Enhancing light control with complex frequency excitations

NEW YORK, April 1, 2025 — Researchers at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) and at Florida International University report in the journal Science their insights on the emerging field of complex frequencies excitations, a recently introduced scheme to control light, sound and other wave phenomena beyond conventional limits. Based on this approach, they outline opportunities that advance fundamental understanding of wave-matter interactions and usher wave-based technologies into a new era.

In conventional light wave- and sound wave- based systems such as wireless cell phone technologies, microscopes, speakers and earphones, the control over wave phenomena is limited by constraints, which stem from the fundamental properties of the materials used in these technologies. Overcoming these constraints typically requires the use of exotic materials, adding energy to the system, and/or making the devices more complex and cumbersome. Complex frequency excitations offer an alternative approach to enhance wave control using conventional materials. By tailoring the excitation form, rather than the materials themselves, to oscillate at complex-valued frequencies, it is possible to emulate the presence of gain and loss in the system, unlocking exotic effects such as perfect absorption, super-resolution imaging, surpass passivity limitations in wave-matter interactions, and access non-Hermitian responses, without having to rely on active complex components that require energy and are prone to instabilities.

“This approach provides a fundamentally new strategy for wave control,” said the study’s principal investigator Andrea Alù, Distinguished Professor and Einstein Professor of Physics at The City University of New York Graduate Center and founding director of the CUNY ASRC Photonics Initiative. “We are no longer limited by the material platform to enhance the device performance. We can now shape how wave-based systems respond simply by designing the right kinds of excitations.”

A New Frontier in Wave Physics

The research team’s work discusses how signal excitations with amplitudes that exponentially grow or decay over time can engage, under suitable conditions, the natural resonances and anti-resonances of a given system, mimicking the effects of adding precise distributions of material gain and/or loss. Applications range from dynamic light control, absorption and amplification of signals, directional wave transport, and enhanced quantum state control.

Alù’s group has pioneered some of the initial explorations in this area of research, demonstrating controllable and enhanced energy storage, super-resolution imaging, enhanced wireless power transfer, and wave manipulation beyond the passivity limits. Among the possible transitions, the enhanced wave control could lead to higher-resolution medical imaging, more efficient wireless communication systems, and improved control over wave-based quantum states for applications, including quantum sensing and computing.

“While the initial demonstrations of complex-frequency excitations have been limited to radio and acoustic frequencies, scaling this technique to higher frequencies, such as optical systems, remains a challenge,” said the study’s first author Seunghwi Kim, postdoctoral researcher at ASRC. “Our work lays the foundation for future breakthroughs by providing a roadmap for researchers across various wave physics domains to explore the untapped potential of complex frequency excitations.”

The study was conducted by researchers from the CUNY ASRC Photonics Initiative and the Department of Electrical and Computer Engineering at Florida International University.

About the Graduate Center of The City University of New York

The CUNY Graduate Center is a leader in public graduate education devoted to enhancing the public good through pioneering research, serious learning, and reasoned debate. The Graduate Center offers ambitious students nearly 50 doctoral and master’s programs of the highest caliber, taught by top faculty from throughout CUNY — the nation’s largest urban public university. Through its nearly 40 centers, institutes, initiatives, and the Advanced Science Research Center, the Graduate Center influences public policy and discourse and shapes innovation. The Graduate Center’s extensive public programs make it a home for culture and conversation.

About the Advanced Science Research Center at the CUNY Graduate Center

The Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) is a world-leading center of scientific excellence that elevates STEM inquiry and education at CUNY and beyond. The CUNY ASRC’s research initiatives span five distinctive, but broadly interconnected disciplines: nanoscience, photonics, neuroscience, structural biology, and environmental sciences. The center promotes a collaborative, interdisciplinary research culture where renowned and emerging scientists advance their discoveries using state-of-the-art equipment and cutting-edge core facilities.