Antisymmetric localization of terahertz defect modes in a planar waveguide with undulated walls

Although various terahertz (THz) functional devices based on artificial materials have been widely proposed, their performance is still unsatisfactory due to the limitation of the involved guided wave modes. The introduction of defects can result in a strong localization effect, which has been found in applications of improving device performance. Due to Bragg resonances, the localization is usually symmetrical about the center of defects. Here, based on multiple mode resonances, we demonstrate an antisymmetric localization of THz waves in a periodic parallel plate waveguide with non-Bragg nature resonances. Unexpectedly, such resonances can produce two extremely narrow transmissions with a transmittance close to 1, and the narrowest linewidth can reach 2 MHz and the Q-factor is close to 7.44 x 10(5), which would be good candidates for THz filtering and sensing. Referring to the field distributions, we employ the mathematical operation symbols of the equal sign '=' and the multiplication sign 'x' to intuitively mark these two antisymmetric localizations with different characteristics. The dispersion curves and mode analysis reveal that the observed antisymmetric localizations caused by non-Bragg resonances are induced by the first- and second-order transverse modes. Furthermore, the frequency of antisymmetric localizations can be manipulated by changing the geometry of defects. Our findings on extremely narrow transmission peaks and antisymmetric localizations pave a way for creating high performance THz functional devices, such as switches, filters, and sensors.