(Invited) Amplified Spontaneous Emission Dynamics of a Lead Halide Perovskite Crystal as Revealed by Femtosecond Transient Absorption Microscopy

Abstract

Organic-inorganic lead halide perovskite materials (e.g. CH3NH3PbX3, CsPbX3, X=Cl, Br, I) have various attractive properties not only for solar cells but also for LED and nanoscale lasers because of their wavelength tunability and low lasing threshold. Such an efficient lasing is indispensable for their applications, and the essential needs are miniaturization and low threshold. While this material was reported in 2009 for use in solar cells, it is also expected to be used in light-emitting and laser devices due to its easy tunability of emission wavelength and low-cost fabrication. However, the low-threshold lasing mechanism of lead halide perovskites has not been established due to various theories on the interaction state between light and carriers generated in the cavity at the initial stage and the carrier-carrier interaction state, which has greatly hindered progress in improving performance. In the present study, we have measured transient absorption spectra of a single CH₃NH₃PbBr₃ microcrystal using a femtosecond transient absorption microscopy to reveal the initial carrier process that induces nonlinear luminescence[1].CH3NH3PbBr3 crystals were prepared by a previously reported method. Aqueous lead(II) acetate solution (100 mg/mL) was dropped onto a glass slide and dried at 60°C for 30 min to obtain a film of lead acetate. Microcrystals of CH3NH3PbBr3 were then obtained by immersing the film in methylammonium bromide solution (5 mg/mL) with isopropanol as a solvent for 20 hours at room temperature in air.For transient absorption measurements, a regeneratively amplified femtosecond pulse (Spectra-Physics, Solstice, 795 nm, 1 kHz) was split in two; the second harmonic of one beam was used as the excitation light, and the white light generated with the other beam by introducing it into an optical parametric amplifier (Light Conversion, TOPAS) and focusing the obtained signal light (1.3 mm) into a CaF2 plate was used as the probe light.Fig. 1(a) shows the transient absorption spectra of lead halide perovskite excited at the intensities over the lasing threshold. The spectral signals that were not observed before the time origin (-1 ps) were significantly observed with modulated spectral shape after the time origin (≥0 fs). The spectral bleaching around 540 nm indicates carrier generation by the excitation light, and the relaxation of the bleaching indicates the recombination of electrons and holes. Furthermore, at around 557 nm, a bleaching signal appeared as shown in Fig. 1(b), that was not represented in the transient absorption spectrum excited at the intensity below the lasing threshold. This signal is suggested to be due to amplified stimulated emission. Spectral modulation is often observed as interference of light propagating in thin-film systems. The thickness of the perovskite crystals is around 200-300 nm, which is suitable to observe the optical interference. However, generally, this interference tends to become lower in frequency as one observes longer wavelengths, which is essentially different from the modulation observed here. The spectra modulation appeared immediately after the carrier was generated, and the modulation continued for up to 1ns. These results suggest that the modulation is caused by the electric field effect of the generated carriers on the electron spectrum. Franz-Keldysh’s theory is often used for spectral modulation in the presence of an electric field. In reflectance transient absorption spectroscopy, the electric field modulated spectrum has been explained in the GaInP2 system by Franz-Keldysh’s theory in systems where charges are generated on the semiconductor surfaces[2]. These results indicate the spectral modulation occurred from the charges separated on the surface of the crystal. We will discuss the mechanism of amplified spontaneous emission and low threshold lasing dynamics at the conference site. Funding: This study was partly supported by KAKENHI (Grant Number JP22H04755, JP19K05190) and JST FOREST (JPMJFR213G).