A collaborative research team led by Interim Head of Physics Professor Shuang ZHANG from The University of Hong Kong (HKU), along with National Center for Nanoscience and Technology, Imperial College London and University of California, Berkeley, has proposed a new synthetic complex frequency wave (CFW) approach to address optical loss in superimaging demonstration. The research findings were recently published in the prestigious academic journal Science.

Imaging plays an important role in many fields, including biology, medicine and material science. Optical microscopes use light to obtain imaging of miniscule objects. However, conventional microscopes can only resolve feature sizes in the order of the optical wavelength at best, known as the diffraction limit.

To overcome the diffraction limit, Sir John Pendry from Imperial College London introduced the concept of superlenses, which can be constructed from negative index media or noble metals like silver. Subsequently, Professor Xiang ZHANG, the current President and Vice-Chancellor of HKU, along with his then team at the University of California, Berkeley, experimentally demonstrated superimaging using both a silver thin film and a silver/dielectric multilayer stack. These works have extensively promoted the development and application of superlens technology. Unfortunately, all superlenses suffer from inevitable optical loss, which converts optical energy into heat. This significantly affects the performance of optical devices, such as superimaging lenses, which rely on the faithful delivery of information carried by light waves.

Optical loss has been the main limiting factor that has constrained the development of nanophotonics for 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 Interim Head of HKU Department of Physics, explained the research foci, ‘To solve the optical loss problem in some important applications, we have proposed a practical solution — using a novel synthetic complex wave excitation to obtain virtual gain, and then offset the intrinsic loss of the optical system. As a verification, we applied this approach to the superlens imaging mechanism and theoretically improved imaging resolution significantly.’

‘We further demonstrated our theory by conducting experiments using hyperlenses made of hyperbolic metamaterials in the microwave frequency range and polariton metamaterials in the optical frequency range. As expected, we obtained excellent imaging results consistent with our theoretical predictions,’ added Dr Fuxin GUAN, the paper’s first author and a Postdoctoral Fellow at HKU.