Generation and applications of strong THz fields driven by intense mid-IR pulses

Abstract

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 scientific fields 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 such as crystal lattice vibrations, molecular rotations or collective spin excitations, leading to a plethora of applications. It can be utilized for biological tissue discrimination, pharmaceutical quality control, astronomy or high-speed communication, as well as for fundamental research to investigate carrier transport processes in solids, coherent control of quantum states or molecular alignment, to name only a few. The THz pulse is thereby primarily used as a linear 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 and engineer new dynamic states in a wider range of materials. Nonetheless, current table-top THz sources remain rather weak, with the most promising being optical rectification in nonlinear crystals and two-color plasma filaments pumped by near-infrared 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 overcome with intense long wavelength driving pulses. However, until recently, such high-power mid-infrared sources were simply not available. In this work, we exploit the recently developed mid-IR optical parametric chirped pulse amplifier (OPCPA) system to drive efficient THz generation in the organic crystal DAST(4-N,N-dimethylamino-4’-N’-methylstilbazolium tosylate) by optical rectification 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.