A terahertz laser was developed with high constant power, tight beam pattern, and broad electric frequency tuning for a wide range of applications in chemical sensing and imaging. The optimized laser can be used to detect interstellar elements in an upcoming NASA mission that aims to learn more about the galaxy’s origins. The photonic wire laser could also be used for improved skin and breast cancer imaging, detecting drugs and explosives, and more.
The laser’s design pairs multiple semiconductor-based, efficient wire lasers and forces them to “phase lock,” or sync oscillations. Combining the output of the pairs along the array produces a single, high-power beam with minimal beam divergence. Adjustments to the individual coupled lasers allow for broad frequency tuning to improve resolution and fidelity in the measurements. Achieving all three performance metrics means less noise and higher resolution for more reliable and cost-effective chemical detection and medical imaging.
Terahertz lasers can send coherent radiation into a material to extract the material’s spectral “fingerprint.” Different materials absorb terahertz radiation to different degrees, meaning each has a unique fingerprint that appears as a spectral line. This is especially valuable in the 1-5 terahertz range; for example, for contraband detection, heroin’s signature is seen around 1.42 and 3.94 terahertz, and cocaine’s at around 1.54 terahertz.
Photonic wire lasers are bidirectional, meaning they emit light in opposite directions, which makes them less powerful. In traditional lasers, that issue is easily remedied with carefully positioned mirrors inside the laser’s body. But it’s very difficult to fix in terahertz lasers, because terahertz radiation is so long, and the laser so small, that most of the light travels outside the laser’s body.
To achieve frequency tuning, tiny “knobs” were used to change the current of each wire laser, which slightly changes how light travels through the laser — called the refractive index. That refractive index change, when applied to coupled lasers, creates a continuous frequency shift to the pair’s center frequency.
The researchers are also building a system for imaging with high dynamic range — greater than 110 decibels — that can be used in applications such as skin cancer imaging. Skin cancer cells absorb terahertz waves more strongly than healthy cells, so terahertz lasers could potentially detect them. The lasers previously used for the task, however, are massive and inefficient, and not frequency-tunable. The new chip-sized device matches or outstrips those lasers in output power and offers tuning capabilities.