polarization control

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 (see photo).

polarization control

W. Conard Holton

One optical field coherently controls another

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 (see photo).

Researchers Stephan Wielandy and Alexander Gaeta demontrated 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.

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