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.