Dr. Ling Chen

Abstract:

Birefringent crystals serve as crucial elements in optical devices, as they exhibit anisotropic refractive indices along different crystal directions. This optical anisotropy stems from the anisotropies of both structural geometry and spatial electron distribution. Consequently, the planar structural building units are excellent choices for constructing birefringent crystals. However, achieving an anisotropic crystal structure (especially a coplanar geometry) poses a significant challenge. Herein, we propose a novel hydrogen bond-click reaction concept to unravel the giant birefringence in (C₅H₆ON)⁺(NO₃)^–, (4HPN) for the first time. We demonstrate that the interactions between the planar hydrogen bond donor (4-hydroxypyridinium, C₅H₆ON⁺ cation) and planar hydrogen bond acceptor (NO₃^– anion) ensure the coplanarity during the crystal packing, generating the desired optical anisotropy. At 546 nm, several as-obtained (001)-single crystal wafers (#1–4) measure varying from 0.331 to 0.358; and two manually cut chips (#5,6) read = 0.469, 0.494, respectively. These values are smaller than the DFT calculated maximal value ( = n_Y – n_X = 0.593 at 546 nm). Since 4HPN has heavy (001)-growth habit, the maximal ∆n has not been observed yet. Nevertheless, the observed ∆n values on 4HPN with an E_g = 3.70 eV already surpass that of the commercialized benchmark crystals, e.g., YVO₄ ( = 0.232, = 3.1 eV) and CaCO₃ (∆n^obv = 0.174, = 5.4 eV), commonly used in the UV to visible and near IR spectral range. 4HPN also exhibits a strong second harmonic generation (SHG = 9.55 × KDP measured at 1064 nm). This unique concept offers a promising avenue for the design and development of birefringent crystals with potential applications in optical communication, sensing and signal processing devices.