Photomixing

Photomixing is a technique in photonics where two optical fields of different frequencies are combined in a fast photodetector or photoconductive material to produce an electrical signal whose frequency equals the difference of the optical frequencies. When two lasers with frequencies f1 and f2 illuminate a photoconductive device biased with a DC voltage, the resulting photocurrent contains a component at Δf = |f1 − f2|. If Δf lies in the terahertz range, the process can generate or detect terahertz radiation.

Mechanism: The two lasers create a time-varying carrier density through interference, producing an optical beat at Δf. In a nonlinear or square-law detector, this beat induces a corresponding current modulation. The strength of the beat signal depends on the carrier lifetime, optical powers, and device geometry. In terahertz photomixers, a photoconductive antenna or similar structure converts the beating current into THz radiation, which is emitted by the antenna. The emitted frequency is tunable by adjusting the laser frequencies, and the usable bandwidth is influenced by the material’s ultrafast response and the detector design.

Applications: Photomixing provides compact, tunable sources of terahertz radiation and enables coherent THz detection for spectroscopy, imaging, and communications. It is used in terahertz time-domain and continuous-wave systems, where two lasers act as a local oscillator to mix with a THz signal. Materials commonly employed include low-temperature-grown GaAs and other ultrafast semiconductors. Effective photomixing requires careful stabilization of laser frequencies and precise device fabrication.

Limitations: Output power is typically modest and rises with optical power and carrier mobility, while coherence is sensitive to laser phase noise and drift. The performance is constrained by carrier lifetime and the speed of the photomixer antenna.