According to the SHG formula, under non-depleted pump approximation, the SHG intensity I 2 can be written as 10: For a high-efficiency SHG output, the NLO crystals are required to meet strict criteria 21, 22, 23. When using DUV NLO crystals, SHG is the preferred method since it is technically efficient and convenient to produce DUV coherent light 20, exhibiting more advantages in practical applications compared with other NLO processes, e.g., sum-frequency generation 10. Especially in the deep-ultraviolet (DUV, λ 2 < 200 nm) spectral region, NLO crystals are the core components for generating stable and high-power DUV lasers, which have important applications in cutting-edge technologies such as medical, micromachining, lithography, photochemistry, spectroscopy, and microscopy 11, 12, 13, 14, 15, 16, 17, 18, 19. Practical NLO technologies depend heavily on the availability of NLO crystals 10. Up to now, the NLO technology is the most mature method to shift or extend the limited wavelength range that practical laser sources can directly access. After >60 years of development, nonlinear optics has penetrated various fields of modern optics and laser technology, which plays an increasingly important role in many scientific and high-tech fields such as all-solid-state lasers, ultrafast lasers, spectrometers, optical storage, and computing, by means of frequency conversion, electro-optic modulation, photorefractive effect, etc 3, 4, 5, 6, 7, 8, 9. carried out the first laser-driven NLO experiment in 1961 2, in which the second harmonic generation (SHG, with the wavelength λ 2 = 347 nm) of light by a ruby laser pulse (with the fundamental wavelength λ 1 = 694 nm) in a quartz crystal was observed. Shortly after Maiman demonstrated the first working laser in 1960 1, Franken et al. Nonlinear optics is closely related to laser technology since nonlinear optical (NLO) effects are usually evident in intense light.
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