During the past decades, breakthroughs in nanophotonics and nanofabrication technologies have vigorously promoted the development of optical metastructures. With the help of precise design on metastructures, incident light can be effectively manipulated. However, the difficulty in finding high-index and low-loss dielectrics in visible range limits the application of all-dielectric metastructures for visible wavelengths. Besides, the edge and surface roughness of fabricated metastructure also have more significant effects on its performance.
Here, we report the design of high contrast all-dielectric metastructure for visible range applications using the switchable all-dielectric metastructure an example. The physics behind the high contrast all dielectric metastructure is studied and analyzed. Based on this, the effect of edge and surface roughness on fabricated high contrast all-dielectric metastructure is explained. A method that can optimize the metastructure performance effectively is also proposed.
We present a technology to fabricate large-area gapped plasmonic structures deterministically with atomic precision, high throughput and high reliability at low cost. The technology is based on collapsible nano-fingers fabricated using nanoimprint lithography and ALD. A pair of metallic nanoparticles is placed on top of two nano-fingers in flexible polymer with high aspect ratio. ALD is then used to coat a thin conformal dielectric layer. By collapsing the pair of nano-fingers, two metallic nanoparticles with dielectric coating contact each other. Therefore, the gap size between two metallic nanoparticles is well defined by twice the thickness of the ALD-coated dielectric layers.
As metallic nanoparticles are known to dramatically modify the spontaneous emission of nearby fluorescent molecules and materials, here we examine the role of the gap plasmon resonance on the molecular fluorescence enhancement. Considering quenching effect, the distance between fluorescent molecules and gold nanoparticles should not be too small in order to obtain strongest enhancement. In that sense, to fully exploit plasmonic enhancement on the fluorescent molecules, an appropriate gap size should be kept between the molecule and each metallic nanoparticle, which separates molecules away from the metal to avoid quenching effect. The ALD-defined gap plasmonic nano-finger structure facilitate direct and precise control on the gap size between the molecule and metallic nanoparticle by simply changing ALD film thickness that has atomic precision. This makes collapsible nano-fingers the ideal structure for the optimization of molecular fluorescence enhancement. With the optimally engineered collapsible nano-fingers plasmonic structure, field enhancement and fluorescence quenching at hot spots can be studied in detail, which paves the way for optimal design on strongest plasmonic enhancement of molecular fluorescence.
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.
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