Abstract
The many fundamental roto-vibrational resonances of chemical compounds result in strong absorption lines in the mid-infrared region (λ ∼ 2–20 μm). For this reason, mid-infrared spectroscopy plays a key role in label-free sensing, in particular, for chemical recognition, but often lacks the required sensitivity to probe small numbers of molecules. In this work, we propose a vibrational sensing scheme based on Bloch surface waves (BSWs) on 1D photonic crystals to increase the sensitivity of mid-infrared sensors. We report on the design and deposition of CaF2/ZnS 1D photonic crystals. Moreover, we theoretically and experimentally demonstrate the possibility to sustain narrow σ-polarized BSW modes together with broader π-polarized modes in the range of 3–8 μm by means of a customized Fourier transform infrared spectroscopy setup. The multilayer stacks are deposited directly on CaF2 prisms, reducing the number of unnecessary interfaces when exciting in the Kretschmann–Raether configuration. Finally, we compare the performance of mid-IR sensors based on surface plasmon polaritons with the BSW-based sensor. The figures of merit found for BSWs in terms of confinement of the electromagnetic field and propagation length puts them as forefrontrunners for label-free and polarization-dependent sensing devices.
Highlights
The many fundamental roto-vibrational resonances of chemical compounds result in strong absorption lines in the mid-infrared region (λ ∼ 2−20 μm)
One of the most widely spread label-free sensors of molecular monolayers is based on functionalized Au surfaces sustaining surface plasmon polaritons (SPPs), that is, propagating plasmons at the interface between a metal and a dielectric material.[1−3] SPPs provide electromagnetic fields evanescently decaying in both the metal and the dielectric half space with decay lengths of a few tens of nanometers, which results in an enhanced sensitivity to very thin molecular layers
Proposed strategies to obtain stronger field confinement at a surface in the mid-IR range have made use of SPPs supported by doped semiconductors instead of metals[5−8] or by non-neutral graphene,[9,10] or have exploited different hybrid polaritonic modes instead of SPPs, such as phonon polaritons in hexagonal boron nitride and in silicon carbide (SiC).[11−13] Approaches alternative to surface wave
Summary
The many fundamental roto-vibrational resonances of chemical compounds result in strong absorption lines in the mid-infrared region (λ ∼ 2−20 μm) For this reason, mid-infrared spectroscopy plays a key role in label-free sensing, in particular, for chemical recognition, but often lacks the required sensitivity to probe small numbers of molecules. One of the most widely spread label-free sensors of molecular monolayers is based on functionalized Au surfaces sustaining surface plasmon polaritons (SPPs), that is, propagating plasmons at the interface between a metal and a dielectric material.[1−3] SPPs provide electromagnetic fields evanescently decaying in both the metal and the dielectric half space with decay lengths of a few tens of nanometers, which results in an enhanced sensitivity to very thin molecular layers. It has been shown that, in the PBG, photonic crystals can be described as metal-like composite metamaterials with a negative value of the real part of the effective dielectric function, a condition that enables photonic crystals sustaining surface electromagnetic waves at certain frequencies within the PBG.[22]
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