New research out of MIT could lead to smarter airport scanners able to detect the presence of drugs and explosives. At the heart of the development is a new approach to laser tuning designed to harness terahertz rays so that they can be used to determine an object's chemical composition.
Thought to be safer than x-rays, terahertz rays (or T-rays) can pass through things like clothing, wood, masonry, plastic, ceramics and human tissue. Because åthey are absorbed to different degrees by different molecules, they can be used to tell chemicals apart.
The problem is that to achieve this with accuracy, the rays need to be applied in a continuous range of terahertz frequencies, something that has proved difficult to achieve using current laser tuning methods.
The solution proposed by ProfessorQing Hu and his colleagues at MIT’s Research Laboratory of Electronics involves a the first practical method for tuning the lasers used to produce terahertz radiation (known as quantum cascade lasers).
The analogy of a guitar string is used to explain the process:
Tuning a laser usually requires either changing the length of its light-emitting cavity or changing its temperature. Hu compares these two approaches to changing the pitch of a guitar string by pressing down on it — changing its length — or screwing its tuning peg — changing its tension. Neither approach, however, works very well with terahertz quantum cascade lasers.
A third way to change the pitch of a guitar string, however, is to change its diameter: the lower-pitched strings on a guitar are thicker than the higher-pitched ones. And Hu’s tuning technique is, roughly speaking, to change the diameter of the light beam.
A light beam confined in, say, an optical fiber or a long, thin, quantum cascade laser exhibits an electromagnetic-field pattern called a “transverse mode.” The transverse mode is kind of like another electromagnetic wave that’s perpendicular to the light beam, except that unlike a light beam, it dies off very quickly.
Using a particular type of quantum cascade laser called a wire laser, Hu's team have demonstrated that the wavelength of the emitted light can be changed by deforming the transverse mode with a block of a material like metal (which shortens the wavelength) or silicon (which lengthens it). How close the blocks are brought to the laser also varies the extent of the shift, therefore providing an accurate control mechanism.
The next stage is to design an electronically controlled tuning device that can control the metal and silicon blocks in order to deliver the continuous range of terahertz frequencies required.
The Paper “Tuning a terahertz wire laser” by Qi Qin, Benjamin S. Williams, Sushil Kumar, John L. Reno and Qing Hu, was published in Nature Photonics on November 22.