Metasurfaces for label-free biosensing mid-infrared optics and active photonic devices

Novel two-dimensional metamaterials, known as metasurfaces, have emerged as a breakthrough platform for controlling electromagnetic wave properties at the nanoscale. These metasurfaces consist of subwavelength nanoantennas or so-called meta-atoms, which can be engineered at will to obtain the desired optical functionalities. The doctoral thesis aims to advance the current state-of-the-art metasurface technology for mid-infrared (mid-IR) applications. The first part of the thesis focuses on surface-enhanced infrared absorption spectroscopy with metasurfaces supporting high-quality (high-Q) resonances. Specifically, it introduces an imaging-based nanophotonic method for detecting mid-infrared molecular fingerprints and its' implementation in chemical identification and compositional analysis of surface-bound analytes. This technique features a two-dimensional pixelated dielectric metasurface with a range of spectrally selective resonances, each tuned to a discrete frequency. The method enables a molecular absorption signature read-out at multiple spectral points and the translation of resulting information into a barcode-like spatial absorption map. Furthermore, the thesis demonstrates high-Q angle-multiplexed metasurfaces, which deliver a large number of on-demand resonances in the mid-IR. This method combines chemically specific broadband IR detection with device-level simplicity and spectrometer-less operation of angle-scanning refractometry. Strikingly, these novel metasurface-based chemical detection methods are capable of resolving absorption fingerprints without the need for spectrometry, thereby paving the way toward sensitive and versatile miniaturized mid-IR spectroscopy devices. Yet another major contribution of the thesis includes a universal method for large-scale nanofabrication of various mid-IR metasurfaces. The core of the approach is based on CMOS-compatible processes where the metasurfaces are fabricated on free-standing metal-oxide membranes. To demonstrate the versatility of our method, we realized metasurfaces for a diverse range of applications in the mid-IR, ranging from highly efficient optical wavefront and polarization control to plasmonic metasurfaces for label-free biochemical detection in aqueous solutions. The membrane-based metasurface concept overcomes the limitations of currently used materials in the mid-IR, therefore enabling mass-production of diverse photonic devices with applications in key areas such as biosensing, optical communications, thermal imaging and spectroscopy. The last chapter of the doctoral thesis shows programmable all-dielectric Huygens' metasurfaces consisting of multi-layer Ge disk meta-units with strategically incorporated nonvolatile phase change material Ge3Sb2Te6. Switching the phase-change material between its' amorphous and crystalline structural states enables nearly full dynamic light phase control with high transmittance in the mid-IR. The versatility of the method is demonstrated by optically programming the spatial light phase distribution of the metasurface with single meta-unit precision and retrieving high-resolution phase-encoded images using hyperspectral measurements. The programmable metasurface concept overcomes the static limitations of previous dielectric metasurfaces and moves the metasurface-based technology one step closer to ultra-compact active optical elements encompassing tunable lenses, dynamic holograms, and solid-state spatial light modulators.