Scientists demonstrate multibeam, multi-functional lasersAn international team of applied scientists from Harvard, Hamamatsu Photonics, and ETH Zürich have demonstrated compact, multibeam, and multi-wavelength lasers emitting in the invisible part of the light spectrum (infrared). By contrast, typical lasers emit a single light beam of a well-defined wavelength. The innovative multibeam lasers have potential use in applications related to remote chemical sensing pollution monitoring, optical wireless, and interferometry.
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. The findings appeared online in the October 23 issue of Applied Physics Letters and will appear as a December 7 cover story.
“We have demonstrated devices that can create highly directional laser beams pointing in different directions either at the same or at different wavelengths,” says 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 ultra-thin 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 multi-wavelength 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,” explains Kan. “Profiles of these atmospheric components—as a function of altitude or location—are critically important for environmental monitoring, weather forecasting, and climate modeling.”
The team’s co-authors included graduate students Mikhail A. Kats and Markus Geiser, research associates 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, 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 (CNS), a member of the National Nanotechnology Infrastructure Network.
Supplementary Technical Information
In one of the prototypes demonstrated by the team the new laser emits several highly directional beams with the same wavelength near 8 microns, 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 laser emits multiple small divergence beams with different wavelengths (9.3 and 10.5 microns) into different directions.
The two functionalities are realized by sculpting on the laser facet metallic structures consisting of sub-wavelength size apertures and multiple sets of gratings with different periods. The aperture couples part of the emitted light into surface electromagnetic waves (so-called surface plasmons) on the laser facet. As the surface waves propagate on the facet, they are progressively scattered by the grating grooves and are reemitted. The period and the number of grooves in one specific grating control, respectively, the direction and the intensity of the beam originating from the grating. Therefore, without resorting to external optical elements such as lenses, collimators and beam splitters, the researchers have demonstrated compact highly versatile lasers emitting multiple beams. Depending on the design of the quantum wells in the active region of the quantum cascade laser and of the gratings patterned on the laser facet the lasers can be made to emit simultaneously multiple wavelengths in different directions.
Though the researchers demonstrated the idea using mid-infrared semiconductor lasers emitting wavelengths in the 8-10 micron 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 micro-endoscopes for in-vivo diagnostics.
