Nonbonding Electrons Driven Strong SHG Effect in Hg₂GeSe₄: Experimental and Theoretical Investigations

A Hg-based ternary infrared nonlinear optical (NLO) material, Hg₂GeSe₄, with the defect diamond-like (DL) structure was systematically investigated for the first time. The experimental results show that Hg₂GeSe₄ exhibits an enhanced second harmonic generation (SHG) response about 2.1 times that of the normal DL selenide AgGaSe₂ ( d₃₆ = 33 pm/V) at the particle size of 150-200 μm, as well as good phase-matchable ability. Moreover, theoretical analysis reveals that the nonbonding electrons around Se atoms in the defect DL structure make a dominant contribution to the improvement of the NLO property: d₃₆ = 78.83 pm/V and Δ n = 0.11. This study highlights the promise of electronic engineering strategies and opens new avenues toward the design of new infrared NLO crystals with high performance.