|Title:||Amplified spontaneous emission and lasing from lead halide perovskites under multiple-photon excitation|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Pages:||xxvii, 178 pages : color illustrations|
|Abstract:||Multiphoton absorption (MPA) is a vital branch of nonlinear optics. This process features long excitation wavelength and nonlinear excitation intensity dependence of the fluorescence. Hence, it brings about the advantages of deeper penetration depth, higher damage threshold, higher image contrast and lesser scattering effects. To date, five-photon pumped amplified spontaneous emission (ASE) from an organic fluorophore has been reported. However, the organic material suffers from intrinsic instability and its practical application is limited by the requirement of certain liquid solutions. In contrast, these drawbacks can be readily overcome by inorganic materials. Thus, it would be highly desirable if higher order MPA pumped ASE and lasing can be obtained from inorganic materials. As a potential candidate, lead halide perovskites (LHPs) attract intensive interests in the field of photovoltaics and optoelectronics. Due to their large light absorption coefficients and long photo-carrier lifetime, the efficiency of LHPs based solar cells climbed over 20% in just a few years. Besides, because of low-cost fabrication procedure and facile bandgap tuning by elements blending, wavelength-tunable light emitting diodes (LEDs) and lasers have been invented. Moreover, initial reports covering MPA induced ASE and lasing also emerged. It seems the LHPs can be a potential candidate for excellent high-order MPA materials. Thus, this thesis aims at the investigation of high-order MPA induced ASE and lasing from the LHPs. Initially, three-photon absorption induced ASE was observed from the CH3NH3PbI3 bulk single crystal. This material has been a rising star in material science with excellent performance both as solar cells and optical gain medium. However, most of the investigations are focused on the nano- or film-structures, which can sensitively interact with environmental factors resulting in detrimental effects like material destruction and performance degrading. In contrast, bulk (millimeter size) single crystals possess substantially increased stability due to lower surface-to-volume ratio and reduced grain boundaries. Besides, the bulk crystal has the advantage of feasible optical characterization due to high uniformity and optical quality. Hence, it is meaningful to investigate MPA property by using a bulk single crystal. The MPA property is investigated in three steps. Firstly, the sample is excited by a femtosecond laser at several wavelengths to prove the supporting of ASE by two- and three-photon absorption. It is exciting to find that the sample supports ASE even at room temperature by three-photon excitation. Secondly, the influence of temperature on the emission property is investigated at a temperature range of 77 to 300 K. The photoluminescence (PL) peak is found to be shifting in an abnormal way with temperature. Thirdly, the nonlinear absorption coefficients of a polished CH₃NH₃PbI₃ crystal are measured by the Z-scan technique.|
In order to obtain a higher order MPA, CH3NH3PbCl3 single crystals were chosen for investigation. Because this material distinguishes from the other LHPs with a wider band gap (2.9 eV) and is relatively rarely studied. In this part, seven-photon absorption induced ASE and lasing are obtained from the microcrystals of CH3NH3PbCl3 at cryogenic temperature. In addition, by systematically investigate the influence of temperature and pressure on PL property, a series of intriguing optical properties were discovered. Firstly, strong photon reabsorption effect was discovered. Under single-photon excitation, both band edge emission and reabsorption induced emission can be observed. In contrast, under multiphoton excitation, the emission mainly comes from the reabsorption effect. Secondly, the emission property of CH₃NH₃PbCl₃ is found to be determined by the crystal phase. In the cubic phase, only weak spontaneous emission can be obtained. However, in the orthorhombic phase, ASE is supported by both single- and multiphoton excitation. Interestingly, lasing can be obtained from the micro-crystals only by multiphoton excitation due to longer penetration depth. Although excellent high order MPA property is found from CH3NH3PbI3 and CH₃NH₃PbCl₃, these materials are intrinsically less stable than the all-inorganic perovskites. Hence, CsPbCl₃ is exploited for further investigation. In addition to enhanced stability and supporting six-photon absorption induced ASE and lasing at 213 K, this material shows unique optical properties including excitonic emission and Varshni type bandgap. By studying the emission property under multiphoton excitation, the excitation power dependence of fluorescence is found to be highly related to the recombination mechanism of the material. Finally, multiphoton pumped ASE and lasing from this material are systematically investigated. By studying the representing samples of the lead halide perovskite material, this thesis has proved extraordinary optical property of the material, including high nonlinear absorption coefficients, high optical gain and the supporting of ASE and lasing by higher order multiphoton absorption. Those findings suggest promising application of the material in photonics, nonlinear optics and multiphoton microscopy.
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