Fractal Growth of 2D NbSe2 for Broadband Nonlinear Optical Limiting

The compelling demand for laser protection both for civil and defense use calls for new-generation nonlinear optical materials. Chemical vapor deposition (CVD) techniques provide extra tricks to modulate crystal optical nonlinearity through fractal growth. The synthesized 2D NbSe2 and its fractal structures exhibit giant, broadband laser attenuation behaviors extending into the near Infrared (NIR) range. Particularly, the optical limiting performance generally correlates with the fractal dimension, where Fractal NbSe2 demonstrates enhanced third-order optical nonlinearity at a huge nonlinear absorption coefficient of 9.7 x 10(-4) m W-1 and an ultralow starting threshold of 5 mJ cm(-2) at 532 nm. Various techniques include femtosecond spectroscopy, Density functional theory (DFT) calculation and Kelvin probe force microscopy disclose the origin of the strong nonlinearity of the 2D NbSe2 crystals, and suggest the formation of edge states and overall faster carrier dynamics, larger surface contact potential difference NbSe2 fractals contribute to their even augmented nonlinear responses. Fractal engineering thus opens new avenues to fabricate highly efficient laser protection materials, and the blocking of intense beam (a large attenuation factor over 13.3 dB at 532 nm) while allowing transmission of weak one (a high linear optical transmittance over 80%) fulfills the much desired "smart" defense.