The 'trapped rainbow' mechanism of graded gratings has been demonstrated only as a type of groove reflection. In this study, we investigate the physical mechanism of the 'trapped rainbow' and demonstrate that a small step increase (∼1/50λ) in the depth of a single obstructed groove can terminate the propagation of surface terahertz (THz) waves and reflect them back. A single obstructed groove can easily manipulate the properties of THz waves by changing the depth or the refractive index of the groove because the cutoff frequency is highly sensitive to the property of the groove. In addition, waves transmitted and reflected by a single groove can be controlled periodically over a period of a 1/2 - λ increase in depth owing to the interference of surface spoof plasmonics and Fabry-Perot resonance. This is an easy means of controlling surface THz waves and fabricating more compact integrated optical devices such as THz branch waveguides, band pass filters, reflectors, and splitters.
- trapped rainbow