| This thesis is about the exploration of the optical properties of perovskite compound CsSnI3 (CSI), a newly identified semiconductor material. Based on what have been discovered so far, we believe that it has a great potential for photonic device applications. The exploration starts with the determination of the atomic and electronic structures of CSI and continues with the fundamental understanding of the optical properties revealed by spectroscipic measurments. One of the most fascinating optical properties associated with the unique atomic structuture of CSI is the superfluorecence from the correlated two-dimensional excitons naturally formed in the planes of SnI4 tetragons. After a brief introduction on the prior and recent research activities on CSI, the atomic structures and structural phase transitions of CSI were investigated using the first-principles approach. With the detailed structural information, the full electronic eigen states of CSI in its gamma phase, commonly accessible by the full optical spectrum from near infrared to ultraviolet, have been calculated. A few key charcteristics of the electronic structure were identified and discussed in view of their optical consequences, such as the much larger effective mass of electrons than that of holes, and the existence of the two lowest parallel conduction bands with an energy separation of 64 meV. In the chapters of 4, 5 and 6, the exploration continues with the understanding of interesting optical properties and the associated physics processes. The abnormal temperature dependence of the energy band gap of CSI is explained by the two combined effects: 1) the neglegible contribution of direct electron-phonon interactions to the band gap change due to the unusual large electron effective mass, and 2) the positive thermal expansion effect to the band gap change calculated by the first-principle approach. Pronounced two-LO-phonon features in both Raman scattering and photoluminescence excitation spectra are interpreted as the resonantly enhanced two-LO-phonon emission processes, originated by the unique electronic band structure of CSI: the two lowest parallel conduction bands with the energy separation close to the energy of two LO phonons. The final part of my thesis in the chapters of 7 and 8 is devoted to the one of most exciting and abstruse phenomena in photonics: superfluorescence (SF). After revisiting Dicke's initial superradiance theory and combining the characteristics of SF, we have developed a model to capture the essential physics, especially on the dynamic time evolution of SF. This model predicts the bi-exponential decay behavior when considerable dephasing is present. Meanwhile, the intensity of SF burst, delay time, and decay rate are also studied with the model. The SF in CSI is revealed through the power and temperature dependences of time resolved photoluminescence. The measured photoluminescence characteristics are shown to match all the SF features predicted by our model, such as the bi-exponential decay, the inverse relation of delay time over the number of exciton (N), the linear relation of decay rate over N, and the temperature dependence of decay rate. The natural formation of two dimensional excitons in the parallel planes of SnI4 tetragons is argued to be the reason for the SF to occur in CSI. |
