By optimizing the material growth and manufacturing process, researchers at the China Academy of Engineering Physics' Research Center of Laser Fusion and the Institute of Applied Physics and Computational Mathematics in Beijing have nearly doubled the continuous-wave (cw) output power of terahertz quantum cascade lasers (QCLs) from the previous record of 138mW to a new record of 230mW (Xuemin Wang et al, AIP Advances 6, 075210 (2016)). Boosting the continuous-wave output power is an important step toward increasing the range of practical applications, which include medical imaging and airport security.
Sitting between microwaves and infrared light on the electromagnetic spectrum, terahertz radiation is relatively low energy and can penetrate materials such as clothing, wood, plastic and ceramics. Its unique qualities make it an attractive candidate for imaging, but the ability to produce and control terahertz waves has lagged behind technology for radio, microwave and visible light.
Recently, researchers have made rapid progress on technology to produce terahertz QCLs. The thin layers of material in a quantum cascade laser give it the valuable property of tunability, so the laser can be designed to emit at a chosen wavelength. The output power is also relatively high compared to other terahertz sources, notes the paper's first author Xuemin Wang of the Research Center of Laser Fusion at the China Academy of Engineering Physics.
In the new terahertz QCL, the optimal 2.9mm-long device operating at a frequency of 3.11THz has a low threshold current density of 270A/cm2 at a temperature of ∼15K. The maximum operating temperature was ∼65K in cw mode
Picture: Scanning electron microscope image of terahertz quantum cascade laser. (Credit: Wang, et al/AIP Advances.)
The research team is now focusing on future improvements to boost terahertz QCLs output power in cw mode to more than 1W (a level that has previously been reached in terahertz QCLs operating in pulsed-wave mode). "In engineering, biomechanics and medical science, the applications require continuous-wave mode," Wang says, adding that scientists and engineers could use the new laser as a flexible source of terahertz radiation in applications including spectroscopy, medical imaging, and remote sensing. In particular, the new terahertz QCL can be used in air, which is a challenge for lower-powered lasers since particles in the air can scatter or absorb the laser light before it reaches its target.
