Fig. 4: Intersubband polariton confined in WO_x/4L-WSe₂/Au nanoresonators.

a Schematics of WO_x/4L-WSe₂/Au nanoresonators fabricated by focused ion beam on gold substrate. b Near-field scattering amplitude s(ω) images of an array of WO_x/4L-WSe₂/Au nanoresonators with different diameters (100 nm to 800 nm) and different shapes. The images are measured at a series of excitation energies from 139 meV to 161 meV. Scale bar: 1 μm. c Near-field scattering amplitude s(ω) image of WO_x/4L-WSe₂/Au nanoresonator with diameter D = 600 nm. The excitation energy is E_ph = 146 meV. Scale bar: 250 nm. d The corresponding Fourier transform image of (c). k_x and k_y are wavevectors in x and y direction. Scale bar: 40 k₀, where k₀ is the momentum of light in free space. e Normalized near-field scattering amplitude as a function of diameters of the resonators with excitation energy at E_ph = 151 meV (top panel), E_ph = 144 meV (middle panel) and E_ph = 139 meV (bottom panel). Red arrows help to identify the peak value of s(ω). f Normalized amplitude as a function of excitation energy for the WO_x/4L-WSe₂/Au nanoresonator with diameter D = 600 nm (green circles). Gray solid line is the Gaussian fitting. g Finite-difference time-domain simulation of the out-of-plane field E_z in WO_x/4L-WSe₂/Au nanoresonator with different excitation energies of E_ph = 144 meV, E_ph = 146 meV, E_ph = 148 meV and E_ph = 151 meV, respectively. The diameter of the nanoresonator is D = 600 nm.