Nonlinear optical responses of α-Fe2O3 nanosheets and application as a saturable absorber in the wide near-infrared region

Abstract In the present work, we report a strategy to synthesize α-Fe2O3 nanoflakes as a nonlinear optical (NLO) material for the optical pulse generation in the near-infrared (NIR) band. With the excitation wavelength of 1 µm, the effective nonlinear absorption coefficients βeff was −0.501 cm/GW and the nonlinear refractive index n2 was 9.33 × 10-4 cm2/GW. While at 1.34 µm, βeff was −0.713 cm/GW and n2 was 5.01 × 10-4 cm2/GW. The two-photon absorption cross sections at 1 and 1.34 µm were as high as 9.6 × 105 and 5.7 × 105 GM, respectively, indicating that the hematite nanosheets could be used as the nonlinear optical devices. The third-order NLO susceptibilities were 6.3 × 10-11 and 3.75 × 10-11 esu at 1 and 1.3 µm, respectively. Using the ultrafast spectroscopy, the excited state lifetime was ~2.62 ps, suitable for the (ultra)short pulse generation. Based on the marvelous nonlinear absorption properties of the α-Fe2O3 nanoflakes saturable absorber, passively Q-switched Nd:GdVO4 lasers at 4F3/2 → 2I11/2 and 4F3/2 → 2I13/2 lasing transitions were demonstrated, producing the shortest pulse duration of 135 ns. The experimental results confirmed that α-Fe2O3 nanoflakes were an excellent candidate for the short NIR pulse generation in a broad band.