Silicon Etching Technique Could Be Used for Integrated Photonics Applications

A novel approach for fabricating 3D gradient refractive index (GRIN) micro-optics using shape-defined porous silicon (Psi) could be a significant development for the fields of integrated optoelectronics, imaging and photovoltaics.

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 microlens under the two different polarizations is illustrated using the color map overlaid on the lens (blue is low refractive index, and orange is high refractive index). Courtesy of University of Illinois.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3D birefringent GRIN micro-optics by electrochemically etching preformed Si microstructures.

“Because the etching process enables modulation of the refractive index, this approach makes it possible to decouple the optical performance and the physical shape of the optical element,” said professor Paul Braun. “Thus, for example, a lens can be formed without having to conform to the shape that we think of for a lens, opening up new opportunities in the design of integrated silicon optics.

“The key is that the optical properties are a function of the etch current. If you change the etch current, you change the refractive index,” he said. “We also think that the fact that we can create the structures in silicon is important, as silicon is important for photovoltaic, imaging, and integrated optics applications.”

“Our demonstration using a three-dimensional, lithographically-defined silicon platform not only displayed the power of GRIN optics, but it also illustrated it in a promising form factor and material for integration within photonic integrated circuits,” said researcher Neil Krueger.

“The real novelty of our work is that we are doing this in a three-dimensional optical element,” added Krueger. “This gives added control over the behavior of our structures given that light follows curvilinear optical paths in optically inhomogeneous media such as GRIN elements. The birefringent nature of these structures is an added bonus because coupled birefringent/GRIN effects provide an opportunity for a GRIN element to perform distinct, polarization-selective operations.”

According to the researchers, PSi was initially studied due to its visible luminescence at room temperature, but more recently has proven to be a versatile optical material, as its nanoscale porosity (and thus refractive index) can be modulated during its electrochemical fabrication.

“The emergence and growth of transformation optics over the past decade has revitalized interest in using GRIN optics to control light propagation,” said Braun. “In this work, we have figured out how to couple the starting shape of the silicon microstructure and the etch conditions to realize a unique set of desirable optical qualities. For example, these elements exhibit novel polarization-dependent optical functions, including splitting and focusing, expanding the use of porous silicon for a wide range of integrated photonics applications.”

“The beauty of this 3D fabrication process is that it is fast and scalable,” said Weijun Zhou at Dow. “Large scale, nanostructured GRIN components can be readily made to enable a variety of new industry applications such as advanced imaging, microscopy, and beam shaping.”

The research was published in Nano Letters (doi: 10.1021/acs.nanolett.6b02939).