Until the nineties, the domain of terahertz waves (THz), which extends between 100 GHz and a few tens of THz in the electromagnetic spectrum, was difficult to access. This situation radically changed with the advent of the THz time domain spectroscopy. Terahertz spectroscopy has indeed become an important tool for studying molecules in the condensed phase. In particular, many chemical, biological, radiological, nuclear and explosive agents exhibit characteristic spectral features in this frequency region, which has thus a strong potential for security applications. However, because a current challenge for atmospheric THz spectroscopy is to overcome the high absorption of ambient humidity that can extinguish a THz signal over 1-m-long distances, there is a growing demand for intense THz sources. Also an important constraint is to be able to cover large spectral bandwidths, in order to collect many molecular signatures.
Whereas conventional THz systems usually offer a narrow bandwidth limited to a few THz only, a terahertz emitter based on an air plasma created by femtosecond laser pulses can supply broad bandwidths above 20 THz. Exploiting this technique is the core of the project named ALTESSE (“Air Laser-based TErahertz SpectroScopy of Explosives”), the main objective of which is to test a THz spectroscopy using a plasma source created by means of optical pulses coupling a fundamental and its second harmonic in the air.
Initially, the scientific tasks of the ALTESSE project aimed at:
(i) Optimizing THz emission in a spectral window extending up to 50 THz using femtosecond pulses in either focused or filamentary propagation,
(ii) Carrying out THz spectroscopy of materials by the Air-Biased Coherent Detection (ABCD) method,
(iii) Examining the amplification of the detected THz signal at long laser wavelengths (1.5 - 2 µm) compared to that delivered by fundamental 800-nm pulses,
(iv) Performing the THz/ABCD spectroscopy of explosives, in transmission then in reflection, over distances far away from the laser source,
(v) Modeling numerically the nonlinear propagation of intense laser pulses in the air together with the emitted THz radiation, in support to the experiments.
After three years of intensive work, the major highlights of ALTESSE which will be presented during the talk are:
(i) The acquisition of spectra extending up to 60 THz and exploited to directly identify solid powders. Systematic comparisons between the measured THz spectra and ab-initio simulations allowed us to countercheck the molecular absorption coefficients and distinguish between the inter- and intra-molecular motions of the probed samples.
(ii) The strong increase, by a factor close to 10, in the THz energy yield measured for laser fundamental wavelengths operating in the mid-infrared,
(iii) The first spectral measurements of explosives capturing molecular fingerprints up to 20-THz bandwidths and obtained at long distances (15 m) from the laser source,
(iv) The first compelling comparisons of absorption spectra both in transmission or reflection geometries,
(v) The optimization of a unidirectional propagation code in terms of computational performance, completed by the incorporation of the HITRAN database to describe absorption of air in a broad spectral range > 10 µm.