Abstract: In our laboratory, we generate intense, single-cycle THz pulses with a central frequency ν ~ 0.2 THz and a maximum field strength (at focus) F ~ 0.5 MV/cm via optical rectification of femtosecond laser pulses. The THz pulses are used to explore strong-field ionization of low-lying Rydberg atoms in the low-frequency regime. A novel F∝n ionization threshold field scaling is found. In addition, the energy transfer in the single-cycle limit shows differences as compared to that in a multi-cycle field. We have also studied field emission from metallic nano-tips using the THz pulses. Electrons with energies easily exceeding 5 keV are observed, substantially greater than previously attained using THz, infrared, or near infrared driving fields. The maximum electron energies are proportional to the peak THz field and are only weakly dependent on the tip radius, for 10 nm < R < 1 μm. The observations are in good agreement with the predictions of a sub-cycle energy transfer model.
