Surface enhanced infrared absorption with highly doped InAsSb plasmonic nano-antenna arrays

Introduction Resonant surface enhanced infrared absorption (SEIRA) spectroscopy relies on advanced nanofabrication to tailor plasmonic antenna resonances at targeted wavelengths.[1] Strongest signal enhancement is obtained... [ view full abstract ]

Introduction

Resonant surface enhanced infrared absorption (SEIRA) spectroscopy relies on advanced nanofabrication to tailor plasmonic antenna resonances at targeted wavelengths.[1] Strongest signal enhancement is obtained with plasmonic resonances tuned to molecular absorption features. This helps lower the IR spectroscopy detection limit and enables the minute detection of analyte quantities.

The tunability of the plasmonic antenna resonances is achieved by their geometry, size or interaction. Doped semiconductors go beyond, as the doping level is an additional parameter to tune the plasma frequency, contrariwise to noble metals where it is a material constant. In the IR, doped semiconductors are advantageous due to their lower carrier density, which engenders low Ohmic losses and resonances close to their plasma frequency.[2]

In this work, we investigate SEIRA performance of highly doped InAs_0.91Sb_0.09 plasmonic nanoantenna arrays with different models and molecular layers.

Methods

Plasmonic resonators were fabricated by large-area surface patterning (interferential or UV lithography) and selective chemical etching from epitaxial highly doped InAs_0.91Sb_0.09 films grown on GaSb substrates.  Analyte layers were deposited by spin- or drop casting. SEIRA signals were evaluated using a normalization to the plasmonic background, to extract the vibrational signals from FTIR spectroscopy in reflectance mode. Additionally, finite-difference time-domain (FDTD) simulations were used to calculate far-field spectra and near-field profiles.

Results and discussion

Firstly, a higher doping level was correlated to a higher SEIRA signal using PDMS layers in line nanoantenna arrays. The highest doping induces in FDTD simulations an electric field less pinned inside the nanoantennas. Secondly, broadband resonant SEIRA of vanillin molecules is demonstrated using rectangular nanoantenna arrays with low aspect ratio. The Fano line vibrational shape features spectrally covered by the large LSPR evidences the coupling. Due to their anisotropy, the nanoantennas display polarization switchable mid-IR plasmonic resonances in two spectral bands. SEIRA signals were measured on characteristic benzene ring fingerprint vibrations, interesting for pharmaceutical applications or environmental monitoring. Finally, recent results using SiO₂ layers deposited by e-beam are under study for industrial coating controls. In sum, we will present SEIRA for different materials using semiconductor nanoantennna arrays.

References

[1] F.Neubrech et al. Chem.Rev.2017,117,5110-5145.

[2] A.Boltassevva et al. Science,2011,331,290-291.