CAMBRIDGE, Mass., Dec. 20, 2018 — A new terahertz (THz) laser designed by researchers at Massachusetts Institute of Technology (MIT) has demonstrated high constant power, tight beam pattern, and broad electric frequency tuning — three performance metrics that could mean less noise and higher resolution, for more reliable and cost-effective chemical detection and medical imaging. The laser has been selected by NASA for a 2021 mission to detect chemical emissions in the “interstellar medium” — that is, the cosmic material between stars.

A tiny terahertz laser designed by MIT researchers is the first to reach three key performance goals at once: high power, tight beam, and broad frequency tuning. Courtesy of A. Khalatpour, J. Reno, and Q. Hu.

The laser design pairs multiple semiconductor-based wire lasers and directs the lasers to synchronize oscillations (phase-lock). By creating close connections between otherwise independent wire lasers, along an array, the researchers allowed phase-locking of two or more wire lasers.

To build a laser with the frequency tuning capabilities required by NASA, the researchers took inspiration from an unlikely source: organic chemistry. The team knew that polymer chains with atoms lined along two sides are “pi-bonded,” meaning their molecular orbitals overlap to make the bond more stable. It applied the concept of pi-bonding to its lasers, phase-locking multiple lasers by pi (π) coupling.

The researchers used tiny “knobs” to change the current of each wire laser, which in turn caused a slight change to how light traveled through the laser. A change in the refractive index, when applied to the coupled lasers, created a continuous frequency shift to the pair’s center frequency. Adjustments to the individual coupled lasers allowed for broad frequency tuning to improve resolution and fidelity in the measurements. The output of the pairs combines to produce a single, high-power beam with minimal beam divergence.

To test the lasers, the researchers fabricated an array of 10 π-coupled wire lasers. The lasers operated with continuous frequency tuning in a span of about 10 gigahertz (GHz), and with a power output of roughly 50 to 90 milliwatts (mW), depending on how many π-coupled laser pairs were on the array. The beam demonstrated a low beam divergence of 10 degrees.

In addition to readying their lasers for the NASA mission, the researchers are using their laser technology to build a system for imaging with high dynamic range — greater than 110 decibels — which could be used for skin cancer imaging and other applications. The team said its chip-size device matches or outstrips larger, less efficient THz lasers in output power, in addition to offering tuning capabilities.

According to the researchers, theirs is the first THz laser to achieve continuous wave power, high beam quality, and frequency tuning all in one laser. “People have done frequency tuning in lasers, or made a laser with high beam quality, or with high continuous wave power. But each design lacks in the other two factors,” said researcher Ali Khalatpour. “This is the first time we've achieved all three metrics at the same time in chip-based terahertz lasers. Having a platform with all those performance metrics together could significantly improve imaging capabilities and extend its applications.”

about THz lasers: Terahertz (THz) lasers can send coherent radiation into a material to extract the material’s spectral fingerprint. Different materials absorb THz radiation to different degrees, meaning each has a unique fingerprint that appears as a spectral line. This is especially valuable in the 1- to 5-THz range. For contraband detection, for example, heroin’s signature is seen around 1.42 and 3.94 THz, and cocaine’s at around 1.54 THz.