These works have extensively promoted the development and application of superlens technology. Sadly, all superlenses experience inescapable 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 shipment of details carried by light waves.
Imaging patterns in several real frequencies and complex frequency of the letter H. Credit: HKU
Optical loss has been the main limiting aspect that has actually constrained the advancement of nanophotonics for the previous 3 years. Lots of applications, consisting of noticing, superimaging, and nanophotonic circuits, would considerably benefit if this problem might be resolved.
Professor Shuang Zhang, corresponding author of the paper and likewise Interim Head of HKU Department of Physics, explained the research foci, To fix the optical loss issue in some essential applications, we have actually proposed a useful solution– utilizing an unique artificial complex wave excitation to get virtual gain, and then balanced out the intrinsic loss of the optical system. As a confirmation, we applied this method to the superlens imaging mechanism and in theory enhanced imaging resolution substantially.
We even more demonstrated our theory by performing experiments using hyperlenses made of hyperbolic metamaterials in the microwave frequency variety and polariton metamaterials in the optical frequency variety. As expected, we acquired exceptional imaging results consistent with our theoretical forecasts, included Dr Fuxin Guan, the papers very first author and a Postdoctoral Fellow at HKU.
Multi-frequency method to overcome optical loss
In this research study, the researchers presented an unique multiple-frequency approach to get rid of the unfavorable impacts of loss on superimaging. Intricate frequency waves can be used to supply virtual gain to compensate for the loss in an optical system.
What does complicated frequency indicate? The frequency of a wave refers to how fast it oscillates in time.
It is natural to consider frequency a real number. Surprisingly, the concept of frequency can be extended into the complicated domain, where the fictional part of the frequency also has a well-defined physical meaning, i.e., how quickly a wave magnifies or decomposes in time. For this reason, for an intricate frequency wave, both oscillation and amplification of the wave occurs all at once. For a complex frequency with a negative (positive) imaginary part, the wave decomposes (magnifies) in time.
Electric field profile of waves in real frequency (a), intricate frequency (b), and truncated complicated frequency (c). Truncated complex frequency wave synthesized by the direct combination of multiple genuine frequencies (d). Credit: HKU
Obviously, an ideal complex wave is not physical due to the fact that it would diverge when time goes to either negative or favorable infinity, depending upon the indication of its fictional part. Any practical application of intricate frequency waves requires to be truncated in time to avoid the divergence. The optical measurement straight based on complex frequency waves requires to be performed in the time domain and it would include complicated time-gated measurements for that reason it has actually not been experimentally recognized so far.
The group made use of the mathematical tool Fourier Transformation to break down a truncated CFW into lots of components of various real frequencies, greatly facilitating the application of CFWs for numerous applications, such as superimaging. By performing optical measurements at several genuine frequencies at a repaired period, it is possible to build the optical action of the system at a complex frequency by mathematically combining that of genuine frequencies.
Super-imaging using a SiC superlens operating at optical frequency. Intricate frequency measurement offer a much better spatial resolution than that of a real frequency.
As an evidence of idea, the group started with superimaging at microwave frequencies utilizing a hyperbolic metamaterial. The hyperbolic metamaterial can bring waves with huge wavevectors (or equivalently very little wavelengths), that can sending the information of very little function sizes. The bigger the wavevector, the more sensitive the waves are to optical loss. Therefore, in the existence of loss, the information of those little feature sizes gets lost during the propagation inside the hyperbolic metamaterial. The scientists showed that, by properly combining the blurred images determined at different genuine frequencies, a clear image at a complicated frequency was formed with a deep-subwavelength resolution.
The team further extended the concept to optical frequencies, utilizing an optical superlens made of a phononic crystal called silicon carbide, which operates at the far-infrared wavelength of around 10 micrometers. The spatial resolutions of imaging at all the real frequencies were restricted by the loss, as shown by the blurred images of the nano-scale holes, ultrahigh-resolution imaging can be obtained with synthesized CFWs that consist of several frequency parts.
The work has provided an option to overcome optical loss in optical systems, a long-standing issue in nanophotonics. The manufactured complex-frequency technique can be easily encompassed other applications, consisting of molecular noticing and nanophotonic incorporated circuits, said Professor Xiang ZHANG, another corresponding author of the paper, the President and Vice-Chancellor of HKU, and likewise Chair of Physics and Engineering. He hailed this as a impressive and generally appropriate approach, This can be leveraged to tackle loss in other wave systems, consisting of acoustic waves, elastic waves, and quantum waves, elevating imaging quality to a brand-new height.
Referral: “Overcoming losses in superlenses with synthetic waves of complex frequency” by Fuxin Guan, Xiangdong Guo, Kebo Zeng, Shu Zhang, Zhaoyu Nie, Shaojie Ma, Qing Dai, John Pendry, Xiang Zhang and Shuang Zhang, 17 August 2023, Science.DOI: 10.1126/ science.adi1267.
This work was supported by the New Cornerstone Science Foundation, the Research Grants Council of Hong Kong.
Interestingly, the concept of frequency can be extended into the complicated domain, where the imaginary part of the frequency likewise has a distinct physical meaning, i.e., how fast a wave decays or magnifies in time. Electric field profile of waves in real frequency (a), complex frequency (b), and truncated complicated frequency (c). Truncated complicated frequency wave synthesized by the direct mix of numerous real frequencies (d). Complex frequency measurement supply a much better spatial resolution than that of a genuine frequency. The scientists showed that, by appropriately combining the blurred images measured at various real frequencies, a clear image at a complex frequency was formed with a deep-subwavelength resolution.
Schematic of imaging under real-frequency and synthesized complex frequency excitation in a superlens. The very same item, when imaged through a superlens under different real-frequency lighting, leads to images with differing degrees of blurriness, and none of the real-frequency images can recognize the true look of the item. By combining the field amplitudes and phases of multiple single-frequency images, a clear image can finally be gotten. Credit: HKU
A collective research study group led by Interim Head of Physics Professor Shuang Zhang from The University of Hong Kong (HKU), along with the National Center for Nanoscience and Technology, Imperial College London, and the University of California, Berkeley, has actually proposed a brand-new artificial complicated frequency wave (CFW) approach to attend to optical loss in superimaging demonstration. The research study findings were recently published in the distinguished academic journal Science.
Imaging plays a crucial function in lots of fields, consisting of biology, medication, and material science. Optical microscopic lens utilize light to acquire imaging of minuscule objects. Traditional microscopic lens can only resolve function sizes in the order of the optical wavelength at best, known as the diffraction limitation.
To conquer the diffraction limit, Sir John Pendry from Imperial College London presented the principle of superlenses, which can be built from unfavorable index media or noble metals like silver. Subsequently, Professor Xiang Zhang, the current President and Vice-Chancellor of HKU, together with his then group at the University of California, Berkeley, experimentally demonstrated superimaging utilizing both a silver thin film and a silver/dielectric multilayer stack.