Bright pulses of coherent terahertz radiation have been created by firing a laser at specially designed targets. Developed by physicists in China and the UK, the new technique could lead to the development of compact yet intense terahertz sources with a wide range of applications, including condensed-matter physics, biomedical imaging and even wireless communications.
Terahertz radiation falls between the infrared and microwave regions of the electromagnetic spectrum. As well as being a useful probe of collective excitations in solids, terahertz radiation is able to pass through materials such as clothing and packaging, and therefore could be used for security scanning. However, it has proven to be very difficult to create practical terahertz sources and detectors – so applications have so far been limited.
Coherent terahertz radiation can be created using quantum-cascade lasers, but the best devices today are relatively low power. Much more powerful terahertz sources can be made by slamming beams of high-energy electrons into solid targets – but this must be done at large accelerator facilities.

Ripped electrons

Now, Yutong Li of the Institute of Physics of the Chinese Academy of Sciences and Xiaohui Yuan of Shanghai Jiao Tong University, and colleagues, have generated pulses of coherent terahertz radiation using a laser-driven plasma wakefield accelerator. This is a relatively new accelerator technology whereby an intense laser pulse is fired at a target. The pulse rips electrons from their atoms to create huge electric-field gradients, which then accelerate the electrons to energies as high as several gigaelectronvolts.
Physicists already know that terahertz radiation is emitted when laser pulses are fired at a metal-coated plastic target. While there are several different physical processes involved in this emission, Li and colleagues focussed on a specific process called "coherent transition radiation" (CTR). This occurs when the accelerated electrons pass through the metal coating on their way out of the target. As they move from the metal into the air, the electric field associated with the electrons changes rapidly and this leads to the emission of electromagnetic radiation.
Li and colleagues developed their terahertz source at the Laboratory for Laser Plasmas at Shanghai Jiao Tong University. They used 30 fs-long pulses of infrared light that each deliver 2 J of energy. Their design involved firing the pulses at a number of different metal and plastic targets. They found that the plastic targets with microns-thick metal coatings generated about 10 times more terahertz radiation than did similarly sized bare plastic targets.

Energetic emissions

The team reckons that each laser pulse produces a pulse of coherent terahertz radiation that has about 400 μJ of energy. This is comparable to sources based on electron accelerators, which have delivered about 600 μJ per pulse.
Potential applications of an intense laser-based coherent terahertz source include the study of charge-carrier dynamics in semiconductors, lattice vibrations and spin precession. Sources could also be used for medical imaging of the skin and teeth. Because terahertz signals are at a higher frequency than microwave communications, it could also be possible to develop terahertz-based wireless-communication systems capable of much higher data-transmission rates than are possible today.
The terahertz source is described in Physical Review Letters.