Metasurface Opens World of Polarization

An SEM image of the device shows the irregular nanostructures created during the inverse design process. (Source: Z. Shi, Harvard SEAS)

Polarization is invisible to the human eye. Yet, so much of our optical world relies on the control and mani­pulation of this hidden quality of light. Materials that can manipulate the polari­zation of light are used in everything from digital alarm clocks to medical diag­nostics, communi­cations and astronomy. Just as light’s polarization can vibrate along a straight line or an ellipse, materials can also be linearly or ellip­tically bire­fringent. Today, most bire­fringent materials are intrinsi­cally linear, meaning they can only manipulate the polari­zation of light in a limited way. If you want to achieve broad polari­zation mani­pulation, you need to stack multiple bire­fringent materials on top of one another, making these devices bulky and ineffi­cient.

Now, researchers from the Harvard John A. Paulson School of Engi­neering and Applied Sciences have designed a meta­surface that can be continuously tuned from linear to elliptical bire­fringence, opening up the entire space of polari­zation control with just one device. This single metasurface can operate as many bire­fringent materials in parallel, enabling more compact polari­zation mani­pulation, which could have far-reaching applications in polarization imaging, quantum optics, and other areas.

“It is a new type of bire­fringent material,” said Zhujun Shi, a former graduate student at SEAS. “We are able to tailor broad polari­zation behavior of a material beyond what naturally exists, which has a lot of practical benefits. What used to require three separate conventional bire­fringent components now only takes one”. “The ability to manipulate a funda­mental property of light like polari­zation in completely new ways with a device that is compact and multi­functional will have important appli­cations for quantum optics and optical communi­cations,” said Federico Capasso.

Metasurfaces are arrays of nano­pillars spaced less than a wavelength apart that can perform a range of tasks, including mani­pulating the phase, amplitude and polari­zation of light. In the past, Capasso and his team have designed these highly ordered surfaces from the ground up, using simple geometric shapes with only a few design parameters. In this research, however, the team turned to a new type of design technique known as topological optimi­zation. “Topo­logical optimi­zation is an inverse approach,” said Shi. “You start with what you want the meta­surface to do and then you allow the algorithm to explore the huge parameter space to develop a pattern that can best deliver that function.”

The result was surprising. Instead of neatly ordered rectan­gular pillars standing like toy soldiers, this meta­surface is composed of nested half circles reminiscent of crooked smiley faces – more like something a toddler would draw than a computer. But these odd shapes have opened up a whole new world of bire­fringence. Not only can they achieve broad polari­zation mani­pulations like transforming linear polari­zation into any desired elliptical polari­zation but the polari­zation can also be tuned by changing the angle of the incoming light. “Our approach has a wide range of potential appli­cations across industry and scientific research, including polari­zation aberra­tion correction in advanced optical systems,” said Capasso. (Source: Harvard SEAS)

Reference: Z. Shi et al.: Continuous angle-tunable birefringence with freeform metasurfaces for arbitrary polarization conversion, Sci. Adv. 6, eaba3367 (2020); DOI: 10.1126/sciadv.aba3367

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Mass., USA

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