In our experiments, the investigated pulse widths fall above the low-femtosecond regime where the combination of both mechanisms is believed to be responsible for the breakdown. Multiphoton ionization is responsible for the initial generation of electrons which are further heated by incoming portion of the

pulse resulting in avalanche ionization and rapid plasma formation [18]. The initial part of the pulse produces free-electron plasma which can absorb the later part more efficiently and/or behave as a mirror and reflect most of the incident energy [17, https://www.selleckchem.com/products/BI-2536.html 19, 20]. Every material has its unique optical damage fluence, but all the pure dielectrics demonstrate similar behavior in all ranges of pulse width as observed for SiO2[21]. Stuart et al. investigated the threshold fluence for fused silica and CaF2 with laser

pulses in the range 270 fs ≤ τ ≤ 1 ns [21]. They discovered that the damage threshold decreased with the decrease of the pulse width. Fan and Longtin developed a femtosecond breakdown model which gives the time at which the laser Torin 1 intensity reaches the breakdown threshold at a given check details position [17], T B (Z). (1) where Z is the axial location in the focal region (Z = 0 at focal point), τ p is the full width at half-maximum pulse duration, c is the speed of light in a medium, β is the ratio of peak pulse power to the breakdown threshold of a material (P max/P th), and Z R is the Rayleigh range or focal region, Equation 1 gives the time at which the breakdown starts after the laser pulse has started interacting with the target surface at a given position in the focal region. From this point onward, the plasma starts to grow and expand, and covers the irradiated spot for few nanoseconds during

CYTH4 which the second part of the laser pulse is still traveling toward the target surface. Using this equation, the time required for the breakdown to initiate is calculated to be 77, 189, and 325 fs for pulse widths of 214, 428, and 714 fs, respectively. The schematic representation of this time is shown in Figure 2. The amount of energy lost to the plasma before reaching the target surface depends on the amount of time the remaining portion, after breakdown initiation, of the pulse spends on traveling through the plasma. Shorter laser pulses (214 fs) reach threshold fluence very early since they possess high intensity, as depicted in Figure 2. However, they are very short and thus spend less amount of time in the plasma and thus loose less energy to the plasma and remove target material more efficiently compared to longer pulses (>214 fs). Hence, as can been seen from Figure 3a, the hole (approximately 12 μm in diameter) drilled by 214-fs pulse is closer in size to the laser beam spot diameter of 10 μm. Although we just worked with pulses in femtosecond regime (214 to 714 fs), the findings in the investigation by Stuart et al.