Recently we have improved the efficiency and the output power of our optically pumped continuous-wave THz sources.
These sources are based on the beating of two laser lines in a wide bandwidth photodetector. Its intrinsic nonlinear
behaviour is used to produce a beatnote at the frequency difference between the two laser lines (photomixing). These
photomixers are continuously tunable THz sources working at room temperature. We have developed two kinds of
photomixers: GaAs-based for 0.8 μm pumping and InP-based for 1.5 μm pumping. On GaAs the best results has been
obtained thanks to low-temperature-grown GaAs (LTG-GaAs) photoconductors (PC). Efficiency and power were
optimized by designing a new type of thin PC placed in a Fabry-Pérot resonator. The high impedance of the PC is a wellknown
limitation of this device but with our approach it was possible to reduce its impedance by a factor 100. Moreover
by designing an impedance matching network it was possible to obtain 1.8 mW at 252 GHz with a total efficiency of 0.5
%. On InP the best results are obtained with uni-travelling-carrier photodiodes (UTC-PD). The device was improved by
designing a new heterostructure and new semi-transparent contacts with sub-wavelength apertures. The active layer was
also bonded to a silicon substrate thanks to metal thermocompression. It is demonstrated that with this approach it is
possible to obtain a power of 0.7 mW at 300 GHz with a total efficiency of 0.7 %. More generally the efficiency of
optically pumped terahertz sources will be discussed.
THz has become a wide field of investigation opening new opportunities in a growing number of domains of physics,
chemistry, and biology. Among the different techniques existing today to generate THz fields, heterodyning two optical
frequencies is a useful approach when tunability is required. Moreover, to address high-resolution spectroscopy or
metrology applications, a key point is the achievement of a narrow linewidth source. To this aim, two-propagation-axis
dual-frequency lasers have been already shown to provide narrow linewidth tunable beat notes up to 2 THz. We report in
this paper the demonstration of a narrow linewidth THz radiation source based upon this laser. Indeed the beat note
provided by the laser is sent into a unitravelling carrier photodiode (UTC-PD), and radiated by a transverseelectromagnetic-
horn antenna (TEM-HA). All components operate at room temperature. The emitted THz signal is
detected by a subharmonic mixer coupled to an electrical spectrum analyzer. The THz signal is observed and analyzed
thanks to a heterodyne detection. The measured dynamic range is 75 dB at 282 GHz, 50 dB at 500 GHz, 35 dB at
700 GHz and decreases to 20 dB at 1 THz. The decrease is due to the UTC-PD efficiency and conversion losses in the
sub-harmonic mixer. The measured linewidth is better than 30 kHz at any frequency from DC to 1 THz.
We have developed a new generation of optoelectronic large bandwidth terahertz sources based on TEM horn antennas
monolithically integrated with several types of photodetectors: low-temperature grown GaAs (LTG-GaAs) planar
photoconductors, vertically integrated LTG-GaAs photoconductors on silicon substrate and uni-travelling-carrier
photodiodes. Results of pulsed (time-domain) and photomixing (CW, frequency domain) experiments are presented.