Mid-infrared bifunctional high-Q plasmonic metasurfaces with strong intrinsic chirality and imaging-based biosensing

Abstract

Multi- or bi-functional metasurfaces have gained wide attention for their capabilities in addressing the increasing complexity of application scenarios. In this paper, we numerically and theoretically demonstrate a high Q-factor plasmonic metasurface that realizes strong intrinsic chiral response and imaging-based molecular-fingerprint detection in the mid-infrared (MIR) spectral region. A stereo asymmetric dimer unit structure is chosen, which consists of two gold trapezoidal prisms with unequal sizes. The plasmonic metasurface sits on a gold substrate with a silica spacer layer, forming a typical metal-insulator-metal (MIM) configuration. A symmetry-protected bound state in the continuum (BIC) can be realized around 4.9 μm. By twisting one of the trapezoidal prisms and adjusting the thickness of the spacer layer, the intrinsic chiral response can be enhanced significantly. The maximum circular dichroism signal (CD = 0.87), preserving the handedness, can maintain a high Q-factor of 125. By scaling the unit cell, the high-Q quasi-BIC mode can cover a wide MIR range. As proof of principle, we also demonstrate a compact imaging-based MIR device composed of a 5×5 pixelated metasurface array. The fingerprint characteristics of A/G-IgG protein bilayer can be recognized by an imaging-based barcode map without the need for spectrometry. The proposed metasurface-based platform promises to facilitate the development of a versatile and multifunctional label-free MIR biosensing system for complex sensing scenarios.