Nonlinear optical properties of CsPbClxBr3-x nanocrystals embedded glass

Chenjing Quan; Xiao Xing; Sihao Huang; Mengfeifei Jin; Tongchao Shi; Zeyu Zhang; Weidong Xiang; Zhanshan Wang; Yuxin Leng

doi:10.1364/PRJ.427155

Abstract

All-inorganic perovskite has attracted significant attention due to its excellent nonlinear optical characteristics. Stable and low-toxic perovskite materials have great application prospects in optoelectronic devices. Here, we study the nonlinear optical properties of

{CsPbCl}_{x} {Br}_{3 - x}

(

x = 1

, 1.5, 2) nanocrystals (NCs) glass by open-aperture Z-scan. It is found that the two- (2PA) and three-photon absorption (3PA) intensity can be adjusted by the treatment temperature and the ratio of halide anions. The perovskite NCs glass treated at a high temperature has better crystallinity, resulting in stronger nonlinear absorption performance. In addition, the value of the 2PA parameter of

{CsPbCl}_{1.5} {Br}_{1.5}

NCs glasses decreases when the incident pump intensity increases, which is ascribed to the saturation of 2PA and population inversion. Finally, the research results show that the 2PA coefficient (

0.127 cm {GW}^{- 1}

) and 3PA coefficient (

1.21 \times 10^{- 5} {cm}^{3} {GW}^{- 2}

) of

{CsPbCl}_{1} {Br}_{2}

NCs glass with high Br anion content are larger than those of

{CsPbCl}_{2} {Br}_{1}

and

{CsPbCl}_{1.5} {Br}_{1.5}

NCs glasses. This is mainly due to the greater influence of Br anions on the symmetry of the perovskite structure, which leads to the redistribution of delocalized electrons. The revealed adjustable nonlinear optical properties of perovskite NCs glass are essential for developing stable and high-performance nonlinear optical devices.

1. INTRODUCTION

All-inorganic perovskite has received widespread attention recently, owing to its tunable light-emitting bandgap [1,2], large exciton binding energy [3], and other excellent photoelectric properties [4,5]. Hence, perovskite materials are mostly used in light-emitting diodes (LEDs) [6,7], solar cells [8], photodetectors [9,10], lasers [4,11], and other devices. Halide perovskite nanocrystals have become one of the most promising optoelectronic materials due to their low-cost and easy synthesis [1]. Compared with bulk and layered materials, the specific surface area of ${CsPbCl}_{x} {Br}_{3 - x}$ nanocrystals (NCs) is greatly increased, thereby enhancing the NC optical performance [1,2,12]. Due to the strong multiphoton absorption (MPA) characteristics of halide perovskite NCs, they are very promising as a material for the development of multiphoton pump lasers [13]. The spherical NCs not only overcome the difficulty of large area growth of thin films, but also can combine with a variety of substrates or solutions for incorporation into optoelectronic devices [14].

Owing to the poor stability of bare perovskite and toxicity of lead halide perovskite, the application of perovskite materials in optoelectronic devices is greatly restricted [15,16]. So far, diverse approaches to improve the stability of perovskite materials have been reported, such as bonding of the organic ligands [17], establishing core/shell nanostructure [18], Mn-doping [19], silica coating [20,21], and infiltrating ${CsPbX}_{3}$ NCs into mesoporous matrices [22]. However, combining the thermal, chemical, and mechanical stability of the glass has been proved an effective method to improve the stability of perovskite NCs [23 –25]. Hu et al. reported the well-designed arrangement of ${CsPbBr}_{3}$ NCs glasses with reduced self-absorption emission and enhanced the quantum efficiency of solar cells [26]. Ye et al. investigated the versatile precipitation of ${CsPbX}_{3}$ NCs glasses, in which photoluminescence (PL) covering the whole visible range with high efficiency could be achieved [24]. Meanwhile, some researches indicated that the relative PL intensity of ${CsPbBr}_{3}$ quantum dots glass is still $\sim 85 % \sim 90 %$ after being immersed in water for 120 h or exposed to UV light for 100 h, and even about 60% of PL intensity still remained for storage up to 45 days [27,28]. All these previous studies reveal that embedding ${CsPbX}_{3}$ quantum dots or NCs into glass has a great potential in improving its stability and fluorescence performance [26 –28].

However, the nonlinear optical aspect of perovskite glass, especially the study of MPA, which is important to the application of these optoelectronic devices, has been rarely investigated [13,29,30]. Many studies have been reported on the nonlinear optical properties of pure perovskite [31]. For example, a two-photon absorption (2PA) cross section of ${CsPbBr}_{3}$ NCs in toluene as high as $10^{6} GW$ has been reported [32]. Chen et al. demonstrated that the 2PA cross section of ${CsPbBr}_{3}$ NCs depends on the particle size [33]. Furthermore, due to the symmetry breaking of the perovskite octahedron structure, Li et al. confirmed that all-inorganic perovskites with different proportions of halogen atoms show greater MPA intensity than ${CsPbCl}_{3}$ and specially designed organic molecules [34,35]. Not only that, Chen et al. reported that the five-photon absorption cross section of Type-I core-shell halide perovskite NCs is 9 orders of magnitude higher than that of specially designed organic molecules [30]. However, the nonlinear optical aspect of perovskite glass, which is highly desired for applications in low-threshold lasing [36], optical data storage [37], photodetectors [38], and other nonlinear optical photoelectric devices [32], has been rarely investigated.

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In the present work, the three-order nonlinear optical characteristics of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses have been investigated by open-aperture (OA) Z-scan measurements using femtosecond laser pulses. We observe the 2PA and three-photon absorption (3PA) phenomena of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses at wavelengths of 800 and 1300 nm. The magnitude of the 2PA coefficient ( $β$ ) of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses is about $10^{- 1} − 10^{- 2} cm {GW}^{- 1}$ , and the magnitude of the 3PA coefficient ( $γ$ ) is $10^{- 5} − 10^{- 6} {cm}^{3} {GW}^{- 2}$ . The observed MPA behavior of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses could play a great role in the development of perovskite-based optoelectronic devices.

2. EXPERIMENT SETUP

In the OA Z-scan measurement, as shown as Fig. 1, the perovskite NCs glasses with thickness L are moved along the Gaussian light propagation direction, and the laser beam transmittance after the sample is measured. For 2PA measurement, we used a Ti:sapphire femtosecond laser (Spectra-Physics) operating at a central wavelength of 800 nm with a repetition frequency of 1 kHz and a pulse width of 35 fs. For 3PA measurements, the laser source consisted of an optical parametric amplifier (TOPAS-Prime), delivering a wavelength of 1300 nm with a pulse width of 200 fs. The repetition frequency of laser was 1 kHz. In order to amplify weak signals, a chopper (Thorlabs MC2000B) is inserted into the optical path. The focal length of the front lens of the sample is 10 cm, and the spot radius at the focal point is about 30 μm. The transmitted intensity of each pulse after passing through the sample is measured by a Si-biased detector (Thorlabs DET10A2) using the lock-in amplifier (Signal Recovery Model 7270) technique.

Experimental setup for the Z-scan technique.

Figure 1.Experimental setup for the Z-scan technique.

3. RESULTS AND DISCUSSION

In this work, ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs are successfully embedded inside a glass sheet ( $radius = 10 mm$ , $thickness = 0.56 mm$ ) matrix via the melt-quenching and in situ crystallization method. The ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs are spherical structures with a diameter of $\sim 35 - 45 nm$ [see the Appendix A, Fig. 6(a)] [39]. Figure 2(a) shows the X-ray diffraction (XRD) patterns of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass under different treatment temperatures. The diffraction peaks of perovskite glass are at 15.5°, 22°, 31°, 38.5°, and 44.6°, corresponding to (100), (110), (200), (211), and (220) phase, respectively [40,41]. Referring to the previous research results, these narrow diffraction peaks demonstrate that the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs have good crystallization [25]. Figure 2(b) displays the PL emission spectra of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass. The PL emission peak of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass displays a slight redshift with the treatment temperature from 470°C to 530°C. The reason for the slight redshift of PL emission peak in the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass is the increase of the crystal grains size, which is caused by the increase of the heat treatment temperature [41]. However, the surface defects increase due to the continuous increase in temperature, which affects the fluorescence quantum yield and causes the PL emission intensity of the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass to decrease [41].

Figure 2.(a) XRD patterns and (b) PL emission spectra of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses under different treatment temperature excited by femtosecond pulses at 365 nm.

Figures 3(a) and 3(b) show the OA Z-scan curves of the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass ( $bandgap \sim 2.58 eV$ ) at the different treatment temperature excited by 800 nm ( $\sim 1.55 eV$ ) with pump intensity of $25.5 GW / {cm}^{2}$ and by 1300 nm ( $\sim 0.95 eV$ ) with pump intensity of $217 GW / {cm}^{2}$ , respectively (see the Appendix A, Fig. 8). As shown in Figs. 3(a) and 3(b), the Z-scan curves are all valley shapes. When the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass is close to the focus, the normalized transmission of the incident laser decreases. As shown in Figs. 3(a) and 3(b), the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass with a heat treatment temperature of 530°C has the strongest nonlinear response.

Figure 3.OA Z-scan results of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses with different treatment temperatures. (a) Under pump intensity $25.5 GW / {cm}^{2}$ at a wavelength of 800 nm and (b) under pump intensity $217 GW / {cm}^{2}$ at a wavelength of 1300 nm. (The balls are the experimental data, and the solid lines are fitting curves.) The fitting results of (c) $β$ and (d) $γ$ of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass with different treatment temperatures. (Inset, the schematic of a two-level model.)

In theory, the measured normalized transmission ( $T$ ) for OA Z-scan results is given by the expression [42,43] $T_{OA (n PA)} = \frac{1}{{1 + (n - 1) α_{NL} L_{eff} {I_{0} / [1 + {(z / z_{0})}^{2}]}^{n - 1}}^{1 / n - 1}} (n = 1, 2, 3),$ (1)where $L_{eff}$ is the effective length of the sample. $α_{NL}$ is the nonlinear optical coefficient. $z$ is the position of the sample in the light path, $z_{0} = π ω_{0}^{2} / λ$ is the Rayleigh range of the Gaussian beam, and $ω_{0}$ is the beam waist at the focal point ( $z = 0$ ).

Among them, the 2PA coefficient is represented by $β$ . The imaginary part of third-order nonlinear susceptibility [44,45], $Im χ^{(3)} = \frac{c^{2} ε_{0} n_{0}^{2} β}{ω},$ (2)where $c$ is the speed of light, $n_{0}$ is the linear refractive index, and $ε_{0}$ and $ω$ are the vacuum permittivity and angular frequency of the laser beam, respectively. The figures of merit (FOMs) are used to describe nonlinear absorption characteristics: $FOM = | Im χ^{(3)} / α_{0} |$ , where $α_{0}$ is the linear absorption coefficient. The 3PA coefficient is represented by $γ$ . The effective thicknesses of 2PA and 3PA are $L_{eff} = (1 - e^{- α_{0} L}) / α_{0}$ and $L_{eff}^{'} = (1 - e^{- 2 α_{0} L}) / 2 α_{0}$ , respectively [42,43].

Figures 3(c) and 3(d) display the 2PA and 3PA coefficients of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses at the different treatment temperatures obtained by fitting Eq. (1). The insets in Figs. 3(c) and 3(d) show the schematic diagram of the 2PA and 3PA processes, respectively. When ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass is excited by the femtosecond laser, electrons in the valence band need to absorb two (or three) photons at the same time to transition to the conduction band. By fitting with a Z-scan theory, 2PA coefficients of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass with different treatment temperatures were calculated: $β = 0.87 cm {GW}^{- 1}$ for 470°C treatment temperature, $β = 0.97 cm {GW}^{- 1}$ for 500°C treatment temperature, and $β = 1.23 cm {GW}^{- 1}$ for 530°C treatment temperature, respectively. Under the pump intensity of $25 GW / {cm}^{2}$ , the $Im χ^{(3)}$ for ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses are in the range of $(1.99 − 2.81) \times 10^{3} esu$ and the magnitudes of FOM are all at $10^{3} esu$ cm obtained by fitting Eq. (2) (see the Appendix A, Table 1). 3PA coefficients of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses with different treatment temperatures were calculated: $γ = 2 \times 10^{- 5} {cm}^{3} {GW}^{- 2}$ for 470°C, $γ = 2.17 \times 10^{- 5} {cm}^{3} {GW}^{- 2}$ for 500°C, and $γ = 2.86 \times 10^{- 5} {cm}^{3} {GW}^{- 2}$ for 530°C, respectively. As the processing temperature increases, the crystallinity is better. The larger the nanocrystal particles, the stronger the 2PA and 3PA performance of the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses [39].

Furthermore, the nonlinear properties of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses with different pump intensities are measured. The OA Z-scan curves of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass under 500°C treatment temperature with various incident pump intensities at the wavelength of 800 nm are shown in Fig. 4(a). The normalized transmittance decreases when either increasing the pump intensity or placing the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass closer to the focus point, $z = 0$ , while as the pump intensity increases, the normalized transmittance curve deepens. These results suggest the potential application of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass in optoelectronic devices, such as optical limiting devices [46]. Figure 4(b) summarizes the dependence of 2PA fitting results as a function of pump intensity. (Fitting results are summarized in Table 2). The $β$ is 0.096 cm/GW of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass at the pump intensity of $255 GW / {cm}^{2}$ and 0.089 cm/GW at the pump intensity of $332 GW / {cm}^{2}$ , respectively. The result of the 2PA coefficient we obtained is with the same order of magnitude as the ${CsPbBr}_{3}$ NCs [36] and ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 2) quantum dots [47]. The perovskite NCs encapsulated in perovskite glass are isolated from the external environment. This gives them better stability and greatly improves the service life of perovskite NCs. The $Im χ^{(3)}$ for ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glasses are in the range of $(2.03 - 2.88) \times 10^{2} esu$ and the FOM is in the range of $(2.3 - 3.2) \times 10^{2} esu cm$ . It is shown that as the pump intensity increased, the value of $β$ , $Im χ^{(3)}$ , and FOM decreased. The same phenomenon has been observed in other materials, such as PbS/glue nanocomposite [48] and few-layer ${WS}_{2}$ films [49]. As the incident light energy increases, a large number of carriers gather in the excited state, resulting in population inversion [50]. Hence, the electrons in the ground state cannot further absorb the photon and transition to the excited state [51]. The $β$ of ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs decreases as the pump intensity increases.

Figure 4.(a) OA Z-scan curves and (b) corresponding fitting results of $β$ (black ball), $Im χ^{(3)}$ (pink), and FOM (blue) of the ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass at the wavelength of 800 nm with different incident pump intensity.

To further explore the influence of different doped halogen anion ratios on the MPA and consider the influence of treatment temperature on the PL performance of all-inorganic perovskite glasses, we choose ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses with a treatment temperature of 500°C to explore their nonlinear response [52]. Figures 5(a) and 5(b) display the OA Z-scan response of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses under femtosecond laser of 800 nm and 1300 nm, respectively.

Figure 5.OA Z-scan results of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses. (a) Under pump intensity $178 GW / {cm}^{2}$ at a wavelength of 800 nm and (b) under pump intensity $535 GW / {cm}^{2}$ at a wavelength of 1300 nm. (The balls are the experimental data, and the solid lines are fitting curves.) (c) PL emission spectra of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses for 500°C treatment temperature excited by femtosecond pulses at 365 nm; (d) fitting the results of 2PA coefficient ( $β$ , purple bar) and 3PA coefficient ( $γ$ , red bar) obtained in (a) and (b), respectively.

Figure 5(a) shows the OA Z-scan curves of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses under pump intensity $178 GW / {cm}^{2}$ at a wavelength of 800 nm. Comparing the normalized transmittance curves of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses with different ${Cl}^{-}$ and ${Br}^{-}$ ion ratios, it is found that the ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses with higher ${Br}^{-}$ ions doping ratio have a greater nonlinear response in the process of 2PA and 3PA. The bandgap width of ${CsPbCl}_{1} {Br}_{2}$ NCs glasses is 2.46 eV, slightly smaller than that of ${CsPbCl}_{1.5} {Br}_{1.5}$ (2.58 eV) and ${CsPbCl}_{2} {Br}_{1}$ (2.7 eV) (see the Appendix A, Fig. 8). Due to the increase of ${Br}^{-}$ ion content, the bandgap is narrowed, thereby promoting the carrier transition rate [34,53]. ${CsPbCl}_{1} {Br}_{2}$ NCs glasses exhibit a large MPA phenomenon. Correspondingly, the PL emission peak of ${CsPbCl}_{2} {Br}_{1}$ , ${CsPbCl}_{1.5} {Br}_{1.5}$ , and ${CsPbCl}_{1} {Br}_{2}$ NCs glasses with emission wavelength peaks at 454, 470, and 490 nm is shown in Fig. 5(c), respectively. The PL peak shifts to the lower energy direction as the proportion of ${Br}^{-}$ ions increases. It is shown that the luminescence can be effectively tuned by introducing the ${Cl}^{-}$ and ${Br}^{-}$ ion ratios. The width of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses at half-height (FWHM) of the PL emission is less than 30 nm (see the Appendix A, Fig. 7).

By fitting the Z-scan data in Figs. 5(a) and 5(b), the $β$ of ${CsPbCl}_{1} {Br}_{2}$ NCs glass is $\sim 0.087 cm {GW}^{- 1}$ , $0.1 cm {GW}^{- 1}$ for ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass, and $0.127 cm {GW}^{- 1}$ for ${CsPbCl}_{2} {Br}_{1}$ NCs glass. The 2PA coefficient of ${CsPbCl}_{1} {Br}_{2}$ NCs glass is 1 order of magnitude higher than that of ${CsPbCl}_{1} {Br}_{2}$ quantum dots [47]. Combined with Eq. (2), the $Im χ^{(3)}$ for ${CsPbCl}_{x} {Br}_{3 - x}$ NCs glasses is in a range of $(1.99 − 2.9) \times 10^{2} esu$ , which is larger than that of ${CsPbBr}_{3}$ NC (see the Appendix A). The FOM value used to describe the nonlinear absorption characteristics is in the range of $(2.2 − 2.53) \times 10^{2} esu cm$ (see the Appendix A, Table 3). Meanwhile, we obtained the $γ$ : $0.54 \times 10^{- 5} {cm}^{3} {GW}^{- 2}$ is for ${CsPbCl}_{2} {Br}_{1}$ NCs glass, $0.82 \times 10^{- 5} {cm}^{3} {GW}^{- 2}$ is for ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass, and $1.21 \times 10^{- 5} {cm}^{3} {GW}^{- 2}$ is for ${CsPbCl}_{1} {Br}_{2}$ NCs glass. The 3PA coefficient of ${CsPbCl}_{1} {Br}_{2}$ NCs glass is over 1 order of magnitude larger than that of the ${CsPbCl}_{1.5} {Br}_{1.5}$ and ${CsPbCl}_{2} {Br}_{1}$ NCs glasses. This is mainly because the ${CsPbCl}_{1} {Br}_{2}$ NCs glass has a narrower bandgap and higher structural destabilization that will lead to the easier carrier transition and delocalized electrons redistribution compared with ${CsPbCl}_{1.5} {Br}_{1.5}$ and ${CsPbCl}_{2} {Br}_{1}$ NCs glasses [24]. Due to the narrower bandgap and structural destabilization of the perovskite, the electron cloud is distorted, which promotes the transition of electrons from the ground state to the excited state [34]. Therefore, the carrier transition rate is further increased.

4. CONCLUSION

In summary, we measure the 2PA and 3PA properties of ${CsPbCl}_{x} {Br}_{3 - x}$ ( $x = 1$ , 1.5, 2) NCs glasses using the OA Z-scan method. The ${CsPbCl}_{1.5} {Br}_{1.5}$ NCs glass under 530°C treatment temperature exhibited the strongest 2PA and 3PA coefficients, which is mainly due to the better crystallization properties of perovskite NCs at higher temperatures. Furthermore, the dependence of incident pump intensity shows that the nonlinear coefficient decreases when the incident pump intensity increases. We also found that the larger the proportion of Br anions, the stronger the MPA performance. These results for ${CsPbCl}_{x} {Br}_{3 - x}$ NCs glass may guide designs for their potential use in applications.

Appendix A

The CsPbCl x Br 3- x (x = 1, 1.5, 2) nanocrystals are spherical structures with a diameter of \sim35-45 nm enclosed in a glass sheet, as shown in Fig. 6 (a).

Figure 6.(a) Transmission electron microscopy (TEM) images of CsPbCl_xBr_3-x (x = 1, 1.5, 2) NCs and (b) high-resolution TEM images of CsPbCl_1.5Br_1.5 NCs.

Figure 7.(a) Amplified spontaneous emission (ASE) measurement on CsPbCl_1.5Br_1.5 NCs glass under an 800 nm pulsed laser at room temperature and (b) corresponding full-width at half-maxima (FWHM) and output as a function of incident pump intensity.

Figure 8.( $α h υ$ )²- $h υ$ plot of CsPbCl_xBr_3-x ( $x = 1, 2, 3$ ) NCs glass.

Figure 9.Open-aperture Z-scan curves of the (a) CsPbCl₁Br₂ and (b) CsPbCl₂Br₁ NCs glasses at the wavelength of 800 nm with different incident pump intensity.

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