Terahertz produced in Ionized Air by Filamentation – TIAF

Properties of plasma generated in air through laser breakdown and filamentation have been investigated as a THz source and THz detection medium. A new technique has been developed to amplify and direct the radiation from filaments. It is based on the coherent synthesis of THz radiations from an array of femtosecond laser filaments. Several terahertz coherent detection systems have also been built to characterize the THz pulse shape and spectrum. Using the ABCD technique (Air Breakdown Coherent Detection) proposed by the group of X.C. Zhang, we were able to characterize THz pulses with a detection bandwidth significantly larger than with classical electro-optic techniques.

The mechanisms for THz generation have been studied with three methods involving generation of laser filament in air: the classical single color laser filamentation at 800 nm, the two color filamentation (800 nm + 400 nm) and filamentation in a static electric field. For this purpose we have developed a THz detection system based on the technique of Air Breakdown Coherent Detection. With two color filamentation we achieved THz field amplitude higher than 150 kV/cm with a spectrum spanning over 30 THz. This method has the higher conversion efficiency, but the laser pulse has to be strongly focused to generate intense THz pulse in air. Classic single color filamentation is less efficient, but it can be generated up to a distance of 1 km from the laser source, while two color filamentation is almost impossible at this distance. We have also demonstrated that the coherent synthesis of the THz radiations from an array of filaments could scale up the THz energy produce in the microjoule regime with a terawatt laser source. With appropriate delay between the filaments, the emission can be directed along preferential directions, which is of high interest for remote THz applications.

Transposition of the laboratory experiments of coherent detection in ionized air and organization of multiple filaments at long distance would be an interesting perspective for remote sensing THz applications.

The results obtained have been published in several international refereed journals. Among these articles one can cite “Measurement and control of plasma oscillations in femtosecond filaments”, Phys. Rev. Lett. 106, 255002 (2011), where we measured the electronic currents inside filament responsible for the THz emission in different gases. A second article dealt with the coherent synthesis of several filaments to amplify and control the directivity of THz radiation (« Coherent synthesis of THz fields from filament antenna array”, Appl. Phys. Lett. 102, 221107 (2013).

At the cross between optic and electronic, Terahertz photonics is experiencing an intense activity for the last 15 years. THz radiation allows non-invasive imaging methods, because most non polar and non metallic materials are transparent in this frequency range. Numerous applications of THz can be mentioned in the domains of spectroscopy, astronomy, security, communication and medicine, where the unique properties of non ionizing THz radiation can complement the usual techniques employing X rays or IR radiation.
The development in the late 80s of electro-optical detection methods based on femtosecond lasers has largely contributed to the surge of THz activity. This THz Time Domain Spectroscopy (TDS) technique allows the direct measurement of both amplitude and phase of the THz pulse electric field, which is not feasible in the visible domain. Over the past decade, many research laboratories in optics, biology or physics of materials have equipped themselves with THz TDS systems based on the non-linear crystals (ZnTe, AsGa..) or biased photoconductive antennas.
The new challenge of coherent THz photonics is now is now to increase the energy, peak intensity and spectrum of THz pulses in order to reach a non-linear regime, and also to propagate THz pulses over long distances in atmosphere. This requires new techniques of generation and detection. Among the new techniques proposed, the most promising and simplest to implement consists in focusing in air a femtosecond pulse of a few millijoules energy to create a plasma. The plasma free electrons driven by the laser field radiate a picosecond THz pulse. By clever manipulation of the local laser electric field, it is possible to amplify this current so that THz pulses of several tens of nanojoules of energy are obtained with a spectrum extending up to the infrared. Furthermore, by exploiting the well known phenomenon of laser filamentation, the produced plasma can be extended into a long plasma column able to illuminate distant targets. This THz source thus overcomes the severe problem of THz attenuation by water vapor during propagation in the atmosphere.
This project aims to study and develop different methods using air as a non linear medium to both generate intense and broadband terahertz pulses by means of laser filamentation in air, and to fully characterize the waveform of the produced THz pulses. The THz detection method in air consists in measuring the increase of the characteristic plasma luminescence of a filament in the presence of the THz electric field. It is particularly well suited for remote sensing of THz pulses.
To summarize, the final objective of this project is the realization of a system capable of remotely generating and detecting THz in the atmosphere, while allowing time domain spectroscopy covering the entire THz domain. The originality of the project relies on exploiting the singular properties of filaments generated in air by femtosecond laser pulses.