MEMS-Integrated Crystals Dynamically Control Light Chirality on a Chip

Harvard engineers have developed a chip-scale device capable of dynamically controlling the “handedness” of light – a property known as optical chirality – with a simple twist. The breakthrough, led by graduate student Fan Du in the lab of Eric Mazur at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), could open fresh avenues for advanced chiral sensing, optical communication and quantum photonics. The team’s function centers on a reconfigurable twisted bilayer photonic crystal, tuned in real time using an integrated micro-electromechanical system (MEMS).

How the Twist Controls Chirality

The concept of “handedness,” or chirality, isn’t limited to biology. It applies to light as well, where it describes the helical rotation of the electric field as light propagates. Light can rotate clockwise (right-circular polarization) or counterclockwise (left-circular polarization). While subtle, these differences are significant. For example, in chemistry, mirror-image molecules – called chiral molecules – can have drastically different biological effects, as famously demonstrated by the thalidomide tragedy of the 1950s.

Photonic crystals, nanofabricated structures designed to manipulate light at the nanoscale, are central to this innovation. These crystals are already used in optical technologies like computing and high-speed communications. Mazur’s group has been exploring ways to enhance photonic crystal engineering, drawing inspiration from “twistronics” – a field that gained prominence with the discovery of twisted bilayer graphene. The Harvard team created their device by stacking two patterned silicon nitride membranes and rotating one relative to the other. This twist introduces an inherent asymmetry, allowing the structure to interact differently with left- and right-circularly polarized light.

The key to the device’s tunability lies in the integration of a MEMS actuator. By continuously varying the twist angle and the space between the layers, the researchers can precisely control the device’s ability to distinguish between different chiral light modes. As detailed in a recent study published in Optica, the team achieved levels of selectivity approaching theoretical extremes.

Beyond Static Polarization: The Limitations of Current Methods

Traditionally, scientists have relied on static polarization optics – wave plates and linear polarizers – to analyze chiral molecules, and materials. But, these methods have limitations. They can only detect a limited range of polarization states and lack the dynamic control offered by the new Harvard device. The ability to tune the device’s response to different types of chiral light, without physically changing components, represents a significant advancement. What we have is particularly important in chiral sensing, where researchers often need to probe different chiral molecules at different wavelengths.

Implications for Sensing, Communications, and Quantum Technologies

The potential applications of this technology are broad. In chiral sensing, the tunable device could be used to identify and analyze chiral molecules with greater precision and flexibility. This has implications for pharmaceutical development, materials science, and environmental monitoring. The device could also be used in optical communications to encode and decode information using the chirality of light, potentially leading to faster and more secure communication systems. The ability to control the “handedness” of light is crucial for certain quantum photonics applications, where the polarization of photons is used to encode quantum information.

A Twist Inspired by Graphene

The research builds on the growing field of twistronics, which explores the unique properties that emerge when two-dimensional materials are stacked and twisted relative to each other. The discovery of unusual superconductivity in twisted bilayer graphene sparked intense interest in this area. By applying similar principles to silicon photonics, the Harvard team has created a new platform for manipulating light with unprecedented control. Eric Mazur explained that integrating twisted photonic crystals with MEMS creates a platform that is “not only powerful from a physics standpoint but also compatible with the way modern photonics are manufactured.”

What Comes Next: From Proof-of-Concept to Practical Applications

While the current device is a proof of concept, the researchers believe it lays the foundation for future advancements. The published paper provides a general design framework for twisted bilayer crystals exhibiting optical chirality, allowing for further optimization and customization. The next steps involve refining the device’s performance, exploring different materials and designs, and developing practical applications. Further research will focus on scaling up the manufacturing process and integrating the device with other photonic components. The team also plans to investigate the utilize of this technology for dynamic light modulation in optical communications, enabling on-chip control of light.