Charting the Exciton–Polariton Landscape of WSe₂ Thin Flakes by Cathodoluminescence Spectroscopy

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adpr.202100124.

Revealing Nanoscale Confinement Effects on Hyperbolic Phonon Polaritons with an Electron Beam

“…The momentum selectivity afforded through careful probe positioning can go a step further to directly acquire HPhP dispersions with polariton interferometry. [ 34,36,41–43 ] This is exemplified for the bulk polariton dispersion in Figure . The experiment is shown schematically in Figure 2a: a line profile is acquired normal to the flake edge, in which polaritons that propagate directly toward the edge of the flake can reflect back resulting in interference.…”

“…The electron beam can access a number of low-energy excitations including localized phonon modes [19][20][21][22] , molecular vibrations [23][24][25][26] , and the HPhPs [27][28][29] . Moreover, the precise control of the probe enables on-demand excitation of polarization-dependent modes 30 , directly induced localized quasiparticle coupling [31][32][33] , and measurements of polariton dispersions 29,[34][35][36] . Consequently, while EELS lacks the spectral resolution of SNOM, its combined spatial-and spectral resolution provides access into phenomena that cannot be explored in optical experiments.…”

“…These excitations are only accessed through impact scattering, and are not part of the polaritonic response, but they are still important as they define the limits of the Reststrahlen band in which the HPhPs are active. The momentum selectivity afforded through careful probe positioning can go a step further to directly acquire HPhP dispersions with polariton interferometry 27,29,[34][35][36] , also frequently used in SNOM 9,11,12 . This is exemplified for the bulk polariton dispersion in Figure 2.…”

“…[ 34–36 ] Moreover, the precise control of the probe enables on‐demand excitation of polarization‐dependent modes, [ 37 ] directly induced localized quasiparticle coupling, [ 38–40 ] and measurements of polariton dispersions. [ 36,41–43 ] Consequently, while EELS lacks the spectral resolution of SNOM, it possesses a key advantage in terms of spatial resolution, since the geometry of the tip and the sample fundamentally limit the maximum achievable resolution in SNOM. Beyond spatial resolution, monochromated EELS has several other benefits.…”

Self-Hybridized Exciton-Polaritons in Sub-10-nm-Thick WS2 Flakes: Roles of Optical Phase Shifts at WS2/Au Interfaces

“…Transition metal dichalcogenides (TMDs) have long attracted the attention of researchers due to their fascinating physical properties for optoelectronic device applications, including sizable bandgap energies, large absorption coefficients, high electron mobility, and superior mechanical flexibility [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. In particular, it has been noted that the exciton–photon interactions dominate the optical responses of TMDs even at room temperature.…”