• Harvard engineers have built a chip-scale, twisted bilayer photonic crystal whose twist angle and spacing can be dynamically adjusted with a MEMS actuator to tune the chirality of light.
• The geometrically chiral device can distinguish between left- and right-circular polarized light that hits perpendicular to its surface and offers a design framework for tunable, integrable chiral photonic devices.

Researchers in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have created a chip-scale device that can dynamically control the “handedness” of light as it passes through – also known as its optical chirality – with a simple twist of two specially designed photonic crystals.

The work, led by graduate student Fan Du in the lab of Eric Mazur, the Balkanski Professor of Physics and Applied Physics, describes a reconfigurable twisted bilayer photonic crystal that can be tuned in real time using an integrated micro-electromechanical system (MEMS). The breakthrough opens new possibilities for advanced chiral sensing, optical communication, and quantum photonics.

“Chirality is very important in many fields of science – from pharma to chemistry, biology, and of course, physics and photonics,” Mazur said. “By integrating twisted photonic crystals with MEMS, we have a platform that is not only powerful from a physics standpoint but also compatible with the way modern photonics are manufactured.”

Photonic crystals are nanofabricated materials that fit on the head of a pin and are used to manipulate light at nanoscale wavelengths, important in many optical elements today for computing, sensing, and high-speed communications. Mazur’s lab has been pushing photonic crystal engineering into new territory by borrowing from principles of twistronics, made famous by the discovery of twisted bilayer graphene. In recent years, Mazur’s team has pioneered twisted bilayer photonic crystals by stacking two patterned membranes of silicon nitride and rotating them to garner new properties.

In their new work published in Optica, they show that a twisted bilayer photonic crystal is a powerful platform to control chirality of light, because the twist naturally introduces a built-in left–right asymmetry to the device. Chirality describes objects that cannot be superimposed on their mirror images, like left and right hands. In optics, chirality manifests in both chiral materials and structures, as well as in chiral light, in which the light traces a helix in a particular direction as it propagates. Chiral light can rotate clockwise – exhibiting right-circular polarization – or counter-clockwise – exhibiting left-circular polarization.

These differences in how light travels are tiny, but important. For example, organic chemists must be able to distinguish between mirror-image pharmaceutical molecules that are chemically identical but have different effects in the body. Among the best-known examples is thalidomide, a 1950s pharmaceutical whose right-handed molecular structure was a treatment for morning sickness in pregnant women, but whose left-handed mirror image caused birth defects.

Chiral light is used to probe chiral structures, typically by using conventional polarization optics such as wave plates and linear polarizers. These items are static and can only detect a small range of polarization.

By contrast, the Harvard researchers’ new device is elegantly designed to be tunable – that is, the device’s response to different types of chiral light can be dialed up or down without switching out any parts. The secret is the bilayer design: When the two photonic crystals are brought close together and twisted, the combined structure becomes geometrically chiral and able to “read” chiral light. Strong coupling between the layers’ optical modes leads to dramatically different transmissions for left- or right-circular polarized light under “normal incidence,” or polarized light that hits perpendicular to the surface.

By using the MEMS device to continuously vary the twist angle and interlayer spacing, the team showed they could tune the device’s intrinsic ability to read different chiral light modes that approach theoretical extremes of perfect selectivity for distinguishing “handedness” of light.

The paper provides a general design framework for twisted bilayer crystals that exhibit optical chirality. Though currently a proof of concept, the research could pave the way for future applications in chiral sensing, where devices are tuned to probe different chiral molecules at different wavelengths, or dynamic light modulators for optical communications, enabling on-chip control of light.

The paper, “Dynamic Control of Intrinsic Optical Chirality via MEMS-Integrated Photonic Crystals,” was co-authored by Haoning Tang, Yifan Liu, Mingjie Zhang, Beicheng Lou, Guangqi Gao, Xuyang Li, Alsyl Enriquez, and Shanhui Fan.