Titanium Nitride Nanotrench Metasurfaces for Mid-infrared Chemical Sensing

Mid-infrared vibrational spectroscopy exploits characteristic infrared absorption features of molecules and chemical bondings, providing a powerful sensing tool to detect molecules. However, the challenge lies in the relatively weak light–matter interaction, leading to small absorption signatures. Plasmonic metasurfaces made of metal nanostructures are able to localize light, enhance light–matter interactions, and thus enable increasing mid-infrared light absorption by molecules. We demonstrate the enhancement of infrared absorption of vanillin molecules by plasmonic titanium nitride trench-based metasurfaces with a high aspect ratio, which outperforms that of silicon samples with similar geometrical parameters. A 70% extra absorption signal by plasmonic structures is due to stronger electric fields within the deep subwavelength trenches. Titanium nitride trench nanostructures may serve as a highly sensitive chemical sensing platform for mid-infrared absorption spectroscopy.