Significance As an important research method, spectroscopy exhibits versatile and unique advantages, such as contactless measurement, high sensitivity, and convenience, and thus is widely used in material science and engineering. With increasing progress of science and technology, the branches of spectroscopy have gradually broadened, and a variety of complex functional spectral analysis technologies have emerged. Among those, ultrafast spectroscopy is an important subject that has developed rapidly in recent years. It introduces the time degree of freedom on the basis of traditional steady-state spectroscopy, subdivides the interaction between light and matter at the picosecond time scale, and studies the time -resolved dynamics of such quasi -particles as hot carriers, phonons, polarons, and excitons in matter. At present, the wavelengths of light used in ultrafast spectroscopy have covered most of the bands in the electromagnetic spectrum, and its scope of application has also extended deeply into condensed matter physics, material science, biomedicine, military, and national defense. Terahertz spectroscopy is an important branch of ultrafast spectroscopy used in technology that developed in the 1980s that has important scientific research and application prospects. Generally, as commonly defined, terahertz (THz) waves refer to electromagnetic waves with frequencies in the range 0.1-10 THz (wavelength 3 mm-30 mu m), also known as submillimeter waves or terahertz radiation. Electromagnetic waves in this band exhibit many useful properties, such as low photon energy, high penetration, a close match with molecular vibration and rotational energy levels, and no harmful radiation. Thus, THz radiation has found a wide range of use nowadays. Terahertz technology therefore has become a major emerging field of science and technology in the 21st century. It is rated as one of the top 10 technologies that will change the world of the future by the United States, and has been highly evaluated by governments around the world.With the continued progress of terahertz technology, the terahertz emission efficiency of materials has continually increased, and the corresponding electromagnetic field intensity has also gradually increased. The electric field component of intense terahertz waves can easily reach MV/cm magnitudes, and its corresponding magnetic field component can reach Tesla magnitudes. When a sample is irradiated with such intense terahertz waves, the strong electromagnetic field can apparently regulate the internal physical properties of matter, such as its spin/electron/lattice structure, dielectric property, and susceptibility, resulting in a series of nonlinear responses, such as collision ionization, valley scattering, and the terahertz Kerr effect. Research on these nonlinear effects can clarify these phenomena, promoting the development of ultrafast optoelectronic devices.Progress Intense terahertz waves exhibit high peak power and correspondingly large amplitudes of the electric and magnetic field components, whereby they can induce numerous novel anomalous phenomena. In this paper, we first introduce some frequency used intense terahertz emission sources, including photoconductive antennas, optical rectification crystals, solid and gas plasma emissions, metamaterials, and tip enhancement. Some typical applications of intense terahertz technology in material science are also introduced, including collision ionization, intervalley scattering, coherent modulation, spin regulation, terahertz fluorescence, terahertz high order harmonic generation, terahertz Kerr effect, and biomedical uses.Conclusions and Prospects With advances in technology, intense terahertz sources have become increasingly readily available, and terahertz detection techniques are also continuously growing. Currently, a wide range of experimental methods are being applied to detect a variety of nonlinear terahertz phenomena. In addition, the range of application of intense terahertz spectroscopy is also expanding, whereby measurements of physical properties of substances under extreme environmental conditions such as extremely low temperatures, strong electric and magnetic fields, and high pressure can now be performed. However, a series of problems also exists, such as the need for stable and efficient intense terahertz sources and a deep understanding of the physical mechanism underlying the interaction between intense terahertz waves and matter. In addition, a new terahertz theory to account for major breakthroughs in basic physics has not yet appeared, and there remains a so-called "terahertz gap" in theory. A combination of electromagnetic theory and quantum theory to solve new terahertz physics problems is needed. The answers to these questions are important not only for the progress of basic science, but also for the development of applied technology. We believe that in the near future, terahertz technology is bound to see major breakthroughs, whereupon terahertz devices will be as widely used in every corner of life as visible and near -infrared devices.
