Mapping optical Bloch modes of a plasmonic square lattice in real and reciprocal spaces using cathodoluminescence spectroscopy

Abstract

Strong electron-light interactions supported by the surface plasmon polaritons excited in metallic thin films can lead to faster optoelectronic devices. Merging surface polaritons with photonic crystals leads to the formation of Bloch plasmons, allowing for the molding of the flow of polaritons and the controlling of the optical density of states for even stronger electron-light interactions. Here, we use a two-dimensional square lattice of holes incorporated inside a plasmonic gold layer to investigate the interaction of surface plasmon polaritons with the square lattice and the formation of plasmonic Bloch modes. Cathodoluminescence spectroscopy and hyperspectral imaging are used for imaging the spatio-spectral near-field distribution of the optical Bloch modes in the visible to near infrared spectral ranges. In addition, the higher-order Brillouin zones of the plasmonic lattice are demonstrated by using angle-resolved cathodoluminescence mapping. We further complement our experimental results with numerical simulations of the optical modes supported by the plasmonic lattice that helps to better resolve the superposition of the various modes excited by the electron beam. Next to previous works in this context, our results thus place cathodoluminescence scanning spectroscopy and angle-resolved mapping as complementary techniques to uncover the spatio-spectral distribution of optical Bloch modes in real and reciprocal spaces.

Tailoring the Band Structure of Plexcitonic Crystals by Strong Coupling

“…The CL spectra acquired at different electron impact positions for the Au lattice demonstrate a rather broad peak (Figure e), centered at the wavelength of λ = 800 nm and shifted by ∼60 nm with respect to the A exciton energy. The CL spectra sustain an inhomogeneous broadening that is due to the excitation of a combination of plasmonic modes with different momenta and symmetries, as confirmed by previously discussed simulation results . The optical response of individual WSe 2 flakes and the formation of the exciton polaritons are thoroughly explored using CL spectroscopy elsewhere .…”

“…For both the plasmonic crystal structure and the hybrid WSe 2 /Au-lattice structure, the hyperspectral images demonstrate a ring-like asymmetric maximum CL intensity surrounding the holes . The asymmetric pattern is due to the reflection of the quasi-propagating PBMs from the boundaries of the truncated photonic crystal.…”

“…The decay rate corresponds to the optical lifetime (Figure b–d). At some specific energy ranges, one can clearly identify the high-quality factor PBM resonances; in general though, a damping ratio of approximately 0.01 to 0.3 eV, corresponding to a lifetime in the range of 3.9 fs to 113.5 fs, is recognized.…”

“…The optical response of individual WSe 2 flakes and the formation of the exciton polaritons are thoroughly explored using CL spectroscopy elsewhere . PBMs supported by the same plasmonic lattice have also been investigated with CL spectroscopy and angle-resolved mapping previously. ,, Here, we focus on the interaction of the PBMs with exciton polaritons.…”

“…For both the plasmonic crystal structure and the hybrid WSe 2 /Au-lattice structure, the hyperspectral images demonstrate a ring-like asymmetric maximum CL intensity surrounding the holes. 83 The asymmetric pattern is due to the reflection of the quasi-propagating PBMs from the boundaries of the truncated photonic crystal. CL spectra of the 70 nm-thick WSe 2 flake at selected electron impact positions demonstrate a wavelength splitting in the order of 160 nm centered at the Aexciton wavelength as a result of the exciton−photon interactions in the WSe 2 flake (Figure 3d), which is in good agreement with the recently reported values 42,51 and confirms exciton−polariton formation in WSe 2 thin films.…”