Highly doped semiconductor plasmonic resonators for surface enhanced infrared absorption spectroscopy

Introduction Infrared spectroscopy is an efficient technique for the selective detection of molecules by their vibrational spectra. The electric field enhancement effect from plasmonic nanostructures in resonance with... [ view full abstract ]

Introduction
Infrared spectroscopy is an efficient technique for the selective detection of molecules by their vibrational spectra. The electric field enhancement effect from plasmonic nanostructures in resonance with vibrational modes of molecules increases the signal strength. Surface enhanced infrared absorption (SEIRA) spectroscopy facilitates therefore the detection of vibrational active molecules using adequately designed plasmonic resonator substrates. We propose highly doped semiconductor material as an alternative to gold or silver for plasmonic resonators. A plasma frequency in the mid-infrared can be reached with highly n-doped InAs(Sb), allowing for higher electric field enhancement compared to gold, considering the respective dielectric functions [1, 2]. We demonstrate SEIRA with a 1-dimensional periodic grating made of InAsSb on GaSb substrate. Furthermore, we investigate 2-dimensional periodic arrays of rectangular resonators. They offer an additional degree of freedom to tailor their optical response.

Methods
Plasmonic resonators were fabricated from epitaxial, highly doped InAs_{0.91}Sb_{0.09} films grown on GaSb substrate by large-area surface patterning methods as interferential or UV lithography and chemical etching with C_6H_8O_7 : H_2O_2. Reflection spectra were obtained with an IR-microscope coupled to an FTIR interferometer.

Results and Discussion
The 1-dimensional periodic gratings feature a dominant spectral signature linked to a localized surface plasmon resonance (LSPR) at the resonator-substrate interface, when excited with light perpendicularly polarized with respect to the ribbons’ long axis. We observe the enhancement of an originally rather weak absorption feature of polymethylmethacrylate deposited on the plasmonic resonators displaying their resonance around the spectral range of the absorption line.
In 2-dimensional periodic arrays, transverse and longitudinal LSPR modes can be distinguished when the polarization direction is oriented along the short respectively the long axis of the rectangular-shaped structures. This gives access to two spectral bands of interest within the mid-infrared with one SEIRA substrate by adjustment of resonator size and aspect ratio. When working with light polarized along the diagonal of resonators or with unpolarized light, we simultaneously excite both modes covering consequently a large range of the mid-infrared spectrum.

References
[1] S. Law et al. Nano Lett. 13, 4569-4574, 2013.
[2] F. B. Barho et al. Opt. Express 24 (14), 16175-16190, 2016.