Terahertz (THz) waves refer to electromagnetic waves within the frequency range of 0.1‒10 THz, corresponding to wavelengths from 3 mm to 0.03 mm. THz techniques have found wide applications in fields such as materials science, non-destructive testing, biomedicine, security imaging, and next-generation communications, thus propelling the rapid development of THz photonics. However, whether in fundamental research or practical applications, effective manipulation of THz waves is essential to achieve functionalities such as frequency conversion, directional transmission, mode conversion, and phase control. Moreover, THz nonlinear effects are usually constrained by the electric field intensity and interaction distance.
For ionic crystals, incident THz waves couple with optical phonons in the material to form stimulated phonon polaritons, which introduces a novel mechanism for the interaction between THz waves and crystal materials. Stimulated phonon polaritons hold the potential to effectively control and dominate the interaction between THz waves and crystals, and offer a theoretical basis and technical means for studying the nonlinear effects of THz waves in crystals, thereby opening up new possibilities for the development of strong-field THz science and technology. Therefore, it is crucial and necessary to summarize research advances of stimulated phonon polaritons.
This paper reviews the research progress of THz waves transmission modulation and nonlinear effects based on stimulated phonon polaritons. Firstly, after a brief introduction, the basic physical concepts and optical properties of phonon polaritons and stimulated phonon polaritons are introduced. With the influences of external input THz waves, stimulated phonon polaritons are excited, which are described by nonlinear Huang equations. In comparison with the spontaneous phonon polaritons described by classic Huang equations, the stimulated ones show mainly three differences: stronger intensity, more coherence, and delocalization. This is mathematically described by nonlinear Huang equations.
Secondly, the three excitation methods of stimulated phonon polaritons and corresponding diagrams of experimental setups (Fig. 2) are introduced. Femtosecond laser pulse pumping ferroelectric crystals such as lithium niobate (LN) is one of the most popular methods of the excitation of stimulated phonon polaritons. Tilted pulse fronts and lateral excitation can meet the velocity matching conditions, improve the excitation efficiency of the excited stimulated phonon polaritons, and modify their center frequency and pulse width. Then, the detection of stimulated phonon polaritons is described. Spatiotemporal super resolution quantitative imaging system (Fig.3) can obtain the complete spatiotemporal evolution process of stimulated phonon polaritons by using pump-probe and phase contrast technique.
Thirdly, THz waves transmission modulation based on stimulated phonon polaritons is mainly achieved by three methods: topological valley transport (Fig.4), asymmetric transmission (Fig.6), and “frozen-phase” propagation (Fig.8). In the topologically protected state, THz waves exhibit valley Hall effect and make smooth detours when encountered with wide angle (120°) bends, while the trivial ones are majorly scattered. In the subwavelength waveguide with phase gradient metasurfaces, THz waves are capable of asymmetric propagation with bandwidth up to 100 GHz by mode conversion. Under lateral excitation, the first order dispersion of THz waves is eliminated, resulting in a phase-invariant propagation. These results lay the foundation for on-chip directional transmission, mode conversion and phase control of THz waves on chip, promoting the practical development of THz integrated devices.
Fourthly, in the ionic crystal, the delocalized stimulated phonon polaritons would lead to a giant enhancement of the optical nonlinearity at THz frequency by increasing the ionic polarization. Different from the heat-excited spontaneous phonon polaritons, the specialty of the stimulated phonon polaritons lies in the attendance of external coherent THz driving and the strong delocalization, which breaks the traditional light-matter interaction mechanism. Once THz waves are employed in the polar material, stimulated phonon polaritons are generated (Fig.10). They transport the ionic states by electromagnetic fields in the whole material, indicating a strong delocalization of the stimulated phonon polaritons and ionic states. Furthermore, the external driving field makes the noncoherent ionic oscillations in spontaneous phonon polaritons become coherent, which is guaranteed to confirm the external driving THz field and behaves in a regular temporal phase evolution. Moreover, the spatial coherence of the stimulated phonon polaritons is protected by the temporal coherence and strong delocalization. Therefore, THz waves can directly excite the ionic polarization via stimulated phonon polaritons-mediated light-matter interaction. This results in a significant nonlinear light-matter interaction and induces a series of phenomena at the THz frequencies. Such high nonlinearities may prove valuable in practical applications such as on-chip integration of THz waves.
Stimulated phonon polaritons in ionic crystals represent not only a continuation of elementary excitations in condensed matter physics, but also a crucial branch in the future development of THz nonlinear physics. Stimulated phonon polaritons exceed Born-Oppenheimer approximation in physics, enabling a novel mechanism of interaction between light and matter. This mechanism turns phonon polaritons, originally affecting material spectra and heat capacity, into active participants in the interaction between light and matter. Through synergizing with strong-field THz radiation, stimulated phonon polaritons can modify the light-matter interaction process, enhancing nonlinear susceptibility and potentially further boosting THz nonlinear effects. Moreover, nonlinear Huang equations suggest that stimulated phonon polaritons can achieve comprehensive control over materials, extending beyond the modulation of optical properties in the high-frequency range (visible and near-infrared) of crystalline materials. This control encompasses properties such as optical-thermal, opto-mechanical, energy levels, and polarization, thus promising significant advancements and breakthroughs in the development of strong-field THz science and technology in the future.
