A new paper explains how signals oscillating at complex-valued frequencies could transform sensing, imaging, and communication technologies.
Researchers from the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) and Florida International University have published new findings in the journal Science on the emerging field of complex frequency excitations. Their new approach aims to control light, sound, and other wave phenomena beyond traditional limitations. The work highlights the potential of this technique to enhance our understanding of wave-matter interactions and to pave the way for advancements in wave-based technologies.
In conventional systems that use light and sound waves, such as wireless communication, microscopes, speakers, and earphones, the control over wave phenomena is constrained by the inherent properties of the materials used. To overcome these limitations, researchers often turn to exotic materials, additional energy input, or more complex and bulky devices.
Complex frequency excitations, however, provide an alternative solution by manipulating the wave’s excitation form rather than relying on advanced materials. By using complex-valued frequencies, this approach can mimic the effects of gain and loss in a system, unlocking novel effects such as perfect absorption, super-resolution imaging, overcoming passivity constraints in wave-matter interactions, and enabling non-Hermitian responses—without the need for active, energy-demanding components that can lead to instability.
“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.”
Reference: “Complex-frequency excitations in photonics and wave physics” by Seunghwi Kim, Alex Krasnok and Andrea Alù, 28 March 2025, Science.
DOI: 10.1126/science.ado4128
The study was conducted by researchers from the CUNY ASRC Photonics Initiative and the Department of Electrical and Computer Engineering at Florida International University.
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