Probing Polaritons in 2D Materials with Synchrotron Infrared Nanospectroscopy

Abstract

Polaritons, which are quasiparticles composed of a photon coupled to an electric or magnetic dipole, are a major focus in nanophotonic research of van der Waals (vdW) crystals and their derived 2D materials. For the variety of existing vdW materials, polaritons can be active in a broad range of the electromagnetic spectrum (meVs to eVs) and exhibit momenta much higher than the corresponding free‐space radiation. Hence, the use of high momentum broadband sources or probes is imperative to excite those quasiparticles and measure the frequency‐momentum dispersion relations, which provide insights into polariton dynamics. Synchrotron infrared nanospectroscopy (SINS) is a technique that combines the nanoscale spatial resolution of scattering‐type scanning near‐field optical microscopy with ultrabroadband synchrotron infrared radiation, making it highly suitable to probe and characterize a variety of vdW polaritons. Here, the advances enabled by SINS on the study of key photonic attributes of far‐ and mid‐infrared plasmon‐ and phonon‐polaritons in vdW and 2D crystals are reviewed. In that context the SINS technique is comprehensively described and it is demonstrated how fundamental polaritonic properties are retrieved for a range of atomically thin systems including hBN, MoS2, graphene and 2D heterostructures.

Nanoscale‐Confined Terahertz Polaritons in a van der Waals Crystal

“…[ 39,40 ] Synchrotrons generate broadband radiation extending down to IR and THz frequencies, where they have been applied for s‐SNOM. [ 41,42 ] Here, restrictions still exist in the detection of the weak tip‐scattered signals in s‐SNOM, which has so far limited broadband nanospectroscopy with synchrotrons to frequencies >9.6 THz. [ 41 ] Free electron lasers (FELs) offer the advantage of broad continuous tunability combined with an extremely narrow spectral bandwidth (≈0.5–2.5% FWHM ).…”

Synchrotron infrared nano-spectroscopy and -imaging

“…Surface polaritons in solids are quasiparticles resulting from strong coupling of photons with collective modes in a crystal, such as plasmons in conductors or optical phonons in polar crystals, that have negative real parts of the dielectric function. [126,127] In s-SNOM, the large near-field momentum provided by the spatial field localization at the apex of the AFM tip is sufficient to optically excite these surface modes, as was demonstrated with surface plasmon polaritons (SPPs) in graphene [128][129][130] and surface phonon polaritons (SPhPs) in hexagonal boron nitride (hBN) [40], using laser-based s-SNOM nano-imaging. Unlike plasmon polaritons that usually span a very broad energy range, SPhPs provide a spectrally selective response associated with optical phonon modes.…”

Hybridized Radial and Edge Coupled 3D Plasmon Modes in Self‐Assembled Graphene Nanocylinders

“…The SINS technique was used to measure and experimentally visualize the plasmon‐induced near‐field enhancement in graphene nanocylinders ( Figure a) because of unique advantages from the AFM‐tip (atomic force microscopy) enhanced probing of the coupled edge plasmons and spectral range, high spectral brightness, and coherence of the source. [ 38–41 ] The SINS data reveals an overlap between graphene nanocylinder topography (Figure 5b) and IR scattering images (Figure 5c) to validate the optical properties of graphene. The scattering images reveal hybridized plasmons being launched along the surface of the graphene cylinders and being reflected by the edge.…”