K+-Doped Cs2NaInCl6 Nanocrystals with Enhanced Nonlinear Absorption and Optical Limiting for Laser Protection Applications

Abstract To address intrinsic parity-forbidden transitions, strong electron-phonon coupling, and defect-assisted nonradiative recombination in Cs2NaInCl6 nanocrystals, K-doped Cs2NaInCl6:xK+ (x = 0−1) nanocrystal powders were synthesized via a hot-injection method. The powders were dispersed in methyl methacrylate to prepare organic glasses. The x = 0.2 sample exhibits the best performance. Nonlinear absorption was evaluated by open-aperture Z-scan measurements. Both undoped and K+-doped nanocrystals exhibit a transition from saturable absorption to reverse saturable absorption with increasing excitation energy. K+ doping significantly enhances the nonlinear absorption response. At 50 μJ, the nonlinear absorption coefficient of the K+-doped nanocrystals reaches 40 cm·GW−1, approximately 4.2 times that of the undoped nanocrystals (9.5 cm·GW−1). Meanwhile, the optical limiting threshold decreases from 4.69 to 3.53 J·cm−2. This enhancement is attributed to reduced nonradiative loss and prolonged excited-state lifetime induced by K+ incorporation, thereby facilitating carrier accumulation and strengthening excited-state absorption. In addition, the (Cs2NaInCl6:0.2K+)6/poly(methyl methacrylate) organic glass retains 97.14% of its initial nonlinear absorption response after five months. These results demonstrate that K+ doping is an effective strategy for regulating the nonlinear absorption performance of lead-free double perovskites and provide insights into the design of high-performance optical limiting and laser protection materials.