For some time, laboratory spectroscopy has used long-wavelength infrared light to identify and characterize target molecules. The sources in the mid- to the far-IR that can excite the molecular modes are cumbersome, however, ensuring that the far-IR regime that is so useful in the laboratory is inaccessible in the field. Now a research team at Lucent Technologies Inc.'s Bell Labs has created a 19-µm quantum cascade laser, making the possibility of field-deployable far-IR devices quite visible. The laser is a semiconductor superlattice of charge-injector quantum and active-area wells. The injector region comprises quantum wells of varying thickness, with barriers that are thin enough to allow significant overlap between the wave functions of each well and that create a nonlocalized electron state. The novelty of quantum cascade lasers is that the energy difference determining the wavelength of the laser is a function of the thickness of the layers. To some extent, controlling the thickness during molecular beam epitaxy can tune the wavelength of the emission, but the materials in the device must be transparent at the desired wavelength. The GaInAs, AlGaInAs and AlInAs in the new device support 19-µm operation, but only with an additional design modification. Waveguide is key If the long wavelength were transmitted through the stacked layers, absorption would weaken the lasing action. The scientists, therefore, incorporated a surface plasmon waveguide along the interface of the metal and the semiconductor, through which the radiation from the active regions of the quantum cascade superlattice would propagate. The result is a semiconductor laser that emits 14-mW pulses of far-IR light at —124 °C. Several issues must be addressed to increase the operating temperature, said Claire Gmachl, a member of the team, which described the device in the Oct. 9, 2000, issue of Applied Physics Letters. Factors such as the temperature dependence of electron lifetimes and thermal management of the laser will determine its feasibility. For now, the researchers say the surface plasmon waveguide is opening the way to new applications. "This is the key to making long-wavelength lasers," Gmachl said.