## 3 Results and Discussion  
The X-ray emission spectra in the energy region below 20 keV, detected by the X-ray CCD camera (CCD1), look rather similar for the different target morphologies (Figure 4). The spectra contain the K-shell emission lines of Zn (K\({}_{\alpha}\) and K\({}_{\beta}\)) and low-energy bremsstrahlung. The emitted flux from the nanostructured samples, especially from the disordered sample, was barely sensitive to the laser field polarization. In contrast, the signal from the flat sample strongly varied as a function of the polarization  
Figure 3: Images of the ordered NW target (**a**) before and (**b**) after interaction with the intense laser pulses. (**c**) optical microscope \(50\times\) magnification image of single-shot damage spots on the disordered NW target. and reached its maximum for p-polarization. The integrated photon flux at Zn K\({}_{\alpha}\) was about 1-4 \(\times\) 10\({}^{6}\) photons/pulse/sr, which at a 1 kHz repetition rate yielded about 10\({}^{10}\) photons/s. Comparable X-ray fluxes obtained in similar experimental conditions have been reported [9; 22]. Independent of the polarization, the nanostructured targets emitted only slightly more K-shell photons (a factor of 1.3) than the maximum flux value from a polished, flat ZnO target. Following the approach suggested in [32], we applied an exponential fit (Maxwell distribution) to the low-energy bremsstrahlung spectrum in Figure 4 and retrieved an electron temperature of 350-435 eV. For the high-energy region above 50 keV measured by the Timepix detector, the flux obtained from the ordered and disordered NW samples was significantly stronger than that from the flat sample (see Figure 5). For instance, in the range from 150-350 keV, as marked in Figure 5, we measured a 15 and 22 times higher integrated flux for p-polarized and s-polarized laser pulses, respectively. Also, for these photon energies, the spectra could be fitted using a two-temperature electron energy distribution with corresponding temperatures of 12-15 keV and 40-50 keV (Figure 5). Such several-temperature energy distributions of \"hot\" electrons in laser generated plasmas have been reported in the past [33; 34; 35; 36]. Under conditions of high temporal pulse contrast, as in our experiment, the formation of the pre-plasma is suppressed, which leads to a sharp density profile. In this situation, for oblique incidence of the laser beam, Brunel (or vacuum) heating is the dominant mechanism of laser energy absorption [37; 38; 39; 40]. The polarization dependence of the hard X-ray flux in the high-energy region (Figure 5) was similar to the dependence observed for the low-energy region shown in Figure 4. The efficiency of bremsstrahlung X-ray generation in the energy range 50-800 keV is insensitive to the laser polarization in the disordered NW array, weakly sensitive in the ordered NW array, and strongly sensitive in the flat target. This difference can be understood by considering the geometry of interaction schematically shown in Figure 2. When the electric field of the pump laser has only a tangential component (s-polarization), the vacuum heating is suppressed, which explains the experimental results for the flat surface. Meanwhile, for NW morphologies, for any laser beam polarization there is always a component of the field perpendicular to the surface of a NW. Thus, it is obvious that for s-polarized light, the vertically standing and ordered NWs were more efficient emitters of hard X-ray photons than the flat bulk sample (6-fold enhancement for 150-350 keV). Despite the observed electron heating with s-polarized light, for the ordered morphology the p-polarized light could still transfer energy to the electrons more efficiently, resulting in a higher flux.