An international team of applied scientists has demonstrated compact, multibeam and multiwavelength lasers emitting in the infrared. Typically, lasers emit a single light beam of a well-defined wavelength; with their multibeam abilities, the new lasers have potential uses in chemical detection, climate monitoring and communications. The research was led by postdoctoral researcher Nanfang Yu and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, both at the Harvard School of Engineering and Applied Sciences (SEAS); Hirofumi Kan, general manager of the Laser Group at Hamamatsu Photonics; and Jérôme Faist, professor at ETH Zürich. A computer rendering of one of the prototype multibeam, multifunctional lasers demonstrated by the team. The new laser emits several highly directional beams with the same wavelength near 8 µm, which requires two coherent beams. (Image: Capasso and Yu)In one of the prototypes demonstrated by the team, the new laser emits several highly directional beams with the same wavelength near 8 µm, a function very useful for interferometry, which requires two coherent beams: a probe beam and a reference beam. The probe beam interacts with a sample and recombines with the reference beam to reveal optical properties of the sample. A second type of laser emits multiple small divergence beams with different wavelengths (9.3 and 10.5 µm) into different directions. “We have demonstrated devices that can create highly directional laser beams pointing in different directions either at the same or at different wavelengths,” said Capasso. “This could have major implications for parallel high-throughput monitoring of multiple chemicals in the atmosphere or on the ground and be used, for example, for studying hazardous trace gases and aerosols, monitoring greenhouse gases, detecting chemical agents on the battlefield and mapping biomass levels in forests.” The more versatile laser is a descendant of the quantum cascade laser (QCL), invented and first demonstrated by Capasso, Faist and their collaborators at Bell Labs in 1994. Commercially available QCLs, made by stacking ultrathin atomic layers of semiconductor materials on top of one another, can be custom designed to emit a well-defined infrared wavelength for a specific application or be made to emit simultaneously multiple wavelengths. To achieve multiple beams, the researchers patterned the laser facet with metallic structures that behave as highly directional antennas and then beam the light in different directions. “Having multibeam and multiwavelength options will provide unprecedented flexibility. The ability to emit multiple wavelengths is ideal for generating a quantitative map of the concentration of multiple chemicals in the atmosphere,” said Kan. “Profiles of these atmospheric components – as a function of altitude or location – are critically important for environmental monitoring, weather forecasting and climate modeling.” Though the researchers demonstrated the idea using mid-infrared semiconductor lasers emitting wavelengths in the 8- to 10-µm range, the concept can be generalized to lasers emitting other wavelengths in the near-infrared and terahertz spectrum, or to passive optical components such as optical fibers. For example, nanostructures can be patterned on the facet of optical fibers to help build microendoscopes for in vivo diagnostics, they said. The findings appeared online in the Oct. 23 issue of Applied Physics Letters and will appear as a Dec. 7 cover story. The team’s co-authors included graduate students Mikhail A. Kats and Markus Geiser, research associate Christian Pflügl, all from SEAS; and Qi Jie Wang, now an assistant professor at Nanyang Technical University in Singapore; researchers Tadataka Edamura, Shinichi Furuta and Masamichi Yamanishi, all from Hamamatsu Photonics; and researchers Milan Fischer and Andreas Wittmann, both from the Institute of Quantum Electronics, ETH Zürich. The work was partially supported by Air Force Office of Scientific Research and Harvard’s Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network. For more information, visit: www.seas.harvard.edu