We characterized the temperature dependence of the complex dielectric susceptibility spectrum of a perovskite $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{Br}}_3$ single crystal by using heterodyne interference spectroscopy. Owing to a time-domain measurement that is not influenced by incoherent lattice relaxations, we were able to detect a low-energy indirect-gap band. We observed two peaks in the energy region below the exciton transition at the $R$ point in the imaginary part of the spectrum. The temperature dependences of the energies of these peaks were different from the dependence of the energy of the direct-transition exciton. The trends of both the splitting width and peak intensities can be explained by assuming a Rashba spin-split band.

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• Photoconductivity
• Rashba coupling
• Ultrafast optics
• Perovskites
• Homodyne & heterodyne detection