Cool terahertz quantum cascade laser speeds chemical fingerprinting

May 23, 2016 // By Julien Happich
Researchers from MIT’s Research Laboratory of Electronics have proven a new terahertz spectroscopy system that uses a quantum cascade laser (QCL) to extract a material’s spectroscopic signature in just 100 microseconds.

While traditional terahertz spectroscopy requires a radiation source that’s heavy and about the size of a large suitcase, and multiple measures at different manually adjusted frequencies, the new chip-based system could perform fully automated absorption spectrums and chemical identification under milliseconds.

Lead author of the paper "Terahertz multiheterodyne spectroscopy using laser frequency combs" in the last issue of Optica, Yang Yang, a graduate student in electrical engineering and computer science explains that QCL-based frequency combs allow to mathematically reconstruct a material’s absorption fingerprint from just a few measurements, without any mechanical adjustments, hence the fast automation enabled through electronic integration.

The main breakthrough here was to even out the spacing in the comb of the QCLs so as to emit radiation at multiple frequencies determined by the length of the gain medium.

To even out their lasers’ frequencies, the MIT researchers and their colleagues used an oddly shaped gain medium, with regular, symmetrical indentations in its sides that alter the medium’s refractive index and restore uniformity to the distribution of the emitted frequencies. They developed a new gain medium that produces a single, unbroken frequency comb, consisting of hundreds of alternating layers of gallium arsenide and aluminum gallium arsenide, with different but precisely calibrated thicknesses.

Another interesting feature of this new QCL implementation is that it was demonstrated to extract a reliable spectroscopic signature from a target using only very short bursts of terahertz radiation. Being approximately on only 1 percent of the time, the laser does not need as much cooling as other terahertz sources would require, enabling a more compact solution.


An artist’s rendering of the “gain medium” used to produce terahertz frequency combs, with different colours indicating different wavelengths of oscillating terahertz radiation travelling different distances through the medium, which has a different refractive index for each of them. Image: Yan Liang/L2Molecule.com

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