Observation of large negative thermo-optic coefficients in lead halide perovskites

Lead halide perovskites are a new class of semiconductor materials for photonic devices [1]. Despite intensive research in the last decade, some of their fundamental physical properties still remain unclear. Here, we report that the halide perovskite CH₃NH₃PbCl₃ shows a large negative thermo-optic coefficient, i.e., a distinct decrease of the refractive index upon temperature rise [2]. We demonstrate that this unique response provides new photonic applications [2,3]. Furthermore, the negative thermo-optic property indicates unusual thermally-induced changes in the electronic states. We investigated the origin of the negative coefficient through the temperature dependence of the absorption spectra of a CH₃NH₃PbCl₃ thin film. With increasing temperature, a slight blueshift in the absorption peak energy was observed. Our analysis suggests that both the absorption blueshift and large thermal expansion contribute to the large negative thermo-optic coefficients of halide perovskites.

References
[1] Y. Kanemitsu and T. Handa, Jpn. J. Appl. Phys. 57, 090101 (2018).
[2] T. Handa et al., Sci. Adv. 5, eaax0786 (2019).
[3] H. Tahara et al., Adv. Opt. Mater. 6, 1701366 (2018).