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The Terahertz (THz) domain lies at the crossroads between the worlds of photonics and electronics. THz waves are characterized by a frequency of between 100 GHz and 10 THz, corresponding to the frequency of the natural modes of vibration of many molecules, a wavelength of between 30µm and 3mm, giving them strong penetrating power, and a low energy (0.4 to 4 meV) making them totally harmless, especially to living organisms. In addition, a large number of materials are transparent to THz radiation, also known as T-rays. However, they are reflected by metals and absorbed by polar liquids, including water. These specific properties give rise to a wide range of applications for THz spectroscopy and THz imaging, in fields as varied as cosmology, non-destructive testing, security, heritage studies, etc.

But THz spectroscopy also enables us to probe a large number of fundamental physical phenomena in condensed and living matter. Indeed, the energy of THz waves corresponds to that of numerous elementary excitations, such as plasmons, magnons, phonons, Landau transitions and electron spin resonances.

The activities of the TOP platform include fundamental research based on THz spectroscopy techniques, as well as research into the development of new sensors and transmitters, and more applied research, combining THz imaging and spectroscopy with agronomy, for example.

THz research activity at the Montpellier site began in 1983 with the publication of a theoretical paper by M. Dyakonov [1], a professor at the University. In 2002, W. Knap demonstrated the first detection of THz radiation by a nanotransistor [2]. The team comprising F. Teppe, N. Dyakonova, D. Coquillat and W. Knap developed this activity at the site from 2003 to 2013. Following the filing of several patents related to the development and integration of THz sources and detectors, the start-up T-Waves Technologies, which has since become Terakalis, was created in 2013. Terakalis develops non-destructive testing systems for industrial materials, based on THz imagers derived from the team's patents.

At the same time, the Institut d'Electronique et des Systèmes (IES UMR 5214) launched an activity in 2001 [3] to simulate physical phenomena in electronic components in the THz frequency domain. The development of an experimental activity at IES, initially led by L. Chusseau and J. Torres, brought the two laboratories closer together, notably through the creation of the "TeraLab" Scientific Interest Group in 2010. With the support of the CNRS, the University of Montpellier and the Occitanie region; and in a federating and structuring spirit for the regional cluster; the Terahertz Occitanie Platform (TOP), at the interface between the L2C and the IES, was created in 2017.

Since 2012, new research activities have been developed within the TOP platform. In solid state physics, a magneto-spectroscopy system for probing the band structure of topological and Dirac materials has been developed by S. Ruffenach and F. Teppe [5]. More recently, a unique European Landau spectroscopy system, developed in 1993 by W. Knap, has been revived by C. Consejo and F. Teppe to study the cyclotron emission of Dirac materials [6, 7]. And more recently, a THz magneto-photoconductivity spectroscopy system has been developed by C. Bray, C. Consejo, K. Maussang and F. Teppe [8] to study electron spin resonances in two-dimensional materials such as graphene. In biophysics, S. Ruffenach, L. Varani and J. Torres are developing near-field THz spectroscopy systems to study quantum electrodynamic interactions between proteins [9]. Finally, C. Bray, N. Diakonova and D. Coquillat have developed an imaging system to study plants [10].

All these tools, transmitters, detectors and spectrometers have been pooled to create the Terahertz Occitanie Platform (TOP).

[1] M. Dyakonov and M. Shur, Phys. Rev. Lett. 71, 2465 (1993).
[2] W. Knap and others, Appl. Phys. Lett. 81, 4637 (2002).
[3] E. Starikov et al. J. Appl. Phys. 89, 1161 (2001).
[4] V. Gruzinskis et al. Phys. Stat. Sol. 204, 77 (1997).
[5] F. Teppe et al, Nature Communications 7, 12576 (2016).
[6] D. But et al. Nature Photonics 13, 783-787 (2019).
[7] S. Gebert, C. Consejo et al, Nature Photonics 17, 244-249 (2023).
[8] C. Bray, K. Maussang et al, Phys. Rev. B 106, 245141 (2022).
[9] M. Lechelon, et al, Science Adv. 8(7): eabl5855 (2022)
[10] Y. Abautret, et al, OPTICS EXPRESS 28, 35018-35037 (2020).