Hybridization of Multi-Plasmon Modes on Coupled Nanoridge Array Metasurfaces for Super-Resolution Imaging

Abstract

Abstract Body: Metamaterials and metasurfaces are promising optical materials for many applications in optics and optoelectronics as the optical characteristics can be engineered via sub-wavelength features. Hyperbolic metamaterials and metasurfaces—a particular class of these designer materials—have been widely studied due their uncommon optical characteristics that enable effects such as negative refraction, light beam steering, subdiffraction imaging, spontaneous and thermal emission engineering, and broadband optical absorption. These materials have been demonstrated across much of the optical spectrum using myriad approaches. Recently, negative refraction of optical surface modes was accomplished using a hyperbolic metasurface comprising coupled nanoridge arrays. For applications in super-resolution imaging, hyperbolic metasurfaces are of particular interest because a larger fraction of the electromagnetic field will interact with the object being imaged, rather than residing inside of the bulk of the material. One approach to imaging using metasurfaces is to leverage structured illumination imaging (SIM), a paradigm where an object is illuminated via a sub-diffraction pattern that can be externally controlled, such as by varying the angle of the incident light. In SIM, a super-resolution image is computed from several diffraction-limited images. Exciting and localizing modes with large effective modal indices provide a path towards increasing the resolution beyond conventional SIM approaches, such as localized surface plasmon SIM. However, coupling to these high-index modes is challenging. In this talk, we will demonstrate hyperbolic metasurfaces appropriate for SIM. These metasurfaces utilize finite-length coupled nanoridge arrays. The coupling between nanoridges is used to engineer the effective modal index of the supported optical modes. These optical modes are then confined by introducing small gaps (90 nm) along the nanoridges, resulting in standing waves that can serve as an illumination pattern for SIM. Large arrays of coupled-nanoridge metasurfaces are fabricated using silver on Si via electron beam lithography and standard metal lift-off. The arrays are characterized using angle-, polarization-, and wavelength-dependent reflection spectroscopy. Optical modes corresponding to multipolar plasmon resonances (n = 2 and n = 3) are identified, including a mode that is identified as a localized hyperbolic mode. The energy and dispersion of all the observed modes agree well with numerical models of the effective modal index. Numerical simulations of the reflection of free-space light also agree well with the measurements. A numerical demonstration of the imaging capabilities for these modes at 458 nm of 9 nm quantum dots, including realistic material losses, predicts a 3.3X improvement in the resolution over diffraction-limited images.