Near-perfect molecular absorption enabled by critical coupling in metamaterial

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 near-perfect absorption due to molecular vibration enabled by critical coupling in the metamaterials. Our molecular metamaterial design consists of a thin molecular 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 vibrational absorption can be enhanced to near unity by tuning the structure's molecular film thickness and periodicity. Our finding offers a systematic approach to the development of ultra-sensitive molecular sensors, high-Q thermal emitters, and molecular photonic devices.