Mid-infrared to terahertz spectrum, ranging from 5 μm to 5 mm in wavelength, is the focus of basic research in solid-state physics, chemistry, materials science, as well as applications in next-generation communications, homeland security, biosensing, and medical imaging. This frequency range is at the intersection between photonics and electronics, presenting tremendous opportunities to probe and harness fundamental light-matter interactions. The emerging fields of plasmonics and metamaterials have shown new possibilities to boost such light-matter interactions using engineered nanostructures. To overcome the dimension mismatch between the long-wavelength radiations and the nanostructures, new lithography techniques that fabricate an array of metal gaps of nanometer-to-Ångstrom scale have been recently developed. These next-generation nanostructures will have a great impact on the advancement of field enhancement and confinement toward the next level of applications as the following:

(1) Terahertz (THz) absorption spectroscopy for sensing
– Ultra-sensitive THz detection and fingerprinting of biomolecules in water
– THz dynamics of two-dimensional water layer

(2) Nanofabrication for large-area nano cavity array operating at broad wavelength range
– Machine learning-based inverse design of terahertz nano-cavities
– Wafer-scale nanofabrication for metal nano-cavities at sub-nanometer resolution

(3) Ultrafast spectroscopy with optical (or terahertz) pump-probe measurements
– Nanogap-enhanced magneto-plasmonic device
– Ultrafast dynamics of ferroelectric polarization in hafnia
– Ultrafast dynamics of the phase-transition materials combined with nano-cavities
– 2D & 3D topological insulators combined with THz nanogap systems