In a conventional wave-based system, control over wave phenomena is limited by the properties of the materials used to build the system. Limitations on bandwidth, efficiency, and other performance metrics are typically resolved by using exotic materials to build the device, adding energy to the system, or increasing the complexity of the device. An alternative approach, explored by researchers at City University of New York (CUNY) and Florida International University, is to use complex frequency excitations to enhance wave control in conventional materials. By tailoring the excitation form to oscillate at complex-valued frequencies, the researchers were able to imitate the presence of gain and loss in a system and achieve effects like perfect absorption and superresolution imaging. They could surpass passivity limitations in wave-matter interactions and access non-Hermitian responses without relying on complex, active, potentially unstable components. Enhanced light-wave control using complex frequency excitations could lead, for example, to higher resolution medical imaging, more efficient wireless communication systems, and improved control over wave-based quantum states used in quantum sensing and computing. Researchers explored how signals oscillating at complex valued frequencies could be tailored to achieve exotic effects and enhance sensing, imaging, and communication technologies. Courtesy of City University of New York. “This approach provides a fundamentally new strategy for wave control,” professor Andrea Alù, principal investigator for the study, said. “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.” The researchers used excitation signals with tailored waveforms, whose amplitudes grew or decayed exponentially over time, to mimic the effect of gain and loss in passive systems. Under suitable conditions, the signal excitations could engage the natural resonances and anti-resonances of the system. This dynamical behavior facilitated access to non-Hermitian responses without the need to modify the systems’ material properties, and enabled exotic optical effects. After tailoring the excitation form to oscillate at complex-valued frequencies and emulate the existence of gain and loss, the team was able to demonstrate controllable, enhanced energy storage, superresolution imaging, enhanced wireless power transfer, and wave manipulation beyond the passivity limits. “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,” researcher Seunghwi Kim said. The experimental use of complex frequency excitations has enabled phenomena previously thought unattainable in passive systems. By bridging theoretical concepts with experimental implementations, these advances demonstrate the feasibility of accessing non-Hermitian responses in passive linear systems. In optics and photonics, this approach could offer opportunities to alter how light interacts with matter in a highly dynamic and tunable fashion, enabling enhanced control over light emission and transport. Applicable to wave systems in general, complex excitations could be applied to developments in metamaterials, optical computing, sensing, and image processing. “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,” Kim said. The research was published in Science (www.doi.org/10.1126/science.ado4128).