Near-field Measurements of Terahertz Meta-atoms with Localized Terahertz Sources

Abstract

Terahertz (THz) metamaterials is very attractive due to their fascinating electromagnetic properties and potentials for building unique optical components and devices[1]. The meta-atom, an elementary unit of the metamaterial, is an electromagnetic resonator with non-zero magnetic and electric dipoles. Coupling between the meta-atoms and THz waves has great attention to investigate the dynamics of the metamaterials. However, the majority of THz metamaterial studies so far has focused on investigating the response of the meta-atom when a few hundred or thousands of its neighbor meta-atoms are excited with far-field THz waves. Then one or several of the meta-atoms are measured with far-field THz-wave and near-field probe. Therefore, those studies using the near-field probe are overlooking the details of the interaction between local THz electromagnetic field and a meta-atom. On the other hand, we have developed a scanning laser THz near-field imaging system[2]. In the system, a nonlinear optical crystal was employed as a THz emitter and THz waves were locally generated in the process of optical rectification at the irradiation spots of femtosecond laser beams. By setting samples in the vicinity of the THz source, THz-TDS and THz imaging of samples of sub-THz wavelength scale have been achieved. Since the size of the generated THz radiation source is smaller than that of meta-atom, the technique allows us to investigate the near-field coupling effects between a single excited meta-atom and its neighboring non-excited meta-atoms[3]. Therefore, in this study, we present direct measurements of the interaction of THz radiation with the meta-atoms of a THz metamaterial by using this system.