Lead Halide Perovskites and Its Applications in Whispering Gallery Mode Micro-Lasing

Ultrahigh luminescence quantum yield (∼100%), low non-radiative recombination rates, and tunable spectral emissions from the ultraviolet to the visible range can be achieved through lead halide perovskites (LHP) (APbX₃; A= Rb⁺, Cs⁺, and X= Br^-, I^-); hence they are promising candidates for optoelectronics and photonics applications. However, a lack of atmospheric stability impedes their effective use as photovoltaic absorbers. Highly emissive perovskite quantum dots (PQDs) are being investigated to develop solar cells, efficient lasers, LEDs, etc. Here, we report the bandgap tunability and stability of APbX₃ PQDs synthesized through the hot-injection method. Structural characterization through X-ray diffraction (XRD) reveals highly phase pure cubic polymorph of PQDs. The A-site ions have a significant impact on the photophysical and chemical properties. Cs⁺ cations are substituted with Rb⁺ at the A site, and anionic X sites are substituted either with Br^- or I^- or a combination of halide ions in these PQDs to study the changes in structure and optical properties. With increasing Rb⁺ concentration, the bandgap of Cs_1-xRb_xPbX₃is shown to vary between 1.7 eV to 3.2 eV, as evident from the blueshift observed in the photoluminescence peak. Bandgap is also shown to be tunable by alloying I^- with Br^- in CsPbX₃ and Cs_0.5Rb_0.5PbX₃. The latter is found to be more stable than the former when stored in laboratory conditions. A detailed analysis will be presented on the effect of cationic and anionic substitution on the absorbance and emission spectra of PQDs. These quantum dots are further used for whispering gallery mode (WGM) lasing from PQDs coated on TiO₂ spherical microcavities. The photoluminescence emission from a CsPbI₃-QDs gain medium strongly couples with a TiO₂ microspherical resonating optical cavity. Spontaneous emission in these microcavities switches to a stimulated emission above a distinct threshold point of 708.7 W/cm². Quality factor as high as Q∼1195 for WGM micro-lasing is demonstrated at room temperature. CsPbI₃-QDs/TiO₂ microcavities are found to be photostable even after continuous laser excitation for 75 minutes. [1] The CsPbI₃-QDs/TiO₂ microspheres are promising as WGM-based tunable microlasers. Results on WGM micro-lasing using Cs_0.5Rb_0.5PbX₃will be presented.<br/><br/>Reference:<br/>[1] S Laskar et al. "Whispering gallery mode micro-lasing in CsPbI₃ quantum dots coated on TiO₂ microspherical resonating cavities." <i>Optics Letters</i> 48.10 (2023): 2643-2646.