A New Silicon Etching Technique That Builds 3-D Gradient Refractive Index Micro-Optics

Multi-institutional research collaboration recently led to a novel approach that includes fabrication of three-dimensional fiber-optics through a shape-defined creation of porous silicon (PSi) that comes with broad level effect in the integrated optoelectronics, photovoltaics, and imaging. A research team from University of Illinois at Urbana-Champaign worked with colleagues from The Dow Chemical Company and the Stanford to fabricate a 3-D birefrigerent gradient refractive index (GRIN) micro-optics by etching performed Si micro-structures, PSi structures electrochemically along with specified refractive index profiles.

Paul Braun, the Material Science and Engineering Professor from Ivan Racheff at Illinois, says,“The emergence and growth of transformation optics over the past decade has revitalized interest in using GRIN optics to control light propagation. In this work, we have figured out how to couple the starting shape of the silicon micro-structure 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.“

Braun further adds, “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. 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.” Neil Krueger, a former student of PhD working in Braun’s lab and also the first author of this paper, also adds, “”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.”

Building 3 dimensional micro-optics

According to these researchers, PSi was studied for its visible luminescence at normal temperature initially. However, a few recent reports suggest that it has proven its worth with respect to versatile optical material as the nanoscale modularity of this material can be modified while it is fabricated electrochemically. Krueger, who also joined the Honeywell Aerospace as an Advanced Technology Scientist, says, “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.”