Award Title: PFI-TT: Development of high-performance nanostructured polarizers

Federal Award ID: 1826966

Report Type: Outcome Report

This project provides new concepts for controlling the state of polarization of light. Polarizers are universal components needed in diverse application fields including imaging, display, microscopy, astrophysical observation, laser machining, quantum information processing, sensors, interferometry, ellipsometry, instrumentation, optical communications systems, and many others. Common polarizers on the market all have some degree of insertion loss. In contrast, we provide subwavelength resonant nanogrids that render near-complete transmission of one polarization state while efficiently reflecting the orthogonal state. Our low-loss polarizers function simultaneously in transmission and reflection at normal incidence, an attribute not possessed by other polarizer technologies. Due to this property, all incident light is converted to a polarized state. The project delivered compact prototype polarizers exploiting the fundamental physics of resonant nanogrids. We found that stacked nanogrid layers achieve performance specifications exceeding those of current polarizers. A chief technical challenge is to develop cost-effective fabrication methodology that retains the inherent low insertion loss and bandwidth of the basic nanogrid polarizer while greatly improving the extinction ratios attainable. Additional investments are needed to develop commercial polarizers in this class; this project was a valuable first step to prove the concept.

With focus on the telecommunication spectral region, we demonstrated design and fabrication of this new class of polarizers that are extremely compact and efficient. Based on an elemental low-loss single resonant grating, we developed multilayer modules providing ultra-high extinction ratio polarizers. The elemental polarizer contains a subwavelength periodic pattern of crystalline silicon on a quartz substrate. A stack of two dual-grating modules exhibited a measured extinction ratio of ~100,000 in a sparse 2-mm-thick device across a bandwidth of ~50 nm in the telecommunications spectral region. Theoretical computations indicate that extreme values of extinction are possible.

Operation in the visible spectral region is more challenging due to the extremely small features required. Nevertheless, we provided design and fabrication of compact high-efficiency resonant polarizers in the crystalline silicon-on-quartz material system. We experimentally verified the improved efficiency attained by a cascaded dual-module polarizer assembled with building blocks of elemental subwavelength grating structures. We obtained a measured extinction ratio of ~3000 in a 2-mm-thick stacked prototype device across a bandwidth of ~110 nm in the 570-680 nm spectral domain. The ridge width of the constituent nanograting was ~84 nm. Computed results show high performance in spite of the lossy nature of crystalline silicon in the visible region, enabling cascaded metasurfaces while preserving high transmission.

The intellectual merit of this technology lies in the novelty of our discovery that a grid of dielectric nanowires is nearly completely invisible to one polarization state while being opaque to the orthogonal polarization state with this property existing over significantly wide spectral bands. Such devices are remarkable in their effective functionality while being exceedingly sparse materially. Thus, our polarizers are based on original new ideas in photonic device engineering. It is scientifically extremely significant that the high-efficiency, wideband spectra presented can be generated in these minimal resonance systems. In addition to its fundamental contributions to science and technology, the project provided substantial analytical and experimental experience for graduate and undergraduate students. They applied excellent nanofabrication facilities for experimental device realization that students were trained to use. The students learned device design with numerous modeling and simulation computer codes. Regular group meetings allowed students and research scientists to hone their presentation and discussion skills. The results of the research were reported in the scientific literature and in conferences.

Last Modified: 06/19/2021
Modified by: Robert Magnusson