The research was led at Harvard by graduate student Nanfang Yu and Federico Capasso, a professor of applied physics and a senior research fellow in electrical engineering, and at Hamamatsu by Hirofumi Kan, general manager of the company's laser group.
Polarization is one of the key features defining a laser beam. "Controlling [polarization] represents an important new step toward beam engineering of lasers with unprecedented flexibility, tailored for specific applications," explains Capasso. "The novelty of our approach is that instead of being conducted externally, which requires bulky and expensive optical components, manipulation of the beam polarization is achieved by directly integrating the polarizer on the laser facet." Capasso notes that the compact solution is applicable to semiconductor lasers and other solid-state lasers, all the way from communication wavelengths to the mid-infrared and terahertz spectrum.
Light sources with a desirable polarization state are useful for a wide variety of applications. For example, satellite communications use two orthogonal polarizations to double a channel's capacity. Lasers featuring a variety of polarization states also have relevance for quantum cryptography.
To achieve their results, the researchers sculpted a metallic structure, dubbed a "plasmonic polarizer," directly on the facet of a quantum cascade (QC) laser. The QC laser emitted light at a wavelength of ten microns (in the invisible part of the spectrum known as the mid-infrared, where the atmosphere is transparent). The team was able to control the state of polarization by generating both linearly polarized light in an arbitrary direction and circularly polarized light.
The research teams' other members are postdoctoral researcher Qijie Wang and research associates Christian Pflügl and Laurent Diehl (all from Harvard) and researchers Tadataka Edamura, Sninichi Furuta and Masamichi Yamanishi (all from Hamamatsu Photonics).
The research was partially supported by the Air Force Office of Scientific Research. The project also received the support of two Harvard-based centers, the NSF-funded Nanoscale Science and Engineering Center (NSEC) and the Center for Nanoscale Systems (CNS), which is a member of the National Nanotechnology Infrastructure Network (NNIN).
Harvard says it has filed for a broad patent on the invention.
