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.