LAUSR

The nonlinear optical properties of Bi2Se3 nanosheets of different diameters were studied using the femtosecond Z-scan technique. As the excitation wavelength increased from that for resonant absorption to non-resonant absorption, the nonlinear absorption of the smaller nanosheets with diameter of 30 or 80 nm changed from saturation to reverse saturation and the corresponding relaxation times decreased, unlike for larger nanosheets with diameter of 2000nm. The more sensitive nonlinear refractive index changed from positive to negative for all the nanosheets, when the excitation wavelengths were near-resonance absorption wavelengths. A simplified model similar to noble metal nanomaterials explains the observations suitably. The reversal in optical nonlinearities of Bi2Se3 nanosheets vastly enhances their properties.