Benefiting from high transmittance of PO₄ tetrahedra in the deep-ultraviolet (deep-UV) region, phosphates have been investigated as deep-UV nonlinear optical (NLO) candidates for years. So far, reported deep-UV NLO phosphates are orthophosphates and polyphosphates while cyclophosphates, important members of the phosphates family, have usually been neglected for use as deep-UV NLO crystals. Here, we first report a novel cyclotetraphosphate K₂SrP₄O₁₂ (KSPO) as a deep-UV transparent NLO crystal. In its structure, the fundamental building groups are ring [P₄O₁₂]⁴⁻ groups. KSPO exhibits a moderate second-harmonic generation (SHG) response that is 0.5 times that of KH₂PO₄ (KDP) and a short deep-ultraviolet absorption edge smaller than 200 nm.

An ultraviolet nonlinear optical material benefited from benzene-like planar π-conjugated (C ₃ N ₆ ) groups, achieved significant balance between large birefringence (Δ n = 0.277 at 546 nm) and strong second-harmonic generation intensity ∼4.3 × KDP.

An ultraviolet organic–inorganic nonlinear optical material benefited from organic π-conjugated [C ₃ N ₆ ] groups and inorganic [ZnNCl ₃ ] groups, revealing excellent optical performance and moderate thermal stability.

We report on the crystal growth, spectroscopic investigation and laser performance of Tm³⁺-doped monoclinic magnesium tungstate (Tm:MgWO₄), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 μm spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.