formation on the surface of irradiated media can shield the processed material from the laser light, subsequently reducing the processing efficiency. _fs_-lasers, due to their ultra-short pulse duration, often avoid significant plasma formation, leading to more efficient material processing [21; 22]. Additionally, _fs_-lasers enable rapid material processing due to their extremely short interaction time with the material, leading to faster and improved material removal rates. The short interaction time helps to suppress thermal effects within the focal volume, preventing melting and phonon propagation into the bulk of the treated solids, even at the highest laser fluences [20].