A tiny terahertz laser is the first to reach three key performance goals at once: high power, tight beam, and broad frequency tuning. Credit: Ali Khalatpour, MIT.

Researchers have achieved a tiny high-power narrow-beam laser that operates in the terahertz frequencies. These frequencies are beyond visible light, and the lasers have potential in many imaging and scanning applications. But previous terahertz lasers required bulky laboratory equipment to stay cool enough to function. The new devices are the first terahertz laser devices to reach three key performance goals at once—high power, tight beam, and broad frequency tuning—in a design that can work outside a laboratory.

The laser achieves three key performance metrics simultaneously. As a result, it offers increased power, reduced noise, and increased resolution. This enables more reliable and lower cost applications in chemical sensing and medical imaging. The laser works outside of laboratory conditions, enabling new remote applications. For example, NASA has selected lasers from this research to fly on the Galactic/Extragalactic Spectroscopic Terahertz Observatory. In this mission, the laser will help NASA

A photonic wire laser (PWL) is a type of laser built on a semiconductor chip, has nanometer-sized bore and a millimeter length cavity. Several can be side-by-side on a chip integrated with surrounding high-speed electronics. Coupling multiple adjacent PWLs can synchronize the light beams to emit at the same or multiple wavelengths and combine their power.

Many applications require the ability to electrically tune laser frequency, at high output power with a tight optical beam pattern. Realizing all three of these performance metrics at the same time is a challenging task because the width of a PWL is much smaller than its wavelength. This results in a large fraction of the propagating waves moving outside the solid core of the wire and coupling with an adjacent laser. Scientists at the Center for Integrated Nanotechnologies, a DOE Office of Science user facility, were able to exploit this unique feature of photonic laser wires to achieve the elusive combination of performance features simultaneously. They used multiple wire lasers which were phase locked, meaning that the lasers’ oscillations were synchronized. This approach was inspired by a type of conjugation in chemistry where adjacent molecules are coupled. By placing pairs of these photonic wires in an array, the researchers combined the output of the pairs to produce a single, high-power beam with minimal beam divergence. Adjusting the individual coupled lasers allows tuning the laser over a broad frequency range.

The new scheme achieved three critical performance metrics simultaneously: tunable laser frequency, high power output, and tight beam pattern. This ability can improve resolution and fidelity in measurements in chemical sensing and medical imaging (e.g., cancer imaging and brain imaging). There are much broader applications as well. For example, the NASA Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) will fly with selected lasers from this collaboration. The observatory will detect and measure carbon, oxygen, and nitrogen emissions from the interstellar medium, the matter and radiation between stars, to provide insight into star birth and evolution and help map more of the Milky Way and nearby Large Magellanic Cloud galaxies.