Fig. 1. Phonon polariton dispersion in a cubic ion lattice (one-dimensional) with one optical mode. (a) No dissipation; (b) dissipation caused by material absorption

Fig. 2. Experimental results of nonlinear terahertz wave generation^[14]. (a) Selection of speed matching; (b) experimental observation of spatiotemporal propagation of terahertz waves in lithium niobate waveguides; (c) dispersion relationship of terahertz waves, with the difference frequency signal marked by a dashed circle; (d) oscillation of terahertz waves in the excitation region X1=1.18mm,X2=1.82mm,X3=2.39mm, and X4=3.44mm; (e) frequency ν0, ν1, and Fourier spectra of differential frequency signals at four positions

Fig. 3. Enhancement of second harmonic generation in near-infrared laser pulses by light matter interaction mediated by stimulated phonon polaritons^[19]. (a) Schematic diagram of experimental equipment; (b) second harmonic spectrum during the movement of lithium niobate crystals; (c) peak intensity and total energy of second harmonic signals during the movement of lithium niobate crystals

Fig. 4. Experimental results of stimulated phonon polaritons in microcavity^[19]. (a) Schematic diagram of experimental configuration of pump laser and microcavity, where lattice constant a=170μm, r=50μm, s=0.15r; (b) time evolution of stimulated phonon polaritons in microcavities; (c) energy band of lithium niobate photonic crystal and the electric field oscillation of the reference point in the microcavity; (d) spectral information at the lines shown in the illustration in the microcavity; (e) spectral information of local stimulated phonon polaritons in the microcavity is illustrated as a comparison of the spectra of broadband and local stimulated phonon polaritons; (f) spectra along the reference line and reference point in the microcavity

Fig. 5. Schematic diagram of stimulated phonon polaritons excited by Cherenkov radiation in lithium niobate crystal^[37]

Fig. 6. Schematic diagram of strong dispersion suppression Cherenkov radiation in subwavelength waveguides

Fig. 7. Schematic diagram of stimulated phonon polaritons generated by tilted pulse wavefront in lithium niobate crystal^[14]. (a) Schematic diagram of velocity matching for tilted pulse wavefront; (b) schematic diagram of inclined pulse wavefront experimental device

Fig. 8. Schematic diagram of stimulated phonon polaritons generated by lateral excitation in lithium niobate crystals^[39]

Fig. 9. Schematic diagram of the experimental device (pump-detection system) for generating and detecting phonon polaritons

Fig. 10. Schematic diagram of the electro-optic sampling optical path

Fig. 11. Topological valley transmission of terahertz phonon polaritons^[47]. (a) Schematic diagrams of two types of valley photonic crystal structures; (b) schematic diagram of topological valley photonic crystal waveguide structure; (c) propagation of phonon polaritons in V-type and Z-type topological valley photonic crystal waveguides; (d) transmission spectra of the center and edge before and after the first corner in a V-type topological valley photonic crystal waveguide

Fig. 12. Conversion of terahertz guided waves to surface waves based on metasurface structure. (a) Schematic diagram of metal metasurface structure^[48]; (b) (c) spatial distribution of electric field intensity when terahertz waves with frequencies of 0.27 THz and 0.43 THz propagate on lithium niobate crystals with metasurface structures^[48]; (d) coupling efficiency of terahertz waves from lithium niobate subwavelength waveguides to surface waves^[49]

Fig. 13. Enhanced terahertz sensing on a metal microrod array induced by metasurface enhancement^[50]. (a) Schematic diagram of metasurface structure of metal microrod array; (b) enhanced field confinement of metal micro rod array metasurfaces; (c) (d) transmission spectra of different thicknesses of lactose layers with or without metal microrod array metasurface structure; (e) (f) transmission spectra of metasurface structures of metal microrod arrays with different spacing and length

Fig. 14. Unidirectional transmission of terahertz waves based on phase gradient metasurfaces^[51]. (a) Schematic diagram of phase gradient metasurface structure; (b) forward propagation of terahertz waves on phase gradient metasurfaces; (c) reverse propagation of terahertz waves on phase gradient metasurfaces

Fig. 15. Surface enhancement of terahertz waves based on a composite antenna structure^[52]. (a) Schematic diagram of two types of composite antenna structures; (b) spectral maps of flat end antenna structure and tip antenna structure; (c) spatial distribution of terahertz electric field intensity under different experimental conditions

Fig. 16. Light confinement and standing wave formation in a terahertz Fabry Perot resonator^[53]. (a) Schematic diagram of the Fabry Perot resonator experimental setup; (b) spatiotemporal distribution and spectral map of the terahertz field in the resonant cavity; (c) spatial distribution of standing wave modes in a terahertz field within a resonant cavity

Fig. 17. Coupling interaction between multiple resonant structures. (a) Schematic diagram of the coupling structure composed of a metal cutting line and a metal double open ring resonator^[54]; (b) experimental and simulated transmission spectra for a single double open ring resonator, a single metal cutting line, and an overall coupled structure^[54]; (c) schematic diagram of cavity coupling structure^[55]; (d) transmission curves of single cavity and cavity structures^[55]; (e) experimental and simulated dispersion curves of cavity structure transmission process^[55]