Anisotropic High-Harmonic Generation in Ferroelectric Materials

High harmonic generation (HHG) serves as a source of soft X-rays and a powerful probe of ultrafast electron dynamics. In ferroelectric materials, HHG spectra are influenced by spontaneous electric polarization, and even-order harmonics can appear due to the materials’ broken inversion symmetry. In this talk, we focus on HHG spectra from ferroelectric materials, including NbOI₂. We calculate HHG spectra using real-time time-dependent density functional theory (RT-TDDFT) under external fields of varying frequencies and polarizations. Our simulations reveal that the HHG yield depends strongly on the laser polarization relative to the ferroelectric axes. We connect the observed HHG signals to the anisotropy of the band gap and effective mass, to the anharmonic motion of electrons within the band structure, and to specific transitions among low-energy bands. These results establish a microscopic link between ferroelectric crystal anisotropy and nonlinear optical emission. They also provide guidance for polarization-controlled HHG in ferroelectric materials, offering insights for future studies of ultrafast carrier dynamics.