Terahertz laser for sensing, imaging outperforms predecessors
As a result, say the researchers, the laser could prove valuable for a wide range of applications in chemical sensing and imaging, ranging from detecting interstellar elements in an upcoming NASA mission to improved skin and breast cancer imaging, detecting drugs and explosives, and much more.
Terahertz lasers can send coherent radiation into a material to extract a material’s spectral “fingerprint,” as different materials absorb terahertz radiation in different degrees, resulting in a unique fingerprint that appears as a spectral line. This is especially valuable in the 1 to 5-terahertz range. For contraband detection, for example, heroin’s signature is seen around 1.42 and 3.94 terahertz, and cocaine’s is at around 1.54 terahertz.
In previous work, the researchers had developed a photonic wire laser – a laser with transverse dimensions much smaller than the wavelength – that was able to achieve high efficiency and beam quality, though it lacked frequency tuning. It was selected as a component of the chemical detector on NASA’s Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO), a high-altitude balloon-based telescope carrying photonic wire lasers for detecting oxygen, carbon, and nitrogen emissions from the “interstellar medium,” the cosmic material between stars.
Building on the previous design, the researchers’ new laser 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, achieving two of the key performance goals.
Adjustments to the individual coupled lasers achieves the third performance metric by allowing for broad frequency tuning to improve resolution and fidelity in the measurements. Achieving all of the three performance metrics at once, say the researchers, means less noise and higher resolution, for more reliable and cost-effective chemical detection and medical imaging.
“People have done frequency tuning in lasers, or made a laser with high beam quality, or with high continuous wave power,” says Ali Khalatpour, a graduate student in electrical engineering and computer science and first author on a paper describing the laser. “But each design lacks in the other two factors. This is the first time we’ve achieved all three metrics at the same time in chip-based terahertz lasers.”
The inspiration for the new design came from organic chemistry, says Khalatpour. The researchers applied the concept of “pi-bonding” – a chemical bond where molecular orbitals overlap to make the bond more stable – to their lasers, creating close connections between otherwise-independent wire lasers along an array. The novel coupling scheme allows phase-locking of two or multiple wire lasers.
To achieve frequency tuning, the researchers used tiny “knobs” 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.
In experiments, the researchers fabricated an array of 10 pi-coupled wire lasers. The laser array operated with continuous frequency tuning in a span of about 10 gigahertz, and a power output of roughly 50 to 90 milliwatts, depending on how many pi-coupled laser pairs were on the array. The beam had a low beam divergence – a measure of how much the beam strays from its focus over distances – of 10 degrees.
The researchers are also currently building a system for imaging with a high dynamic range – greater than 110 decibels – which can be used in many applications, including 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 such an application, however, are massive and inefficient, say the researchers, and not frequency-tunable. Their new chip-sized device, however, matches or outstrips those lasers in output power, and offers tuning capabilities.
“Having a platform with all those performance metrics together,” says Khalatpour, “could significantly improve imaging capabilities and extend its applications.”
For more, see “Phase-locked photonic wire lasers by pi coupling.”
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