Porous Silicon Gradient Refractive Index Micro-Optics

Neil A. Krueger, Aaron L. Holsteen, Seung Kyun Kang, Christian R. Ocier, Weijun Zhou, Glennys Mensing, John A. Rogers, Mark L. Brongersma, Paul V. Braun*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


The emergence and growth of transformation optics over the past decade has revitalized interest in how a gradient refractive index (GRIN) can be used to control light propagation. Two-dimensional demonstrations with lithographically defined silicon (Si) have displayed the power of GRIN optics and also represent a promising opportunity for integrating compact optical elements within Si photonic integrated circuits. Here, we demonstrate the fabrication of three-dimensional Si-based GRIN micro-optics through the shape-defined formation of porous Si (PSi). Conventional microfabrication creates Si square microcolumns (SMCs) that can be electrochemically etched into PSi elements with nanoscale porosity along the shape-defined etching pathway, which imparts the geometry with structural birefringence. Free-space characterization of the transmitted intensity distribution through a homogeneously etched PSi SMC exhibits polarization splitting behavior resembling that of dielectric metasurfaces that require considerably more laborious fabrication. Coupled birefringence/GRIN effects are studied by way of PSi SMCs etched with a linear (increasing from edge to center) GRIN profile. The transmitted intensity distribution shows polarization-selective focusing behavior with one polarization focused to a diffraction-limited spot and the orthogonal polarization focused into two laterally displaced foci. Optical thickness-based analysis readily predicts the experimentally observed phenomena, which strongly match finite-element electromagnetic simulations.

Original languageEnglish (US)
Pages (from-to)7402-7407
Number of pages6
JournalNano letters
Issue number12
StatePublished - Dec 14 2016


  • Microlenses
  • birefringence
  • silicon photonics
  • transformation optics

ASJC Scopus subject areas

  • Bioengineering
  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanical Engineering


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