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Abstract
The nonlinear optical absorption effects of perovskite quantum dots (QDs) in nanosecond and femtosecond regimes are studied. By tuning the absorption edge via adjusting the halogen elements in the perovskite QDs, the materials could exhibit excellent saturable absorption (SA) effects, both for 400 nm femtosecond laser-pulses and 532 nm nanosecond laser-pulses. Femtosecond transient absorption spectroscopy measurements reveal that photoinduced bleaching of the valance band and filling of the conduction band was responsible for the strong SA effect of the QDs. The ultra-low SA intensity of the QDs indicates that the materials have important application potential in mode-locking and Q-switched lasers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optical materials with high nonlinearity have aroused great concern, owing to their important applications in optical switch, mode locking, optical limiting, and lasing. In the past few decades, different kinds of nonlinear materials such as semiconductors, organic molecules, inorganic glasses and crystals, and two-dimensional nanomaterials (such as graphene and transition metal sulfides nanosheets) have been widely studied [1–3]. Developing new materials with strong nonlinear optical effect and ultrafast response time has always been the focus for researchers.
Cesium lead halide perovskite (CsPbX3, X: I, Br, and Cl) quantum dots (QDs) have garnered much interest in the field of optoelectronics due to their great optical and photoelectric characteristics [4,5]. Benefiting from the unique properties of tunable emission bandgap and bandwidth, and excellent photoluminescence (PL) quantum yield [5–7], perovskite QDs have been widely used in solar cells, light-emitting diode, photodetector and laser [8–13]. Recently, nonlinear optical properties of perovskite QDs, such as nonlinear absorption effect, have been noticed by researchers [14,15]. For example, electrons in the valence band (VB) could be readily stimulated into the conductive band (CB) when the QDs are irradiated by ultrashort laser pulses, leading to the bleaching of the VB and weakening the absorption of the incident pulses. This phenomena, namely saturable absorption (SA), has been widely used in Q-switched or mode-locking lasers for generating ultrashort laser pulses [5,15]. Although some previous studies have reported on the nonlinear optical response of perovskite QDs, deeper study of the tunable nonlinear absorption for different wavelength and pulse durations are still lacked. Especially, it is necessary to further study the carrier dynamics in the nonlinear response, which is essential for lasers with the designing and the preparation of materials with high optical nonlinearity and ultrafast response time [16].
In the research, we detect the nonlinear absorption effect of perovskite QDs using 532 nm nanosecond laser and 400 nm femtosecond laser. By adjusting the halogen elements in the perovskite QDs, the absorption edge of the materials are changed, and exhibit excellent saturable absorption (SA) effect for laser pulses with different wavelength, respectively. Femtosecond transient absorption spectroscopy (fs-TA) measurements are performed to study the SA mechanisms of the materials. The results reveal that, photoinduced bleaching of the VB was responsible for the strong SA effect of the QDs.
2. Experiments
CsPb(Br/I)3 and CsPb(Br/Cl)3 nanocubes were prepared following the procedure reported by Pan et al. [17]. The solid precipitate was finally dispersed in hexane for further characterization. Transmission electron microscopy (TEM, JEM-2100Plus) was used to characterize the morphology of the materials. PL and absorption spectra of the QDs were conducted applying the FLS920 (Edinburgh) spectrophotometer and UV-2600. X-ray diffraction (XRD) spectrum was conducted via X-ray diffractometer (Bruker D8 ADVANCE).
To measure the nonlinear absorption effect of the QDs, conventional laser Z-scan apparatus was used. Two laser sources were used in the Z-scan experiments respectively: A Q-switched Nd3+: YAG laser operating at the second harmonic of 532 nm with pulse width of 10 ns and a 10 Hz repetition rate; A mode-locked Ti: sapphire amplifier program operating at second harmonic of 400 nm. (Vitesse, Coherent, 800 nm center wavelength, 100 fs pulse width, 1000 Hz repetition rate). The laser beam was focused into the sample by a convex lens with a focal length of 20 cm. The nonlinear transmittance of the materials was recorded simultaneously when the distance between the sample and the focus of the laser beam was changed by a translation stage. In the Z-scan measurements, the incident laser pulse power density was kept lower than the damage threshold of the samples.
To understand the carrier dynamics in the nonlinear response of the material, fs-TA measurements were performed using a home-built setup. The output of the femtosecond laser working at 800 nm was divided into two parts, the powerful one of which was frequency-doubled to produce a 400 nm pump beam, and the other one was focused on a sapphire plate to produce a wideband supercontinuum probe beam. The transmitted probe light was recorded by a spectrometer. By adjusting the delay time between the pump and probing beams, the transient absorption change of the materials i.e. the TA spectra dependence on the delay time were obtained.
3. Results and discussion
At first, we measured the chemical and morphological structure of the perovskite QDs. Figure 1(a) presents the XRD patterns of CsPb(Br/I)3 QDs. Compared with the standard PDF card (shown by the black curve in the Fig. 1(a)), four obvious peaks at 21.54°, 30.64°, 37.76° and 43.88° can be assigned to the (110), (200), (211), (220) planes, respectively. The single peak at ∼30° indicates the cubic phase of CsPb(Br/I)3 QDs [18–20]. The sharp peaks indicate good crystallinity of the QDs. Figure 1(b) exhibits the TEM photograph of the QDs, indicating good monodispersity in non-polar solvent and a shape close to a cubic. The averaged edge length of CsPb(Br/I)3 QDs is concentrated at 11.6 nm [21]. The HRTEM images of QDs given in Fig. 1(c) show the highly crystalline nature with a lattice distance of 0.29nm, agreeing with the (2 0 0) planes [22].
Figure 2(a) presents the PL and the absorption spectra of CsPb(Br/I)3 QDs. In the experiments, the absorption and emission property of the CsPb(Br/I)3 QDs can be effectively adjusted by changing the mole ratio of bromine and iodine elements. To maintain the absorption edge of the material close to incident laser wavelength used in the Z-scan measurements, the absorption edge of the QDs was adjusted to around 532 nm, and the fluorescence emission peak was centered at 540 nm, as shown in Fig. 2(a). The nonlinear optical characteristics of the CsPb(Br/I)3 QDs were determined through open aperture (OA) Z-scan technique with 532 nm and 10 ns pulses as the laser source. Figure 2(b) presents the normalized nonlinear transmittances as functions of the Z-position in CsPb(Br/I)3 QDs with different incident pulse energy. It can be seen that the transmittances of the sample increase obviously when the incident pulse energy increases, exhibiting a powerful SA behavior.
Fig. 2. (a) PL and absorption spectra and (b) Z-scan results with different incident light energy of CsPb(Br/I)3 QDs
The Z-scan results in Fig. 2(b) can be fitted by the following nonlinear propagation formula, which including the effects of SA and reverse saturable absorption (RSA) [23,24].
Here, fs-TA measurements was conducted to clarify nonlinear optical response mechanism contour plot of the TA spectra of CsPb(Br/I)3 QDs, where the horizontal and vertical axis indicate the delay time and wavelength respectively, and the TA change was indicated by different color as given by the color bar. Figure 3(b) shows the TA spectra of CsPb(Br/I)3 QDs at different delay time with a pump energy of 0.5 mW. A negative absorption peak corresponding to the photobleaching (PB) effect around 532 nm was observed. Electrons in VB of CsPb(Br/I)3 QDs are readily stimulated to CB by light excitation, leading to the bleaching of VB, as well as the decrease of the light absorption near the band-edge. The observed PB effect can be used to well explain the Z-scan results given in Fig. 2. When an intense laser pulse incidents into the material, the QDs absorb the light in the pulse-front edge, and electrons are excited from the VB to CB. As the decay process of the excited electrons could take a long time, when the following parts of the pulse arrive at the material, absorption of the light could be suppressed, resulting in the SA effect.
Fig. 3. (a) 2D contour plot of TA spectrum. (b) Time dependent TA spectra at distinct delay times. (c) Dependence of the TA curves observed at 532 nm on the excitation effect.
Figure 3(c) shows the PB signal intensities as a function of the delay time with distinct pump energy. Table 1 summarizes the fitted lifetime of the time-resolved TA signal at different pump energy. Three different processes are visualized: two fast decay processes with the lifetime of about ∼40 ps and ∼250 ps, and a slow relaxation process of more than 5 ns. The average lifetime are estimated to be about 1.15 ns, 0.58 ns, 0.30 ns for pump power of 0.08 mW, 0.1 mW, 0.5 mW, respectively. As given in the previous report, these two fast process could be attributed to the nonradiative Auger recombination of double excitons and charged excitons, while the slower one could be due to the relaxation of single exciton [16]. The dependence of the TA decay curves on excitation energy indicates that, the ratio of fast components increases with increasing the pump energy, owing to the increment of light induced double excitons and charged excitons.
Table 1. Three-exponential fitting for TA spectra of the CsPb(Br/I)3 QDs with distinct pump energy
As the bandgap of the perovskite could be tuned by changing the halogen elements, perovskite QDs with SA effect for lasers with different wavelength can be easily designed. For this reason, we prepared CsPb(Br/Cl)3 QDs with an absorption edge near 400 nm, and tested their nonlinear response in femtosecond regime using Z-scan method with 400 nm femtosecond laser pulses. As shown in Fig. 4(a), the absorption edge of the QDs was adjusted to around 400 nm, and the fluorescence emission peak was centered at 425 nm. Figure 4(b) presents the normalized nonlinear transmittances in dependence on of the Z-position in CsPb(Br/Cl)3 QDs. It can be seen that similar with CsPb(Br/I)3 QDs, the transmittances of the sample raises significantly as the incident light intensity raises, exhibiting a powerful SA influence. By fitting the Z-scan curves, we can obtain the Is values of CsPb(Br/Cl)3 QDs samples which is 130 GW cm−2 for femtosecond laser pulses, and the FOM of the optical nonlinearity is estimated to be about 3×10−14 esu cm. By comparing the nonlinear optical behavior of the materials for nanosecond and femtosecond lasers, we can find that SA effect can be easily induced by femtosecond pulses with much lower energy than nanosecond laser. This could be explained by the lifetime of excited excitons. When excited by femtosecond pulses, the excited electrons stay in the excited state and have no time to relax back to the ground state in the ultra-short duration of pulses, and cannot absorb the incident light causing a strong SA effect. For nanosecond pulses, the electrons decayed back to the CB could cause the absorption of the back-edge of the pulse. Therefore, the materials show a weaker SA effect for nanosecond pulses than femtosecond pulses. Even so, as the femtosecond pulse duration is about 5 orders of magnitude shorter nanosecond laser, the pulse peak power and the calculated Is values is much larger than nanosecond laser pulses.
Fig. 4. (a) PL and absorption spectra and (b) Z-scan results with different incident light energy of CsPb(Br/Cl)3 QDs.
4. Conclusion
In this paper, we measured the nonlinear absorption effect in perovskite QDs using 532 nm nanosecond laser and 400 nm femtosecond laser pulses, and the absorption coefficients were estimated to be 0.2 GW/cm−2 and 130 GW/cm−2, respectively. Fs-TA measurements revealed that, when the QDs were irradiated by ultrashort laser pulses, electrons in the VB could be readily stimulated into the CB, leading to the bleaching of the VB and blocking the absorption of the incident laser pulses. The advantages of powerful nonlinear optical absorption, fast response, and tunable nonlinear absorption indicated that the perovskite QDs could be well candidate for nonlinear saturable absorbers.
Funding
National R&D Program of China (2017YFA0207400); National Natural Science Foundation of China (11674260, 61690221); the Key research and Development Program of Shaanxi province (2017ZDXM-GY-120).
Acknowledgments
This work was supported by the National R&D Program of China (2017YFA0207400); National Natural Science Foundation of China (11674260 and 61690221); the Key research and Development Program of Shaanxi province (2017ZDXM-GY-120).
Disclosures
The authors declare no conflicts of interest.
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