Metamaterial-enhanced infrared attenuated total reflection spectroscopy.

Cheng Shi,Isaac J Luxmoore,Prarthana Gowda,Geoffrey R Nash,Jaqueline E Pitts,Callum Penrose

doi:10.1039/c8na00279g

Abstract

The use of Fourier transform infrared spectroscopy with attenuated total reflection (FTIR-ATR) allows solid or liquid samples to be characterised directly without specific sample preparation. In such a system, the evanescent waves generated through total internal reflection within a crystal interact with the sample under test. In this work we explore the use of a mid-infrared metasurface to enhance the interaction between molecular vibrations and the evanescent waves. A complementary ring-resonator structure was patterned onto both silicon and SiO2/Si substrates, and the spectral properties of both devices were characterised using a FTIR-ATR system. Minima in reflectance were observed corresponding to the resonance of the metasurface on the silicon substrate, and to the hybrid resonance of phonon modes and metasurface resonances on the SiO2/Si substrate, in good agreement with simulations. Preliminary experiments were undertaken using mixtures containing trace amounts of butyl acetate diluted with oleic acid. Without the use of a metasurface, the minimum concentration of butyl acetate that could be clearly detected was 10%, whereas the use of the metasurface on the SiO2/Si substrate allowed the detection of 1% butyl acetate. This demonstrates the potential of using metasurfaces to enhance trace chemical detection in FTIR-ATR systems.

Highlights

Infrared spectroscopy is a powerful analytical technique, providing a label-free, non-destructive method to identify chemicals.[1,2,3] As the mid-infrared photon, whose energy ranges from 0.06 to 0.4 eV, can be absorbed by the unique vibrationaland rotational-modes of speci c chemical bonds, infrared absorption spectra allow the identi cation of the chemical components of a sample by measuring the amount of IR light absorbed at speci c frequencies
In this work we explore the use of a mid-infrared metasurface to enhance the interaction between molecular vibrations and the evanescent waves
We explore the use of a mid-infrared metasurface to enhance the interaction between molecular vibrations and the evanescent waves within an ATR crystal

Summary

Introduction

Infrared spectroscopy is a powerful analytical technique, providing a label-free, non-destructive method to identify chemicals.[1,2,3] As the mid-infrared photon, whose energy ranges from 0.06 to 0.4 eV, can be absorbed by the unique vibrationaland rotational-modes of speci c chemical bonds, infrared absorption spectra allow the identi cation of the chemical components of a sample by measuring the amount of IR light absorbed at speci c frequencies. One approach to improving the sensitivity of ATR is to directly attach resonant nanoparticles to ATR crystals,[14,15] whereas Adato and Altug[16] developed the concept of plasmonic internal re ection for biological sensing. In this manuscript we demonstrate how metasurfaces[17,18,19,20,21] can be used in a standard FTIR-ATR instrument, with no modi cations required, to improve the sensitivity of the measurements. Using mixtures of butyl acetate diluted with oleic acid, we show that a metasurface can be used in a standard FTIR-ATR instrument to enhance trace chemical detection, an area which has not been explored previously

Methods

Results

Conclusion