Third-Order Harmonic Generation in Bulk Topological and Non-Topological Crystals*

Solid-state harmonic generation (HG) offers insights into sub-femtosecond phenomena within condensed matter systems. We select a series of crystals with different electronic band-structure features to work toward elucidating the effect of topological surface states on third-order harmonic generation (THG). We compare Al₂O₃, a direct band-gap material, and Fe₂O₃, an indirect band-gap material, and in the future will add Cr:Al₂O₃, a direct band-gap material, and Bi₂Se₃, a topological insulator. We use a 20-TW Ti:Sapphire laser (𝜆_central=800nm, 25 fs, 10 Hz) inside a vacuum chamber at 10^-6 Torr, in the reflection geometry with the sample at 45° relative to the incident beam. The THG signal was detected by either a UV CCD camera or spectrometer which was in the atmosphere. We first studied the THG dependence on laser polarization and then on driving laser intensity, from ~0.2 TW/cm² to ~1.2 TW/cm². The THG dependence on laser polarization was studied and no such dependence was observed. We have experimentally compared the THG energy in Al₂O₃ and Fe₂O₃ through laser intensity scans on these materials and observed differences in generated THG energy between both materials.