Three-dimensional bonding anisotropy of bulk hexagonal metal titanium demonstrated by high harmonic generation

High harmonic generation (HHG) in solid-state materials is an emerging field of photonics research that can unveil the detailed electronic structure of materials, bond strengths and scattering processes of electrons. Although HHG in semiconducting and insulating materials has been intensively investigated both experimentally and theoretically, metals have rarely been explored because the strong screening effect of high-density free electrons is considered to significantly weaken the HHG signal. Here, we investigated HHG upon infrared excitation in bulk hexagonal metal titanium (Ti), a typical building block for practical lightweight structural materials. By analyzing the polarization dependence, the approach revealed the three-dimensional (3D) anisotropy in the electronic states. The results demonstrated the potential of HHG spectroscopy for characterizing 3D bonding anisotropy in metallic systems that are of fundamental importance for designing lightweight and strong structural materials. High harmonics generation (HHG) is a promising way of investigating electronic structures and anisotropy in materials. The authors demonstrate the observation of HHG in simple structural material, hexagonal metal titanium, and experimentally clarified the anisotropy in the electronic states from the polarization dependence.