Generation and Applications of strong THz Fields driven by intense mid-IR Pulses

Although the THz spectral range experiences a tremendous grow of interest for at least two decades, it is still one of the least explored but most exciting areas to study light-matter interaction. Due to the small photon energy of THz radiation, it can propagate through non-conductive materials and resonantly interact with low-energy excitations, leading to a plethora of applications. The THz pulse is thereby primarily used as a probe while an optical pulse excites the material that is examined. In contrast, intense strong-field THz transients would allow on-demand control of the properties of matter. Current table-top THz sources remain rather weak, with the most promising being optical rectification (OR) in nonlinear crystals and two-color plasma filaments pumped by near-IR sources. While the former is mainly restricted by multi-photon absorption of the short wavelength driving pulse, causing crystal damage, the latter suffers from pump pulse scattering in dense plasma and limited laser field-asymmetry. These limitations can be surpassed with intense long wavelength driving pulses. Until recently, such high-power mid-IR sources were simply not available. In this work, we exploit the recently developed mid-IR optical parametric chirped pulse amplifier system to drive efficient THz generation in the organic crystal DAST by OR and in two-color plasma filaments, wherein we boost the optical- to THz conversion efficiency by almost an order of magnitude, compared to previous works with near-IR drivers. The intense THz source is further applied to heterostructure quantum dots without any field enhancing structures to investigate the pure quantum confined Stark effect. The possibility to manipulate optical properties of nano-scale semiconductors by direct THz radiation demonstrates the feasibility for an all-optical electro-absorption modulator with data rates in the range of Tbit/s.