Perfect absorption of molecular vibration enabled by critical coupling in molecular metamaterial

TL;DR

This paper demonstrates perfect molecular vibration absorption using a specially designed metamaterial that achieves critical coupling, significantly enhancing infrared absorption for chemical sensing applications.

Contribution

It introduces a novel molecular metamaterial design enabling near-unity and narrowband vibrational absorption through critical coupling and structural tuning.

Findings

Achieved near-unity vibrational absorption in the infrared

Demonstrated control of absorption bandwidth via structural parameters

Enabled selective excitation of molecular vibrations without metamaterial resonance

Abstract

The absorption and emission spectrum arising from the vibrational motion of a molecule is mostly in the infrared region. These fingerprint absorptions of polar bonds enable us to acquire bond-specific chemical information from specimens. However, the mode mismatch between the atomic-scale dimensions of the chemical bonds and the resonance wavelength limits the direct detection of tiny amounts of samples such as self-assembled monolayers or biological membranes. To overcome this limitation, surface-enhanced infrared absorption spectroscopy (SEIRA) has been proposed to enhance infrared absorption directly via local field enhancement. Here, we report on the perfect absorption of molecular vibration enabled by critical coupling in the metamaterials. Our molecular metamaterial design consists of a thin polymer layer sandwiched between a structured metal layer on top and a continuous metal layer at the bottom that supports the gap plasmon mode. The measured and simulated infrared spectra of the molecular metamaterial show broad and narrow absorption bands corresponding to the metamaterial and molecular vibration modes. We show that by tuning the structure's molecular film thickness and periodicity, vibrational absorption can be enhanced to near unity. We also show that for a particular periodicity of the array, metamaterial resonance can be completely suppressed, and only molecular vibrational absorption is excited, giving rise to an extremely narrow absorption band.