Nonlinear optical crystals have important applications in laser science and technology fields such as lithography, communications, micromachining, and laser display. Phase matching is a necessary condition for nonlinear optical crystals to realize efficient conversion, and traditional nonlinear optical crystals are usually based on the principle of birefringence to realize phase matching. However, in the deep ultraviolet (UV) band, where the wavelength is less than 200 nm, a large number of nonlinear optical crystals are difficult to realize birefringent phase matching due to their small birefringence. The quasi-phase matching technique realizes the efficient output of octave light by constructing a structure in which the nonlinear coefficients are periodically reversed in the crystal, so that the energy continuously flows from the fundamental frequency light to the octave light. Compared with birefringent phase matching, this technique has the advantages of not relying on the birefringence of the material, matching a wide wavelength band, and being able to utilize the maximum nonlinear coefficient of the material. However, nonlinear optical crystals suitable for quasi-phase-matched output in the deep ultraviolet band are still very rare.
Recently, researcher Zhao Sangen & Luo Junhua from Fujian Institute of Materials and Structures, Chinese Academy of Sciences (FIMSTEC) successfully grew inch-scale transparent LiNH4SO4 single crystals in aqueous solution and confirmed the ferroelectricity of LiNH4SO4 crystals by using electric hysteresis loop and variable temperature nonlinear optical test, etc. The LiNH4SO4 crystals are characterized by a high degree of ferroelectricity and a high degree of nonlinearity. Single-domain samples of LiNH4SO4 were successfully obtained by applying a unidirectional polarization voltage, and the LiNH4SO4 crystals have a transmission range as short as 171 nm, moderate second-order nonlinear optical coefficients (0.33 pm/V), and can withstand laser irradiation of up to 1.47 GW/cm-2 without damage. The wavelength-dependent refractive index of LiNH4SO4 was accurately determined and the dispersion equation of LiNH4SO4 was fitted by the minimum angle of deflection method, and the results show that LiNH4SO4 has a very low refractive index dispersion, which results in a first-order quasi-phase-matching period of the crystal of 1.4 µm at the doubled light wavelength of 177.3 nm. The above results indicate that LiNH4SO4 is a strong candidate for deep-ultraviolet laser frequency conversion. The results of first-principles calculations indicate that the nonlinear optical response and wide transmission range of LiNH4SO4 mainly originate from the contribution of SO42-tetrahedral motifs, whereas its lower refractive index dispersion is mainly due to the highly localized nature of the Li+ and NH4+ cations and the electrons of SO42- motifs in the LiNH4SO4 crystal. This finding provides an effective way to develop deep-ultraviolet quasi-phase-matched nonlinear optical crystals.
Dr. Yipeng Song, a PhD student at the University of Chinese Academy of Sciences, is the first author of the paper, and Associate Researcher Bingxuan Li at the Institute of Physics and Structures, Fujian, China, is the co-corresponding author of the paper.
Figure 1 (a) Comparison of birefringent phase-matched and quasi-phase-matched; (b) LiNH4SO4 crystal in ferroelectric phase; (c) crystal structure of cis-electric phase
Fig. 2 (a) LiNH4SO4 crystals grown by seed crystals in the [011] direction (b) [001] direction; (c) LiNH4SO4 crystals with variable-temperature nonlinear optics test; (d) variable-temperature nonlinear optics cycling test; (e) P-E and J-E curves of LiNH4SO4 crystals at 413 K; (g) 180° image of ferroelectric domains of LiNH4SO4 crystals; (h) Single-domained LiNH4SO4 crystals
Fig. 3 (a) Deep UV transmission spectrum of LiNH4SO4 crystal; (b) LiNH4SO4 crystal Maker streak; (c) Optical microscopy image of LiNH4SO4 crystal after being damaged by a nanosecond laser (d) before and after (e); Triangular prism used for the refractive index of LiNH4SO4 (e) Passing of light in (100) direction; (f) Passing of light in (001) direction; (g) (h) Refractive index dispersion equation of LiNH4SO4 crystal; (i) Optical index body at 532 nm of LiNH4SO4 crystal
Fig. 4 First-order quasi-phase-matching cycles of the sum- and difference-frequency processes of LiNH4SO4 crystals
Fig. 5 Electronic energy band structure of LiNH4SO4; (b) density of states/partial density of states diagram of LiNH4SO4; (c) HOMO of LiNH4SO4; (d) LUMO of LiNH4SO4
