Terahertz Sources

Microwave technology presents a number of options for high-frequency oscillators, such as Gunn diodes, Impatt diodes and resonant tunneling diodes. Some of these have been optimized long ago for emitting at particularly high frequencies up to several terahertz. In that regime, however, the performance in terms of output power and power conversion efficiency is normally much lower than at lower frequencies.

Another way to obtain higher frequencies is harmonic generation in nonlinear electronic devices. This requires high-power pump sources and typically delivers rather low output powers.

Generally, the performance of such microwave technology sources is quite modest in terms of output power and spectral coverage.

Free electron lasers as well as synchrotron light sources can be constructed which emit very high powers in the terahertz spectral regions. They are useful for various research purposes, but are very large and expensive. Therefore, they have quite limited use for general terahertz technology.

Certain molecular gas lasers can generate terahertz radiation. (They are also sometimes called far infrared lasers.) They exploit transitions of certain molecules (e.g. of methanol) between molecular rotational states, with which discrete frequencies in a wide range can be generated, typically with output powers of a few milliwatts or some tens of milliwatts. Such gas lasers are usually optically pumped, e.g. with a CO₂ laser. For example, there are CO₂-pumped methanol lasers, emitting at 2.5 THz. The conversion efficiency is very low.

Quantum cascade lasers are semiconductor lasers which have originally been developed for emitting in the mid- and far-infrared spectral region. Optimization for particularly long emission wavelength has lead to emission frequencies of only a few terahertz [7, 10, 14, 17, 19], which may be tuned in some limited range. Such lasers are very compact, but need a cryogenic cooling system.

In the area of optical sampling technology, photoconductive dipole antennas have been developed which are suitable both for generation and detection of high-frequency electromagnetic signals. Miniature versions of such antennas allowed their use also in the terahertz region. Essentially, a sender antenna consists of two short metallic stripes with a small gap in between, made on a semiconductor material with a short charge carrier lifetime. A DC bias voltage is applied to the stripes, and an intense ultrashort laser pulse from a mode-locked laser focused on the region between the metallic stripes generates a short circuit for a short time. (The semiconductor gap serves as a photoconductive switch.) The fast potential change induces fast oscillations in the antenna, which in turn lead to terahertz radiation emitted in a wide range of angles.

The decay is often so fast that one obtains a single-cycle source, i.e., a source emitting only about a single cycle of the electromagnetic oscillation. The emission spectrum may then roughly cover a substantial part of an octave or even more. This can be useful for spectroscopy, for example, as it allows one to cover a large frequency range without needing a tunable source.

For higher output powers, devices with larger areas have been constructed with interdigitated electrodes as part of a metal–semiconductor–metal (MSM) structure.

Photoconductive antennas can also be operated in a continuous-wave mode, where irradiation is done with two single-frequency laser diodes (or with a single two-color laser [9]), having a terahertz frequency difference. Particularly in that continuous-wave regime, photoconductive antennas are also called terahertz photomixers. They can produce spectrally very pure terahertz radiation, which can also be frequency-tunable.

Modulation of the resulting terahertz signal is easily possible by modulating one of the involved optical waves.

One can use difference frequency generation of two optical waves with similar frequency to obtain terahertz radiation. Alternatively, one may have two frequency components of a broadband ultrashort pulse interacting nonlinearly. Such processes are possible not only in photomixers as discussed above, but also in various nonlinear crystal materials and essentially work like difference frequency generation for mid-infrared laser sources, for example.

If only a single input beam (not a dual-frequency source) is used, the method is called optical rectification. This is explained in detail in the corresponding encyclopedia article.

Compared with nonlinear frequency conversion processes involving only optical beams, terahertz sources based on that technology are generally much less efficient. A fundamental reason for that is the low photon energy of terahertz radiation, which is far below that of optical beams. In addition, there are technical challenges related to the strong divergence of terahertz beams, which results from the relatively long wavelengths and limits the realizable interaction length in a nonlinear crystal.

Somewhat surprisingly, optical rectification of femtosecond optical pulses, leading to terahertz wave emission, can also occur in a gas (e.g. air). Here, a plasma is generated by the superposition of an infrared beam with its second harmonic [18]. Careful phase control of the involved waves is necessary for a high conversion efficiency. Compared with optical rectification in crystals, the emission bandwidth is typically higher, and higher pulse energies can be obtained.

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