Large‐Area Metal Gaps and Their Optical Applications

Abstract

Recent technological advances in fabrication methods have allowed researchers to manipulate light–matter interactions in the subwavelength region and develop a wide variety of innovative optical applications from the visible to the microwave region. Metal patterning at a subwavelength scale plays a crucial role in realizing these optical applications. Various standard lithography techniques including laser beam machining, focused ion beam, photolithography, and electron‐beam lithography are used for the subwavelength feature size of the metal patterns. Many recent studies have demonstrated that funneling light into nanometer‐wide gaps in metals gives rise to strong field enhancements and nonlocal electromagnetic effects. However, these standard methods encounter difficulties when one tries to fabricate nanometer feature sizes with macroscopic circumferences, crucial for long‐wavelength applications, over a large area. Here, new lithography techniques that fabricate an array of metal gaps of nanometer‐to‐Ångstrom scale are covered. The corresponding photonic applications in the terahertz and microwave regions are also introduced. These next‐generation metal gaps will have a great impact on the advancement of field enhancement and confinement toward the next level of applications such as metamaterials, quantum tunneling, active switching devices, and ultrasensitive chemical/biological sensors.

Massively Parallel Arrays of Size‐Controlled Metallic Nanogaps with Gap‐Widths Down to the Sub‐3‐nm Level

“…It is well known that regular arrays of sub-wavelength holes or slits in metal films allow optical energy to be efficiently coupled into SPPs-electromagnetic excitations that propagate in a wave-like manner along the planar interface between a metal and a dielectric-or into localized surface plasmons. [7,46,60] The confinement of the electromagnetic wave to the vicinity of the metal/dielectric interface results in a substantial enhancement of the electromagnetic field and accounts for many useful surface-enhanced optical properties, for example increased absorption, fluorescence, Raman scattering, second-harmonic generation, and chiroptical behavior. [6,10,46,60] The RSN arrays are therefore of potential interest for a variety of plasmonic applications.…”

“…In addition, there have been no reported methods for systematically tuning the width of the nanogaps or for fabricating massively parallel arrays of identical nanogap structures (as required e.g. for surface-enhanced spectroscopy or catalysis [46] ).…”

Nanoantenna enhanced terahertz interaction of biomolecules

“…We also showed how a molecular dynamics (MD) simulation can be a versatile tool to estimate the terahertz absorption and vibrational density of states (VDOS). Then we 17,23 (Fig. 2) is one of the most highthroughput methods for fabricating nanogap structures.…”

Fabrication of Size-Controlled Metallic Nanogaps down to the Sub 3-Nm Level

“…MNGs are essential components of molecular electronic devices, where conductive molecules are attached across the gap (individually or in groups) and serve as functional semiconductors in highly miniaturised rectifiers, switches and transistors 1,6 . They also permit the manipulation of light via plasmonic interactions, with illumination of the nanogap inducing resonant oscillations of the free electrons inside the metal electrodes (surface plasmon polaritons) [7][8][9][10][11] . The oscillating electrons act as electric dipoles that re-emit light coherently at the same frequency as the incident radiation, and additionally allow for channelling of a significant fraction of electromagnetic energy from the far field to highly confined near-field regions within the nanogaps.…”

“…In addition, there have been no reported methods for systematically tuning the width of the nanogaps or for fabricating massively parallel arrays of identical nanogap structures (as e.g. required for surface-enhanced spectroscopy or catalysis 10 ).…”