Terahertz (THz) radiation, which occupies a crucial region between electronics and photonics, has become a transformative tool in strong-field physics, ultrafast spectroscopy, and materials control. This review offers a comprehensive survey of cutting-edge advances in THz generation via optical frequency downconversion and its emerging applications. We delve into the principles and significant progress underlying various THz sources, including optical rectification in lithium niobate and organic crystals, optical different frequency generation, intense laser-induced electron radiation, spintronic THz emission, and quantum cascade lasers (QCLs). The review also highlights the profound impact of these intense THz sources in material manipulation, such as polarization control, phonon polariton excitation, and nonlinear phononics, and in revolutionizing electron dynamics through compact acceleration. Furthermore, we cover advances in THz QCL frequency combs and their role in high-precision dual-comb spectroscopy. Finally, we discuss remaining challenges and future directions, offering a roadmap for the development of next-generation high-field THz technologies and their applications in fundamental research and critical applications.
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