Fig. 11

Orientation-dependent HHG emission for (a) MgO, and (b) quartz. In (a), a free-standing, 50 \(\upmu\)m thick MgO single crystal (\(<100>\)) was driven at 2.1 \(\upmu\)m, 20 fs pulses from the SWIR OPCPA. The incident pulse energy was 27 \(\upmu\)J, the repetition rate was 50 kHz, and the beam was focused by an f \(=40\) cm CaF\(\phantom{0}_{2}\) lens. The peak intensity is estimated as \(\sim 5 \times 10^{12}\) W/cm\(\phantom{0}^{2}\). The plot shows the three strongest harmonic orders: 23, 25, and 27. In (b), a free-standing, 20 \(\upmu\)m thick z-cut, \(\alpha\)-quartz single crystal was driven by 2.1 \(\upmu\)m, 20 fs, 230 \(\upmu\)J pulses at 10 kHz, and focused with a f \(=20\) cm CaF\(\phantom{0}_{2}\) lens, to an intensity \(\sim 2 \times 10^{13}\) W/cm\(\phantom{0}^{2}\). The plot shows two pairs of adjacent odd and even harmonic orders: one in the plateau region (H22, H23) and another near the cutoff (H35, H36). For both crystals, the orientation of the crystal axes with respect to the laser polarization was controlled by a rotation motor perpendicular to the beam propagation direction. For MgO, the zero-degree (45-degree) alignment corresponds to the laser polarization aligned along the \(\Gamma - X\) (\(\Gamma -K\)) direction. For \(\alpha\)-quartz, the zero-degree (30-degree) alignment corresponds to the laser polarization along the \(\Gamma - K\) (\(\Gamma -M\)) direction. The HHG emissions were measured in transmission with a flat field spectrometer consisting of an aberration-corrected laminar-type replica diffraction grating (Shimadzu) that focuses the harmonics onto a micro channel plate (MCP) coupled to a phosphor screen, which are then imaged with a CCD camera. With fixed crystal orientation, HHG RMS stability is around 5 \(\%\) for MgO and 15\(\%\) for \(\alpha\)-quartz.