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Flat lens could create new opportunities in electronics, telecommunications and microscopy

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have created a new optical lens that is flat, rather than curved like
traditional glass lenses. The unique optical properties available from
the flat lens could help reduce the size of computer hard drives and
create exceptionally small microscopes, among other applications.

"We've shown a
new way to control light," said Ruben Maas, who carried out the
research in Albert Polman's research group at the Center for
Nanophotonics, FOM Institute AMOLF, The Netherlands. "This new type of
optical element will hopefully enable new types of optical devices that
are much smaller than what we've seen up until now."

In The Optical Society's journal for high impact research, Optica,
the researchers detail the fabrication and characterization of their
new lens, which is made of extremely thin layers of silver and titanium
dioxide. The flat lens offers properties not available from traditional
lenses including a larger field of view and a very short working
distance that allows it to be placed very close to an object of

Building a flat lens

Traditional lenses use curved glass to force light to converge or
focus. The key to the team's new flat lens is alternating layers of
silver and titanium dioxide. It may seem counterintuitive to use silver
to form a lens, since metals tend to act like a mirror by reflecting all
the light hitting the surface. However, for very thin layers of
metal—thinner than the wavelength of light—a portion of the light will
transmit through the material. This transmitted light undergoes an
unusual phenomenon called negative refraction that allows the silver and
titanium dioxide layers to act as a lens, focusing light coming from
many directions.

The researchers created the thin layers of silver and titanium
dioxide using physical vapor deposition, a method commonly used in
industrial settings to create coatings or protective layers. The
difficult task was optimizing the layer thicknesses with sub-nanometer
precision. The researchers found that a 10-layer structure alternating
between 53.2-nanometer (nm) thick layers of silver and 25.0-nm thick
layers of titanium dioxide produced the best flat lens.

Maas and his colleagues made a lens that operates in the ultraviolet
(UV) part of the spectrum because silver and titanium dioxide show low
optical absorption in the UV and because this wavelength produces a
higher resolution. However, other materials can be used to create a flat
lens that works at other wavelengths. "The resolution we can achieve is
still bound by the diffraction limit of light, which scales with the
wavelength," said Maas. "The short UV wavelength automatically gives it
higher resolution than for visible light."

Small optics make small devices

The new flat lens could be very useful for lab-on-a-chip devices,
which integrate several laboratory processes onto a chip that is up to a
few square centimeters in size. Many of the processes performed by
these devices require optical signals, but it is challenging to
fabricate a curved lens small enough to integrate into these devices,
and it can be difficult to maintain the necessary optical alignment. A
flat lens could more easily fit the size requirements, and because it
can focus light coming from a range of angles, up to 55 degrees from
normal, it wouldn't require precise alignment.

The new flat lens could also be used with optical recording
techniques such as magneto-optical recording or heat-assisted magnetic
recording to allow even more information storage in a smaller space in
computer hard drives.

The researchers are also examining the possibility of tuning the
optical properties of the lens using an electrical signal, which could
bring promising applications in telecommunications. "Information is
transmitted through optical fibers with optical signals," said Maas.
"Connecting this optical signal to an electrical signal, or imprinting
an electrical signal onto an optical signal is a relatively slow and
tedious process at the moment, but we envision that a flat lens could be
used for electro-optical coupling by applying a voltage over the
multilayer structure, which then modulates the transmission."

Creating an image

In the paper, the researchers show that their flat lens produced a
clear image of a 100 nm slit placed about 350 nm from the lens. Although
the demonstration lens had lateral dimensions of 25x25 microns, the
researchers say that it could easily be enlarged to centimeters or more.

"By applying the multilayer structure over a very large surface, it
acts as a lens over this entire surface," said Maas. "In a normal
microscope you would have to scan across the sample to image it because
the field of view is so limited. With a large-diameter flat lens,
scanning wouldn't be necessary because the field of view would be as
large as the lens."

The lens does show significant loss of light due to reflection, but
the ability to use it in ways that are impossible for traditional lenses
– such as on a very small scale, where alignment is critical, or very
close to the object – will override this drawback for those

The researchers previously designed a similar flat lens
where all the titanium oxide layers were of equal thickness but the
metal layer alternated between thinner and thicker layers. "We figured
out that we actually don't need to fabricate the lens in this
complicated manner," said Maas. "In this paper we present a geometry
where the thickness of the metal and the titanium dioxide layers are
different, but all the metal layers have the same thickness and all the
titanium dioxide layers have the same thickness. You can achieve exactly
the same result with this more simple design."

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