Near-field observation of enhanced resonance in metasurface by cavity mode coupling.

Metasurfaces have demonstrated significant potential for applications in electromagnetic emission, regulation, imaging, and wireless communication, particularly in the rapidly expanding terahertz frequency range, due to their highly flexible and tunable functionalities. However, to better understand their underlying mechanisms, detection methods capable of surpassing the diffraction limit are required. As near-field detection technologies are still developing, direct observation of surface resonance modes remains limited, with most research focusing on the detection of intrinsic modes. In this study, we propose utilizing a waveguide layer structure as the substrate for metasurfaces to enhance the intensity of surface resonance and employ terahertz time-domain spectroscopy in conjunction with scattering-type scanning near-field optical microscope (THz-TDS s-SNOM) to achieve experimental observation of this enhanced resonance on an individual resonant element. Building on this stronger resonance, we further investigated the coupling effects between modes from a microscopic perspective by fine-tuning the structural parameters of the resonant unit to modulate the resonance strength. These findings contribute to a deeper understanding of metasurface operation and coupling mechanisms, while the enhanced near-field resonance also opens new possibilities for micro and nano sensing.