Researchers at Cornell University (Ithaca, NY) have demonstrated that the polarization of a probe laser beam can be completely controlled by a second laser tuned to a slightly different wavelength. With further development, the technique could control polarization at deep ultraviolet or infrared wavelengths—where no such control currently exists—or be used in conjunction with a polarizing beamsplitter as a two-port optical switch.
Researchers Stephan Wielandy and Alexander Gaeta demonstrated that an isotropic atomic vapor, in this case a cell of rubidium gas, can be made to exhibit either linear or circular birefringence near one atomic transition by applying a "control" laser field tuned near another optical transition. Using a quantum coherence effect closely related to electromagnetically induced transparency, a control laser changed the refractive index of rubidium vapor for one polarization component of a "probe" laser beam while leaving the refractive index for the other component unchanged. This process caused the vapor to behave as a birefringent material that could be used to rotate the polarization of the probe laser beam.
Previous experiments have demonstrated laser-induced birefringence but either produced an effect too small to substantially rotate a laser beam's polarization or required the use of high-intensity pulsed lasers. By contrast, this new method allows a laser beam's polarization to be placed in any arbitrary state with only minimal loss from absorption. Wielandy says this work is a qualitative improvement because it allows light-induced polarization control at intensities accessible by continuous-wave lasers and because the high efficiency makes feasible the construction of practical optical devices.
He adds, "The coherence-induced birefringence we have observed can also be demonstrated in systems with different energy-level schemes. This flexibility is important in adapting our technique to a variety of applications."
The initial research was funded by the US Office of Naval Research (Washington, DC). With additional funding, Gaeta says he would like to investigate how this technique could be applied to the fabrication of a high-bandwidth optical switch.
Conard Holton
Conard Holton has 25 years of science and technology editing and writing experience. He was formerly a staff member and consultant for government agencies such as the New York State Energy Research and Development Authority and the International Atomic Energy Agency, and engineering companies such as Bechtel. He joined Laser Focus World in 1997 as senior editor, becoming editor in chief of WDM Solutions, which he founded in 1999. In 2003 he joined Vision Systems Design as editor in chief, while continuing as contributing editor at Laser Focus World. Conard became editor in chief of Laser Focus World in August 2011, a role in which he served through August 2018. He then served as Editor at Large for Laser Focus World and Co-Chair of the Lasers & Photonics Marketplace Seminar from August 2018 through January 2022. He received his B.A. from the University of Pennsylvania, with additional studies at the Colorado School of Mines and Medill School of Journalism at Northwestern University.