Nanofocusing of acoustic graphene plasmon polaritons for enhancing mid-infrared molecular fingerprints

Abstract

<jats:title>Abstract</jats:title> <jats:p>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.</jats:p>

Real-space imaging of acoustic plasmons in large-area graphene grown by chemical vapor deposition

“…We note that the probed structure with t ≪ λ AGP does not support propagating GSP due to the proximity of graphene to the gold layer. For the MIR frequencies of interest and E F ≈ –0.5 eV, it has been analytically demonstrated 8 that the AGP dispersion approaches that of the GSP when t ≳ 100 nm.…”

“…Therefore, our near-field measurements indicate that, while the observed AGP is ×1.7 (×2.3) times more compressed in terms of the wavenumber, its FOM is 1.4 times higher than that of the GSP in the same graphene sheet. The better FOM of AGP has been predicted 8 due to the larger compression factor of the plasmonic wavelength compared to the decrease in the propagation distance. However, dispersion analysis also indicates that the AGP is in general less sensitive to the loss in graphene if considered in local limit 42 (see Supplementary Information section S-1 ).…”

“…This mode is supported by a structure comprising of a metal, a dielectric spacer, and a graphene layer, where the image charges in the metal effectively “mirror” the charge density oscillations in the doped graphene layer. The acoustic graphene plasmon (AGP) supported by the structure is mostly confined in the dielectric spacer and does not experience cutoff as the spacer thickness decreases 8 , thus resembling the fundamental plasmonic mode of a narrow metal gap. The AGP excited at the important MIR frequencies 9 , 10 does not exhibit significant loss and is detectable even when the spacer is reduced to a single atomic layer of hexagonal boron nitride 5 .…”

Spatiotemporal imaging of 2D polariton wave packet dynamics using free electrons

“…Looking forward, the unique optical properties of 2D polaritons can enable many novel phenomena concealed in their dynamics. For example, we could explore diffractionless polariton wavepackets (7) or polariton focusing and continuous wavepacket deceleration in cases where the polariton propagates in a flake with a gradually reducing thickness (74). The current challenge is to improve the in-coupling of the external laser excitation into the polaritonic mode, which can be overcome by using metallic nano-antennas (75) or nanoparticles (76) instead of only the sample edge, as in our study.…”

Enhanced Molecular Infrared Spectroscopy Employing Bilayer Graphene Acoustic Plasmon Resonator

“…In such configuration, an ultra-tightly confined plasmon mode with linear dispersion at small frequencies, so-called acoustic GPs (AGPs), can be excited [ 23 , 24 , 25 , 26 ]. The infrared photons are squeezed into extremely confined graphene/nano-gap/metal plasmon cavity down to a sub-nanometric scale and achieve strong interaction between free-space light and atomically thin molecules, which is promising for demonstrating ultimate plasmon confinement limits and ultrasensitive infrared spectroscopy [ 27 , 28 , 29 , 30 , 31 ]. However, the fabrication process of such structure is relatively complex and the metal surface roughness significantly attenuate the excitation far-field signals [ 30 ].…”