Indeed, KBBF exhibits an excellent DUV NLO balance with E g ~8.45 eV, d 22 ~0.47 pm V −1, and Δ n ~0.088 10. Meanwhile, the parallel-aligned (Be 2BO 3F 2) ∞ layers exhibit strong structural anisotropy for sufficiently large ∆ n. In KBBF, all planar anionic (BO 3) 3− groups are aligned in the same orientation to achieve a strong d ij, and their dangling bonds are saturated with Be 2+ cations to enlarge E g. KBBF is the first DUV SHG crystal that can break the “200-nm-wall” as its λ PM ~161 nm 20. It is no exaggeration to say that without beryllium there would be no discovery of KBe 2BO 3F 2 (KBBF) and no development of DUV NLO crystals 27. Beryllium is really a magic element that can not only saturate the dangling bonds in boron-oxygen groups well to increase E g, but also rarely deteriorate the Δ n and d ij of the layered borate groups, thus enabling balanced DUV NLO performance. introduced beryllium into the borate structures 20. To further explore DUV NLO crystals, Chuangtian Chen et al. For LBO, its λ PM can only reach 277 nm due to the small Δ n (~0.04), which is also DUV-SHG unavailable 10. However, due to the phase mismatch induced by insufficient refractive dispersion near the absorption edge ( λ UV ~185 nm), its shortest PM SHG output wavelength λ PM ~205 nm > 200 nm so it cannot achieve available DUV SHG output 10. BBO exhibits excellent UV NLO performance with wide E g (~6.7 eV), strong d ij (~4×KDP) and large Δ n (~0.10). β-BaB 2O 4 (BBO) and LiB 3O 5 (LBO) are two important UV NLO crystals thus discovered 10. Guided by this theory, a set of concise and efficient structure-property relationships were summarized, and excellent predictability has been obtained especially in borate systems 27. Historically, the anionic group theory proposed by Chuangtian Chen has answered this question well, which tells us that the UV/DUV NLO properties of a crystal mainly depend on the composition and polarization arrangement of microscopic anionic groups and can be accurately evaluated by quantum chemical methods 27. The question then is which structural motifs in crystals can have wide E g, strong d ij, and large Δ n simultaneously, as E g is in general inversely proportional to Δ n and d ij. Clearly, the SHG conversion efficiency I 2/ I 1 is mainly determined by the transparent wavelength λ 2 (DUV crystal requires the shortest λ 2, i.e., UV absorption edge λ UV, 6.3 eV to transmit DUV light) and a large SHG coefficient ( d ij > d 36 of KDP) as well as a sufficient Δ n (preferably >0.06 default at 400 nm) 22. Where ε 0 is the vacuum dielectric constant, c is the light velocity in vacuum, n 1 and n 2 are the refractive indices at the fundamental and SHG frequencies ω and 2ω, respectively, and I 1 is the intensity of fundamental light. 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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