This position is within the THz Photonics Group at the Institute of Electronics, Microelectronics, and Nanotechnology (IEMN, https://www.iemn.fr/), located near the University of Lille campus. The engineer will collaborate with the technical staff of the cleanroom and the characterization platform under the supervision of Emilien Peytavit.

-------Résumé :

Terahertz (THz) radiation, which encompasses electromagnetic waves in the frequency range 0.1 to 10 THz, plays a key role in many advances in fundamental research and engineering [1]. They open the way to a multitude of applications, including gas spectroscopy, THz imaging and wireless telecommunications [2]. The field of THz spectroscopy has seen significant progress in terms of pulse energy, tunability and bandwidth. THz pulses can be generated by converting ultrafast laser pulses via nonlinear interactions in liquids, gases or crystals. In parallel, THz systems have been developed using ultra-fast photoconductors (PCs), in which a semiconductor (SC) is integrated with a THz metal antenna. When photon energy exceeds the bandgap of the SC, mobile electron-hole pairs are generated and accelerated by a static electric field applied to the SC via polarizing electrodes, producing an oscillating current at the THz frequency responsible for the radiation.

Femtosecond lasers are generally used as the optical source to produce sub-picosecond broadband THz pulses. It is also possible to generate a THz wave in the continuous wave (CW) regime by producing an optical beat from two superimposed CW lasers which is converted by the PC, called a photomixer. In this regime, ultra-fast photodiodes (PDs) outperform PCs as THz transmitters [3]. PC/PD technology, which can be seamlessly integrated with optical fibers, offers substantial promise in terms of efficiency and integration into compact systems. The performance of THz systems is determined by the properties of the optoelectronic converter and the laser. Most THz PCs and PDs have been designed to operate at laser wavelengths around 800 nm and 1550 nm, compatible with the most advanced technologies such as Ti:Sa fs lasers and CW telecom lasers.

In recent years, ytterbium-based fiber lasers have demonstrated impressive capabilities by significantly increasing repetition rates and delivering exceptionally high average power, reaching several kW in bulky systems or ~100 W in fiber chains [4]. However, this substantial power available at the 1 µm wavelength has yet to be exploited for the development of high-power THz transmitters using optoelectronic devices featuring high optical-THz conversion efficiency and ease of integration.
To date, research into the development of THz optoelectronic components operating at a wavelength of 1 µm has focused mainly on photoconductors (PCs) using ultrafast materials such as low-temperature GaAs (LTG), LTG-InGaAs and GaBiAs [5]. However, PCs using these materials, designed to operate at a wavelength of 1 µm, have so far shown poor efficiency, because SC alloys with the appropriate bandgap, such as In0.25Ga0.75As, are not mesh-tuned with standard growth substrates like InP and GaAs, or because they are composed of alloys, such as InGaNAs or GaBiAs, that are difficult to grow.
The aim of this thesis is to advance optoelectronic THz transmitters using three distinct strategies: the first approach is based on the use of plasmonic cavities, already developed in the THz Photonics group to improve optical absorption in ultrafast devices [7].
Here, we aim to exploit the low light absorption at 1 µm in LTG-GaAs by integrating it into a high-quality factor optical cavity. The second approach involves the development of a multilayer structure on a GaAs substrate, integrating thin absorption layers of In0.25Ga0.75As between two carrier-capture layers of LTG-GaAs. A layer of phosphorus material would also be integrated at each period to compensate for compressive stress in the InGaAs layers. The final approach concerns the development of ultra-fast PIN photodiodes optimized for operation at 1 µm, using an absorption layer based on the quaternary alloy InGaAsP on an InP platform. Inspired by recent research [8] and capitalizing on previous achievements of photodiodes operating at 1.55 µm within the THz Photonics group, the PhD student will design epitaxial structures and device topologies to achieve high bandwidth and high-power photomixers using InGaAsP/InP heterostructures. Finally, linear photodetector arrays will be studied to achieve THz powers in the mW range, taking advantage of the enormous power reserve of Ytterbium lasers.

REFERENCES :
[1] A. Leitenstorfer et al “The 2023 THz Science and Technology Roadmap” J. Phys. D. 56, 223001 (2023).
[2] D. Bigourd et al. "Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy” Opt. Lett. 31, 2356 (2006); N. V. Petrov et al. "Terahertz phase retrieval imaging in reflection," Optics Letters, vol. 45, p. 4168–4171, 2020; A. Kumar et al. “Phototunable chip-scale topological photonics: 160 Gbps waveguide and demultiplexer for THz 6G communication” Nature communications 13 (1), 5404 (2022).
[3] P. Latzel et al., “Generation of mW Level in the 300-GHz Band Using Resonant-Cavity-Enhanced Unitraveling Carrier Photodiodes,” IEEE Trans. Terahertz Sci. Technol. 7, 800 (2017).
[4] J-P. Negelet al.“Ultrafast thin-disk multipass laser amplifier delivering 1.4 kW (4.7 mJ, 1030 nm) average power converted to 820 W at 515 nm and 234 W at 343 nm” Opt. Express 23, 21064 (2015); F. Röser et al. Opt. Lett. 32, 2230 (2007).
[5] A. Arlauskaset al. “GaAsBi Photoconductive THz Detector Sensitivity at Long Excitation Wavelengths” Appl. Phys. Express 5, 022601 (2012); V. Pačebutas et al. “THz time-domain-spectroscopy system based on femtosecond Yb:fiber laser and GaBiAs photoconducting components” Appl. Phys. Lett 97, 031111 (2010); R. J. B. Dietz et al. “Low Temperature Grown Be-doped InGaAs/InAlAs Photoconductive Antennas Excited at 1030 nm” J. Infrared, Millimeter and Terahertz Waves 34, 231 (2013); K. Kitahara et al. “Development of a high resolution and wide band THz spectrometer based on a 1 μm-band external cavity diode laser,” Rev. Sc. Instr. 84, 126102 (2013).
[6] E. Peytavit et al. “THz Photomixers,” in Fundamentals of THz Devices and Applications, John Wiley&Sons, Ltd, p 137–186 (2021)
[7] E. Peytavit et al., “Milliwatt-level output power in the sub-terahertz range generated by photomixing in a GaAs photoconductor,” Appl. Phys. Lett. 99, 223508 (2011); M. Billet et al. “Resonant cavities for efficient LT-GaAs photoconductors operating at λ = 1550 nm,” APL Photonics 1, 076102 (2016). C.Tannoury et al.,”Photonic THz mixers based on iron-doped InGaAs embedded in a plasmonic microcavity”,APL Photonics 8,116101 (2023)
[8] Y. Peng et al. “High-Speed and High-Power MUTC Photodiode Working at 1064 nm,” Phot. Technol. Lett. 31, 1584 (2019).