Achieving Up-Conversion Amplified Spontaneous Emission through Spin Alignment between Coherent Light-Emitting Excitons in Perovskite Microstructures

Abstract

Metal hybrid perovskites have presented interesting infrared-to-visible up-conversion light-emitting lasing properties through multi-photon absorption. Here, when the optical pumping switches between circular and linear polarization, up-conversion amplified spontaneous emission (ASE) intensity exhibits large and small amplitudes, respectively, leading to a positive up-conversion ΔASE in the CsPbBr₃ perovskite microrods. This observed phenomenon demonstrates that the coherent interaction between coherent light-emitting excitons is indeed established at the up-conversion ASE regime in the CsPbBr₃ perovskite microrods. In addition, the positive up-conversion ΔASE indicates the orbital magnetic dipoles between coherent light-emitting excitons are conserved during up-conversion ASE action. Essentially, the up-conversion ΔASE results provide evidence that shows up-conversion ASE can be realized by the orbit−orbit polarization interaction between light-emitting excitons. Moreover, up-conversion ASE proportionally increased as the pumping fluence increased, which shows that orbit–orbit polarization interaction can be gradually enhanced between coherent light-emitting excitons by increasing pumping density in the CsPbBr₃ perovskite microrods. Substantially, our studies provide a fundamental understanding of the spin alignment between coherent light-emitting excitons towards developing spin-dependent nonlinear lasing actions in metal halide perovskites.

1. Introduction

The metal hybrid perovskites have recently attracted widespread attention as promising candidates for solution-processed optoelectronic applications such as photovoltaics [1,2], lasing [3,4], light-emitting diodes [5,6], and photodetectors [7,8], because of the tunable optical bandgap and remarkable charge transport properties [9,10]. The metal hybrid perovskite APbX₃ (where A = Cs⁺, CH₃NH³⁺, or HC(NH₂)²⁺; X = Br⁻, I⁻, and Cl⁻) materials with all of these characteristics have been realized in various forms, including thin films, single crystals, nanowires, and quantum dots [11,12,13]. In particular, CsPbBr₃ perovskites have demonstrated the capabilities of generating multi-photon up-conversion amplified spontaneous emission (ASE) through multi-photon absorption under infrared laser excitation [14,15]. This indicates high power laser excitation is necessary to realize the simultaneous absorption of multiple photons through the interaction between incident electromagnetic waves and photo-induced polarizations. In contrast, metal halide perovskites are formed with s-p orbital hybridization in their band structures, leading to strong orbital momentum in light-emitting excitons [16,17]. The orbital momentum provides the precondition to realize the interaction between incident electromagnetic waves and photoinduced polarizations when electrons are excited by high-power laser excitation, leading to multi-photon absorption up-conversion ASE. Furthermore, our previous work has found that long-range orbital–orbital polarization interactions occur at the generation of ASE in perovskite, which provides the necessary conditions for coherent interactions between light-emitting excitons through polarization-dependent ASE studies [18,19,20,21,22].

In this work, we study the coherent interaction between light-emitting excitons in the up-conversion ASE regime based on CsPbBr₃ perovskite microrods upon operating orbital magnetic dipoles by using circularly polarized optical pumping.

2. Materials and Methods

2.1. Materials

The CsPbBr₃ precursor solution was prepared by dissolving 0.1 mmol CsBr (Xi’an Polymer Light Technology Corp, Xi’an, China) and PbBr₂ (Xi’an Polymer Light Technology Corp, Xi’an, China) into 1 mL dimethylformamide (Sigma-Aldrich, St. Louis, MO, USA). The solution was then subjected to ultrasonic treatment for 10 min. For CsPbBr₃ microrod growth process, 2 μL solution was dropped on a pre-cleaned glass substrate. The substrate was then placed into a sealed glass container with an isopropyl alcohol atmosphere for 12 h, in which the CsPbBr₃ microrods start to grow. After the growth process was finished, the substrate was rinsed with toluene solvent and dried with a nitrogen gas blow.

2.2. Characterizations

The optical microscopy images were captured using the Horiba Xplora Plus system. The excitation and the photoluminescence (PL) spectra were measured using the Horiba Fluorolog 3 spectrometer with the Xenon lamp as the excitation source. The ASE spectra were recorded using the Oceanoptics FLAME-S-XR1-ES spectrograph. The pump beam was from a pulsed laser beam (1030 nm) generated through a harmonic generator (Ultrafast Systems LLC, Sarasota, FL, USA, third harmonic) with a Pharos laser (Light Conversion, 25 kHz, 1030 nm, 290 fs). The diameter of the focused laser beam was ~100 µm. All the ASE measurements were performed in the transmission geometry with the pump beam and detection direction normal to the sample surface. The time-resolved PL measurements in the ASE regime were taken using the Horiba Fluorolog 3 time-correlated single-photon counting system in combination with the pulsed laser beam (1030 nm, 25 kHz). For the ∆PL measurements in the ASE regime, a linear polarizer combined with the zero-order quarter plate was used to generate a switchable linearly and circularly polarized pump beam with identical intensity.

3. Results

3.1. Optical Characterizations

For optical characterizations, the CsPbBr₃ microrods were prepared by a solution process (see Experimental Method). Figure 1a shows the optical microscopy images of the prepared CsPbBr₃ microrods. Obviously, CsPbBr₃ microrods are rectangular in shape and measure approximately tenths of a millimeter in size. Figure 1b shows the excitation and up-conversion PL spectra of the CsPbBr₃ microrods. The up-conversion PL spectrum exhibited a peak at 533 nm with narrow full width at half maximum (FWHM) of 23 nm under 1030 nm pulse laser with a duration of 290 fs at 8.2 mJ/cm².

3.2. Up-Conversion ASE Emission Spectra

Figure 2a shows the up-conversion emission spectra from the CsPbBr₃ microrods under 1030 nm pulse laser with a duration of 290 fs at different pumping intensities. A narrow up-conversion emission peak at 543 nm was observed when the pumping power reached 14.3 mJ/cm². We should note that a narrow up-conversion red-shifted ASE peak appearing along with the increased excitation power can be attributed to the waveguided photons’ reabsorption with the reduced optical loss [23,24]. Figure 2b demonstrates the spectral intensity and width as a function of pumping power. The up-conversion emission intensity was increased greatly along with the FWHM being decreased to approximately 14 nm when the pumping power was gradually increased above the threshold of 14.3 mJ/cm². Here, it is observed the power dependence with the slope of 1.7 indicates that the up-conversion PL undergoes a two-photon process when the excitation power is below the threshold. Figure 2c shows the time-resolved PL lifetimes of up-conversion ASE peaked at 543 nm and up-conversion spontaneous emission peaked at 533 nm when the pumping power is above the threshold of 14.3 mJ/cm². It can be seen that up-conversion ASE decays much faster than the up-conversion spontaneous emission. The fast decay shows a stimulated light-emitting process within the ASE region when the pumping power exceed a threshold of 14.3 mJ/cm². Especially, we should note that the realization of ASE needs to satisfy two conditions: (i) polarizing the light-emitting excitons before radiative emission occurs and (ii) initiating the coherent interaction between light-emitting excitons, leading to coherent light-emitting states. Therefore, our up-conversion ASE results indicate a coherent interaction between light-emitting excitons is established during up-conversion ASE action in the CsPbBr3 perovskite microrods.

4. Discussion

We now discuss the spin alignment between coherent light-emitting excitons at the up-conversion ASE regime in CsPbBr₃ perovskite microrods. Figure 3 shows the up-conversion ASE (peaked at 543 nm) and up-conversion PL (peaked at 533 nm) intensities when the 1030 nm pumping beam is transitioned between linear and circular polarization at the same intensity in the CsPbBr₃ microrods. Here, the up-conversion ΔASE is defined by $\Delta \mathrm{ASE}=\frac{{\mathrm{I}}_{\mathrm{Circular}}-{\mathrm{I}}_{\mathrm{Linear}}}{{\mathrm{I}}_{\mathrm{Linear}}}$, where ${\mathrm{I}}_{\mathrm{Linear}}$ and ${\mathrm{I}}_{\mathrm{Circular}}$ are the up-conversion ASE intensities under linearly and circularly polarized photoexcitation, respectively. When increasing pumping fluence above the threshold of 15 mJ/cm², the positive up-conversion ΔASE with the amplitude of 11% can be observed by switching circularly and linearly polarized pumping, as shown in Figure 3a. Furthermore, the PL intensity remains virtually unchanged upon switching the pumping between linear and circular polarization in the up-conversion spontaneous emission region. This up-conversion ΔASE phenomenon provides three experimental indications. First, the relaxation of aligned orbital magnetic dipoles between coherent light-emitting excitons reaches 8 ns at room temperature, comparable with the up-conversion ASE. This satisfies the precondition to use the up-conversion ΔASE phenomenon to identify the spin alignment between coherent light-emitting excitons. Second, the observed up-conversion ΔASE phenomenon indicates that the spin alignment is realized by optically generating the same-directional orbital magnetic dipoles under circularly polarized pumping, leading to a coherent interaction between coherent light-emitting excitons through orbit–orbit polarization interaction. Third, the positive ΔASE caused by switching the pumping beam from linear to circular polarization indicates that the spin alignment indeed introduces an additional coherent interaction between transition dipoles through in-phase orbital polarization between coherent light-emitting excitons. Clearly, by monitoring the up-conversion ASE intensity when the pumping beam switched between linear and circular polarization, the spin-polarized coherent light-emitting excitons are indeed established through orbit–orbit polarization interaction during the up-conversion ASE in CsPbBr₃ perovskite microrods. Moreover, up-conversion ΔASE is proportionally increased from 14% to 17% when the pumping power is increased from 16 mJ/cm² to 17 mJ/cm². In addition, the PL intensity keeps constant upon switching the pumping between linear and circular polarization in the up-conversion spontaneous emission region. This indicates that the coherent interaction between coherent light-emitting excitons is largely increased by increasing pumping density within up-conversion ASE regime in the CsPbBr₃ microrods. It should be noted that establishing coherent transition dipoles and spin alignment requires different threshold pumping intensities. The coherent transition dipoles are established at a lower threshold pumping power of 14.7 mJ/cm², initiating a spectral narrowing phenomenon (Figure 2b). When the excitation reached a higher threshold pumping power of 15 mJ/cm², up-conversion ΔASE can be realized by establishing the spin alignment between coherent transition dipoles (Figure 3). Clearly, when the density of coherent transition dipoles is increased, the spin alignment between coherent transition dipoles can be gradually established by increasing the pumping power of circular polarization, leading to increasing coherent interaction between light-emitting excitons in the CsPbBr₃ microrods.

Here, we monitor the up-conversion ASE amplitude by switching the optical pumping beam between linear and circular polarization. Clearly, in the CsPbBr₃ perovskite microrods, circularly polarized excitation produced higher up-conversion ASE than linearly polarized excitation with the same pumping power, leading to positive up-conversion ΔASE phenomenon at room temperature. Furthermore, up-conversion ΔASE magnitude proportionally increased with increasing excitation power. This result indicates that spin-polarized coherent light-emitting excitons are indeed generated and also enhanced with the increasing excitation power in the CsPbBr₃ perovskite microrods, leading to up-conversion ASE. Essentially, this indicates that under the action of a circularly polarized light pump, the aligned magnetic dipole can form an in-phase transition dipole between light-emitting excitons, thus forming spin-polarized coherent optical excitons during nonlinear optical actions in CsPbBr₃ perovskite microrods.

5. Conclusions

In summary, by monitoring up-conversion ΔASE when switching the pump beam between linear and circular polarization, we found that the coherent interaction between coherent light-emitting excitons is indeed established at the up-conversion ASE regime in CsPbBr₃ perovskite microrods. A positive up-conversion ΔASE phenomenon in the CsPbBr₃ microrods can be observed when switching from linear to circular polarization that circularly and linearly polarized with the same pump fluence generate. The positive up-conversion ΔASE phenomenon provides evidence that the coherent interaction between coherent light-emitting excitons generated through the orbit–orbit polarization interaction within coherent transition dipoles in the CsPbBr₃ perovskite microrods. Substantially, this result shows that the spin lifetime of orbital magnetic dipoles can reach the order of nanoseconds, which is similar to the lifetime of coherent transition dipoles in up-conversion ASE. This provides a precondition for the time constant to realize the spin alignment between coherent light-emitting excitons. Upon satisfying the time-constant precondition, spin alignment can conserve the in-phase orbital polarization and consequently introduces an additional cooperative interaction between coherent light-emitting excitons, shown as an increased up-conversion ASE. Thus, our results demonstrate a fundamental understanding of the spin alignment between coherent light-emitting excitons during up-conversion ASE action in metal halide perovskites.