The trick to getting a usable amount of terahertz light out at room temperature, says Capasso, was to take advantage of what the quantum-cascade laser does best: emit mid-infrared light. Thus, the laser was designed to emit two different wavelengths of infrared light into a specially designed region of semiconductor layers within the chip. Due to the properties of this special region, the incoming infrared photons mix with the atoms in the semiconductor layers and are converted into a single outgoing beam of terahertz radiation.
This light-mixing process is not a new phenomenon. In fact, it’s a well-established phenomenon in a branch of science called nonlinear optics. The same procedure–taking two incoming beams and producing a beam with a different wavelength–is actually an established method for producing terahertz radiation in larger systems. For instance, two mid-infrared lasers can shine light into a special nonlinear crystal, and the outcome is a terahertz beam. This setup, however, tends to take up as much space as a dining-room table. By building a region in the semiconductor laser that acts just like a larger nonlinear crystal, Capasso simply miniaturized an established yet bulky terahertz system.
“The significance [of the research] lies both in the fact that room-temperature operation has been achieved … and in the considerable power, up several orders of magnitude from earlier work,” says Claire Gmachl, a professor of electrical engineering at Princeton University. “Room-temperature operation is a big plus for essentially all applications,” she says, because it makes systems simpler to use and more cost effective.
Still, there is room for improvement, says Capasso. At room temperature, the terahertz laser currently emits a microwatt of power, but it would need to produce at least a milliwatt for practical applications. Capasso’s plan for more power is to modify the design. Currently, the laser shines light out of a narrow rectangular face on the side of the chip, which limits the total output. But by forcing light to come out the top of the chip, which has a much larger surface area than the side, the power could be boosted by an order of magnitude. A simple grating, added to the chip when it’s built, would channel the light out the wider surface, says Capasso. What’s more, he notes, adding a relatively small and inexpensive thermoelectric cooler to the laser could eke out even more power.