Picosecond Random Lasing Based on Three-Photon Absorption in

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Picosecond Random Lasing based on Three-Photon-Absorption in Organometallic Halide CH3NH3PbBr3 Perovskite Thin Films Guo-En Weng, Juanjuan Xue, Jiao Tian, Xiaobo Hu, Xumin Bao, Hechun Lin, Shaoqiang Chen, Zi-Qiang Zhu, and Junhao Chu ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Picosecond Random Lasing based on Three-Photon-Absorption in Organometallic Halide CH3NH3PbBr3 Perovskite Thin Films Guoen Weng,* Juanjuan Xue,* Jiao Tian,* Xiaobo Hu, Xumin Bao, Hechun Lin, Shaoqiang Chen,** Ziqiang Zhu, and Junhao Chu Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China

ABSTRACT: Organometallic halide perovskites have been demonstrated very promising for nonlinear optics and practical frequency upconversion devices in integrated photonics. In this work, high quality organometallic halide CH3NH3PbBr3 perovskite thin films were synthesized through a solution-based one-step spin-coating method. With femtosecond optical pumping at 1300 nm, frequency-upconverted random lasing (RL) from the bromide perovskite films were achieved via three-photon (3P) absorption processes. The RL spectra show no spikes due to the large scattering mean-free path in the perovskite crystals, meaning the incoherent RL emission with incoherent feedback. In comparison with the one-photon pumped situation, it is found that both the two-photon and 3P excitations are more effective in reducing the RL threshold despite the low conversion efficiency of their nonlinear multi-photon schemes. Moreover, the time- and spectral-resolved lasing characteristics of the laser pulses were systematically explored by time-resolved photoluminescence based on an optical Kerr-gate method. The measured ultra-short 3.1 ps output pulse is the shortest one that has been observed so far in bromide perovskite random lasers, without any post-processing. In addition, wavelength dependence of the pulse width and delay time of the RL pulses were clearly demonstrated, and could be unravelled by intraband carrier relaxation dynamics, which is an important physical mechanism in ultrafast lasers. Our results demonstrate that organometallic halide perovskites are excellent gain medium for high-performance frequency upconversion random lasers, and, have great potential for use in gain-switched semiconductor lasers with ultra-short output pulses and tunable emission wavelengths across the entire visible spectrum. KEYWORDS: random lasing, nonlinear, three-photon excitation, halide perovskite, frequency upconversion Over the past few years, organic-inorganic halide perovskites have attracted tremendous attention due to their excellent optical properties, such as high absorption coefficient,1 low trap-state density,2 long carrier lifetime and diffusion length,3,4 large carrier mobility,5 and broad luminescence windows covering the entire visible spectral range,6,7 which make them a promising alternative for both light-harvesting and light-emitting applications.8−11 Recently, the amplified spontaneous emission and/or lasing from organometallic and all-inorganic halide perovskite (MPbX3, where M = CH3NH3 or Cs, and X = Cl, Br, I, or their mixture systems) thin films and colloidal nanocrystals have been experimentally demonstrated with a low threshold under one-photon (1P),12,13 two-photon (2P),13,14−17 and three-photon (3P)16,18,19 pumping, showing a great potential of halide perovskites for applications in optoelectronics and nonlinear optics. Moreover, the random lasing (RL) actions have also been observed in the nano/micro-structured halide perovskite films due to the strong multiple light scattering and their self-constructed random cavities,20−25 which are extensively explored in closely packed semiconducting quantum dots (CdSe/CdS core-shell)26 and ZnO powders.27,28 Until now, however, RL in halide perovskites has only been rarely demonstrated via linear 1P and low-order nonlinear 2P absorption processes, while investigation on frequency upconversion 3P absorption effects and resultant RL behaviors is still lacking, but of great importance in higher-order nonlinear optical applications. As to 1P pumped cases, ultraviolet (UV) excitations have destructive effects to samples and suffer the problem of short penetration depth due to the strong light absorption coefficient, which limits their developments especially in biomedical imaging and sensing.29 In contrast to 1P pumped situations, namely linear absorption and emission, multi-photon analogues must be achieved through the simultaneous absorption of two or more photons. Such frequency upconversion nonlinear absorption processes feature several merits including large penetration depth, high spatial resolution, as well as little photodamage and photobleaching.30,31 The larger penetration length within the samples offers more opportunities for investigating the inner physical nature of materials, and also effectively suppresses nonradiative recombination due to surface defects.32 Moreover, the multi-photon-pumped process has fascinating characteristics such as repression of unwanted absorption losses.33 Consequently, multi-photon-pumping is a feasible and important technique to generate the nonlinear luminescence for halide perovskites due to their large absorption cross sections, stable excitons at room temperature, and strong oscillator strength,34,35 leading to further applications in optical data storage,36 characterization of

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ultrafast optical signals,37 and optical limiting.38 In this study, high quality CH3NH3PbBr3 perovskite thin films were synthesized through a solution-based one-step spin-coating method. Frequency-upconverted RL from the bromide perovskite films were demonstrated by direct nonlinear 3P absorption processes. Compared to the 1P pumped situation, the 2P and 3P excitations are found to be more effective in reducing the RL threshold despite the low conversion efficiency of their nonlinear multi-photon schemes, while the 3P excitation exhibits a relatively higher threshold than the 2P excitation. To gain more insight into the emission dynamics, the Kerr-gate-based time-resolved photoluminescence (TRPL) were carried out. The measured 3.1 ps pulse width, to our knowledge, is the shortest one that has been observed in bromide perovskite random lasers, without any post-processing. The observed asymmetry of the pulse images with respect to wavelength were satisfactorily unravelled by the proposed carrier relaxation mechanism. Moreover, the wavelength dependence of the pulse width and delay time of the RL pulses were clearly illustrated, showing very good agreement with the intraband relaxation mechanism, which plays an important role in ultrafast lasers. These results would provide a new platform for organometallic halide perovskites in nonlinear photonics and frequency upconversion lasing devices.

RESULTS AND DISCUSSION The investigated CH3NH3PbBr3 thin films were synthesized by a one-step solution approach following the procedure reported by Zhao et al.39 with slight adjustment. Briefly, the CH3NH3Br (MABr) was firstly synthesized by reacting methylamine and hydrobromic acid with a stoichiometric ratio of 1:1, followed by a vacuum drying process. The perovskite CH3NH3PbBr3 precursor solutions were then prepared by mixing equimolar MABr and lead bromide (PbBr2) powders in dimethylformamide (DMF) solutions. After that, the perovskite CH3NH3PbBr3 precursor solutions were spin-coated onto a fluorine-doped tin oxide (FTO) transparent conducting glass substrate. Upon drying at a temperature of 100 ˚C for 10 min, the coated films darkened in color, indicating the formation of CH3NH3PbBr3 perovskite films in the solid state (see Methods section for details). The average thickness of the CH3NH3PbBr3 film was measured to be about ~4.3 µm.

Figure 1. (a-c) Top-view SEM images of the CH3NH3PbBr3 perovskite thin films. Scale bars are 5 µm, 1 µm, and 300 nm, respectively. (d) XRD pattern of the CH3NH3PbBr3 films. The two main peaks at 14.92° and 30.12° represent the different crystallographic planes of (100) and (200), respectively. (e) UV-visible absorption spectrum (black dashed line) and SE spectrum (green hollow circle) excited by 1300 nm fs laser pulses. Inset shows the photograph of CH3NH3PbBr3 film illuminated by 1300 nm laser beam. The visibly green emission is a direct signature of 3P absorption.

Figure 1(a)-(c) presents top-view scanning electron microscope (SEM) images that show the morphology of the synthesized perovskite microstructures. We can clearly see the typical crystalline


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faces of the CH3NH3PbBr3 particles and the average size of their crystallite dimensions ranging from several hundred nanometers to a few microns. Furthermore, X-ray diffraction (XRD) measurements were performed to reveal the crystal phase of the investigated samples. As shown in Fig. 1(d), two main diffraction peaks at 14.92° and 30.12° are distinctly exhibited, corresponding to (100) and (200) planes of the cubic phase CH3NH3PbBr3, respectively. Figure 1(e) shows the UV-visible absorption and room temperature spontaneous emission (SE) spectra of the bromide perovskite films. Absorption onset occurs at about 550 nm, which is consistent with previous report.40 The non-zero baseline of the absorption spectrum is attributed to light scattering and interference effects.41 The SE spectrum was recorded by femtosecond (fs) optical excitation at 1300 nm via 3P absorption processes, with a peak position at about 545 nm which corresponds to a bandgap energy of 2.27 eV. The visibly observed green emission from the sample shown in the inset of Fig. 1(e) provides direct evidence of the upconversion nonlinear below-bandgap absorption.

Figure 2. (a) Excitation-power-dependent emission spectra under 3P (λexc = 1300 nm) excitation. Inset illustrates the 3P absorption and emission processes. (b) FWHM and peak position of the emission spectra at various excitation intensities under 3P pumping. (c) Emission intensity of the SE and RL versus the excitation power via 3P absorption. Inset shows the normalized emission spectra of the SE and RL for comparison purpose. (d) Log-log plot of the integrated RL emission intensity as a function of the excitation power via 3P absorption. The red linear fitting line presents a slope (k) of 2.4.

Figure 2(a) shows the excitation-power-dependent SE and RL spectra of the CH3NH3PbBr3 thin films pumped at a wavelength of 1300 nm. The inset of Fig. 2(a) illustrates its photophysical absorption and fluorescence processes under such 3P pumped condition. Since there is no specifically defined laser cavities in these perovskite films, the optical feedback for lasing could be formed via multiple random scattering provided by the polycrystalline grain boundaries.20 Here, it should be noted that the RL spectra present no spikes or so-called laser modes. Generally, according to feedback mechanisms, random lasers are classified into two categories:42,43 (i) coherent random lasers with coherent feedback which exhibit a typical spiky spectrum; (ii) incoherent random lasers with incoherent feedback which exhibit a smooth spectrum. In our case, such spikes were not detected at varied excitation powers, meaning the incoherent RL emission. This also indicates that the light propagation is diffusive due to the large scattering mean-free path in the CH3NH3PbBr3 particles, and hence, interference contributes negligibly to the feedback process, which is incoherent.44 As shown in Fig. 2(b) and (c), clear signatures of the RL, including abrupt narrowing of the spectral linewidth and threshold behavior with a steep rise in intensity above the threshold, can be readily obtained. The pump threshold is derived to be about 27 mJ/cm2. For


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the excitation intensity lower than the threshold, a broad SE spectrum was observed with a peak wavelength of ~545 nm and a full width at half maximum (FWHM) of ~22 nm. With increasing excitation intensity, a distinguishable RL generated at 547 nm when the excitation intensity reached the threshold. As the excitation intensity was further increased, the peak wavelength gradually redshifted to 550 nm, accompanied by a remarkable increase in emission intensity and a precipitate decrease in spectral linewidth, while the broad SE experienced saturation. The normalized SE and RL spectra are presented in the inset of Fig. 2(c) to facilitate comparison. Similar lasing behaviors have been observed previously by Kao et al.20 in perovskite CH3NH3PbI3 films under 1P excitation. To judge the origin of the observed fluorescence, the spectrally integrated RL emission intensity versus excitation intensity was replotted on log-log scales in Figure 2(d). The slope, k, of the linearly-fitted solid red line to the measured data physically revealed the number of photons involved in the excitation process.16,45 Here, k is found to be 2.4, which is in excellent agreement with the ratio between the emitting photon energy (2.28 eV, @ 545 nm) and the pumping photon energy (0.95 eV, @ 1300 nm). Noteworthily, it is still considered to be 3P absorption due to the fact that the energy of two 1300 nm photons is less than that of the bandgap energy. The surplus of energy (∆E = 3 × 0.95 - 2.28 = 0.57 eV) will contribute to heat instead of upconversion emission after the 3P absorption processes.

Figure 3. Excitation-power dependence of emission spectra under 1P (λexc = 400 nm) (a) and 2P (λexc = 800 nm) (b) excitation, respectively. Insets show the absorption and emission mechanisms (left) and the evolution of FWHM and peak position at various excitation powers (right). Emission intensity of the SE and RL versus the excitation power via 1P (c) and 2P (d) absorption, respectively. Insets show the normalized emission spectra of the SE and RL to facilitate comparison. (e) RL thresholds of CH3NH3PbBr3 perovskite thin films via 1P, 2P, and 3P absorption processes.

To further explore the spectral evolution at varied excitation fluences and the dependence of RL threshold (Eth) on excitation wavelength (λexc), 1P and 2P pumped RL experiments were performed with 400 and 800 nm fs pulsed laser beams. Similar to the 3P pumped situation, clear RL was observed at the longer wavelength side of the photoluminescence (PL) spectra, as shown in Fig. 3(a) and (b), with evidence of gain-induced linewidth narrowing (see the inset of Fig. 3(a) and (b)) and threshold behavior of excitation-power dependence of the emission intensity shown in Fig. 3(c) and (d). It is noted that for the 1P absorption process, the RL spectra above the threshold present negligible changes of bandwidth and slight sub-linear red-shifts of peak position with increased excitation power. Nevertheless, for the 2P and 3P absorption processes, the corresponding RL presents an identifiable spectral broadening and a relatively larger linear red-shift of the peak position with elevated excitation intensity, which has also been observed by Z. Y. Zhang et al.46 Generally, excitation-power-dependent spectral broadening and peak shifts can be ascribed to multiple mechanisms, such as the band filling effect, many-body


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interactions, thermally induced bandgap/refractive index changes, the interplay between the effect of the electron-phonon renormalization and the lattice expansion, the stabilization of the valence band maximum, and optical density fluctuations.47−49 In our cases, the many-body interactions in the perovskite particles should be the most plausible mechanism accounting for the red-shifts of the RL peak since any other effects mentioned above would ultimately result in the peak blue-shifts. Such many-body effect should play a more important role in multi-photon absorption processes. However, despite the tremendous efforts that have been made,47,50 it is still elusive of many-body interactions in perovskites and further investigations are highly desired. Besides, the spectral broadening resulting from charge carrier accumulation can be interpreted by the dynamic Burstein-Moss shift,51 that is, the broadened intensity of the RL linewidth is sensitively related to carrier densities. As a result, the 3P pumped RL emission shows a maximum red-shift of 2.8 nm (from 548.0 to 550.8 nm) and the 1P pumped counterpart shows a minimum red-shift of 1 nm (from 550.0 to 551.0 nm) above the threshold. Figure 3(e) plots the RL threshold of the CH3NH3PbBr3 perovskite thin films with different pumping wavelengths of 400, 800, and 1300 nm, corresponding to 1P, 2P, and 3P pumped situations, respectively. The respective thresholds were estimated to be 105, 0.75, and 27 mJ/cm2. As we can see, both the 2P and 3P excitations are more effective in reducing the RL threshold than the 1P excitation, despite the low conversion efficiency of their nonlinear multi-photon schemes. The RL threshold of 0.75 mJ/cm2 via 2P absorption is rather consistent with Xu’s result of ~0.8 mJ/cm2 for all-inorganic CsPbBr3 perovskite films,17 which is about one-order of magnitude lower than that of CdSe nanocrystal films with similar emitting color under 2P excitation condition.52 As is well known, the 1P pumped RL belongs to conventional Stokes fluorescence, and the penetration depth of the 400-nm pumping light is rather small (< 300 nm) due to its strong light absorption coefficient.53 While the frequency upconversion 2P and 3P absorption give rise to another RL style, namely, anti-Stokes upconverted fluorescence. The upconversion pumping light can penetrate much deeper into the investigated thin films and thus many more internal perovskite lattices can be excited due to their much weaker absorption intensity.30 According to the Beer-Lambert law,54 e.g., the 800 nm lasers can punch into the crystal with 30~60 µm, indicating that the line gain length is nearly 100-times longer than that of 400 nm lasers. Hence, if the perovskite films are thin enough that the integral films are all run through under either 1P or multi-photon excitation, then the RL threshold for multi-photon absorption processes should be much higher than that of the 1P excitation paradigm because of the low conversion efficiency of their nonlinear multi-photon effects.15 However, when the thickness of the perovskite film is increased to several microns or even larger (~4.3 µm for our sample), only the lattices close to surface can be excited under 1P excitation, then multi-photon pumping will show great advantages due to its larger penetration depth and thus contributes to a lower lasing threshold.46 Note that the 3P pumping case presents a relatively higher threshold of 27 mJ/cm2 compared to the 2P pumped situation in our experiments. This phenomenon can be similarly explained as above mentioned, that is, the lower conversion efficiency of higher order nonlinear 3P absorption give rise to a relatively higher threshold since the integral perovskite film was effectively excited by both 2P and 3P pumping.


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Figure 4. TRPL images of the SE (a) and RL (b) from the CH3NH3PbBr3 perovskite films at excitation intensities of 0.6 and 2.0 Eth, respectively. Inset presents the enlarged time-resolved RL image. (c) Schematic illustrating the carrier relaxation after the transient fs pumping. The red dashed arrow shows the electron relaxation from high-energy excited states to emissive lower energy states. Rise (d) and decay (e) processes of the RL emission. Insets show the normalized emission spectra plotted in a logarithmic scale.

To confirm the RL behaviors and gain more insight into the emission dynamics, excitation-power-dependent TRPL experiments were carried out using the optical Kerr-gate method.55 As shown in Fig. 4, a typical wide SE decay with a broad emission band and long decay time is observed (Fig. 4(a)) below the threshold (0.6 Eth). In contrast, when the excitation power is above the threshold (2.0 Eth), a narrow emission peak with a short decay time appears at the longer-wavelength side of the image (Fig. 4(b)), featuring the ultrafast lasing action.16 The inset of Fig. 4(b) shows an enlarged view of the time-resolved pulse image, which is obviously asymmetrical with respect to wavelength. To better understand this asymmetry, a possible mechanism was proposed as follows: just after the above threshold excitation, large amount of carriers are pumped into high-energy excited states and the population inversion is build up, and then, the initial high-energy stimulated emission happens rapidly. After that, excess carriers relax down rapidly to the emissive lower energy states and present the following low-energy stimulated emission, as shown in Fig. 4(c), within several ps. Such a short time provides additional unambiguous evidence for the RL actions because the removal processes like Auger recombination and other recombination pathways should require much more decay time. Consequently, fast initial components with ultra-short durations emerged at the short-wavelength side while gradually increased durations for the long-wavelength counterparts, resulting in the asymmetry of the RL pulses with respect to wavelength. To verify this transient carrier dynamics, the spectral evolutions during pulse rise and decay processes are exhibited in Fig. 4(d) and (e). It shows analogous spectral shape with negligible shifts of peak position (see the inset of Fig. 4(d)) during the ultrafast rise process for t < 1.8 ps. This also indicates the shifts of the RL emission are not ascribed to the carrier-density-related refractive index changes. In contrast, when the pulse starts to decay for t > 1.8 ps, the spectra present a more rapid decrease in emission intensity at short-wavelength sides with the time going on, resulting in the spectral narrowing and a peak redshift of 2 nm, as shown in the inset of Fig. 4(e). This result is in very good agreement with the proposed carrier relaxation mechanism.


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Figure 5. (a) Waveforms of the SE and RL pulses from the CH3NH3PbBr3 perovskite films at excitation intensities of 0.6 and 2.0 Eth, respectively. The solid lines shows the fitting curves of the pulses. (b) Time-integrated emission spectra of the CH3NH3PbBr3 perovskite films at excitation intensities of 0.6 and 2.0 Eth, respectively. Inset shows the replotted 3P-pumped emission intensity versus excitation intensity on a log-log scale. The red dash lines are guides for the eye. (c) Wavelength dependence of the pulse width and delay time of the RL pulses extracted from the TRPL image in Fig. 4(b). (d) Waveform of the RL pulses at excitation intensity of 2.0 Eth plotted in a logarithmic scale. The rise and decay times were obtained to be 0.7 and 2.1 ps, respectively, from a fitting analysis.

Figure 5(a) shows the time-resolved waveforms of the output pulses from the perovskite films with excitation intensities of 0.6 and 2.0 Eth, respectively. By well-fitting the experimental data, their pulse widths were respectively found to be 3.1 and 37.4 ps. Actually, it is a big challenge even for high-performance semiconductor lasers to obtain ps pulses. As to an ASE process with similar spectral evolution, it is plausibly considered to be too difficult to achieve such ultra-short ps pulses. To our knowledge, the 3.1-ps pulse width is so far the shortest one that has been observed in organometallic bromide perovskite random lasers. Figure 5(b) shows the normalized time-integrated emission spectra, which are consistent with the spectroscopy shown in Fig. 2. The inset of Fig. 5(b) is a replot of the emission intensity as a function of excitation intensity on a log-log scale under 3P excitation, showing the typical “S-shaped” L-I characteristic of a laser, which further demonstrates the RL behaviors in the perovskite films rather than the ASE process. Figure 5(c) presents the wavelength dependence of the pulse width and delay time of the RL pulses extracted from the TRPL images with excitation intensity of 2.0 Eth. It is clearly disclosed that over the wavelength rang from 543 to 550 nm, the delay time increases almost linearly with increasing wavelength with a slope of 0.147 ps/nm, corresponding to the rapid carrier relaxation processes from high-energy excited states to emissive lower energy states. In the meanwhile, the pulse width increases linearly from 1.9 to 3.5 ps. This indicates that the pulses at long wavelength side have longer pulse width and delay time than the pulses at short wavelength side, which accordingly gives rise to the asymmetry of the pulse images with respect to wavelength, as discussed above. In Fig. 5(d), the waveform of the RL pulses is replotted in a logarithmic scale. Clear exponential rise and decay of the pulses are apparent. The rise and decay times were obtained to be 0.7 and 2.1 ps, respectively, from a fitting analysis. Actually, such exponential rise and decay of the pulses, as well as the dependence of the delay time and pulse width on the wavelength are the typical characteristics of gain-switched pulses in semiconductor lasers.56,57 Gain switching is a very simple technique for generating ultra-short pulses through direct modulation of optical gain in semiconductor lasers, which enables one to fabricate compact semiconductor pulse lasers with tunable repetition rates and durations for various applications. Herein, the


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lasing pulses with ps durations demonstrate the great potential of organometallic halide perovskites for applications in ultrafast gain-switched lasers. The ultrafast rise process of the RL pulse is benefited from the high material gain of organic-inorganic halide perovskites, while the shortest decay time is limited by the photon lifetime in the optical path,58 which is related to the length of the scattering gain medium and/or the photon mean free path in the perovskite particles. Therefore, by synthesizing the perovskite particles with smaller and homogeneous nanometer-scale size and decreasing the excitation point size, the shorter photon scattering length and hence even shorter pulse width could be possible. Another strategy towards even shorter pulses is synthesizing the perovskite particles with regular cubic or sphere shapes in larger micrometer-scale, where Fabry-Pérot mode or whispering-gallery mode could be formed in the micro-cavity of the particles producing even shorter pulses. Additionally, the spectral filtering technique59 could be utilized to

generate even shorter pulses by cutting off the slow component at longer wavelengths.

CONCLUSIONS In summary, high quality organometallic halide CH3NH3PbBr3 perovskite thin films were synthesized through a solution-based one-step spin-coating method. With fs optical pumping at 1300 nm, the frequency-upconverted RL emission was demonstrated by direct nonlinear 3P absorption processes. Compared to the 1P pumped (λexc = 400 nm) situation, the 2P (λexc = 800 nm) and 3P excitations are found to be more effective in reducing the RL threshold despite the low conversion efficiency of their nonlinear multi-photon schemes. This can be well explained by their larger penetration depth and hence more effective excitation of internal perovskite lattices for multi-photon absorption processes. Noteworthily, the 3P excitation exhibits a relatively higher threshold than the 2P excitation due to its relatively low conversion efficiency for higher order nonlinear absorption processes, because the integral perovskite films were all run through in both excitation cases. Above the thresholds, the spectral broadening and peak redshifts with increasing excitation intensity are respectively due to the charge carrier accumulation and thermally-induced decrease in bandgap energy. Additionally, the time- and spectral-resolved lasing characteristics at excitation intensity of 2.0 Eth were systematically explored by TRPL measurements based on an optical Kerr-gate method. The asymmetry of the pulse images with respect to wavelength is considered to be induced by the rapid carrier relaxation after the transient fs pumping. The output ultra-short pulses, as short as 3.1 ps, is the shortest one that has been observed in bromide perovskite random lasers, without any post-processing. Besides, linearly increased delay time with a slope of 0.147 ps/nm, and linearly increased pulse width from 1.9 to 3.5 ps, were extracted from the RL images over the spectral rang from 543 to 550 nm. Such dependence of the delay time and pulse width on the wavelength, as well as the exponential rise and decay of the pulses, present the typical characteristics of gain-switched laser pulses. Our results demonstrate the great potential of organometallic halide perovskites for applications in frequency upconversion lasing devices, and gain-switched semiconductor lasers with ultra-short output pulses and tunable emission wavelengths.

METHODS Synthesis of CH3NH3PbBr3 perovskite films: Typically, CH3NH3Br (MABr) was synthesized by reacting methylamine (33 wt% ethanol solution) and hydrobromic acid (47 wt% in water, Aldrich) with a stoichiometric ratio of 1:1 in an ice bath for 3 h with stirring, followed by vacuum drying at a temperature of 60 ˚C for 24 h. Then, the perovskite CH3NH3PbBr3 precursor solutions were prepared by mixing equimolar MABr (0.7 g) and lead bromide (PbBr2) (2.3 g) powders in dimethylformamide (DMF) solutions (5 mL) at a temperature of 70 ˚C for 4 h. After that, the perovskite CH3NH3PbBr3 precursor solutions were spin-coated onto a fluorine-doped FTO transparent conducting glass substrate at 4000 rpm for 15 s, followed by heat treatment at 100 ˚C for 10 min. The coated films darkened in color, indicating the formation of CH3NH3PbBr3 perovskite films in the solid state, which is confirmed by XRD measurements. Characterization: Structural characterization of the as-synthesized thin films was performed using both SEM (XL30FEG, Philips) and XRD (D8 Advance, Bruker AXS; Cu Kα) operated at room temperature. The UV-visible absorption spectrum was recorded on an UV/Visible/NIR spectrophotometer (Carry 5000, Varian). Optical Measurements: All PL experiments were conducted at room temperature (20 °C) by fs impulsive optical excitations at 400, 800, and 1300 nm, corresponding to 1P, 2P, and 3P absorption processes, respectively. The 1300 nm pulses were generated from an optical parametric amplifier (OPA) system pumped by 800 nm pulses from a mode-locked Ti:sapphire regenerative amplifier system (Verdi G8, Coherent) operating at 35 fs pulse duration and 1 kHz repetition rate, while the 400 nm pulses were generated by frequency doubling the 800 nm fs pulses via the β-barium borate (BBO) crystal. The


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excitation beam was introduced into the confocal system, and focused to a spot of 300 µm in diameter through an optical lens with focal length of 8 cm. The SE and RL behaviors of the sample were identified by a Kerr-gate-based TRPL system with a temporal resolution of about 100 fs. The luminescence signals were dispersed by a triple-grating spectrometer (SR303, Andor) and detected by a cooled charge-coupled device (CCD).

AUTHOR INFORMATION Corresponding Author **Email: [email protected].

Author Contributions S. Q. Chen and G. E. Weng conceived the idea of frequency-upconverted random lasers based on three-photon-absorption in organometallic halide perovskites. The synthesis of the CH3NH3PbBr3 films were carried out by J. J. Xue, J. Tian, G. E. Weng, X. B. Hu and H. C. Lin. The structural characterizations of the samples were carried out by J. J. Xue, J. Tian and X. B. Hu. The optical experimental setup were constructed by G. E. Weng, S. Q. Chen and X. M. Bao. The optical measurements were carried out by G. E. Weng, J. Tian and J. J. Xue. The analysis and discussion of the results were carried out by all the authors. The manuscript was written by G. E. Weng with suggestions from all the authors. *G. E. Weng, J. J. Xue and J. Tian contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Recruitment Program of Global Experts (1000 Talent Plan) of China, the National Natural Science Foundation of China (Grant Nos. 61704055, 61604055), and the Program of Shanghai Science and Technology Committee (No. 17142202500).

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For Table of Contents Use Only Title: Picosecond Random Lasing based on Three-Photon-Absorption in Organometallic Halide CH3NH3PbBr3 Perovskite Thin Films Authors: Guoen Weng, Juanjuan Xue, Jiao Tian, Xiaobo Hu, Xumin Bao, Hechun Lin, Shaoqiang Chen, Ziqiang Zhu, and Junhao Chu

TOC:

Frequency-upconversion incoherent random lasing (RL) from the bromide perovskite films were achieved via three-photon absorption processes. The measured ultra-short 3.1 ps output pulse based on an optical Kerr-gate method is the shortest one that has been observed so far in bromide perovskite random lasers, without any post-processing. The wavelength dependence of the pulse width and delay time of the RL pulses were clearly demonstrated, and could be unravelled by the proposed intraband carrier relaxation dynamics, which is an important physical mechanism in ultrafast lasers. Our results demonstrate that organometallic halide perovskites are excellent gain medium for high-performance frequency upconversion random lasers, and, have great potential for use in gain-switched semiconductor lasers with ultra-short output pulses and tunable emission wavelengths across the entire visible spectrum.


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Figure 1. (a-c) Top-view SEM images of the CH3NH3PbBr3 perovskite thin films. Scale bars are 5 µm, 1 µm, and 300 nm, respectively. (d) XRD pattern of the CH3NH3PbBr3 films. The two main peaks at 14.92° and 30.12° represent the different crystallographic planes of (100) and (200), respectively. (e) UV-visible absorption spectrum (black dashed line) and SE spectrum (green hollow circle) excited by 1300 nm fs laser pulses. Inset shows the photograph of CH3NH3PbBr3 film illuminated by 1300 nm laser beam. The visibly green emission is a direct signature of 3P absorption. 96x61mm (600 x 600 DPI)


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Figure 2. (a) Excitation-power-dependent emission spectra under 3P (λexc = 1300 nm) excitation. Inset illustrates the 3P absorption and emission processes. (b) FWHM and peak position of the emission spectra at various excitation intensities under 3P pumping. (c) Emission intensity of the SE and RL versus the excitation power via 3P absorption. Inset shows the normalized emission spectra of the SE and RL for comparison purpose. (d) Log-log plot of the integrated RL emission intensity as a function of the excitation power via 3P absorption. The red linear fitting line presents a slope (k) of 2.4. 112x87mm (600 x 600 DPI)


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Figure 3. Excitation-power dependence of emission spectra under 1P (λexc = 400 nm) (a) and 2P (λexc = 800 nm) (b) excitation, respectively. Insets show the absorption and emission mechanisms (left) and the evolution of FWHM and peak position at various excitation powers (right). Emission intensity of the SE and RL versus the excitation power via 1P (c) and 2P (d) absorption, respectively. Insets show the normalized emission spectra of the SE and RL to facilitate comparison. (e) RL thresholds of CH3NH3PbBr3 perovskite thin films via 1P, 2P, and 3P absorption processes. 102x70mm (600 x 600 DPI)


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Figure 4. TRPL images of the SE (a) and RL (b) from the CH3NH3PbBr3 perovskite films at excitation intensities of 0.6 and 2.0 Eth, respectively. Inset presents the enlarged time-resolved RL image. (c) Schematic illustrating the carrier relaxation after the transient fs pumping. The red dashed arrow shows the electron relaxation from high-energy excited states to emissive lower energy states. Rise (d) and decay (e) processes of the RL emission. Insets show the normalized emission spectra plotted in a logarithmic scale.


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Figure 5. (a) Waveforms of the SE and RL pulses from the CH3NH3PbBr3 perovskite films at excitation intensities of 0.6 and 2.0 Eth, respectively. The solid lines shows the fitting curves of the pulses. (b) Timeintegrated emission spectra of the CH3NH3PbBr3 perovskite films at excitation intensities of 0.6 and 2.0 Eth, respectively. Inset shows the replotted 3P-pumped emission intensity versus excitation intensity on a log-log scale. The red dash lines are guides for the eye. (c) Wavelength dependence of the pulse width and delay time of the RL pulses extracted from the TRPL image in Fig. 4(b). (d) Waveform of the RL pulses at excitation intensity of 2.0 Eth plotted in a logarithmic scale. The rise and decay times were obtained to be 0.7 and 2.1 ps, respectively, from a fitting analysis. 111x82mm (600 x 600 DPI)


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Frequency-upconversion incoherent random lasing (RL) from the bromide perovskite films were achieved via three-photon absorption processes. The measured ultra-short 3.1 ps output pulse based on an optical Kerrgate method is the shortest one that has been observed so far in bromide perovskite random lasers, without any post-processing. The wavelength dependence of the pulse width and delay time of the RL pulses were clearly demonstrated, and could be unravelled by the proposed intraband carrier relaxation dynamics, which is an important physical mechanism in ultrafast lasers. Our results demonstrate that organometallic halide perovskites are excellent gain medium for high-performance frequency upconversion random lasers, and, have great potential for use in gain-switched semiconductor lasers with ultra-short output pulses and tunable emission wavelengths across the entire visible spectrum. 80x39mm (300 x 300 DPI)


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Picosecond Random Lasing Based on Three-Photon Absorption in

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