Building a Better Polarizer—With Metamaterials

Conventional polarizers work as filters, screening out the undesired polarization direction in incoming, unpolarized light. That means their transmission efficiency has a theoretical upper limit of 50 percent—and real-world polarizers tend to be substantially less efficient than that. A team of engineers at the University of Utah, USA, has now fashioned a new type of metamaterial-based polarizer that reportedly can transmit as much as 80 percent of incoming light as polarized output, well above the theoretical maximum for conventional polarizers (Optica, DOI: 10.1364/OPTICA.1.000356). The scientists believe that the new technique has prospects for improving low-light photography, building energy-efficient displays, and more.
 
Electromagnetic waves such as visible light oscillate in two orthogonal directions; conventional polarizers, operating based on birefringence properties of specific crystals and polymers, transmit only one of those two polarization directions. Various research groups have attempted to develop polarization techniques using alternative approaches, including surface gratings, metasurfaces with subwavelength structures, and perforated metal films. But even these devices have worked by manipulating only one polarization direction, thereby continuing to run up against the 50 percent theoretical limit.
 
The Utah researchers took a different approach, creating an all-dielectric polarizer from silicon that could manipulate both polarization directions. With the aid of a nonlinear optimization algorithm, they designed a structure consisting of nanoscale pillars and holes, and etched the pattern onto a silicon wafer using a focused gallium ion beam. The structure is designed in such a way that light with polarization parallel to the principal axis—that is, in the desired polarization direction—passes through the polarizer unmolested. In addition, however, the material rotates light with polarization perpendicular to the principal axis by 90 degrees, thereby nudging the light to the desired polarization direction.
 
By squeezing polarized output from both oscillation directions of the incoming wave, the new polarizing metamaterial can transmit as much as 30 percent more light than conventional polarizers, according to the researchers. That could be a boon to photographers, who routinely use polarizing filters to achieve certain effects, but are dogged by those filters’ tendency to darken the scene in low-light situations. The material could also allow for more energy-efficient LCD screens on smartphones and tablets, the current designs for which include polarizers that “throw away” much of their light in transmission. The Utah team is now working to scale up the technology, which they believe could be marketable in five to ten years.