Fig. 1. Principle of (a) propagating plasmon polaritons on the interface of the air-gold film and (b) localized surface plasmon resonance of a metal particle and (c) phonon polaritons in polar crystals

Fig. 2. (a) Schematic of graphene; (b) The dispersion curves of graphene plasmon polaritons at two Fermi levels of 0.2 eV (solid blue curve) and 0.4 eV (dashed blue curve); (c) The calculated field distribution (\begin{document}$ \mathrm{R}\mathrm{e}\left({{E}}_{{z}}\right) $\end{document}) of graphene plasmon polaritons at Fermi energy levels of 0.2 eV and 0.4 eV, the green curve indicates half the wavelength of free-space incident light

Fig. 3. (a) The real part of the relative dielectric function of h-BN. The inset shows the schematic diagram of the crystal structure of h-BN; (b) Schematic isofrequency surfaces for type I (upper) and type II (lower) hyperbolic phonon polaritons; (c) The schematic of the s-SNOM experiment for measuring h-BN (upper panel) and the PHPs near-field imaging of h-BN when ω=1420 cm⁻¹(bottom panel); (d) The real part of the relative dielectric functions of α-MoO₃ crystal along three main crystal directions; (e) Isofrequency surfaces at the band 1, band 2 and band 3 in (d); (f) Electric field intensity distribution and (g) E_z component diagrams of α-MoO₃ at frequencies of 623 cm⁻¹, 925 cm⁻¹, and 1000 cm⁻¹, with an α-MoO₃ thickness of 160 nm

Fig. 4. (a) Schematic of the FTIR setup; (b) Schematic of the THz TDS setup; (c) Schematic of the THz s-SNOM setup (BS: beam splitter, PM: parabolic mirror, BM: detector); (d) Schematic of the THz TDS s-SNOM setup

Fig. 5. (a) Schematic of photocurrent detected THz s-SNOM imaging acoustic graphene plasmons. The bottom panel show the schematic of samples, that is h-BN encapsulated graphene, in which the distance between graphene and the below metal is d; (b) The photocurrent trajectories of three different devices with d=27 nm, 14.5 nm and 5.5 nm, respectively; (c) The phase velocity of graphene plasmons depends on distance d. The red and grey lines are calculated by non-local and local models, respectively; (d) Phase velocity depending on the carrier density n_s and d

Fig. 6. (a) Schematic of the THz s-SNOM; (b) Top panel-illustration of mapping polaritons (indicated by red sine waves). E_inc and E_sca denote the electric field of the incident and tip-scattered radiation, middle panel-topography image of a 25-nm-thick Bi₂Se₃ film on Al₂O₃, bottom panels-recorded amplitude and phase images at a frequency of 2.52 THz; (c) Top panel is the sketch of Bi₂Se₃. Solid lines show calculated dispersions based on conductivity models considering the contributions of optical phonon (OP) + Dirac carriers (DC)+bulk carriers (BC)+ two dimensional electron gas (2DEG) (red curves), the contributions of OP+DC+BC (pink curve), the contributions of OP+BC+2DEG (grey curve), which are sketched in top panel; (d) Sketch of the s-SNOM measuring Bi₂Se₃ film; (e) Top panel-topography line profile, showing the height h as measured by AFM, bottom panels-experimental s-SNOM amplitude and phase line profiles recorded at 2.52 THz; (f) Amplitude and phase line profiles obtained from the data shown in panel e after subtraction of the complex-valued signal offset C at large distances x

Fig. 7. (a) Schematic diagram of WTe₂ crystal structure (top left), schematic diagram of extinction spectrum measurement (bottom left), far infrared absorption spectra of WTe₂ films polarized along the a-axis and b-axis at 20 K. The solid, dotted and dotted lines are the corresponding fitting curves of the total extinction spectrum, Drude component and interband transition component (right), respectively; (b) The extinction spectra of microdisk arrays prepared on Si/SiO₂ substrates at 10 K along the a and b axes (left), and the temperature dependence of the extinction spectra of microdisk arrays polarized along the a and b axes (right); (c) The schematic diagram of Ag₂Te polariton imaging experiment using THz s-SNOM (left 1), the AFM image of Ag₂Te flake on Si substrate (left 2) and the near-field images under different excited frequencies (right); (d) The AFM image of Ag₂Te flake on SiO₂/Au substrate (left 1) and polariton near-field images under different excited frequencies (right); (e) Light microscopy image of the Ag₂Te platelet on a SiO₂ thin layer on Au, the diagram shows the isofrequency contour of the APPs; (f) APP dispersion perpendicular to the platelet edges e₁ (blue), e₂ (red) and e₃ (violet), as obtained from the near-field line profiles recorded along the dashed lines in e; (g) Calculated near-field distribution (real part of vertical electric field, Re(E_z)) of APPs at 4.25 THz, which propagate radially and perpendicular to platelet edges e₁ and e₂; (h) Solid lines show the calculated isofrequency contours of APPs under different frequencies; (i) Calculated near-field distribution (real part of the vertical electric field, Re(E_z)) of radially propagating PPs at 4.25 THz, which propagate perpendicular to platelet edges e₁ and e₂; (j) Calculated isofrequency contours of PPs under different frequencies

Fig. 8. (a) Schematic diagram of near field characterization of α-MoO₃ terahertz polaritons using THz s-SNOM combined with free-electron laser; (b) Dielectric permittivity tensor of α-MoO₃ in the THz spectral range obtained by correlating ab initio calculations with near- and far-field experiments. The real (solid lines) and imaginary (dashed lines) parts of the permittivity tensor reveal three distinct reststrahlen bands with negative permittivities along different crystal axes, shaded in red ([001]), green ([010]), and blue ([100]); (c) The near-field intensity (S_2Ω) images of α-MoO₃ (left) and the open hyperbolic polariton isofrequency curves (IFC) in momentum space (kx, ky) with the numerical simulations of the electric field distribution (false color plots) obtained at RB₁ (ν=9.22 THz) and RB₃ (ν=11.17 THz) frequencies, respectively; (d) Real (solid lines) and imaginary (dashed lines) components of the complex permittivity ε. The permittivity in the THz regime is governed by four optical phonons and exhibits two in-plane reststrahlen bands RB_y and RB_x. The inset highlights the real part of ε from 8 to 9 THz. The shaded areas A, B, and C identify three spectral regions with a different constitution of Re(ε_i) (i=x, y, z); (e) The optical near-field intensity S_2Ω image of α-GeS and the corresponding S_2Ω profiles extracted at three different excitation frequencies along the GeS [100] (blue) and [010] (red) crystal directions (bottom)

Fig. 9. Manipulation of the polaritonic light field. (a) s-SNOM amplitude images taken at the frequency is 8.67 THz when θ = 0, 50 and 90°, respectively; (b) Anisotropic extinction spectra and the real part (dashed line) and imaginary part (solid line) of the electrical conductivity for the 90°-twisted bilayer structure of WTe₂; (c) Experimental and fitted results of the anisotropic extinction spectra of Mo_xW_1–xTe₂ tuned by Mo doping and the imaginary part of the electrical conductivity along two crystal axes. The shaded area indicates the corresponding hyperbolic frequency range; (d) Polarization dependence of the original extinction spectra of the polarization-resolved plasmons in the oblique band array of WTe₂

Fig. 10. (a) Schematics of the encapsulated BLG FET (top left), schematic diagram of terahertz detector (lower left), conductance of BLG FETs as a function of the gate voltage V_g measured at a few selected temperatures, optical image of terahertz photodetector (lower right); (b) Schematic diagram of the terahertz tuner based on the α-MoO₃ grating metasurface (top left), SEM image of α-MoO₃ grating super surface (top right), polarized extinction spectra of THz notch filters. The long axes of the ribbons are parallel to [100] (left panels) and [001] (right panels) crystallographic directions, respectively (lower left), polar plots of the extinction as a function of excitation polarization at resonances frequencies of 270 cm⁻¹ /362 cm⁻¹ (lower right)