Conventional lasers energize electrons, which then emit a single photon by jumping from the semiconductor's conduction band to its valence band. Quantum cascade lasers, on the other hand, arrange a stair-step of quantum wells--each at a progressively lower energy level--that allow electrons to cascade down an energy staircase, emitting a photon at each step. Today, quantum cascade lasers lose their ability to work in the terahertz gap without supercooling. But by using a heterodyning architecture, the Harvard researchers demonstrated twin quantum cascade lasers, whose mixed output is in the terahertz gap.
The heterodyning principle is well known in nonlinear optics as difference frequency generation (DFG). Most materials act like linear harmonic oscillators when light impinges on them, oscillating only when the frequency matches their own internal natural resonant frequency. Nonlinear materials like vacuum tubes and transistors, on the other hand, can be made to resonate at the sum and difference frequencies of two inputs, enabling radios to move signals between bands, or to encode and decode them.
Others have demonstrated the feasibility of terahertz lasers using DFG, but bulky external "pump" lasers were used just to prove the principle. The Harvard group accomplished the task with semiconductor materials that, if all goes well, eventually could be mass produced for inexpensive room-temperature devices.
"Our device does everything in one small semiconductor crystal with no need for bulky external lasers for pumping; hence, the advantages of compactness, portability and low power consumption," said Capasso. "In essence, the material of the device is designed and grown so that when a bias current is applied to it, not only are laser beams emitting at two different mid-infrared frequencies generated, but also coherent radiation at the difference frequency corresponding, in our case, to 5 Terahertz".
The mechanism by which nonlinear devices perform operations like mixing--generating sum and difference frequencies--depends on the materials used. The quantum cascade laser is fabricated using molecular-beam epitaxy, a layer of atoms at a time, from alternating layers of gallium and aluminum. Each layer is slightly thinner than the one before it.
Next, the Harvard researchers plan to optimize their design in an attempt to increase the output power to milliwatts, from its nanowatt levels today. One way is to add low-cost thermoelectric coolers to the laser's substrate--since the cooler the laser runs, the higher its output power. Secondly, the group plans to switch from edge emission to surface emission for their semiconductor material.
"Our approach will be to greatly increase the surface area used for emission," said Capasso. "Surface emission will be achieved by fabricating a suitable grating to scatter vertically the terahertz radiation generated in the device's active region."
Belkin and Capasso performed the work in cooperation with researchers Feng Xie and Alexey Belyanin, at Texas A&M University (College Station), and researchers Milan Fischer, Andreas Wittmann, and Jrme Faist, at ETH (Zurich, Switzerland). Funding was provided by the Air Force Office of Scientific Research, the National Science Foundation and two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.