Most photonic devices struggle to balance robustness and sensitivity. This challenge stems from a fundamental contradiction in wave dynamics, where robustness is needed to ensure stability against disturbances, while sensitivity is required to detect and respond to external stimuli. Researchers at the University of California, San Diego used subwavelength phase singularity in a chiral medium to resolve this paradox. “Our research addresses this critical challenge,” professor Abdoulaye Ndao, who led the study, said. “We have designed new photonic devices that are both highly sensitive to their environment and robust against fabrication errors and material imperfections.” The durable, yet sensitive, devices could be used to develop next-generation photonic devices that are more precise, more powerful, and easier and less expensive to produce at scale. They could enable robust, highly sensitive applications for medical diagnostics, environmental sensing, and secure communications, for example — all built into chip-scale devices. Researcher Jeongho Ha (center) is part of a team that developed the first photonic devices that are both highly sensitive and robust. Courtesy of University of California, San Diego Jacobs School of Engineering/David Baillot. To achieve subwavelength phase singularity, the researchers engineered a chip-scale device made from two layers of gold nanorods, with an extremely thin layer of polymer sandwiched in between. The bottom layer of the device was embedded in the polymer. The top layer was exposed to the air, allowing it to interact directly with molecules targeted for sensing, and making its surface available for chiral analyte detection. The two different circular polarizations of light (right-handed and left-handed) were incident onto the front side of the bilayer and transmitted to the back side (substrate). The nanorods in the top and bottom layers of the device were arranged in rows that were twisted at specific angles (ie., twisted angle θ) relative to each other. By adjusting the horizontal spacing between these two layers, the researchers found they could precisely control how the layers interacted with light. Subwavelength phase singularity occurs when light is confined to a space that is smaller than its own wavelength. The confinement of light creates a point of total darkness, causing the intensity of the light to drop to zero, while the light phase continues through its full cycle. Phase singularity is highly sensitive to changes in the surrounding environment, making it ideal for sensing applications. It is also inherently durable, making it resistant to imperfections from manufacturing processes. The researchers experimentally demonstrated the sensitivity and durability of the new photonic device by measuring its phase singularities. Historically, devices that are sensitive enough to detect subtle changes in the environment are fragile and tend to break down when imperfections occur during manufacturing, even if the imperfections are small. This makes highly sensitive devices expensive to produce and difficult to scale. Increasing the ruggedness of these devices often compromises their precision. Phase singularity shows significant potential for practical applications because it can alleviate the strict constraints imposed by fabrication tolerances. Devices that use phase singularities have a degree of flexibility in design and fabrication that can be challenging to achieve with conventional methods. Phase singularity makes light extremely sensitive to external changes, which is useful for high-precision detectors, optical communications, and imaging. The combination of subwavelength light confinement and robust phase singularities could advance the development of high-sensitivity, chip-scale photonic devices in both the quantum and classical realms. “This is the first device that’s both sensitive and robust to fabrication imperfections,” Ndao said. “We have developed tiny optical devices that are both tough and highly sensitive at the same time — a combination that was previously thought to be impossible.” The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.7.3.035001).