Here we report the designing of chiral metamaterial with metallic helix array. The effective electric and magnetic dipoles, which originate from the induced surface electric current upon illumination of incident light, are collinear at the resonant frequency. Consequently, for the circularly polarized incident light, negative refractive index is realized. Our design provides a unique approach to tune the optical properties by assembling helices, and demonstrates a different approach in exploring threedimensional chiral metamaterial.

Supporting Information Structure fabrication and measurementsThe sample has been fabricated by photolithography on a double-polished silicon wafer 500 μm in thickness. The marks for alignment in nesting lithography were first prepared on the wafer. Lift-off procedures were used to fabricate the double-layer U-shaped resonators (USRs). A U-shaped pattern was first defined on the substrate using photoresist. A 100-nmthick gold film was blanket-deposited all over the patterned substrate, covering the photoresist and the areas on which the photoresist had been cleared. Thereafter, the photoresist under the film was removed with solvent, leaving the gold U-shaped pattern on silicon substrate. In this way, the lower layer of USRs was generated. Then a layer of 600-nm-thick silicon nitride was deposited over the gold structures as a spacer layer. Thereafter, a layer of photoresist was once again spin-coated, followed by alignment nesting lithography and lift-off procedure. Meanwhile, each USR on the top layer located exactly above the one on the lower layer, yet the orientation had being rotated for 90 o in a specific way. In this way, the double-layered USRs were fabricated.

Recently, plasmonics has been central to the manipulation of photons on the subwavelength scale, and superior infrared imagers have opened novel applications in many fields. Here, we demonstrate the first pixel-level plasmonic microcavity infrared photodetector with a single quantum well integrated between metal patches and a reflection layer. Greater than one order of magnitude enhancement of the peak responsivity has been observed. The significant improvement originates from the highly confined optical mode in the cavity, leading to a strong coupling between photons and the quantum well, resulting in the enhanced photo-electric conversion process. Such strong coupling from the localized surface plasmon mode inside the cavity is independent of incident angles, offering a unique solution to high-performance focal plane array devices. This demonstration paves the way for important infrared optoelectronic devices for sensing and imaging.