Determination of optical nonlinearity with photothermal effect within a layered bismuth telluride

In this work, the photothermal nonlinearity within a layered bismuth telluride (Bi₂Te₃) is studied in detail. The dynamics of the optical nonlinearity with increasing photothermal effect are investigated via pump-probe spectroscopy. The difference between the thermally-induced and photon-induced nonlinearities was clarified and the entire photothermal nonlinearity was therefore sketched. Furthermore, by removing the thermally-induced background signal, the n₂ without accumulated heat can then be estimated. For example, the n₂ value is 2.80 × 10⁻⁸ cm²/W at a peak intensity of 12.81 MW/cm². The results are compared with those obtained from another measurement to validate the reliability of the estimation. The working range of the n₂ value is also discussed. To provide further validation of the feasibility of the proposed method, the photothermal nonlinearity within monolayer graphene is provided and discussed. This work provides an alternative method to deal with optical nonlinearity measurements that are subject to the photothermal effect, paving the way toward further related research.