HONG KONG SAR – Media OutReach – August 21, 2023 – A collaborative research team led by Acting Professor of Physics Shuang ZHANG of the University of Hong Kong (HKU), with the National Center for Nanoscience and Technology, Imperial College London and the University of California, Berkeley, proposed a novel synthetic complex frequency wave (CFW) approach to address optical loss in superimaging demonstration. The research results were recently published in the prestigious academic journal
Science.
Diagram of imaging under excitation in real frequency and in complex frequency synthesized in a superlens. The same object, when imaged through a super lens under different true-frequency illumination, results in images with varying degrees of blur, and none of the true-frequency images can discern the true appearance of the object. By combining the field strengths and phases of several single-frequency images, a clear image can finally be obtained. Image credit: HKU
Imaging plays an important role in many fields, including biology, medicine, and materials science. Optical microscopes use light to image tiny objects. However, conventional microscopes can only best resolve feature size in the order of optical wavelength, known as the diffraction limit.
To overcome the diffraction limit, Sir John Pendry of Imperial College London introduced the concept of superlenses, which can be constructed from negative index media or noble metals such as silver. Subsequently, Professor Xiang ZHANG, the current President and Vice-Chancellor of HKU, and his team at the University of California, Berkeley, experimentally demonstrated superimaging using both silver thin film and a silver/dielectric multilayer stack. This work has greatly promoted the development and application of superlens technology. Unfortunately, all super lenses suffer from unavoidable optical loss, which converts optical energy into heat. This greatly affects the performance of optical devices, such as superimaging lenses, which rely on the faithful transmission of information carried by light waves.
Optical loss has been the main factor limiting the development of nanophotonics over the past three decades. Many applications, including sensing, superimaging, and nanophotonic circuits, would greatly benefit if this problem could be solved.
Professor Shuang Zhang, corresponding author of the paper and also acting head of the physics department of HKU, explained the lines of research: “To solve the problem of optical loss in some important applications, we have proposed a practical solution – using a new synthetic complex wave excitation to achieve a virtual gain and then compensate for the intrinsic loss of the optical system As a verification, we applied this approach to the imaging mechanism of the superlens and theoretically improved so significant imaging resolution.
“We further demonstrated our theory by conducting experiments using hyperlenses made of hyperbolic metamaterials in the microwave frequency range and polaritonic metamaterials in the optical frequency range. As expected, we obtained excellent imaging results consistent with our theoretical predictions,” added Dr. Fuxin GUAN, first author of the paper and postdoctoral fellow at HKU.
Multi-frequency approach to overcome optical loss
In this study, researchers introduced a novel multi-frequency approach to overcome the negative impacts of loss on superimaging. Complex frequency waves can be used to provide virtual gain to compensate for loss in an optical system. What does complex frequency mean? The frequency of a wave refers to the speed at which it oscillates over time. It is natural to think of frequency as a real number. Interestingly, the concept of frequency can be extended to the complex domain, where the imaginary part of frequency also has a well-defined physical meaning, i.e. the rate at which a wave amplifies or decays in the time. Therefore, for a wave of complex frequency, the oscillation and the amplification of the wave occur simultaneously. For a complex frequency with a negative (positive) imaginary part, the wave decays (increases) over time. Of course, an ideal complex wave is not physical because it would diverge when time passed positive or negative infinity, depending on the sign of its imaginary part. Therefore, any realistic implementation of complex frequency waves must be truncated in time to avoid divergence. Optical measurement directly based on complex frequency waves must be performed in the time domain and would involve complex temporal measurements and thus has not been experimentally realized so far.
The team used the mathematical tool Fourier Transform to decompose a truncated CFW into several components of different real frequencies, greatly facilitating the implementation of CFWs for various applications, such as superimaging. By carrying out optical measurements at several real frequencies at a fixed interval, it is possible to construct the optical response of the system at a complex frequency by mathematically combining that of the real frequencies.
As a proof of concept, the team started with superimaging at microwave frequencies using a hyperbolic metamaterial. Hyperbolic metamaterial can carry waves with very large wave vectors (or equivalently very small wavelengths), capable of transmitting the information of very small feature sizes. However, the larger the wave vector, the more susceptible the waves are to optical loss. Therefore, in the presence of loss, the information of these small feature sizes is lost during propagation inside the hyperbolic metamaterial. The researchers showed that by appropriately combining the blurred images measured at different real frequencies, a clear image at a complex frequency was formed with deep sub-wavelength resolution.
The team then extended the principle to optical frequencies, using an optical superlens made of a phononic crystal called silicon carbide, which operates at the far-infrared wavelength of around 10 micrometers. In a phononic crystal, the lattice vibration can couple with light to create a superimaging effect. However, the loss remains a limiting factor in the spatial resolution. Although the spatial resolutions of imaging at all true frequencies were loss-limited, as shown by blurry images of nanoscale holes, ultra-high resolution imaging can be achieved with synthesized CFWs consisting of multiple frequency components.
“This work provided a solution to overcome optical loss in optical systems, a long-standing problem in nanophotonics. The synthesized complex frequency method can be easily extended to other applications, including molecular sensing and nanophotonic integrated circuits,” said Professor Xiang ZHANG, another corresponding author of the paper, President and Vice Chancellor of HKU, and also Chairman of Physics and Engineering. He hailed this as a remarkable and universally applicable method, “It can be harnessed to combat loss in other wave systems, including sound waves, elastic waves and quantum waves, elevating the quality of imagery to a new high.”
This work was supported by the New Cornerstone Science Foundation, the Research Grants Council of Hong Kong.
Journal article: “Overcoming losses in superlenses with synthetic waves of complex frequency”,
Science.
The review can be accessed here:
https://www.science.org/doi/10.1126/science.adi1267
More information about Professor Shuang Zhang:
https://shorturl.at/efCN1
Hashtag: #HKU
The issuer is solely responsible for the content of this announcement.
