A team of researchers from the Center for Nanophotonics, FOM Institute AMOLF, in the Netherlands has created a flat lens that could pave the way for new opportunities in electronics and telecommunications. The unique lens has a large field of view and short working distance, so it could enable the creation of very small optical devices.
The optical lens is flat instead of curved like traditional glass lenses. The optical properties that come along with a flat lens could help reduce the size of computer hard drives and create smaller-than-ever microscopes, along with a plethora of 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.”
The researchers created the lens with extremely thin layers of silver and titanium dioxide. The flat lens offers properties currently unavailable in traditional lenses, such as a larger field of view and a very short working distance, so it can be placed very close to an object of interest.
Traditional lenses use curved glass to force light to converge or focus, but instead the team alternated layers of silver and titanium dioxide. While it may seem counterintuitive to use silver to form a lens— because metals can act like a mirror by reflecting all the light hitting the surface—for very thin layers of metal that are 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, which allows the silver and titanium dioxide layers to act as a lens and focuses the light coming from many directions.
The team employed a method called physical vapor deposition to create the layers, a method commonly used in industrial settings to create coatings or protective layers. One of the challenges encountered was optimizing the layer thicknesses with sub-nanometer precision. The researchers found that a 10-layer structure alternating between 53.2-nm-thick layers of silver and 25.0-nm-thick layers of titanium dioxide produced the best flat lens.
The newly developed lens also 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.”
The new flat lens could prove useful for lab-on-a-chip devices. Many of the lab processes that are integrated onto the chip—just a few square centimeters in size—require optical signals. However it is challenging to fabricate a curved lens small enough to integrate into these devices, and can be difficult to maintain the necessary optical alignment.
A flat lens could more easily fit the size requirements, and would not require precise alignment.
In addition the lens could be used with optical recording techniques, such as magneto-optical recording or heat-assisted magnetic recording, to allow for even more storage in computer hard drives.
Now the researchers are examining the possibility of tuning the optical properties of the lens using an electrical signal, which could provide a future of 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.”