Abstract
Abstract Mid-infrared (mid-IR) optical spectroscopy of molecules is of large interest in physics, chemistry, and biology. However, probing nanometric volumes of molecules is challenging because of the strong mismatch of their mid-infrared absorption and scattering cross-sections with the free-space wavelength. We suggest overcoming this difficulty by nanofocusing acoustic graphene plasmon polaritons (AGPs) – oscillations of Dirac charge carriers coupled to electromagnetic fields with extremely small wavelengths – using a taper formed by a graphene sheet above a metallic surface. We demonstrate that due to the appreciable field enhancement and mode volume reduction, the nanofocused AGPs can efficiently sense molecular fingerprints in nanometric volumes. We illustrate a possible realistic sensing sсenario based on AGP interferometry performed with a near-field microscope. Our results can open new avenues for designing tiny sensors based on graphene and other 2D polaritonic materials.
Highlights
Nondestructive analysis of molecules by means of midinfrared spectroscopy is of high relevance for plenty of vital applications in organic and inorganic chemistry, for gas concentration control, identification of polymer degradation, or determining the blood alcohol content, to name a few
Probing nanometric volumes of molecules is challenging because of the strong mismatch of their mid-infrared absorption and scattering cross-sections with the free-space wavelength. We suggest overcoming this difficulty by nanofocusing acoustic graphene plasmon polaritons (AGPs) – oscillations of Dirac charge carriers coupled to electromagnetic fields with extremely small wavelengths – using a taper formed by a graphene sheet above a metallic surface
We have suggested a method of probing molecular fingerprints with nanofocused AGPs
Summary
Nondestructive analysis of molecules by means of midinfrared (mid-IR) spectroscopy is of high relevance for plenty of vital applications in organic and inorganic chemistry, for gas concentration control, identification of polymer degradation, or determining the blood alcohol content, to name a few. The sample is illuminated by an incident wave of different frequencies, and the transmitted or reflected light is captured by a detector This simple technique, does not allow for studying small amounts of molecules in nanometric volumes as the cross-section of the latter is much smaller than the wavelength of light, and the absorption or scattering signals are too small to be detected. To further increase the field confinement of AGPs and couple more efficiently to the molecules vibrations, one can make use of a nanofocusing concept [25] The latter consists of the propagation of an electromagnetic mode along a tapered waveguide so that both its cross-section and wavelength gradually reduce, leading to a dramatic increase of the electromagnetic field intensity at the taper apex [26, 27]. The molecular fingerprint is encoded into the AGPs propagating properties (the amplitude, phase and wavelength), which can be probed by the tip of a near-field microscope
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