Harvard scientists demonstrate laser with controlled polarization

April 14, 2009 10:17 am 

WASHINGTON, April 14 — U.S. and Japanese scientists have demonstrated, for the first time, lasers in which the direction of oscillation of the emitted radiation, known as polarization, can be designed and controlled at will. The innovation opens the door to a wide range of applications in photonics and communications.

Spearheaded by graduate student Nanfang Yu and Federico Capasso, Robert L. Wallace, Professor of Applied Physics of Harvard School of Engineering and Applied Sciences (SEAS), the findings were published Monday as a cover feature of the Applied Physics Letters. Harvard University has filed a broad patent on the invention.

"Polarization is one of the key features defining a laser beam. Controlling it represents an important new step towards 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. This 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 the capacity of the channel, circularly-polarized light sources are necessary to detect certain biomolecules, and laser sources with a variety of polarization states have relevance for quantum cryptography.

To achieve the 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 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 along an arbitrary direction and circularly polarized light. (PNA/Xinhua) ALM/ebp

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