ABSTRACT
Nowadays there are a few techniques based on ultrashort laser pulses, which are able to generate high energy and high-field THz pulses. These are large-area photoconductive antennas (LAPCA) [1], optical rectification (OR) in organic and inorganic nonlinear crystals [2–4], spintronics emitters [5], and dual-color laser-generated air plasma [6]. From among them OR is able to generate pulses with both larger than 1 mJ energy and higher than 10 MV/cm peak electric field, predestining them to use not only to drive and investigate dynamics in matter [7], but also as sources for compact, THz driven particle accelerators [8–11].
OR is a special form of the well-known nonlinear optical process, the difference frequency generation (DFG). However, in the case of OR, not two beams with well-separated spectra travel through the nonlinear optical material (NM), but an ultrashort pump pulse having a broad bandwidth. The OR process can be considered as the result of a huge number of DFG processes between the frequency components of the ultrashort pump pulse. In the 48absence of frequency dependence of both absorption, refraction, and nonlinearity, this process results in generation of an electromagnetic wave having a temporal shape proportional to the time derivative of the intensity of the ultrashort pump pulse [12], approximating a single-cycle THz pulse.
