Hyperlenses can overcome the diffraction limit but their fabrication is difficult and expensive. A new nanoimprint lithography method can produce hyper lens arrays in a simple fashion and in large scale. The discovery paves the way for practical applications of super-resolution imaging in various scientific fields where resolving the tiniest features is essential.
The resolution of conventional lenses, such as those used in optical microscopes, is restricted by the diffraction limit. The term describes that the smallest distance between two resolved points cannot be smaller than the wavelength of the incoming light, as discovered by German physicist Ernst Abbe in 1873. Since then, scientists have tried - and succeeded - to overcome the diffraction limit in different ways.
One of the newest advances in microscopy technology providing super-resolution are hyperlenses. They make use of so-called evanescent waves that are lost in conventional lenses. In contrast to propagating waves that form the image in a microscope, evanescent waves cannot propagate into, (or pass through) the lens material and fall off exponentially at its surface.
Hyperlenses can convert them into propagating waves so that the super-resolved image can be projected into the far-field. However, the observation area becomes extremely small in hyperlens devices making it difficult to correctly position the subject.
One solution to this problem are arrays of hyperlenses, i.e. ordered arrangements of multiple hyperlenses. Yet, the fabrication of hyperlens arrays has so far been difficult and not been feasible in large-scale with the existing nanofabrication methods.
Nanoimprint lithography for scalable hyperlens array fabrication
Now, researchers from Pohang University of Science and Technology and Korea University have discovered a direct pattern transfer technique for the production of hyperlens arrays in large scale. The research team further demonstrated that the fabricated devices was suitable for sub-diffraction imaging.
The findings, which have been recently published in the Nature journal Scientific Reports, open the door for practical and real-time super-resolution imaging to answer research questions in many different scientific fields .
To fabricate a hyperlens array, Minsueop Byun and the research team, used nanoimprint lithography, which is a technique to produce nanoscale patterns. In particular, they used a photo nanoimprint lithography process with a mould made from poly dimethylsiloxane (PDMS), and a liquid resist that is cured in ultraviolet (UV) light.
The fabrication of the hyperlens arrays was performed in multiple steps: First, the researchers produced a quartz master stamp. This was done by creating the hyperlens array pattern on a quartz substrate in several etching processes.
In the next step, the nanoimprinted hyperlens arrays were produced by transferring the master stamp pattern onto a PDMS mould. The mould was then covered with a liquid resist and the quartz substrate that should exhibit the array pattern was pressed onto the mould with the resist in between. In the following step, the mould was removed and the resist cured in UV light so that it became solid. After annealing of the substrate, the new hyperlens array was obtained.
Whereas, fabricating the master stamp is a time-consuming process, creating a mould that stamps the pattern into a new substrate is quick.
Arrays with super-resolution power
To show its super-resolution power, the researchers used a 5cm x 5 cm hyperlens array in a microscope setup. They were able to resolve sub-diffraction features as low as 160 nm using visible light with a wavelength of 410 nm. Abbe and the diffraction limit seem long forgotten.
The simple and scalable solution for producing hyperlens arrays can be expected to be make a huge impact in various scientific fields. Resolving the tiniest features with the hyperlens devices in biology, medical science or nanotechnology research, will aid to answer many outstanding scientific questions.
 M. Byun (2017). Demonstration of nanoimprinted hyperlens array for high-throuhput sub-diffraction imaging, Scientific Reports 7, 46314, doi: 10.1038/srep46314.
 Science Daily (2017). Nanoimprinted hyperlens array: Paving the way for practical super-resolution imaging, (assessed at 24/04/2017).
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