Field Ionization and Field Emission with Intense Single-cycle THz Pulses

This dissertation focuses on light-matter interactions in the strong-field, low-frequency, single-cycle regime in both gas-phase and solid-state systems through the investigation of field ionization of Rydberg atoms and field emission from metallic nano-tips exposed to intense, single-cycle THz pulses. Field ionization/emission mechanism, as well as post-ionization/emission energy transfer dynamics, have been studied.

In the experiments, single-cycle THz pulses with a central frequency f ≈ 0.2 THz and peak field amplitude Fm ≈ 0.5 MV/cm are used to explore:

1) Field ionization of excited atoms (Na Rydberg states of nd, n = 6-15) in the long-pulse/low-frequency regime. A F ∝ 1/ n^3 over-the-barrier ionization threshold scaling behavior is observed, reflecting the suppression of core-scattering during the ionization pulse and, more importantly, the extended times required for Rydberg electrons to escape over the field-dressed Coulomb potential barrier. We have also examined energy transfer in the single-cycle limit and showed that in contrast to ionization by multi-cycle pulses, electrons that are ionized near the field extrema of a single-cycle pulse acquire substantial energies ( 2Up), and those ionized near the zero-crossing can obtain much larger energies (> 6Up), even in the absence of rescattering.

2) Field ionization of oriented atoms (Na Rydberg-Stark states in an n-manifold, n = 10-12). The ionization threshold fields are found to be orientation-dependent, due to both diabatic population transfer between “uphill” and “downhill” oriented states and an asymmetry in the single-cycle THz waveform.

3) High-energy field-electron-emission from W nano-tips having different cone angles and tip radii, 10 nm < R < 1 μm. Electrons with energies easily exceeding 5 keV are observed. The maximum electron energies are proportional to the peak THz field and are roughly independent of the tip radius. These observations can be attributed to the large field enhancement and spatial localization of the enhanced field in the vicinity of the nano-tips. At low-frequencies/long-wavelengths, such near-field characteristics can fundamentally change the energy transfer process as compared to that which occurs following the ionization of gas-phase atoms or molecules, and allow for the creation of very high energy electrons.