Key considerations for effective optical excitation of low-temperature-grown gallium arsenide terahertz emitters

This work shows the characterization and optimization of the optical excitation conditions for low-temperature-grown gallium arsenide (LT-GaAs) terahertz (THz) emitters, given attention to the numerical aperture (NA) of the microscope objective and its defocusing distance. Our pump-probe transmission and THz emission analyses show that low-NA (0.35) and high-NA (0.65) microscope objectives photoexcite charge carriers within the LT-GaAs epilayer and semi-insulating GaAs (SI-GaAs) substrate. However, only the high-NA (0.65) microscope objective can localize the charge carriers in the LT-GaAs epilayer. The defocusing distance for this objective can then be varied to fully optimize the THz bandwidth and dynamic range.