Finding the First Squarates Nonlinear Optical Crystal NaHC₄O₄·H₂O with Strong Second Harmonic Generation and Giant Birefringence

With the development of laser technology, nonlinear optical (NLO) crystals with optimal performance are urgently required. Finding new NLO-active units is necessary to expand the exploration systems for NLO materials. Herein, a π-conjugated planar hydrogensquarate (HC4O4)− with large hyperpolarizabilities and giant polarizability anisotropies is identified as a new NLO-active unit. On the basis of this idea, a new NLO crystal NaHC4O4·H2O is found by using the first-principles high-throughput screening pipeline for nonlinear optical materials (FHSP-NLO). It is the first NLO crystal found in squarates. Subsequently, the crystal with a size of 30 × 6 × 5 mm was grown by a facile aqueous solution method. NaHC4O4·H2O shows a strong powder SHG effect of 2.3 × KDP and a giant birefringence of 0.52 at 1064 nm. Electronic structure calculation and analysis reveal that the π orbitals of the (HC4O4)− groups are the dominant source of the SHG coefficient. This study provides novel NLO-active units and a materials system to design and find NLO crystals.