## 1 Introduction  
Due to the high precision and low thermal damage offered by ultrashort-pulse micromachining, femtosecond laser systems are widely applied in both industrial and scientific fields [1]. Though the advantages of such systems as compared with longer-pulse micromachining (or nonlaser-based micromachining [2]) are unquestioned, the constantly advancing industry requires higher micromachining throughput, better end-quality, and higher versatility in terms of machining materials [3, 4]. In this case, femtosecond pulse micromachining offers a unique advantage since the energy of the pulses is absorbed through a non-linear interaction mechanisms, therefore, enabling the micromachining of virtually any type of material [5]. Recently, there is a growing trend towards high average power (>100 W) femtosecond laser systems, and in addition, the price/Watt is decreasing [6, 7]. In general, although higher average power leads to an increase in processing throughput, maintaining process efficiency comparable to lower power systems without introducing additional thermal damage is a great challenge [8]. The main issue is that high pulse energies that lead to fluence values which exceed the ablation threshold by more than two orders of magnitude are not particularly preferred for precise micromachining. This is due to the decrease in ablation efficiency, i.e., the energy absorbed in the material is transferred to the kinetic energy of the ablation debris or/and the shock wave development, in which case the excess power is not used for material removal but rather for heating the ablated mass [9]. A possible workaround would be to use spatial light modulators (SLM) or various diffractive optical elements (DOE) that enable parallel processing of multiple samples at once, however such an approach is not always favourable due to its limited use for specific applications only, and its overall complexity. A different scenario may occur if the high average power laser system is realized by means of high repetition rates (>10 MHz). In this case, the fluence may be chosen closer to its optimum setting [10], however the pulse-to-pulse time becomes small enough for heat accumulation to significantly decrease the end-quality of the micromachined samples [11; 12], especially near the tails of the Gaussian beam. This is a typical problem for high average power systems. Outcomes such as these stem from the fundamental properties of light-matter interaction and cannot be easily overcome. Although, for MHz systems, the quality of the micromachined samples is improved by increasing the spatial separation (decreasing pulse overlap), this requires using extremely fast (10-100 m/s) translation systems, which are either specially modified galvo scanner setups or polygon scanners [13]. This presents several disadvantages, for example, the system becomes complex due to the addition of sharp focusing optics and loss of working area cannot be avoided, as well, in the case of polygons, vector scanning is almost impossible, and therefore these systems can only be used for specific patterns, hence for specific applications. Therefore, it is necessary to increase the efficiency (ablated material/unit energy/unit of time) of the micromachining process as much as possible to fulfil the necessary requirements. Several techniques exist for processing materials with femtosecond pulses which include: direct ablation in air [14; 15], back side wet etching [16; 17; 18], spatiotemporally enhanced focusing [19; 20], laser-assisted etching (LAE) [21; 22; 23; 24; 25; 26], and gas-assisted processing [27; 28; 29]. Although the techniques presented above all have their advantages and disadvantages, when talking about cutting/drilling deep (> 500 \(\upmu\)m) structures inside the material, all of the stated techniques fall short (with the exception of LAE), and ultimately fall victim to slow processing and heat-affected zones around the cut.