Graphene-based Devices For Terahertz Technology

THz radiation is extremely attractive for fundamental investigations of matter and emerging applications including,for example,security screening,medical imaging,and spectroscopy.However,the THz spectral range remains one of the least exploited spectral regions,mainly due to the lack of compact powerful THz sources/amplifiers and passive devices.This thesis concerns the development of advanced THz devices based on graphene material.The first project focuses on the carrier lifetime in neutral graphene-h BN van der Waals heterostructures under mid-infrared illumination.The carrier lifetime in such device for photoexcited carriers at low density and energy is reported to be 30 ps using mid-infrared photoconductivity measurements.By igniting the hyperbolic phonon polaritons in h BN via an electrical or optical pump,we demonstrate that the carrier lifetime is switched from 30 ps down to few picoseconds.This project opens interesting perspectives for the development of THz lasers by exploiting graphene/h BN heterostructures,for the realization of highly sensitive THz photodetectors as well as for phonon polariton optics.The second project reports on original devices based on hybrid semiconductor-metal waveguides coupled to the graphene layer.These devices,which are easy to fabricate,can act as THz amplitude modulators and THz amplifiers.First,we numerically investigate the dispersion relation of the two fundamental propagation modes of these hybrid waveguides.Then,we experimentally characterize these graphenecoupled waveguides using THz time-domain spectroscopy.We demonstrate that these devices act as Mach-Zehnder modulators at 2 THz and 5.7 THz,the cut-off frequencies of TE0 and TE1 modes respectively.This project opens interesting perspectives on the modulation of THz waves and represents an important building block for the realization of THz lasers.