A twisted pair of photonic crystals integrated with MEMS can dynamically control the handedness of light on a chip.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a chip scale device that can actively control the “handedness” of light as it moves through it, a property known as optical chirality. The system works by slightly twisting two specially engineered photonic crystals.

The project was led by graduate student Fan Du in the laboratory of Eric Mazur, the Balkanski Professor of Physics and Applied Physics. The team designed a reconfigurable twisted bilayer photonic crystal that can be adjusted in real time using an integrated microelectromechanical system (MEMS). The advance could enable new tools for chiral sensing, optical communications, 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 structures small enough to fit on the head of a pin. They are designed to control how light behaves at nanoscale wavelengths and are used in many optical technologies, including computing, sensing, and high-speed communications.

Mazur’s group has been expanding the possibilities of photonic crystal engineering by drawing inspiration from twistronics, a research area that gained attention after the discovery of twisted bilayer graphene. In recent years, the team has developed twisted bilayer photonic crystals by stacking two patterned silicon nitride membranes and rotating them relative to each other to produce new optical properties.

Twisted photonic crystals

In a new study published in Optica, the researchers show that twisted bilayer photonic crystals provide a powerful way to control the chirality of light. The rotation between the layers introduces an inherent left-right asymmetry in the structure.

Chirality refers to objects that cannot be placed on top of their mirror images. Human hands are a familiar example. In optics, chirality can occur in materials, structures, and even in light itself. Chiral light travels in a helical pattern as it moves forward.

This type of light can rotate clockwise, producing right circular polarization, or counterclockwise, producing left circular polarization.

Although these differences in light propagation are subtle, they can have major consequences. In chemistry, for example, scientists must distinguish between mirror-image molecules that have identical chemical formulas but very different biological effects.

One well-known case involves the drug thalidomide, introduced in the 1950s. Its right-handed molecular form was used to treat morning sickness in pregnant women, while the left-handed version caused severe birth defects.

Scientists often use chiral light to analyze chiral molecules and materials. Conventional methods rely on polarization optics such as wave plates and linear polarizers, but these components are static and only detect a limited range of polarization states.

Tunable properties

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.

Reference: “Dynamic control of intrinsic optical chirality via MEMS-integrated photonic crystals” by 19 March 2026, Optica.
DOI: 10.1364/OPTICA.578880

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