Terahertz Pulse Generation from Laser Induced Plasma in Air

Abstract

The spectral range lying from microwaves to optical spectrum was known as the “terahertz (THz) gap” due to the lack of efficient sources and detectors in this spectral region up to the 90 ‘s. This had prevented any use of THz radiation particularly useful in spectroscopy since vibrational modes of molecules are involved in the THz range. THz spectroscopy provides therefore unique fingerprints of material of great interest (e.g. explosives, pharmaceuticals, drugs …) for security and medical applications. Since the emerging of femtosecond pulsed lasers, this gap has been filling by scientists and industrials. Time resolved spectroscopy setups historically used in THz range 1 take advantage of non-linear effects in crystals and semiconductors to generate and detect THz using laser pulse. Such techniques provide picosecond electromagnetic bursts whose spectrum are limited and spread typically from 0.1 THz to few THz. Indeed, photoconductive antennas based on semiconductors have limited bandwidths due to photo-carrier lifetime, while electro-optic (EO) crystals absorb a part of the generated THz leading to dips in the available THz spectrum. The use of diluted media as gases could therefore leads to larger bandwidth. The first solution was proposed by Dai et al.2 who used plasma as a centro-symmetric nonlinear media for broadband THz generation and detection. This technique offers the possibility to get THz waves 1 to 2 orders of magnitude more powerful than classical methods. It also gives the possibility to get a very broadband spectrum, from 0.1 THz to several tens of THz. To generate such broadband THz radiation, a high power femtosecond laser pulse is focalized in air until breakdown is reached: a plasma is generated. The electrons produced by this ionization process are accelerated by an intense optical field asymmetry. This asymmetry is enhanced by using part of the optical pulse whose frequency was previously doubled in a non-linear crystal. The same principle is used to detect the THz signal. In this configuration, the plasma acts as a non linear medium where a four wave mixing occurs: 2 photons from the optical pulse at pulsation ω and 1 THz photon mix to generate a photon at pulsation 2ω whose intensity is directly proportional to THz signal intensity. The authors will present during the conference, short THz waveforms and their associated broadband spectrum generated and detected by air plasma techniques, and their dependence on the experimental parameters.