THz Laser Tuning Detects Substances at Airports – and Elsewhere

By Joerg Schwartz

Researchers from MIT in Cambridge have demonstrated an important feature of tunability by using a novel technique that also is potentially relevant to other fields of laser technology. Their work was published in a recent issue of Nature Photonics.

Electromagnetic waves in the 1- to 10-THz region – i.e., with wavelengths between 30 and 300 – µm – have lived in the shadows because they fall into the gap between microwaves generated and manipulated by solid-state electronic devices and the higher frequencies covered by infrared photonics. This is unfortunate because terahertz waves not only penetrate clothing and human tissue more easily – and are supposed to be safer than x-rays – but they also could answer important questions because their frequencies correspond to relevant energies in molecular rotation and intermolecular modes of large biomolecules (in gases and liquids). This spectroscopic terahertz fingerprint also offers several potential new applications in pharmacy and medical imaging – not just distinguishing a painkiller from an explosive in a passenger's belongings.

A wire laser's tuning is redshifted by a block ("plunger") of silicon (left), which expands the transverse mode profile and decreases the transverse wave vector. Blueshift can be achieved by a metal plunger (right), which squeezes the transverse mode profile and increases the transverse wave vector. The sinusoidal shape of the cavity ridge structure generates distributed reflection required for laser feedback.

One of the main reasons terahertz waves are not more popular already is that they are not easily produced, particularly in a tunable way, such that the resonance frequency of various materials can be scanned. Nevertheless, recently developed quantum cascade lasers (QCLs) look very promising, although challenges remain, according to professor Qing Hu, research group leader at MIT.

QCLs, first developed in 1994 at Bell Labs, do not use bulk semiconductor material but consist of a periodic structure of thin layers forming a so-called "superlattice" or one-dimensional quantum well, splitting the band of permitted energy bands into various subbands. This means that the electrons do not recombine in one big transition but instead cascade down in several smaller steps, resulting in less required photon energy for terahertz waves.

Although the QCL wavelength can be set within a wide range for a system comprising a specific material – e.g., GaAs/AlGaAs by controlling the layer thickness – continuous tuning in operation remains a challenge. Typical laser tuning methods such as changing the refractive index or applying an external cavity with variable length do not work well. The former achieves only approximately 1 percent tuning, whereas the latter practically has not been achieved because the facet of the laser is much smaller than the wavelength of the emitted light.

However, the somewhat unusual situation where the laser wavelength is much larger than the cross section of the gain medium has now been demonstrated by the researchers, using a "wire laser."

"This is a laser with deep subwavelength cross section," said Hu, "which means that a substantial part of the transversal mode travels outside the solid core of the laser cavity." This unusual geometry gives access to a different approach to tuning: By bringing a block of either metallic (gold) or dielectric material (silicon) into this evanescent field, the output frequency can be changed.

This new tuning method also may have potential for other lasers, but it primarily offers an approach to tunable terahertz radiation. Terahertz lasers are not likely to show up at the airport anytime soon because much further work is needed to demonstrate how successful this application is. Plus, at the moment, terahertzµ QCLs work only at a temperature below 186 K, meaning that they must be cooled by liquid nitrogen. Both Hu and QCL pioneer Jerome Faist believe that this problem can be overcome, perhaps by employing clever quantum design or by moving to another material platform (such as gallium nitride).

Joerg Schwartz
joerg.schwartz@photonics.com