Photomixing

Photomixing is the generation of continuous wave terahertz radiation from two lasers. The beams are mixed together and focused 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 continuously tunable over the frequency range from 300 GHz to 3 THz (10 cm⁻¹ to 100 cm⁻¹) (1 mm to 0.1 mm), and spectral resolutions in the order of 1 MHz can be achieved. However, the achievable power is on the order of 10⁻⁸ W.

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 allows the conductivity variation to be converted into a current which is radiated by a pair of antenna. A typical photoconductive device or 'photomixer' is made from low temperature GaAs with a patterned metalized layer which is used to form an electrode array and radiating antenna.

High resolution spectrometer

The photomixing source can then form the basis of a laser spectrometer which can be used to examine the THz signature of various subjects such as gases, liquids or solid materials.

The instrument can be divided into the following functional units:

• Laser sources which provide a THz beatnote in the optical domain. These are usually two near infrared lasers and maybe an optical amplifier.
• The photomixer device converts the beatnote into THz radiation, often emitted into free space by an integrated antenna.
• A THz propagation path, depending on the application suitable focusing elements are used to collimate the THz beam and allow it to pass through the sample under study.
• Detector, with the relatively low levels of available power, in the order of 1 μW, a sensitive detector is required to ensure a reasonable signal to noise ratio. Si bolometers provide a solution for in-coherent instruments. Alternatively a second photomixer device can be used as a detector and has the advantage of allowing coherent detection.

History of photomixing

A review of the history of photomixing can be found in Fundamentals of THz Devices and Applications by Peytavit et al. ^[1] Some selected milestones in the development of photomixing are:

• 1955 – First experimental demonstration by Forrester et al. ^[2] Because of the lack of coherent sources, they used two Zeeman components of a mercury lamp, resulting in a 10 GHz beat frequency detected with a photoelectric tube whose SbCs₃ photocathode acted as the mixer.

• 1962 – Shortly after demonstrating the first He–Ne laser, Javan et al. performed mixing of two single-mode He–Ne lasers (5 MHz) on the photocathode of a photomultiplier. ^[3] Their goal was to study the spectral width of the laser beam rather than to produce microwaves.

• 1962 – Inaba and Siegman reported the first use of a PIN-junction photodiode as an optical mixer. ^[4]

• 1995 – A major breakthrough by Brown et al. demonstrated photomixing in low-temperature-grown GaAs (LTG-GaAs). ^[5] Using an interdigitated-electrode photoswitch on an LTG-GaAs wafer pumped by two Ti:Al₂O₃ lasers (λ ≈ 0.8 µm), they generated THz radiation up to 3.8 THz, producing 4 µW at 300 GHz and 1 µW at 800 GHz. This was enabled by the material’s sub-picosecond carrier lifetime, high electron mobility (~150 cm²/V·s), and high electric-field breakdown (~300 kV/cm). ^[6]

• 2003 – Ito et al. achieved about 80 µW at 300 GHz and 2.6 µW at 1 THz using InP-based uni-traveling-carrier photodiodes (UTC-PDs). ^[7]

Thanks to these developments, and to advances in laser stability and THz antenna design, commercial frequency-domain THz spectrometers based on photomixing are now available from several manufacturers. ^{[8] [9]}

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

1. Peytavit, E.; Ducournau, G.; Lampin, J.-F. (2021), Pavlidis, D. (ed.), "THz Photomixers", Fundamentals of THz Devices and Applications, Wiley
2. Forrester, A. T.; Gudmundsen, R. A.; Johnson, P. O. (1955), "Photoelectric mixing of incoherent light", Physical Review, 99 (6): 1691
3. Javan, A.; Ballik, E. A.; Bond, W. L. (1962), "Frequency characteristics of a continuous-wave He–Ne optical maser", J. Opt. Soc. Am., 52: 96
4. Inaba, H.; Siegman, A. E. (1962), "Microwave photomixing of optical maser outputs with a PIN-junction photodiode", Proc. IRE, 50 (8): 1823
5. Brown, E. R.; McIntosh, K. A.; Nicholas, G. M.; DiNatale, W. F.; Dennis, K. L. (1995), "Photomixing up to 3.8 THz in low-temperature-grown GaAs", Applied Physics Letters, 66: 285
6. Krotkus, A. (2010), "Semiconductors for terahertz photonics applications", J. Phys. D: Appl. Phys., 43: 273001
7. Ito, H.; Nakajima, F.; Furuta, T.; Yoshino, K. (2003), "Photonic terahertz-wave generation using antenna-integrated uni-traveling-carrier photodiode", Electronics Letters, 39 (24)
8. Bakman Technologies – Frequency-Domain THz Spectrometers, bakmantechnologies.com, retrieved 2025-10-04
9. TOPTICA Photonics – Frequency Domain Terahertz Systems, toptica.com, retrieved 2025-10-04