Deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials for modern laser application are broadly exercised in high-precision fields and confronted imperative requirements. The NLO coefficient, birefringence, and absorption edge are the most critical optical parameters for NLO crystals. How to achieve the modulation and enhancement of three pivotal parameters has always been a magnitude challenge for chemists and material scientists. Previous studies have revealed that microscopic control-mechanisms of second-order NLO coefficient, birefringence, and absorption edge mainly originate from the geometric superposition of NLO-active chromophores, the anisotropic response to incident light, and electron transition near the Fermi surface. These can be quantitatively analyzed by the hyperpolarizability, polarizability anisotropy and energy gap, respectively. Therefore, modulating above three physical quantities can achieve the large enhancement of three pivotal optical parameters for nonlinear optics. In order to generate the deep-UV harmonic coherent light with a direct second harmonic generation (SHG) method, an NLO crystal must have at least a balanced property among them, that is, λcutoff < 200 nm; deff > 0.39 pm/V; Δn ~0.05？0.10@1064 nm to make λSHG, PM < 200 nm. However, how to effectively implement the modulation and make the above critical performance coexist in one NLO crystal is always the hotspot of this field.
In the last few years, the previously reported deep-UV NLO fluorooxoborates by Pan, et al., including AB4O6F (A = NH4, Na, Rb, Cs; abbreviated as ABF series) and MB5O7F3 (M = Ca, Sr; abbreviated as MBF series) have verified the feasibility of cooperative assembly of π-conjugated and non-π-conjugated modules to design novel deep-UV NLO crystals. Recently, a research group at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science, has obtained the first magnesium fluorooxoborate MgB5O7F3 as a new competitive candidate for deep-UV NLO application. It has a sufficiently large nonlinearity and a deep-ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB5O7F3 when compared with its isomorphic phases was discussed. More importantly, we demonstrated that fluorooxoborate system with the general formula of MB5O7F3 (M = divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged.