Pioneering in Superimaging: Novel Approach Overcomes Established Optical Constraints

Pioneering in Superimaging: Novel Approach Overcomes Established Optical Constraints

Imaging via real-frequency and synthesized complex frequency excitation in a superlens displays differing results. While real-frequency illumination offers images of varied clarity, the true nature of the object remains elusive. Merging multiple single-frequency images, however, yields a crystal-clear image. Image Credit: HKU

A joint research endeavor helmed by Professor Shuang Zhang, Interim Head of Physics at The University of Hong Kong (HKU) and supported by the National Center for Nanoscience and Technology, Imperial College London, and the University of California, Berkeley, has advanced a synthetic complex frequency wave (CFW) method to counter optical loss in superimaging. These findings have found their place in the esteemed academic journal, Science.

In numerous sectors like biology, medicine, and material science, imaging holds significant importance. Traditional microscopes utilize light to capture images of tiny objects, but they are confined to the diffraction limit, allowing them to resolve features only as small as the optical wavelength.

Addressing this challenge, Sir John Pendry of Imperial College London introduced superlenses, made from negative index media or metals such as silver. Furthering this, Professor Xiang Zhang, presently the President and Vice-Chancellor of HKU, alongside his team at the University of California, Berkeley, showcased superimaging through both a silver film and a silver/dielectric multilayer. However, a recurring issue with superlenses is the unavoidable optical loss, turning optical energy to heat, thus hindering the performance of devices like superimaging lenses.

Optical loss has long impeded nanophotonics’ progress for over thirty years. The sector would experience a renaissance if a resolution to this issue were found. Professor Shuang Zhang, the study’s lead author, articulated, “To address the persistent optical loss, we’ve suggested a feasible solution by employing synthetic complex wave excitation to achieve virtual gain, compensating for the inherent loss in optical systems. This technique was applied to superlens imaging, markedly enhancing theoretical imaging precision.”

This theory’s application was evidenced through experiments involving hyperlenses made from hyperbolic metamaterials in microwave frequencies and polariton metamaterials in optical frequencies, yielding results in line with theoretical expectations, as noted by Dr. Fuxin Guan, the primary author and Postdoctoral Fellow at HKU.

The research introduced a unique multi-frequency technique to neutralize the adverse effects of loss in superimaging. Complex frequency waves could offer virtual gain to balance the loss experienced in optical systems.

Complex frequency pertains to the oscillation rate of a wave over time. While typically seen as a real number, the notion of frequency can be expanded into the complex domain. Here, the frequency’s imaginary segment carries a clear physical interpretation, indicating the rate at which a wave either amplifies or diminishes. For waves with complex frequencies, both amplification and oscillation occur simultaneously. A negative or positive imaginary component means the wave respectively diminishes or amplifies over time.

Although ideal complex waves aren’t feasible due to potential divergence, any real-world implementation of these waves must be time-constrained. This team employed Fourier Transformation to deconstruct a truncated CFW into multiple real frequency components, simplifying the application of CFWs, including in superimaging. Conducting optical measurements across various real frequencies at fixed intervals can lead to the synthesis of system optical response at a complex frequency.

As a practical test, the group commenced with microwave-frequency superimaging using hyperbolic metamaterials. Despite the larger wavevectors making the waves more susceptible to optical loss, a combination of blurred images taken at various real frequencies resulted in a clear, deep-subwavelength resolution image at a complex frequency.

Building on this, the team applied the principle to optical frequencies using a silicon carbide-based phononic crystal superlens. Although loss did limit the resolution, ultra-high-resolution imaging was achievable using synthesized CFWs composed of multiple frequency components.

Professor Xiang Zhang, co-author and President and Vice-Chancellor of HKU, commented on the significance of this work, stating that it offered a path to counteract optical loss in systems, a persistent issue in nanophotonics. He lauded it as an approach with broad applications, capable of addressing loss across diverse wave systems, ultimately raising imaging standards.

Reference: “Overcoming losses in superlenses with synthetic waves of complex frequency” by Fuxin Guan et al., 17 August 2023, Science. DOI: 10.1126/science.adi1267

This study received backing from the New Cornerstone Science Foundation and the Research Grants Council of Hong Kong.

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• University of Hong Kong (HKU) Research
• Science Journal
• Nanophotonics Overview
• Optical Microscopes
• Superlenses and Negative Index Media
• Imperial College London – Physics Department
• Fourier Transformation in Imaging
• Hyperbolic Metamaterials
• Silicon Carbide Properties
• Phononic Crystals in Imaging