A UCLA-led research team has developed a laser that can be tuned across a broad part of the terahertz band of the light spectrum.

The technology could improve drug screening equipment by making them able to analyze more compounds, or the technology could be incorporated into future telescopes that study dusty regions of space home to stars in the process of being born.

The innovative design could also be the basis for other types of lasers that can vary the wavelength of light they emit. It also produces a very precise wavelength of terahertz light. The study linked here was published in Nature Photonics and led by Benjamin Williams, a professor of electrical and computer engineering at the UCLA Samueli School of Engineering.

The terahertz band of the electromagnetic spectrum lies between the infrared and microwave bands. Its particular advantage is it can reveal chemical composition of many solids, gasses, and molecules by looking at what wavelengths of light are absorbed, reflected, and transmitted – a process analogous to looking at the color of an object using visible light.

“Different molecules absorb terahertz light at very particular wavelengths, and in ways that are specific to each molecule,” said Williams, who leads the Terahertz Devices and Intersubband Nanostructures Laboratory. “In other words, each molecule has a spectroscopic ‘fingerprint’ that can tell you what something is made of.”

For example, a drug screening tool using terahertz light could tell if something is an illegal compound or harmless sugar. Or, terahertz telescopes – such as the proposed Origins Space Telescope – are needed to study star formation processes, histories of galactic evolution, and the signatures of life and planetary formation.

However, the maturity of technologies that use the terahertz band lag far behind their counterparts in other wavelengths – in some ways the terahertz band is the electromagnetic spectrum’s “last frontier.”

One promising terahertz tool, the quantum cascade laser or QCL, has been in development for nearly 20 years. It has the capability of generating the extremely precise wavelength of terahertz light needed to make precise spectral measurements; it has recently reached the level of maturity such that the first terahertz QCLs have been deployed on airborne and balloon observatories for astrophysical measurements of the interstellar medium.

What terahertz QCLs are missing is tunability – that is, the capability to change the wavelength of the light that the laser emits. Current QCLs can only change their wavelength only across 1% – tuning by a little more is possible, but comes at the expense of laser performance. That limits the type of molecules that a single laser can analyze – if more wavelengths are available, then more molecules are able to be identified.

The UCLA researchers’ new QCL design can emit terahertz light across nearly a fifth of that band, the broadest available. That’s equivalent to a lightbulb that changes its color from blue, to green, to yellow, and finally to orange. “Our breakthrough not only achieves a record high fractional tuning of a terahertz laser, at 19% of the terahertz spectrum, it also has a record high continuous output power, and excellent beam patterns,” Williams said. “These qualities will make that will make this a practical terahertz source for a range of applications.”

Their design is called a metasurface “vertical-external-cavity surface-emitting laser”, or “VECSEL.” While VECSELs have been around for a while at much shorter wavelengths in the visible and near-infrared, it is the “metasurface” which is new and allows terahertz operation. What the specialized amplifying metasurface does is eliminate the need for a separate “gain medium” that resides within the laser’s cavity – the material which amplifies the light as it bounces back and forth between two mirrors until it builds up into a strong laser beam. In the new device, the metasurface is now both the mirror and the amplifier. With nothing in between two mirrors, the distance can be made very tiny, as small as just a single terahertz wavelength. Those two new qualities are what increases the tunability of the new terahertz device and creates a very strong laser with a precise wavelength.” While the researchers set out to make a tunable terahertz laser, the design could be adapted to make practical tunable lasers in other parts of the light spectrum.

The study’s lead author is Chris Curwen, a UCLA doctoral student in electrical and computer engineering and member of Williams’ research group. The other author was John Reno, principal member of the technical staff at Sandia National Laboratory in New Mexico.

The study was funded by the National Science Foundation and NASA.

This paper was covered in Nature Photonics News&Views: “A wavelength-size tunable Fabry–Pérot laser,” Nathan Jukam, vol 13, pp. 823-825, 2019. https://doi.org/10.1038/s41566-019-0555-7