Research overview
This paper investigates terahertz nonlinear absorption in a thin Al2O3 (sapphire) crystal using high-field nonlinear THz spectroscopy. Intense THz pulses from an organic-crystal source were focused onto a 430 µm-thick sample, and a pair of wire-grid polarizers was used to vary the THz fluence, replacing a conventional z-scan measurement. The crystal showed broadband saturable absorption from 0.1 to 8 THz, with a high modulation depth of 27.5% and a relatively low saturation intensity of 6.99 µJ/cm². First-principles calculations attribute the effect to free electrons from oxygen vacancies and impurities: strong THz fields scatter them into higher-lying conduction-band valleys, where their mobility is reduced, making the crystal more transparent. Stable, bulk Al2O3 is therefore a promising saturable absorber for mode-locking THz sources, such as quantum cascade lasers, to deliver broadband ultrashort pulses.
Key findings
- Broadband THz saturable absorption in Al2O3 from 0.1 to 8 THz
- High modulation depth (27.5%) with a low saturable intensity (6.99 µJ/cm²)
- Wire-grid polarizer pair used to vary THz fluence in place of a z-scan
- Effect explained by THz-driven intervalley scattering of free electrons
