ABSTRACT
GRANT W. BIDNEY: Fabrication, Numerical Modeling, and Testing of Silicon Micropyramid Arrays and Retroreflectors
(Under the direction of DR. VASILY N. ASTRATOV)
This dissertation is devoted to the optical properties of mesoscale and nanoscale photonic arrays, specifically regarding two different areas: i) silicon (Si) micropyramidal photonics aimed at enhancing photodetectors and emitters, and ii) plasmonic Littrow retroreflectors in the optical regime.
In the first area, we show that Si anisotropic wet etching is attractive for the fabrication of large-scale arrays of micropyramids, or microvoids, with an extraordinary level of uniformity over centimeter-scale wafers. This is related to the self-terminating nature of the etching process when two (111)-type planes meet under the conditions when a surfactant is used to slow down the undercutting rate of the SiO2 layer. Although this technology is generally well studied by the microelectromechanical (MEMS) community, it seems that this particular property did not receive sufficient attention in previous studies. However, it is this property which enables the fabrication of uniform micropyramid arrays suitable for integration with detector and emitter arrays in optoelectronics applications. The optical properties of such arrays are studied by 3-D finite-difference time-domain (FDTD) numerical modeling in two realms represented by different boundary conditions (BCs). Periodic BCs result in Talbot self-images experimentally observed in this work. Perfectly matched layer BCs describe mesoscale interference effects resulting in the subwavelength focusing properties of individual micropyramids. It is proposed that integration with micropyramid arrays can enhance the collection of photons, signal-to-noise ratio, and operational temperatures of mid-wave infrared photodetector focal plane arrays (FPAs). It is also proposed that Si micropyramid arrays can be used to enhance light extraction and directionality of quantum sources and infrared scene projectors. Additionally, micropyramids were monolithically integrated with silicon-platinum silicide (PtSi/p-Si) Schottky barrier photodetectors to experimentally demonstrate an improved signal obtained by these micropyramid arrays. These results were compared with 3-D FDTD numerical modeling, as well as the modeling of a novel resonator cavity micropyramid structure as a way to further increase the enhancement capabilities of these micropyramids based on using a silicon-on-insulator (SOI) wafer. This structure demonstrated increased absorption of up to 11× compared to a planar reference device of the same size.
In the second area devoted to Littrow grating retroreflectors, we tackle the problem of simultaneous and efficient TE and TM polarization retroreflection. We developed the guidelines for designing such retroreflectors. Optimized performance at wavelengths in the vicinity of λ = 633 nm is expected for top metal slot arrays with thickness in the 20-40 nm range. However, this can vary for different metals such as Au, Ag, Al, and Cu. The most interesting development is our proposal to use the experimentally measured index values for thin films with different thicknesses to study and optimize the performance of real physical retroreflector devices. To the best of our knowledge this approach was proposed for the first time in our work. Using this approach, we showed that there is potentially plasmonic enhancement mechanisms involved, caused by their confinement in the metal stripes of the arrays. We demonstrated that, despite presence of absorption, such Au Littrow retroreflectors reach simultaneous ~0.2 and ~0.6 efficiency levels at TE and TM polarizations simultaneously in the same structure.