Toward the Enhancement of Critical Performance for Deep-Ultraviolet Frequency-Doubling Crystals Utilizing Covalent Tetrahedra

ConspectusDeep-ultraviolet (deep-UV, λ < 200 nm) coherent light is emerging as an indispensable driving force behind the innovation of optics and materials science. The deep-UV-driven applications range from laser interference photolithography to precise micromachining to futuristic ideas such as space propulsion using remotely controlled positioning lasers. Unlike conventional approaches to obtaining deep-UV light, for instance, synchrotron radiation, direct laser excitation, and gas discharge, nonlinear frequency conversion can be regarded as a more attractive way to endow such resource with high photon energy, high photon flux, and high spectral resolution. Actually, the nonlinear frequency conversion can be efficient only with the use of high-performing frequency-doubling crystals, which should be well-suited to the physics of nonlinear optical process. However, the necessary prerequisites for a practical frequency-doubling crystal are extremely strict, and thus very few crystals can be used to generate the deep-UV light. Faced with this, sustained effort has been expended by chemists and materials scientists toward discovering novel deep-UV frequency-doubling crystals. Studies have so far indicated that the main difficulty in finding a perfect candidate comes from the combination of three critical properties (absorption edge, nonlinear optical coefficients, and birefringence) into one crystal because they share the mutual relation of restriction and influence.In this Account, we present recent progress in discovering emergent deep-UV frequency-doubling crystals with the discussion of our efforts to balance the three critical properties by introducing the covalent tetrahedra MO4–nXn (n = 1–3), in which M refers to central atoms such as B, P, Si, S, Al, Zn, and Be and X can be apical atoms such as F, Cl, Br, and N. By analyzing the influence of the covalent tetrahedra on optical properties, we came to the conclusion of how to use the oxidized tetrahedra to achieve the improvement of the absorption edge, nonlinear optical coefficients, and birefringence for deep-UV frequency-doubling crystals. The followings are the key points in achieving the above goals: (i) elimination of dangling bonds with covalent tetrahedra to push the absorption edge of crystals into the deep-UV spectral region; (ii) orbital hybridization enhancement, charge-transfer energy reduction, and symmetry breaking of original tetrahedra with the introduction of X atoms and thereby the achievement of the enhancement of nonlinear optical coefficients; and (iii) uniform alignment of tetrahedral distorted units and the introduction of polarized X atoms containing MO4–nXn tetrahedra with high polarizability anisotropy to cause the large enhancement of birefringence. These findings allow us to understand the microcosmic behaviors of covalent tetrahedra on pushing the current limitations and provide an optional functional group toward the maximum thresholds of three critical parameters for deep-UV frequency-doubling crystals. Finally, we conclude this Account with a better understanding of the positive roles of covalent tetrahedra in enhancing the optical performance and how they can facilitate the construction of high-performing deep-UV crystals.