Researchers at Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a compact device that can actively control the “handedness” of light as it passes through it, also known as optical chirality. This is achieved by slightly rotating two specially designed photonic crystal layers.
The project was led by Van Du, a graduate student in the lab of Eric Mazur, Balkansky Professor of Physics and Applied Physics. The research team designed a reconfigurable twisted bilayer photonic crystal that can be tuned in real time using an integrated microelectromechanical system (MEMS). This advance could enable new capabilities in chiral sensing, optical communications, and quantum photonics.
“Chirality is very important in many scientific fields, from pharmaceuticals to chemistry to biology and, of course, physics and photonics,” Mazur said. “By integrating twisted photonic crystals with MEMS, we get a platform that is not only powerful from a physics perspective, but also compatible with modern photonics fabrication methods.”
Twisted photonic crystals and light manipulation
Photonic crystals are nanoscale materials designed to control the behavior of light. These structures are small enough to fit on the tips of pins and are already used in computing, sensing, and high-speed data transmission technologies.
Mazur’s group expanded the field by applying the idea of twistronics, a concept that gained attention through their work on twisted bilayer graphene. By stacking two patterned silicon nitride layers and rotating them relative to each other, researchers are able to create new optical properties not present in a single layer.
In their study published in opticalThe research team demonstrated that this twisted double-layer structure naturally introduces asymmetry between the left and right sides and is highly effective in controlling optical chirality. Chirality refers to objects that cannot be superimposed into mirror images, such as the left and right hands. In optics, this concept applies both to materials and to light itself, which travels in a spiral pattern.
Light rotates clockwise, known as right-handed circularly polarized light, or counterclockwise, known as left-handed circularly polarized light. Although these differences are subtle, they play an important role in many scientific applications.
Why chirality is important in science
Small differences in chirality can have big consequences. In chemistry and medicine, molecules that are mirror images of each other can behave completely differently in the body. A well-known example is the drug thalidomide from the 1950s. One version of this molecule helped treat morning sickness in pregnant women, but its mirror image caused severe birth defects.
Scientists often use chiral light to study such molecules. Traditional tools such as wave plates and linear polarizers can detect polarized light, but their functionality is fixed and their range limited.
Tunable photonic device with MEMS control
The new Harvard device overcomes these limitations because it is fully adjustable. Instead of relying on static components, the response to different types of chiral light can be continuously tuned without replacing parts.
This flexibility is due to the two-layer structure. When two photonic crystal layers are rotated close together, the structure becomes geometrically chiral, allowing the handedness of the incident light to be detected. Due to the strong interaction between the layers, the transmission behavior of left-handed and right-handed circularly polarized light under “normal incidence”, i.e., polarized light that is incident perpendicular to the surface, is very different.
By using a MEMS system to precisely control both the twist angle and the interlayer spacing, the researchers demonstrated that they could tune the device to near-perfect selectivity in identifying light handedness.
Future applications in sensing and communications
This study also outlines a broader design strategy for creating twisted bilayer photonic crystals with controllable optical chirality. Although the current device serves as a proof of concept, it points towards practical applications.
Future systems could be used for chiral sensing, where devices are tuned to detect specific molecules at different wavelengths. It also serves as a dynamic light modulator for optical communication systems, allowing precise control of light directly on the chip.
The paper “Dynamic control of intrinsic optical chirality by MEMS-integrated photonic crystals” is co-authored by Haoning Tang, Yifan Liu, Mingjie Zhang, Beicheng Lou, Guangqi Gao, Xuyang Li, Alsyl Enriquez, and Shanhui Fan.
