Transparent Lead‐Free Ferroelectric (K, Na)NbO₃ Single Crystal with Giant Second Harmonic Generation and Wide Mid‐Infrared Transparency Window

Abstract Environment‐friendly lead‐free potassium‐sodium niobate (KNN) has become one of the most extensively investigated ferroelectrics in the past decade, while high transparency KNN single crystal has never been realized yet. Here, a transparent KNN single crystal is successfully grown by the top seeded solution growth method. The KNN crystal possesses a very wide transmittance range of 0.37–8 µm, completely covering the important mid‐infrared atmospheric transparent window (3–5 µm). Noticeably, the KNN crystal exhibits the largest second‐harmonic generation efficiency (7.6× that of potassium‐titanyl phosphate (KTP)) with type‐I phase matchability under the fundamental wavelength of 1064 nm among all of the studied mid‐infrared oxide nonlinear optical crystals reported so far. This work demonstrates a novel mid‐infrared nonlinear optical oxide crystal with a giant second‐harmonic generation and wide transparency range, providing a new idea for the exploration of nonlinear oxide crystals.