Fig. 1. Square radius, r2, of the crater produced by the fs pulses as a function of the pulse energy, E, at three different number of pulses namely N = 40, 100, 1000. Solid lines are fits to the expected dependence from which the laser modification threshold of CdZnTe and beam spot size are obtained. The inset reports the variation of the fluence threshold Fth with the number of pulses N in the form:$N{{F}_{\text{th}}}\left( N \right)={{F}_{\text{th}}}\left( 1 \right){{N}^{\xi }}$, with single shot fluence threshold Fth(1)=(14 ± 1) mJ/cm2 and incubation factor ξ=(0.80 ± 0.05).

Fig. 2. (a) SEM image of the CdZnTe surface after the irradiation with N = 20 pulses with energy E = 5 μJ (Fp = 0.64 J/cm2). The red arrow represents the laser polarization direction; (b) Zoomed view of the marked region in (a), clearly showing long cracks with nanometer width formed on the surface (see also inset); (c) 2D-FFT of the crater in (a), indicating that the cracks are formed along direction at angles multiples of π/3.

Fig. 3. SEM images of CdZnTe after irradiation with a sequence of various number of pulses N, at E = 5 μJ (Fp = 0.64 J/cm2) and corresponding zoomed views: (a, b) N = 30; (c-f) N = 50; (g-i) N = 100. The red arrow in panel (a) indicates the laser polarization direction. In the zoomed view of panel (b) cracks directed along three different directions are indicated and termed as type I, II, and III. Panel (d) shows the central region of the spot displayed in panel (c) evidencing periodic bumps separated by cracks, while periodic arrays of subwavelength holes formed close to the spot edges are shown in panels (e, f). The zoomed view of panel (h) addresses the columnar features formed at the center of the spot for N = 100.

Fig. 4. Low frequency and high frequency periodic surface structures formed on CdZnTe sample at an energy of E = 55 μJ (peak fluence Fp≈7.0 J/cm2) for high pulse number N, namely (a) N = 500 and (b) N = 1000. The corresponding FFT spectra are shown in panels (c, d), while panel (e) displays the profiles taken over the white dotted lines shown in panels (c, d). In panels (c-e) the term LF and HF indicate low and high spatial frequency, respectively.

Fig. 5. Sequence of SEM images displaying the evolution of the LIPSS formed on sample surface after irradiation with (a) N = 300, (b) N = 500 and (c) N = 1000 laser pulses at E = 55 μJ. The double headed arrow in panel (a) represents the direction of laser polarization.