Nitrate nonlinear optical crystals: A survey on structure-performance relationships

Nonlinear optical (NLO) crystals are important for laser frequency converters and have been widely applied in key devices in all-solid-state lasers. Based on the structure-property relation of NLO crystals, the planar triangular π-conjugated groups are considered excellent NLO-active units in ultraviolet (UV) and even deep-ultraviolet (deep-UV) spectral areas, such as [BO3] in borates and [CO3] in carbonates. In fact, [NO3] groups possess similar triangular planar π-conjugated configurations but greater microcosmic second-order susceptibility than that of the [CO3] and [BO3] groups. This makes nitrates a promising UV and deep-UV NLO material with a large second harmonic generation (SHG) effect. Recently, dozens of nitrate NLO materials have been reported, but a comprehensive review of the structure-performance relationship in these nitrate NLO materials is still not available. In this review, we classify the nitrate NLO materials according to the coordination environments around the [NO3] groups, and systematically analyze their structure-property relationships. Moreover, the potential applications and directions for future exploration of nitrate NLO materials are discussed.