Researchers have developed a high-power, portable version of a device called a quantum cascade laser that can generate terahertz radiation outside of a laboratory setting. The laser could potentially be used in applications such as pinpointing skin cancer and detecting hidden explosives.
Until now, generation of terahertz radiation powerful enough to perform real-time imaging and fast spectral measurements required temperatures far below 200 kelvin (-100 °F) or lower. These temperatures could only be achieved with bulky equipment that limited the technology’s use to a laboratory setting. The terahertz quantum cascade laser can function at temperatures of up to 250 K (-10 °F), meaning that only a compact portable cooler is required.
The lasers, which measure only a few millimeters in length and are thinner than a human hair, are quantum well structures with meticulously custom-engineered wells and barriers. Within the structure, electrons “cascade” down a kind of staircase, emitting a light particle, or photon, at each step.
One important innovation was the doubling of the height of the barriers within the laser to prevent leakage of the electrons, a phenomenon that tended to increase at high temperatures. Previously, higher barriers were explored sporadically but they yielded inferior results. The researchers developed the correct parameters for the band structure for tall barriers and a conceptually novel optimization scheme for the design.
This innovation was paired with a direct phonon scheme that keeps the laser operating through a configuration in which lower lasing levels of each module, or steps of the structure’s staircase, are quickly depopulated of electrons through phonon (or a unit of vibrational energy) scattering into a ground state, which then serves as the injector of electrons into the next step’s upper level and the process repeats. Such an arrangement of the electrons in the system is essential for lasing to occur.
There are close to 15,000 interfaces between quantum wells and barriers, half of which are not even seven atomic layers thick. The quality and reproducibility of these interfaces are of critical importance to the performance of terahertz lasers.
In a medical setting, the new portable system — which includes a compact camera and detector and can operate anywhere with an electric outlet — could provide real-time imaging during regular skin cancer screenings or even during surgical procedures to excise skin cancer tissues. The cancer cells show up dramatically in terahertz because they have higher water and blood concentrations than normal cells.
The technology could also be applied in many industries where the detection of foreign objects within a product is necessary to assure its safety and quality. Detection of gases, drugs, and explosives could become especially sophisticated with the use of terahertz radiation. Compounds such as hydroxide, an ozone-destruction agent, have a special spectral “fingerprint” within the terahertz frequency range, as do drugs including methamphetamines and heroin, and explosives including TNT.