Photomixing

Photomixing is a technique that uses two lasers to generate Terahertz radiation. The lasers beams are mixed together and focussed onto a photomixer device which generates the Terahertz radiation. It is technologically significant because there are few sources capable of providing radiation in this waveband, others include frequency multiplied electronic/microwave sources, Quantum cascade laser and Ultrashort pulsed lasers with photoconductive switches as used in Terahertz time-domain spectroscopy. The advantages of this technique are that it is continously tuneable over the frequency range from 300GHz to 3THz (10cm^-1 to 100cm^-1) (1mm to 0.1mm), and spectral resolutions in the order of 1MHz can be achieved.

Principle

Two continuous wave lasers with identical polarisation are required, the lasers with frequency ω₁ and ω₂ are spatially overlapped to generate a Terahertz beatnote. The co-linear lasers are then used to illuminate an ultra fast semiconductor material such as GaAs. The photonic absorption and the short charge carrier lifetime results in the modulation of the conductivity at the desired Terahertz frequency ω_THz = ω₁ - ω₂. An applied electric field allow the conductivity variation to be converted into a current which is radiated by a pair of antenna. A typical photoconductive deivce or 'photomixer' is made from Low Temperature GaAs with a patterned metalised layer which is used to form an electrode array and radiating antenna.

References

Francis Hindle, Arnaud Cuisset, Robin Bocquet, Gaël Mouret "Continuous-wave terahertz by photomixing: applications to gas phase pollutant detection and quantification" Comptes Rendus Physique (2007), doi:10.1016/j.crhy.2007.07.009