Photomixed terahertz emitters by multipolar photoconductive terahertz antennas

Longtime restricted to the field of astronomy, the recent exploitation of electromagnetic waves at terahertz frequencies (1 THz = 10^12 Hz) has highlighted an interest in other areas such as THz spectroscopy, industrial non-destructive testing, security and telecommunications. However, the lack of compatible sources for industrial use restricts the development of possible applications at these frequencies. This thesis focuses on continuous photomixed THz sources, whose working principle consists in superimposing two laser beams of close frequencies (frequency difference close to 1 THz) in order to allow the resulting optical beat to excite a photomixer radiating with the help of an antenna a signal at the frequency difference. In order to avoid an excessive frequency noise which would result from a simple transposition of the optical frequency noise, a bi-frequency source is used to ensure a correlation between the noises of the two signals, offering in the presence of the beating a reduction of four orders of magnitude of the frequency noise. The used source is a semiconductor laser (InGaAs/GaAs) with vertical emission in external cavity operating on two transverse Laguerre-Gauss (LG) modes thanks to the integration of loss masks, one of the modes being the fundamental Gaussian mode, the other being a higher order mode without energy at the center (typically LG_{0n} with n = 2, 3 or 4). The realized laser emits a beam at a wavelength of 1064 nm offering several spots (2*n) in the transverse space, where the 2 modes are superimposed, thus generating a THz beat. In order to use all the available spots and to multiply the THz power by the number of spots, the study of multipolar antennas has been performed, showing in addition a possible exaltation of the gain of two crossed dipole antennas compared to a single dipole (6.5 dBi versus 2.2 dBi). However, few photomixers are reported in the literature for excitations at the wavelength of 1064 nm, despite the availability of mature high-power components and lasers. The study of a fast saturable absorber mirror, constituted by an InGaAs/GaAs quantum well on the surface (in order to reduce the lifetime), allowed to show a photoconductive effect encouraging for a possible THz emission, and the study of plasmonic electrodes on InGaAs/GaAs allowed to demonstrate a first continuous THz emission. The continuity of this work may open the way to continuous THz sources 6 to 8 times more powerful with an increased optical-THz conversion efficiency and a radiation gain superior to existing solutions.