Two-photon absorption within layered Bi2Te3 topological insulators and the role of nonlinear transmittance therein

Abstract

In this study, strong two-photon absorption (TPA) in a layered bismuth telluride (Bi₂Te₃) topological insulator (TI) is observed and investigated by the Z-scan method under excitation with a femtosecond laser pulse at a wavelength of 1056 nm. Nonlinear transmission measurements reveal that Bi₂Te₃ possesses not only the property of single photon absorption (SPA) but also the effect of TPA. A saturation TPA coefficient as high as ∼2.29 × 10⁶ cm GW⁻¹ has been obtained, which is at least two orders of magnitude higher than that of other two dimensional (2D) materials, such as MoS₂. The large TPA effect in the Bi₂Te₃ TI is attributed to the existence of high-lying bulk states located above the first bulk state. Meanwhile, it can also be used to explain the discontinuity of the optical nonlinear transmittance between the low and high excitation energy regions.