A research team co-led by Professor Zhang Shuang, interim head of the Department of Physics at the University of Hong Kong, and Professor Dai Qing from China's National Center for Nanoscience and Technology has proposed a solution to a common problem in the field of nanophotonics - studying light at extremely small scales. Their research results were recently published in the famous academic journal "Nature-Materials", and they proposed a synthetic complex frequency wave (CFW) method to solve the problem of optical loss in polaron propagation.
These findings provide practical solutions such as the use of more efficient light-based devices in devices such as computer chips and data storage devices to enable faster and more compact data storage and processing, and to improve the accuracy of sensors, imaging technology and security systems.
Surface plasmon polaritons and phonon polaritons have the advantages of efficient energy storage, local field enhancement, and high sensitivity, thanks to their ability to confine light at small scales. However, their practical applications are hampered by the problem of ohmic losses, which cause energy dissipation when interacting with natural materials.
This limitation has hindered the development of nanophotonics for sensing, ultra-imaging, and nanophotonic circuits over the past three decades. Overcoming ohmic losses will greatly improve device performance, thereby enabling the development of sensing technologies, high-resolution imaging, and advanced nanophotonic circuits.
Professor Zhang Shuang, the corresponding author of the paper, explained the focus of the research: "In order to solve the problem of optical loss in key applications, we proposed a practical solution. By using a novel synthetic complex wave excitation, we can achieve virtual gain to offset the intrinsic loss of the polariton system. To verify this method, we applied it to the phonon polariton propagation system and observed a significant improvement in polariton propagation."
"We demonstrated this method by conducting experiments in the optical frequency range using phonon polariton materials such as boron hydride and molybdenum oxide. As expected, we obtained almost lossless propagation distances, which is consistent with theoretical predictions," added Dr. Guan Fuxin, first author of the paper and a postdoctoral fellow in the Department of Physics at the University of Hong Kong.
Multi-frequency approach to overcome optical loss
In this study, the research team developed a novel multi-frequency method to solve the problem of energy loss in polaron propagation. They used a special type of wave called a "complex frequency wave" to achieve virtual gain and compensate for losses in the optical system. Ordinary waves maintain a constant amplitude or intensity over time, whereas complex frequency waves exhibit both oscillation and amplification. This feature allows for a more complete representation of wave behavior and compensates for energy losses.
Although frequency is usually treated as a real number, it also has an imaginary part. This imaginary part tells us how the wave gets stronger or weaker over time. Complex frequency waves with negative (positive) imaginary parts decay (amplify) over time. However, directly performing measurements under complex frequency wave excitation in optics is challenging because it requires complex timing measurements. To overcome this difficulty, the researchers employed the mathematical tool of the Fourier transform, which decomposes a truncated complex frequency wave (CFW) into components with independent frequencies.
Just like when you cook and need a specific ingredient that's hard to find, researchers followed a similar line of thinking. They break down complex frequency waves into simpler ingredients, like using substitute ingredients in a recipe. Each component represents a different aspect of the frequency wave. It's like making a delicious dish by using substitute ingredients to get the desired flavor. By measuring these components at different frequencies and combining the data, they reconstructed the system's behavior under exposure to complex-frequency waves. This helps them understand and compensate for energy losses. This approach greatly simplifies the practical use of CFW in different applications, including polariton propagation and hyperimaging. By taking optical measurements at different real frequencies at fixed intervals, the optical response of the system at complex frequencies can be constructed. This can be achieved by mathematically combining the optical responses obtained at different real frequencies.
Another corresponding author of the paper, Professor Dai Qing from the National Center for Nanoscience and Technology, pointed out that this work provides a practical solution to the long-standing light loss problem in nanophotonics. He emphasized the significance of the synthetic complex frequency approach, noting that it could easily be applied to various other applications such as molecular sensing and nanophotonic integrated circuits. He further emphasized: "This method is remarkable and universally applicable because it can also be used to solve loss problems in other wave systems, including acoustic waves, elastic waves, and quantum waves, thereby improving imaging quality to unprecedented levels."
Compiled source: ScitechDaily