Spacer‐Less Substrate‐Integrated Metasurface for Near‐Unity Terahertz Absorption

Although conventional metal‐insulator‐metal (MIM) terahertz (THz) absorbers can provide near‐unity absorption, their multilayer stacks inevitably introduce additional thickness, mass, and residual stress, which limit their compatibility with THz thermal sensing platforms. Here, we propose a spacer‐less substrate‐integrated metasurface (SIM) THz absorber for integration with THz thermal detectors. By using the substrate itself as the functional dielectric region through subwavelength staircase structuring, the proposed design realizes a quasi‐MIM architecture without requiring an additional deposited dielectric layer. As a result, the absorber maintains high THz absorption while reducing structural complexity, thickness, thermal/mechanical mass, and residual stress. Finite‐element‐method (FEM) simulations predict THz absorption as high as 98%, with flexible spectral tuning through structural design. THz time‐domain spectroscopy measurements agree well with simulations and confirm an absorption of approximately 95% at the designed frequency band. In addition, Fourier transform infrared spectroscopy reveals Fabry–Pérot‐interference‐induced broadband mid‐infrared absorption of approximately 60%. These results demonstrate that the proposed SIM absorber offers a structurally simple and integration‐friendly platform for high‐performance MEMS‐based THz and infrared thermal detectors.