Spectral Shape Control of Laser-Induced Terahertz Waves from Micro Split-Ring Resonators Made of Metallic Nanostructures

Efficient terahertz sources with controllable characteristics such as frequency range and polarization state are being rapidly researched and developed to suit various practical applications. To address this need, we realized the idea of combining micro- and nano-sized materials by fabricating micrometer-scale split-ring resonators made of a metal nanostructured film. We found that the peak frequencies of the emitted terahertz waves are in good correspondence with the terahertz resonance frequencies of the split-ring resonators. A possible mechanism is that a surge current was induced inside the split-ring resonators as a result of photoexcitation with the help of plasmon resonance around nanostructures, and the induced current emitted terahertz waves reflecting the resonance properties of the split-ring resonators. Although the silver nanostructures constituting the rings are random and homogeneous, the induction of the current parallel to the sample surface is enabled by the oblique incidence excitation, which breaks the symmetry along the sample surface. The present study shows the possibility of making compact terahertz emitters with flexibly tunable spectral shape, potentially leading to the development of terahertz sources optimized for specific spectroscopic uses.