Broadband tunable nanostructures for surface-enhanced infrared absorption spectroscopy

Surface-enhanced infrared absorption (SEIRA) spectroscopy leverages nanostructures to enhance the infrared absorption spectrum. Nanorods with different lengths yield different plasmon resonance wavelengths conducive to broadband SEIRA. Here, we systematically investigated the far-field and near-field behaviors of the nanostructures composed of one to four nanorods. Each multi-nanorod structure is composed of two kinds of nanorods and has dual resonance modes. By intersecting nanorods with different lengths and featuring X shapes, an additional tunable geometric dimension—the angle of X shapes—was introduced into the structure design. A strategy to manipulate the far-field amplitudes of dual-resonance by tuning the angle of the X shape was proposed for broadband SEIRA applications. Meanwhile, we focused on the different angle tuning behaviors of the broadband tunable two-nanorod and three-nanorod structures. Introducing an additional nanorod to the two-nanorod structure resulted in a convergence of the far-field amplitudes of dual-resonance (at wavelengths of 6.48 µm and 8.83 µm) and an increase of the SEIRA hotspots. However, introducing additional nanorods to the three-nanorod structure will sacrifice the tuning range of the far-field amplitudes and the near-field enhancement at the hotspots. On the other hand, the specific resonances may also be selectively excited by controlling the polarization of the light source, thereby providing a more flexible way to modulate the far-field amplitudes. Our broadband tunable nanostructures are helpful for low-noise multi-molecule measurements based on surface-enhanced infrared absorption spectroscopy.