Light-Controlled Disorder-Engineering of Optical Metasurfaces: Aspects of Design and Applications

Metasurfaces have become one of the cornerstones of modern nanotechnology. In planar optics, they enable the precise control of phase, amplitude, and polarization of light at the nanoscale, which has led to advances in numerous application fields, including sensing, nonlinear optics, and optical modulators. However, their implementation often requires surface nanostructuring, based on complex design and fabrication methods such as electron-beam lithography of focused ion-beam milling. In addition, exploiting narrow spectral features, e.g., for sensing, is accompanied by high demands in terms of precise post-process alignments with respect to probing light, which is challenging particularly in the scope of miniaturization and device integration.<br/><br/>Here, we demonstrate the fabrication of optical metasurfaces exploiting the plasmon-mediated growth of silver nanoparticles (AgNPs). The particle growth is based on the reduction of silver ions in an aqueous solution. Incident light interacts with the localized surface plasmon resonances (LSPRs) of the AgNPs, and thus enables to enhance the reduction kinetics of the silver ions and support the particle growth at intensity hotspots. The changing scattering properties of the growing ensemble of particles can further influence the distribution of intensity hotspots and thus influence the growth positions of other AgNPs. The resulting metasurface shows an optical response, which is highly sensitive to changes in the electromagnetic environment.<br/>The morphology of the resulting metasurfaces shows different features of engineered disorder, including disordered hyperuniformity. These features can be linked to the electromagnetic waves involved in the fabrication process.[1,2] By varying the illumination properties during the growth using incident waves with individual wave-front profiles, penetration depths into the solution and different frequencies, we demonstrate the vast design freedom of the presented method. It is even possible to simultaneously incorporate multiple waves into the fabrication process and thus to increase the amount of structural information stored in the metasurfaces.<br/>As a first application scenario, we show that the metasurfaces are directly applicable as self-optimized optical sensors. The sensors are both instantaneously tailored and intrinsically aligned to the probing light source, as the fabrication and probing environment can be identical. The inherent sensitivity of the optical response to any variations of the electromagnetic environment thus enables high-performance nanoplasmonic sensing without the need for any post-process alignments. We found a sensing performance in terms of a Figure of Merit* of 968. [1]<br/>Furthermore, we show that our method also renders it possible to harness short-range surface plasmon polaritons (SR-SPPs) in smooth ultra-thin silver films using a Kretschmann-Raether geometry. [3] Typically, it is challenging to utilize SR-SPPs in planar stack geometries, because of their high effective mode index. Nanostructuring ultra-thin metal films (around 20nm thickness), or placing metallic nanostructures in close proximity to the planar film for coupling is technologically challenging and can strongly influence the SR-SPP properties. The new possibilities given by our method promise great potential for sensing single surface binding events and high resolution imaging applications due to their strong field enhancement and localization. First promising results also suggest the possible use of other noble metals instead of silver, such as gold, for biological and medical applications.<br/><br/>[1] I. Shutsko, M. Buchmüller, M. Meudt, and P. Görrn, <i>Adv. Opt. Mat.</i>, <b>2022</b><br/>[2] I. Shutsko, M. Buchmüller, M. Meudt, and P. Görrn, <i>Adv. Mat. </i><i>Tech.</i>, <b>2022</b><br/>[3] M. Buchmüller, I. Shutsko, S.O. Schumacher, P. Görrn, <i>ACS Appl. Opt. Mat.</i>, <b>2023</b> (accepted)