Yuhan Yao

Full-color reflective display system based on high contrast gratings

A full-color reflective display system with potentially unprecedented performances on brightness, color saturation, and contrast ratio is proposed. A three-layer architecture is employed to achieve maximum brightness, wherein the key components of every layer are reflective color filters based on high-contrast gratings that are designed and fabricated. The reflective filters exhibit both high reflectance and high color saturation. Switching of each color filter using electrowetting is proposed, and the feasibility of switching is proved. Finite-difference time-domain-based simulations are used in the design of the filters as well as to evaluate their performance. The fabrication process, which combines interference lithography, nanoimprint lithography, linewidth tuning, and reactive ion etching, is developed and optimized. The blue and the green filters are both fabricated and characterized experimentally.

Fabrication of high-contrast gratings for a parallel spectrum splitting dispersive element in a concentrated photovoltaic system

High-contrast gratings are designed and fabricated and their application in a parallel spectrum splitting dispersive element is proposed that is found to have the potential to improve the solar conversion efficiency of a concentrated photovoltaic system. This proposed system will also lower solar cell cost in the concentrated photovoltaic systems by replacing the expensive tandem solar cells with cost-effective single-junction solar cells. The structures and parameters of the high-contrast gratings for the dispersive element are numerically optimized, and a large-area fabrication is experimentally demonstrated using nanoimprint lithography and dry etching. Both the quality of the material and the performance of the fabricated device are experimentally characterized and discussed.

Nanoimprint-defined, large-area meta-surfaces for unidirectional optical transmission with superior extinction in the visible-to-infrared range

Optical devices with asymmetric transmission have important applications in optical systems, but optical isolators with the modal asymmetry can only be built using magneto-optical or nonlinear materials, as dictated by the Lorentz reciprocity theorem. However, optical devices with the power asymmetry can be achieved by linear materials such as metals and dielectrics. In this paper, we report a large-area, nanoimprint-defined meta-surface (stacked subwavelength gratings) with high-contrast asymmetric transmittance in the visible-to-infrared wavelength range for TM-polarized light. The physical origin of asymmetric transmission through the meta-surface is studied by analyzing the scattering matrix.

Probing Gap Plasmons Down to Subnanometer Scales Using Collapsible Nanofingers

Gap plasmonic nanostructures are of great interest due to their ability to concentrate light into small volumes. Theoretical studies, considering quantum mechanical effects, have predicted the optimal spatial gap between adjacent nanoparticles to be in the subnanometer regime in order to achieve the strongest possible field enhancement. Here, we present a technology to fabricate gap plasmonic structures with subnanometer resolution, high reliability, and high throughput using collapsible nanofingers. This approach enables us to systematically investigate the effects of gap size and tunneling barrier height. The experimental results are consistent with previous findings as well as with a straightforward theoretical model that is presented here.

Optical metasurface based on hybrid high-contrast dielectric gratings for visible and near-IR ranges (Conference Presentation)

In the past decade, subwavelength high contrast gratings (HCGs) have been developed and studied, which has led to many applications. The broadband reflectance in HCGs mainly comes from the contrast between the grating material and its surrounding environment, so high-index and low-loss materials are required for making HCGs. Compared with infrared (IR) ranges, building HCGs in visible or near-IR wavelength ranges is much harder due to the limitation of optical materials. In order to overcome the challenge of materials in making HCGs in visible to near-IR ranges, hybrid HCGs are proposed. The design of hybrid HCGs is a combination of low-loss and low-index materials and high-loss and high-index materials. In order to reduce the optical loss due to the incorporation of high-loss material, optical modes must be manipulated to be confined in the low-loss region. In our work, the structure and parameters for hybrid HCGs are optimized based on numerical study (both FDTD and RCWA). As a proof-of-principle demonstration, hybrid HCGs composed of amorphous silicon, silicon nitride and silicon dioxide are optimized. The optimal structure has a broadband reflectance (>90%) in visible to near-IR ranges. The design demonstrates a great fabrication tolerance to line width, dielectric thicknesses and sidewall verticality. The hybrid HCGs are patterned by nanoimprint lithography. Reactive ion etching at cryogenic temperature is optimized for the best etching profile. More details on design, fabrication and measurement will be presented at the conference.

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

Stereolithography with variable resolutions using optical filter with high-contrast gratings

A three-dimensional printing approach based on stereolithography with variable printing resolutions was invented to solve the trade-off between throughput and resolution. In this technology, the variable fabrication resolutions are achieved by switching laser wavelength. The key component to enable this technology is an optical filter based on high-contrast gratings. The optical filter has been designed and numerically studied using the finite-difference time-domain method and was fabricated using nanoimprint lithography. The minimum printing resolution of the accordingly constructed stereolithography apparatus is reduced to 37 μm. Variable pixel sizes from 37 to 417 μm have been demonstrated. Using the designed and fabricated optical filter is a promising method to optimize the manufacturing efficiency of the stereolithography process.