Fig. 1. Dispersion curves of phonon polariton. (a) Dispersion curves of phonon polariton in LiNbO₃ crystal; (b) classically derived fraction of lattice vibrational energy in phonon polariton

Fig. 2. Three excitation methods of stimulated phonon polariton. (a) Broadband excitation; (b) tilted wavefront by gratings;(c) lateral excitation

Fig. 3. Schematic of spatiotemporal super resolution quantitative imaging system (lateral excitation)^[43]

Fig. 4. Topological valley transport for stimulated phonon polaritons^[43]. (a) Schematic diagram of LiNbO₃ VPC; (b) dispersion curves of supercell shown in inset; (c),(d) electric intensity distribution of stimulated phonon polaritons inside topological bandgap in V- and Z-type VPCs, with yellow dashed sections indicating interface; (e),(f) electric intensity distribution of stimulated phonon polaritons outside topological bandgap in V- and Z-type VPCs

Fig. 5. Propagating features of topological stimulated phonon polaritons along interface^[43]. (a),(b) Spectra at center and margin of interface before and after the first corner in V-type VPC, respectively, where insets show spectra in exponential coordinates from 0.18 to 0.20 THz; (c),(e) dispersion curves along center of top and bottom edges in Z-type VPC, where white dashed lines indicate light cone of LiNbO₃ slab, green and red dashed lines represent fundamental mode and femtosecond laser in LiNbO₃ slab, respectively; (d),(f) dispersion curves along margin of top and bottom edges in Z-type VPC

Fig. 6. Unidirectional transmission of stimulated phonon polaritons^[49]. (a) Schematic diagram of structural design; (b),(c) profile view of electric field distribution in forward and backward transmission, respectively; (d) transmission spectra in different propagation directions; (e) forward-backward transmittance ratio; (f) scattered field from antennas of different lengths

Fig. 7. Electric field distribution and dispersion curves of unidirectional transmission^[49]. (a),(b) Spatial scattered electric field distribution of antennas with different offsets and rotations; (c) dispersion curves of stimulated phonon polaritons in incidence region; (d),(e) dispersion curves of stimulated phonon polaritons in transmitted region of forward and backward transmission

Fig. 8. Propagation behavior of stimulated phonon polaritons under different excitation methods^[53]. (a),(d) Schematic diagram of experimental setup; (b),(e) visualization images; (c),(f) electric field waveforms

Fig. 9. Dispersion curves of stimulated phonon polaritons under different excitation methods^[53]. (a) Experimental results of lateral excitation; (b) simulation results of lateral excitation; (c) experimental results of broadband excitation; (d) spectra of lateral excitation in experiments and simulations

Fig. 10. Diagram of stimulated phonon polariton-mediated light-matter interaction (LMI) in LiNbO₃ crystal^[21]. (a) Traditional light-matter interaction mechanism; (b) stimulated phonon polariton-mediated light-matter interaction mechanism excited by THz waves; (c) stimulated phonon polariton-mediated light-matter interaction mechanism excited by visible/infrared light with THz waves exciting stimulated phonon polaritons

Fig. 11. Experimental results of generation of THz nonlinear waves^[26]. (a) Selection of velocity matching; (b) experimental observations of spatiotemporal propagation of THz waves in LiNbO₃ waveguide; (c) dispersion relation of THz waves, where difference frequency generation signal is marked by dashed circle

Fig. 12. Evolution of THz optical field with time at different positions and their corresponding Fourier spectra^[26]. (a) Field oscillation of THz waves as function of time at various positions of x₁=1.18 mm, x₂=1.82 mm, x₃=2.39 mm and x₄=3.44 mm relative to position where THz waves are generated; (b) corresponding Fourier spectra of THz waves at frequencies ν₀ and ν₁, as well as their difference frequency generation signal at four positions

Fig. 13. Experimental design of microcavity and temporal evolution of stimulated phonon polaritons^[21]. (a) Illustration of pump laser and layout of microcavity, where a=170 μm, r=50 μm, s=0.15r; (b) temporal evolution of stimulated phonon polaritons in microcavity

Fig. 14. Experimental stimulated phonon polaritons and simulated electromagnetic waves in microcavity^[21]. (a) Energy bands of LiNbO₃ photonic crystal; (b) electric field oscillation of reference point in microcavity; (c) spectral information of microcavity confined stimulated phonon polaritons, where inset presents spectral comparison between broadband and localized phonon polaritons; (d) spectral information at line shown by inset, in microcavity; (e) spectral information along reference line and at reference point in microcavity

Fig. 15. Second-harmonic generation (SHG) enhancement of near-infrared laser pulses due to stimulated phonon polariton-mediated light-matter interaction^[21]. (a) Schematic of experimental setup; (b) spectra of SHG signal obtained by moving LiNbO₃ slab; (c) peak intensity and total energy of SHG signal when sample is moved