Improbable vision


Rebecca Pool

Tuesday, November 6, 2018 - 12:00
Image: Dr Zengbo Wang develops superlenses based on dielectric materials, spider silk and more.
In the last few years, a growing research team at the University of Bangor, Wales, has developed a series of 'superlenses' that defy convention.
Designed to boost the resolution of traditional optical microscopes, the very untraditional lenses have been fabricated from nanoparticles, spider silk and even light-sensitive bacteria.
As research leader, Dr Zengbo Wang, Senior Lecturer in Imaging and Laser Micromachining at the University says: "Our research centres on the development of dielectric superlenses and super-resolution imaging applications."
"We've delivered resolution between 45 nm and 100 nm under white light, have viewed details to a far greater level than has been previously possible, breaking the resolution barrier of the microscope with our superlenses," he adds.
Photonic curiosity
Wang's interest in superlenses and simple, super-resolution microscopy started at the turn of this century, when as part of his PhD, he was looking at the interaction of light with micro-sized beads, and specifically, the sub-wavelength focusing effect of the microbead.
This effect, discovered by Boris Luk’yanchuk in Singapore and dubbed the 'photonic nanojet' by Northwestern University researchers in the US, enhances the backscattering of visible light, provides a novel way to image nanoparticles and has spawned several ultramicroscopy methods to detect proteins, viruses and more. 
As Wang tells Microscopy and Analysis: "I quickly realised that a label-free nanoscope was going to have big, big potential, with a white light super-resolution microscope being what everybody wanted."
Wang was right, and come 2011 when he was a lecturer at the University of Manchester he and colleagues unveiled a powerful white-light microscope called the 'microsphere nanoscope'.
The 50 nm resolution instrument used five micron diameter, silica microsphere superlenses, also known as dielectric superlenses, to magnify underlying near-field objects by up to eight times.
These objects were then projected into a conventional microscope objective lens, typcially 50 to 100X magnification with a numerical aperture of more than 0.6.
From imaging living viruses to to DNA to biomolecules, the potential was clear.
As Wang points out: "This was a simple set-up and the design was beautiful; by using the right-sized microsphere with the necessary refractive index with the objective lens, you could see all the way down to 50 nm."
Indeed, the original method has since been advanced by numerous research groups with start-up companies also having launched super-resolution microscopes.
Wang left the University of Manchester in 2012, moving to the University of Bangor and taking the position of Senior Lecturer in Imaging and Laser Micromachining.
Bangor Super Lenses for label-free super-resolution imaging. (a) Nephila edulis spider in its silk web, the silk was used to make the bio-superlens. (b) SEM image of Blu-ray disk with 200/100 nm groove and lines. (c) Clear magnified image (2.1x) of Blu-ray disk under spider silk bio-superlens. (d) The META-Superlens was made by stacking high-index TiO2 nanoparticles. (e) SEM image of CPU chip with 60 nm features. (f) Clear optical image of CPU chip under META-superlens.
Here, he quickly set to work setting up his own laboratory while developing new superlens-based systems.
In his earlier days at Manchester, he had realised that the micropshere superlens of the nanoscope at the time, had a small sample viewing window, typically a quarter of the size of the microsphere diameter.
"We wanted to have a larger area for imaging so needed to develop a scanning version of the nanoscope," he says. "We knew that if we could integrate the objective lens with the particle lens to give a single piece, than you could scan this to generate a complete image of a sample."
With this in mind, Wang and colleagues first proposed a coverslip superlens, encapsulating high-index microspheres inside a transparent host polymer.
However, synchronisation with the microscope objective proved difficult, so the researchers instead developed a custom-made lens adaptor to integrate the two lenses and form a superlensing microscope objective lens.
As Wang points out, this 'superlensing objective lens' is now designed to fit to any existing conventional microscope; for example, he and colleagues have fitted it to an Olympus BX60 instrument as well as a low-cost ICM 100 microscope.
And in tests, the lens is kept static while an underlying high resolution nanostage scans samples across the objective lens.
"Our lens has been successfully demonstrated for label-free super-resolution imaging of 90-100 nm features in engineering and biological samples, including a Blu-ray disk sample and adenoviruses," highlights Wang.
"We are guiding the live bacteria so they aggregate into a lens shape" Dr Zenbo Wang
At the same time, Wang and colleagues have also been developing a second generation of dielectric superlens, 'META-superlens', a nanoparticle metamaterial superlens.
Using a metamaterial design concept, the researchers stack high-index titanium dioxide nanoparticles into the desired 3D shape to form an all-dielectric metamaterial solid immersion lens.
According to Wang, the assembled superlens generates millions of nanoscale illumination spots on a sample surface during imaging to effectively retrieve nanoscale information and produce a sharp image to 45 nm resolution surpassing previous superlenses.
"The working mechanism is no longer the photonic nanojet but is high-index nanoparticle, super-resolution, evanescent illumination and decoupling, a new nanophotonic effect that we discovered," explains Wang. "[With the lens], we have a more stable imaging process, without varying resolution and magnification bringing quality and clarity to different samples, including nanochips and viruses."
The researchers are now working on commercialising these developments, and as Wang highlights: "Prototypes are complete and we are seeking commercialisation... we hope to launch the first product within three years".
A natural approach
Dielectric superlenses aside, Wang also wanted to adopt a simpler approach to superlenses, and so turned to nature.
"When playing in the back garden with kids, as well as on a beach, I realised that spider silk and fine sand particles could perhaps work as natural super lenses," he says.
Wang opted for spider silk but on retrieving dragline silk from common garden spiders realised that at three to four microns, the silk diameter was too small to make a useful superlens.
Given this, he and colleagues approached Professor Fritz Vollrath of the University of Oxford, UK, renowned for studying the biology of spider silks.
Importantly, Vollrath's research had included studies on golden web spiders which produce larger diameter silk up to eight microns.
Spider silk from Nephila spider provides natural super-resolution lens [Toby Hudson] 
A collaboration ensued, and by applying the dragline silk from a golden web Nephila spider to the surface of the material to be viewed, the researchers boosted magnification by up to three times.
As Wang highlights, with the cylindrical 'silk lens', 100 nm features could be distinctly resolved under a conventional white-light microscope. What's more, this lens provided a wider field of view compared to a microsphere superlens.
The researchers went onto view details on a microchip and blue-ray disk that would not have been visible using an unmodified optical microscope, and Wang is confident the set-up could be used to image biological microstructures including germs and viruses.
"I have had lots of school children write to me, asking how they can make this," he says. "But also importantly, for commercial applications, a spider silk nanoscope would be robust and economical, which could provide excellent manufacturing platforms for a wide range of applications."
But Wang's interest in nature doesn't stop here with his group currently investigating the use of cyanobacteria as a live biological superlens.
As the researcher points out, cyanobacteria is light-senstive, so will group around a laser point.
"We hope to use this phenomena to stack up the bacteria, just like we did with our nanoparticle all-dielectric metamaterial solid immersion lens, but this time we are guiding the live bacteria so they aggregate into a lens shape," he says.
And looking to the future, Wang's interest in superlenses is only set to diversify further. 
Despite the discovery of their new nanophotonic effect with the second generation dielectric META-superlens, Wang and colleagues remain interested in photonic nanojet super-resolution, having used masks with the microsphere to raise resolution further.
The group has also discovered a new type of curved light beam, 'the photonic hook', that is set to be used in nanoparticle guiding and trapping.
And the researchers have also been developing a lab-on-chip microfluidic nanoscope device, based on a microsphere lens, to directly image and analyse moving objects inside a microfludic channel in real time, as well as superlens-based 3D nano printing and fabrication systems.
"We are proud to be pioneering of such different types of super lens from Bangor," says Wang. "Our vision is that one day, every microscope user will have a 'Bangor superlens' product for daily use with their microscopes."
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