Tunable rainbow light trapping in ultrathin resonator arrays

Rainbow light trapping in plasmonic devices allows for field enhancement of multiple wavelengths within a single device. However, many of these devices lack precise control over spatial and spectral enhancement profiles and cannot provide extremely high localised field strengths. Here we present a versatile, analytical design paradigm for rainbow trapping in nanogroove arrays by utilising both the groove-width and groove-length as tuning parameters. We couple this design technique with fabrication through multilayer thin-film deposition and focused ion beam milling, which enables the realisation of unprecedented feature sizes down to 5 nm and corresponding extreme normalised local field enhancements up to 10 3 . We demonstrate rainbow trapping within the devices through hyperspectral microscopy and show agreement between the experimental results and simulation. The combination of expeditious design and precise fabrication underpins the implementation of these nanogroove arrays for manifold applications in sensing and nanoscale optics. Plasmonics: Trapping a rainbow with nanogrooves A new approach facilitates the fast and versatile design and easy fabrication of nanogroove arrays that can trap the full spectrum of visible light, with potential applications in sensing and nanoscale optics. Katelyn Dixon of the University of Toronto and colleagues conducted mathematical analyses followed by software simulations to determine the wavelengths of visible light that elicit ‘resonance’ within a single metal-insulator-metal nanogroove depending on its width and length. When designed within an array, each nanogroove will have a specific optical response depending on the wavelength of light that causes it to resonate. The team used a unique technique to fabricate devices made of arrays of nanogrooves with different lengths and widths. The narrowest reached five nanometres; ten times smaller than previously designed grooves. The rainbow-trapping arrays overcome limitations in previous designs.