We propose 1D periodic, highly doped InAsSb gratings on GaSb substrates as biosensing platforms applicable for surface plasmon resonance and surface enhanced infrared absorption spectroscopies. Based on finite-difference time-domain simulations, the electric field enhancement and the sensitivity on refractive index variations are investigated for different grating geometries. The proposed, optimized system achieves sensitivities of 900 nm RIU-1. A clear red shift of the plasmon resonance as well as the enhancement of an absorption line are presented for 2 nm thin adlayers in simulations. We experimentally confirm the high sensitivity of the InAsSb grating by measurements of the wavelength shift induced by a 200 nm thin polymethylmethacrylate layer and demonstrate an enhancement of vibrational signals. A comparison to a gold grating with equivalent optical properties in the mid-infrared is performed. Our simulations and experimental results underline the interest in the alternative plasmonic material InAsSb for highly sensitive biosensors for the mid-infrared spectral range.
Tailored plasmonic nanoantennas are needed for diverse applications, among those sensing. Surfaceenhanced 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 rectangularshaped, 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.
A metamaterial perfect absorber based on layered, doped and undoped semiconductors is experimentally and theoretically investigated. Design rules are given to control the multispectral, narrow, strong absorption features (>98% absorption) in the mid‐IR spectrum. The proposed sub‐wavelength grating structures support localized surface plasmons and photonic resonances associated to the quarter wavelength optical thickness of the undoped spacer layer. The resonances hybridize depending on the geometric setup of the metamaterial and the material properties of the heavily doped semiconductor. The angular response of the resonances is studied to determine the acceptance angle. While the absorption is best at near‐normal incidence, the spectral position of the absorption bands is weakly dependent on the angular incidence up to angles of 45°. Complementary to the polarization‐dependent grating design of the metamaterial, an alternative based on a crossed grating nanopattern is also investigated for polarization insensitivity. Furthermore, it is demonstrated that the metamaterial perfect absorber provides a tailored thermal emission spectrum. Based on the heavily doped InAsSb/GaSb metamaterial platform, integrable and miniaturized perfect absorbers and thermal sources can be built.
Surface-enhanced spectroscopy techniques using plasmonic nanoantennas or metasurfaces help to reduce the detection limit for biochemical sensing. While infrared spectroscopy is an excellent tool to identify a molecular species, a typically expensive IR light source is needed. We report a surface enhanced spectroscopy technique based on the thermal emission of III–V semiconductor metasurfaces. The presence of a molecular species grafted on the surface modulates the emission spectrum analogously to the modulation achieved in surface-enhanced infrared absorption (SEIRA) spectroscopy. The vibrational fingerprint of the molecular species is detected due to the electromagnetic field enhancement obtained with a plasmonic metasurface. Because the metasurface acts simultaneously as radiation source and sensor chip, the experimental setup is simplified and therefore more compact and potentially more cost-efficient. This novel approach of surface-enhanced thermal emission spectroscopy (SETES) is appealing for miniaturized and integrated molecular sensing devices.
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