Abstract
AbstractTailored plasmonic nanoantennas are needed for diverse applications, among those sensing. Surface-enhanced infrared absorption (SEIRA) spectroscopy using adapted nanoantenna substrates is an efficient technique for the selective detection of molecules by their vibrational spectra, even in small quantity. Highly doped semiconductors have been proposed as innovative materials for plasmonics, especially for more flexibility concerning the targeted spectral range. Here, we report on rectangular-shaped, highly Si-doped InAsSb nanoantennas sustaining polarization switchable longitudinal and transverse plasmonic resonances in the mid-infrared. For small array periodicities, the highest reflectance intensity is obtained. Large periodicities can be used to combine localized surface plasmon resonances (SPR) with array resonances, as shown in electromagnetic calculations. The nanoantenna arrays can be efficiently used for broadband SEIRA spectroscopy, exploiting the spectral overlap between the large longitudinal or transverse plasmonic resonances and narrow infrared active absorption features of an analyte molecule. We demonstrate an increase of the vibrational line intensity up to a factor of 5.7 of infrared-active absorption features of vanillin in the fingerprint spectral region, yielding enhancement factors of three to four orders of magnitude. Moreover, an optimized readout for SPR sensing is proposed based on slightly overlapping longitudinal and transverse localized SPR.
Highlights
Surfaces with adjustable optical properties are extensively studied and of interest for sensing [1,2,3,4], controlled light absorption [5], color printing [6,7,8], among other applications
The original approach in this article is to cover a large spectral range by the polarization switchable plasmonic resonances in simple, gap-free rectangular nanoantennas. This is interesting for sensing applications, like resonant surface-enhanced infrared absorption (SEIRA) spectroscopy, which ideally requires a good spectral overlap between the plasmonic resonance and the narrow IR-active molecular vibrations [15, 32]
We studied the polarization selective resonances of rectangular-shaped plasmonic nanoantennas made of highly doped InAsSb on GaSb substrates
Summary
Plasmonic nanoantenna arrays are fabricated from 100nm-thin epitaxial, Si-doped InAs0.91Sb0.09 films grown lattice matched on a 200-nm-thick GaSb buffer layer on the GaSb substrate. The doping level of the InAsSb film is 5.1 × 1019 cm−3 corresponding to a plasma wavelength λp = 5.35 μm [34]. The buffer layer is non-intentionally doped, and the GaSb substrate is n-type, Te-doped at a level of 8 × 1017 cm−3. IR spectroscopy of plasmonic nanoantenna samples is performed using an Hyperion 3000 IR microscope coupled to a Vertex 70 Fourier-transform IR spectrometer. The light is focused on the sample by a Cassegrain objective (15 × magnification) with NA = 0.4 and detected by a mercury cadmium telluride detector in the spectral range of 5500–450 cm−1. The acquired spectra are normalized to the spectrum of a gold mirror serving as reference
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