Researchers from MIT‘s Research Laboratory of Electronics have created a new terahertz spectroscopy system which they discuss in depth in the latest issue of Optica journal. Their system uses a quantum cascade laser. The device consumes radiation that is merely the size of a computer chip and can extract the spectroscopic signature of a single material in about 100 microseconds.
Terahertz spectroscopy has been a promising security technology due to its ability to extract the spectroscopic fingerprints of a massive range of materials, most importantly the chemicals present in explosives. Terahertz spectroscopy uses the band of electromagnetic radiation in between microwaves and infrared light and up until recently required a lot of radiation to work. Up until the most recent studies, detection would take as long as 30 minutes per sample being tested, making it a step in the right direction for the technology but nothing that could be relied upon for quick and simple results.
Yang Yang, first author on the new paper says this work answers the question of the true application of quantum-cascade laser frequency combs. He goes on to explain that terahertz is a unique region and that spectroscopy is probably its best application. QCL-based frequency combs are a wonderful candidate for spectroscopy to be applied to.
Materials absorb unique frequencies of terahertz radiation and at different degrees, which means they have a unique terahertz-absorption profile. In traditional testing, terahertz spectroscopy required the measurement of a materials response to each frequency on its own. The method was extremely time-consuming because testers were required to readjust the spectroscopic apparatus for each material being tested. Since the frequencies within a frequency comb are the same distance apart from one another, a material’s absorption fingerprint can be mathematically reconstructed with just a few measurements. This removes the need for mechanical adjustments.
Quantum cascade lasers bounce electromagnetic radiation through a gain medium until the radiation builds up enough energy to escape the space. These lasers emit radiation on a wide range of frequencies that depend on the gain medium’s length. Those frequencies depend on the refractive index of the medium (the speed electromagnetic radiation passes through). Since the refractive index varies depending on the frequencies, so do the gaps between frequencies in the comb.
In order to even out laser frequencies, the researchers at MIT used a uniquely shaped gain medium that had both regular and symmetrical indentations on its sides, changing the refractive index to restore overall uniformity to the emitted frequencies. This finding was published in 2014 in Nature Photonics journal. Unfortunately, the first prototype emitted two frequency combs that clustered around different central frequencies, leaving a gap which made them a difficult application in spectroscopy.
During the latest research, first authors Yang and Burghoff along with the Netherlands Institute of Space Research built a new medium that produces unbroken frequency combs. The research was tested by using the new system to measure the spectral signature of an optical device known as the etalon. Etalon is made from a water of gallium arsenide, and has spectral properties that may be calculated in advance. Since the measurements were a great fit, this suggests that the etalon terahertz-transmission profile could be very beneficial in detecting chemicals.
Hu and his group are currently working on designing higher temperature quantum cascade lasers. Thankfully, Yang and his colleagues have shown that reliable spectroscopic signatures can be extracted with short-period use of terahertz radiation. This means, soon terahertz spectroscopy may be helpful even at very low temperatures.