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Abstract
Herein, we report the two-photon pumped amplified spontaneous emission (ASE) in the 2D RPPs flakes at room temperature. We prepared high-quality (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes by mechanical exfoliating from the fabricated crystals. We show that the (BA)2(MA)n-1PbnI3n+1 flakes display a tunable two-photon pumped emission from 527 nm to 680 nm, as n increases from 1 to 5. Furthermore, we demonstrated two-photon pumped ASE from the (BA)2(MA)n-1PbnI3n+1 (n = 3, 4, 5) flakes. The two-photon pumped ASE thresholds of the RPPs are lower than lots of the other semiconductor nanostructures, indicating an excellent performance of the RPPs for two-photon pumped emission. In addition, we investigated the pump-wavelength-dependent two-photon pumped ASE behaviors of the RPPs flakes, which suggest that the near-infrared laser in a wide wavelength range can be converted into visible light by the frequency upconversion process in RPPs. This work has opened new avenues for realizing nonlinearly pumped ASE based on the RPPs, which shows great potential for the applications in wavelength-tunable frequency upconversion.
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1. Introduction
Owing to outstanding optoelectronic properties such as large absorption coefficient, high carrier mobility and tunable bandgaps, organic-inorganic hybrid halide perovskites have been widely utilized for solar cells, light-emitting devices (LEDs) and photodetectors [1–4]. More recently, the two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) with a van der Waals layered structure have attracted increasing attentions. Compared with their 3D counterparts, the 2D RPPs possess stronger quantum and dielectric confinement, thus showing a large exciton binding energy and enhanced optical properties [5,6]. Moreover, the RPPs can be conveniently controllable from pure 2D to quasi-2D structures by adjusting the thickness of the quantum wells, which exhibits an outstanding tunable property among the hybrid perovskites [7,8]. In addition, 2D RPPs also experience remarkable environmental stability under the protection of the dielectric organic layers, showing great promise for overcoming the challenges of device instability [9,10].
As an excellent optical gain material, 2D RPPs have been demonstrated to be good candidates for realizing lasing emissions [11]. For examples, C. M. Raghavan et al. observed low-threshold lasing behaviors from homologous 2D hybrid RPP ((BA)2(MA)n−1PbnI3n+1) single crystals at room temperature [12]. H. Zhang et al. fabricated large-area microrings based on the (BA)2(MA)n−1PbnBr3n+1 films, and demonstrated a high-quality whispering-gallery-mode (WGM) laser array [13]. More recently, Y. Liang et al. achieved multicolor lasing from the mechanically exfoliated 2D homologous (BA)2(MA)n−1PbnI3n+1 flakes by engineering the inorganic layer thickness [14]. However, previous reports dominantly focused on the linearly (one-photon) pumped lasing property [11,15–17]. The nonlinearly pumped regimes, usually with two/multi-photon pumping, possess a larger penetration depth and a small damage effect. Therefore, the nonlinear pumped light emissions are of great potentials for the applications in frequency upconversion, high-resolution microscopy and nonlinear detection [18–22]. Though 2D RPPs have been demonstrated to exhibit strong nonlinear optical responses such as second harmonic generation (SHG), third harmonic generation (THG) and two/multi-photon absorption, the nonlinear pumped light emission especially stimulated emission has still remained unexplored in 2D RPPs [23–26].
In this work, we report the two-photon pumped amplified spontaneous emission (ASE) in the 2D RPPs (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) at room temperature. High-quality (BA)2(MA)n-1PbnI3n+1 flakes were prepared by mechanical exfoliation from the fabricated crystals. The (BA)2(MA)n-1PbnI3n+1 flakes display two-photon pumped emission from 527 nm to 680 nm, tuned by thickness engineering of the inorganic layer. Based on that, two-photon pumped ASE behaviors were demonstrated for the (BA)2(MA)n-1PbnI3n+1 (n = 3, 4, 5) flakes. The ASE threshold decreases from ∼0.61 mJ/cm2 to ∼0.19 mJ/cm2 as n increases from 3 to 5, which are lower than that of the other semiconductor nanostructures. Finally, we investigated the pump-wavelength-dependent two-photon pumped ASE behaviors of the RPPs flakes, which suggest that the near-infrared laser in a wide wavelength range can be converted into visible light by the frequency upconversion process in RPPs.
2. Results and discussion
The quasi-2D Ruddlesden-Popper perovskites in this study can be generally descript by a formula of (BA)2(MA)n-1PbnI3n+1, with BA being the long organic butylammonium cation, and MA being the methylammonium cation. n represents the number of inorganic lead iodide layers. As schemed in Fig. 1(a), the quasi-2D layered structure of the RPPs is formed with n continuous lead iodide layers being sandwiched between two organic layers. In particular, for n = 1, the perovskite has a pure 2D layered structure, in which one inorganic layer is sandwiched between two organic layers. While for n = ∞, the perovskite has a pure 3D structure, corresponding to the 3D counterpart of MAPbI3. As n increases from 1 to ∞, the quantum size confinement effect decreases gradually, which results in a reduction of the bandgap of the perovskites [7,14]. Therefore, the synthetic control of the inorganic layer n would be beneficial for tuning the emission color. Single crystals of (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, and 5) were synthesized using a super-saturation-controlled crystallization growth, as shown in Fig. 1(b). The color of the (BA)2(MA)n-1PbnI3n+1 crystals varies from orange to chocolate and finally to dark as n increases from 1 to 5, due to the gradual decrease of the bandgap [7,26]. After that, monocrystalline flakes were mechanically exfoliated from the single crystals by using Scotch-tape method. Figure 1(c) shows the optical microscopy images of the exfoliated 2D homologous (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes. The lateral size of the 2D RPPs flakes is in the order of 20 µm. The van der Waals layered structure of the exfoliated 2D RPPs flakes can be clearly characterized by the scanning electron microscopy (SEM) (Figure S1, Supplement 1).
Fig. 1. (a) Schematic of the RP perovskite structures. (b) Photographs of as-grown (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) single crystals. (c) Optical microscopy images of exfoliated (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes. (d) XRD patterns, (e) Plots of (αhν)2 versus the photon energy (hν), and (f) One-photon pumped PL spectra of exfoliated (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes.
Figure 1(d) shows the X-ray diffraction (XRD) patterns of the global exfoliated (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes on the glass substrate. For n = 1, the diffraction peaks at 6.39°, 12.81°, 19.27°, 25.80°, and 32.41° correspond to the (00 l) diffraction series, indicating a preferential growth direction of [100]. In contrast, when the 3D counterpart is introduced between the 2D layers (n = 2, 3, 4, and 5), a dominant (0k0) diffraction series constitutes the XRD patterns, suggesting a preferential growth direction along [101] of the quasi-2D RPPs [14]. The diffraction peaks marked with orange and blue stars for n = 5 are consistent with the (040) and (060) peaks in n = 4. The slight impurity is originated from the competition of the BA and MA cations, and indicates a mixed phase at the high n value, which is consistent with the previous reports [27]. To determine the band-edge absorption behavior of the 2D RPPs, the absorption spectra of the samples were measured. The bandgaps can be obtained by Tauc’s plots, as shown in Fig. 1(e), which are determined to be 2.4 eV, 2.16 eV, 2.03 eV, 1.92 eV and 1.87 eV respectively for the (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes [14]. Furthermore, the one-photon pumped PL of the samples was measured by exciting with a 473 nm continuous laser (Fig. 1(f)). As expected, the PL emission peaks can be tuned from ∼527 nm to ∼676 nm as the inorganic layer thickness increases.
To investigate the nonlinearly pumped light emission of the RPPs flakes, a confocal microscopy system is utilized for optical measurement (Fig. 2(a)). A femtosecond-pulsed Ti:sapphire regenerative amplifier (Coherent, Astrella-USP-1 K) combined with an optical parametric amplifier (OPA) is used as the pumping source. The pumping laser beam was focused onto the sample with a lens (focal length 10 cm). The focal spot is estimated to be 200 µm, which is beneficial for covering the whole RPPs flakes. The transmitted signal was collected by a 40× microscopic objective (NA = 0.65), and introduced into a spectrometer (HRS-300-MS, Princeton Instruments) for spectrum measurements or to a CCD camera for microscopic imaging. A short-pass filter was positioned before the detectors to remove the pumping beam.
Fig. 2. (a) Schematic of the set-up for optical measurements. (b) Dark-field microscopic images of the emission from the (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes. (c) Normalized TPL spectra of (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, 5) flakes. (d) Plots of integrated PL intensity versus the pump fluence.
At first, a fs-laser beam with central wavelength at 800 nm was used to pump the RPPs. As the bandgaps of the RPPs are larger than one time (∼1.55 eV) but smaller than two times (∼3.1 eV) of the pumping photon energy, the RPPs will experience a two-photon pumped process. Figure 2(b) presents the dark-field images of the two-photon pumped photoluminescence (TPL) from the (BA)2(MA)n-1PbnI3n+1 (n = 1, 2, 3, 4, and 5) flakes. The TPL color varies from green to orange, and finally to red as n increases, implying a tunable emission tuned by the inorganic layer thickness. The corresponding TPL spectra were measured, as shown in Fig. 2(c). The single peaks of the spectra demonstrate a pure compound of the synthesized RPPs crystals. Moreover, the emission wavelength can be tuned from 528 nm to 680 nm as n increases from 1 to 5, presenting a slight red shift compared with the one-photon pumped results, which can be attributed to the larger penetration depth and self-absorption of the PL signal at nonlinear pumped regime [25,28]. Fig. 2(d) shows the double-logarithmic representations of the pump-fluence-dependent TPL of the (BA)2(MA)n-1PbnI3n+1 flakes. For all the samples, the experimental results can be well fitted with a power-law model, y = Axm. For n = 1, as a pure 2D compound, the TPL intensity is in quadratic dependence on the pump fluence, which suggests a typical two-photon pumped exciton recombination process. However, as n increases, the exciton binding energy of the RPPs decreases with the inorganic layers. As a result, the proportion of free-carrier recombination will increase as the RPP varies from 2D to quasi-2D structures, leading to a coexist of the exciton recombination (monomolecular recombination) and free carrier recombination (bimolecular recombination) in the TPL process [29–33]. Therefore, the pump-fluence-dependent TPL traces exhibit a superquadratic characterizations for the quasi-2D RPPs.
For two-photon pumped ASE measurement, the pump fluence was gradually increased beyond the threshold for stimulated emission. Figure 3(a) shows a microscopy image of the RPP flake, and Fig. 3(b) displays the corresponding dark-field image above the ASE threshold. Bright ASE emission from the boundaries indicates a strong optical confinement effect inside the flake cavity. Figure 3(c) plots the TPL spectra of the (BA)2(MA)2Pb3I10 (n = 3) flake versus the pump fluence. A broad TPL spectra with a dominant peak at ∼640 nm can be observed at low pump fluences. The corresponding full width at half maximum (FWHM) of the spectra is ∼36 nm, which is related to the spontaneous emission process. As the pump fluence gradually increases, a narrow peak located at about ∼652 nm appears, demonstrating the occurrence of ASE. Figure 3(d) plots the evolution of the PL intensity and FWHM as a function of the pump fluence. The “kink” characteristic of the PL intensity represents a typical signal for the transition from spontaneous emission to ASE. Simultaneously, the FWHM of the spectra exhibits a sharp decrease during the evolution. In particular, the transition happens at ∼0.61 mJ/cm2, which is determined as the ASE threshold.
Fig. 3. (a) Microscopic image of a RPP flake. (b) Dark-field image of the RPP flake above the ASE threshold. (c, e, j) Plots of TPL spectra versus the pump fluence, for n = 3, 4, 5 respectively. (d, f, k) Plots of TPL intensities and corresponding FWHM versus the pump fluence, for n = 3, 4, 5 respectively.
We also measured the evolution of the TPL spectra and intensity versus the pump fluence for n = 4 and n = 5, as shown in Figs. 3(e)–3(k). Similar two-photon pumped ASE processes can be observed for these RPPs. Interestingly, the ASE threshold decreases to 0.36 mJ/cm2 for n = 4, and further decreases to 0.35 mJ/cm2 for n = 5. The decreased ASE threshold is originated from the different two-photon absorption of the RPPs. The two-photon absorption coefficient of the RPPs can be calculated using the second-order perturbation approach [26,34]
Table 1. Summary of dominant parameters for two-photon pumped ASE/lasing of different semiconductor nanostructures
Finally, we investigated the pump-wavelength dependent ASE behaviors of the RPPs flakes. Figure 4(a) shows the TPL spectra evolution of the (BA)2(MA)4Pb5I16 (n = 5) flake as functions of the pump fluence, with pump wavelengths locating at 1140 nm, 1200 nm, and 1300 nm, respectively. As bandgaps of the RPPs are larger than one time but smaller than two times of the pumping photon energies, all these processes can be attributed to the two-photon pumped mechanism. One can observe that for each pump wavelength, the spectra experience the typical transition from the spontaneous emission to the ASE processes. Moreover, the corresponding PL intensity and FWHM of the spectra are plotted as a function of the pump fluence, as shown in Fig. 4(b). The ASE thresholds are determined to be 0.28 mJ/cm2, 0.34 mJ/cm2, and 0.54 mJ/cm2 for the pumps at 1140 nm, 1200 nm, and 1300 nm, respectively. According to Eq. (1), the two-photon absorption coefficients of the (BA)2(MA)4Pb5I16 flake can be calculated to be 53.2 cm/GW, 50.6 cm/GW, 40.7 cm/GW for 1140 nm, 1200 nm and 1300 nm respectively. Therefore, the increase of the ASE threshold with pumping wavelength can be attributed to the decreased two-photon absorption. We also noticed that as the pump wavelength further increases and beyond the third-photon wavelength (1360 nm), the TPL signal gradually decrease and almost disappears in the end (Fig. 4(c)). Simultaneously, the THG signal can be observed accompanied in these processes. In particular, as the pump wavelength increases close to 1360 nm, the TPL intensity gradually decreases while that of the THG signal gradually strengthens. Since both the TPL and THG are originated from the third-order nonlinear optical processes, the opposite trend of the wavelength dependent TPL and THG intensity indicates that there exists a competition between these two processes, which will also affect the ASE efficiency of the 2D RPPs at different pumping wavelengths. Similar results are obtained for the RPPs with n = 3 and n = 4 (Fig. S4 and S5, Supplement 1). Though the two-photon pumped ASE threshold increases with the pump wavelength, our results show that the near-infrared pumped laser in a wide wavelength range can be converted to the visible light by the two-photon pumped process, which demonstrates a great potential of the RPPs for the frequency upconversion at room temperature.
Fig. 4. (a) TPL spectra of the RPP flakes pumped at 1140 nm, 1200 nm, and 1300 nm, respectively. (b) Plots of the PL intensities and FWHM as a function of the pump fluence, with pumping at 1140 nm, 1200 nm, and 1300 nm, respectively. (c) Emission spectra of the TPL and THG under various pumped wavelengths.
3. Conclusion
In summary, we demonstrate the two-photon pumped ASE in the (BA)2(MA)n-1PbnI3n+1 (n = 3, 4, 5) flakes. The (BA)2(MA)n-1PbnI3n+1 flakes display a tunable two-photon pumped emission from 527 nm to 680 nm, as n increases from 1 to 5. In particular, two-photon pumped ASE behaviors were demonstrated for n = 3, 4, 5. The ASE threshold decreases from ∼0.61 mJ/cm2 to ∼0.19 mJ/cm2 as n increases from 3 to 5, which are lower than that of the other semiconductor nanostructures. The pump-wavelength-dependent investigations show that the near-infrared laser in a wide wavelength range can be converted into visible light by the frequency upconversion process in RPPs. The results also inspire that, to integrate with high-quality microcavities, high-performance two-photon pumped nanolasing will be realized at room temperature based on the RPPs. Our work has opened new avenues for realizing nonlinearly pumped ASE based on the RPPs, which shows great potential for the applications in wavelength-tunable frequency upconversion.
Funding
Basic and Applied Basic Research Major Program of Guangdong Province (2019B030302003); National Natural Science Foundation of China (11804109, 12021004); Campus Science Foundation Research Project of Wuhan Institute of Technology (K2021078).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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