Metasurfaces, planar arrays of subwavelength resonators, have emerged as powerful tools for versatile light manipulation in compact formats across different spectral ranges. Among these, the mid-infrared (mid-IR) is uniquely characterized by its interaction with vibrational modes, making it especially valuable for biological and material sciences. Yet, the full potential and adoption of mid-IR metasurfaces remains untapped due to persistent material and design challenges. This thesis advances metasurface capabilities in the mid-IR through innovations in design, fabrication, and actuation, developing platforms tailored for enhanced lightâ matter interactions, label-free sensing, nonlinear optics, and chiral control.

We first introduce resonance-gradient metasurfaces, which combine broadband spectral coverage with high quality-factor (high-Q) resonances. By continuously varying the size of dielectric nanoresonators along the metasurface, we achieve a gapless spectrum of local high-Q resonances. This enables label-free molecular detection via surface-enhanced infrared absorption, resolving complex polymer mixtures, probing biomolecular interactions in multistep assays, and exploring vibrational strong coupling.

We then extend this concept to nonlinear optics. Using Germanium-based metasurfaces with bone-like resonator geometries, we demonstrate enhanced and tunable third- and fifth-harmonic generation. By engineering the local resonances to match pump wavelengths, we enable spatially selective and spectrally wide nonlinear responses, illustrating how structural design can be leveraged for broadband high-harmonic tuning.

Second, a symmetry-guided framework is developed for mid-IR chiral metasurfaces, overcoming the limitations of trial-and-error and black-box AI design. Group-theoretical analysis of meta-atom rotation and lattice symmetry led to metasurfaces with tunable circular dichroism across all planar Bravais lattices with predictable zero-chirality anchor points. Applications include dual-channel image encoding in both transmission and chiral response, offering routes to anti-counterfeiting and advanced polarization control.

Finally, the thesis addresses practical challenges in experimentally realizing ultra-high-Q resonances. By employing suspended crystalline silicon membranes, we eliminate losses in substrate and high refractive index resonator material, achieving Q-factors exceeding 2500 with modulation over 50% - an order-of-magnitude improvement over prior results. This advance enables the detection and control of increasingly subtle optical effects and is exemplified through voltage-controlled electro-thermal modulation of the resonances up to 15 kHz. These actively tunable metasurfaces approach the linewidths of gas-phase molecular absorption features, making them particularly promising for trace gas detection. Here, active tuning could facilitate spectral alignment with target absorption lines beyond fabrication limitations, offering a robust platform for reconfigurable mid-IR photonic devices.

Together, these contributions establish a versatile toolbox for designing, fabricating, and applying mid-IR metasurfaces in molecular spectroscopy, nonlinear optics, and chiral light control - advancing the field toward practical, scalable, and active photonic devices.