Dipole-quadrupole hybrid metasurface for broadband surface-enhanced infrared spectroscopy.

The identification of molecules constitutes a fundamental challenge in chemical sensing applications. Surface-enhanced infrared absorption spectroscopy (SEIRA), serving as a complementary technique to surface-enhanced Raman scattering spectroscopy (SERS), is widely employed for molecule identification by resolving characteristic absorption fingerprints. While commonly used nanorods support strong dipole plasmons, their narrow spectral response limits coverage of the broadband molecular fingerprint region. In this Letter, we report a tetra-band metasurface absorber that simultaneously supports both dipole and quadrupole plasmonic modes. This unique configuration generates four distinct absorption bands with uniform resonance strengths across the mid-infrared spectral range. Simulations confirm that the four resonance modes exhibit spatially overlapping "hot spots" at the designed nanogap, guaranteeing that multiband SEIRA enhancements probe the very same molecules. Multiple vibrational bands of PMMA and CO₂ served as spectroscopic probes to characterize the enhancement abilities of the four resonance modes.