Researchers at the University of Illinois at Urbana-Champaign UIUC) in the US have fabricated 3D birefringent gradient refractive index (GRIN) micro-optics by electrochemically etching preformed porous Si (PSi) microstructures with defined refractive index profiles.
PSi was initially studied for visible luminescence at room temperature. More recently, the work of this and other teams has shown PSi’s versatility as optical material, as its nanoscale porosity — and thus refractive index — can be modulated during its electrochemical fabrication.
Figure shows how the PSi square GRIN microlens focuses and splits TM and TE polarized light, respectively TM polarized light is focused to one point and TE polarized light is focused to two different points The refractive index gradient for the square micF
“Prior to our discovery, it was well known that macroscale gradient refractive index (GRIN) optical elements could strongly control the flow of light in interesting ways,” says Paul Braun, the Ivan Racheff Professor of Materials Science and Engineering at UIUC. “However, there was no known route to forming high refractive index contrast microscale GRIN elements, such as those that could be used in integrated optics and photodetectors. We discovered that by first shaping silicon into a 3D microscale object and then using a controlled electrochemical etch process, we could directly print a 3D GRIN structure into the silicon.”
Braun and his team envision their structures will enable enhanced photodetectors and elements for integrated optics. “For example, the birefringence of the GRIN elements enables them to concurrently provide concurrent polarization-based optical splitting and focusing,” the scientist says, adding that their ability to behave like a lens while having flat top and bottom surfaces will enable the GRIN elements to be directly placed on CCD detectors.
Furthermore, because multiple GRIN elements can be interlaced, there is the potential to form miniaturized on-chip optical microscopes.
The novel concept could have broad impact on the fields of integrated optoelectronics, imaging and photovoltaics. “Along with the attributes I’ve described, the fact that the lens elements are directly formed on a silicon wafer greatly simplifies the process of integration with other on-chip optical devices.”
The professor explains that for imaging, along with the aforementioned attributes, flat optical elements enable control of undesirable reflections. In fact, he says antireflection coatings can be directly formed during the electrochemical processing.
“Finally, for PV, it is intriguing to consider using GRIN structures to route and concentrate light on a light harvesting element,” says Braun. “The GRIN elements enable light to be routed in ways not possible with conventional optics, opening up a new space on PV design.”
Designing microstructures with desirable optical qualities
“It is very difficult to form structures with even small gradient in refractive index,” Braun notes. “The structures we form can have a gradient in refractive index exceeding 1 refractive index unit. This dramatically opens up the design space.”
The professor further elaborates that formint the structures in silicon using processing techniques compatible with other microelectronic processing methods enables the structures to have a much broader impact than if they were made out of materials and using processes not common in microelectronics.
New types of applications and miniaturization of optical devices
In general, the research team’s new process enables the creation of microscale optical elements with properties previously only realized in much larger macroscale optics. Braun projects this will enable unexpected miniaturization of optical devices.
Braun and his colleagues have demonstrated that the silicon structures can be converted into silica and other materials that are transparent. “This enables them to impact applications involving visible light,” he says, adding that the silicon structures are only weakly transparent in the visible. “We are now focusing on building integrated optics demonstration devices and working with optical engineers to realize the full potential of our fabrication approach.”
The work is described in "Porous Silicon Gradient Refractive Index Micro-Optics," published in Nano Letters. Professor Braun was the first author of the paper.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today