The high precision and accuracy [1] reachable by ultrashort laser microfabrications is counterbalanced by the low processing speed. Though using high repetition rates is useful to enhance the ablation rate and the processing efficiency, it also causes the substrate temperature to increase [2]. This leads to melting or thermal-induced micro-cracks, thus reducing the quality and the control of the microfabricated structures. One of the strategies developed to overcome these problems has been the exploitation of bursts of pulses, consisting in groups of pulses having time delays ranging from few picoseconds to microseconds. Several studies have been carried out on silicon, due to its wide application in the high tech industry [3-8]. Burst mode (BM) processing with GHz intra-burst frequency has been demonstrated useful to increase the material removal rate compared to normal pulse mode (NPM) [4,6]. Moreover, a cooling effect has been reported when the time needed to transfer heat from the irradiated area to the surroundings is longer than the delay between pulses within the bursts [7].

The use of bursts with THz intra-burst frequency, where the intra-burst delay is comparable with the electron-phonon coupling time, is still poorly investigated. Here, we studied the laser ablation of silicon with bursts of 2, 4, 8 and 16 pulses having 0.25 THz, 0.5 THz, and 2 THz intra-burst frequency. Measuring the depth and the diameters of the craters produced with different burst configurations and comparing them to the NPM case, we showed that the specific ablation rate in NPM is generally higher than in BM. A density-dependent TTM-based simulation revealed that this is due to a smaller reflectivity drop experienced when bursts of increasing number of pulses are used. However, we also found some combinations of processing parameters and burst configurations leading to higher specific removal rate compared to NPM [8].