Ultrafast transient absorption spectroscopy of 2D semiconductors: a review

Despite the decades that have passed since the discovery of ultrafast transient absorption spectroscopy and its apparent simplicity, this method is still often subject to experimental errors and misinterpretations when applied to 2D semiconductors. The reason for this lies not only in the unique nature of these extremely thin samples, but also in the different experimental configurations and data processing methods used. Moreover, since this type of spectroscopy was originally used to characterize the ultrafast relaxation dynamics of photoexcited carriers in chemical compounds, colloidal nanostructures and gas molecules, a purely 'molecular' approach to interpreting transient absorption spectra of 2D semiconductors is often proposed. However, this approach is fundamentally inapplicable to thin-film semiconductors grown or mechanically exfoliated on transparent substrates. This review considers the recent experimental results of ultrafast transient absorption spectroscopy of 2D semiconductors in a wide spectral range from several THz to UV (∼1000 THz) based on the 'solid-state' approach to their interpretation. We also highlight typical errors that arise in measuring, processing and interpreting transient absorption spectra of 2D semiconductors.