KCHO(HO): A Promising UV Nonlinear-Optical Material with Large Birefringence Based on a π-Conjugated (CHO) Group.

In order to meet the growing needs for the laser technology and optics industries, the goal is to find suitable fundamental building blocks with large nonlinear-optical (NLO) coefficients and birefringence for an excellent-performance NLO or birefringent system. Via preliminary investigations and calculations, it has been found that the planar π-conjugated group (CHO) possesses large polarizability anisotropy (δ) and hyperpolarizability (β), comparable to well-known groups such as (BO), (CNO), etc. Herein, we report a new alkali-metal 3,5-dicarboxybenzoate, KCHO(HO) (KHBTC), which crystallized in the acentric space group 2. Second-harmonic-generation (SHG) measurements of KHBTC under 1064 nm laser radiation show that the SHG response of KHBTC is 1.2 times that of KDP with type I phase-matching behavior. Birefringence measurements show that KHBTC owns a large birefringence of about 0.372 at 550 nm. The band gap of KHBTC obtained by ultraviolet (UV) diffuse-reflectance spectroscopy is 3.91 eV, indicating that KHBTC has potential applications as UV NLO or birefringent materials. Theoretical calculation further confirmed that the impressive optical properties of KHBTC are derived from the large polarizability anisotropy of the (CHO) anions.