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Abstract
The third-order nonlinear optical properties of CH3NH3PbI3 perovskite film have been investigated by the Z-scan technique in the mid-infrared (mid-IR) region. We prepared the high-quality CH3NH3PbI3 films with the precursor solutions, and experimentally observed its broadband saturable absorption behavior and large Kerr nonlinearity. The CH3NH3PbI3 perovskite film exhibited a large nonlinear refractive index (~10−12 m2/W) in the mid-IR regime. Our study indicates the great potential of the organic-inorganic perovskite material in the mid-IR nonlinear optics, and might provide some new sights for Kerr nonlinearity based mid-IR nonlinear optical application.
© 2017 Optical Society of America
1. Introduction
Nonlinear optical materials with broadband optical response, especially reaching the mid-infrared (mid-IR) region, have aroused great interest for their potential applications in the spectroscopy, biomolecular sensing, security, etc. During the exploration of optical materials with broadband and excellent nonlinear response, various materials, such as graphene [1], topological insulator (TI) [2] and transition metal dichalcogenides [3], have been deeply investigated and explored in different application areas. However, the materials with significant optical nonlinearity in the mid-infrared (mid-IR) regime have less been reported comparing with those in the visible or near-infrared (NIR) regime.
A group of hybrid organic-inorganic lead halide perovskites (HOIPs), CH3NH3PbX3 (X = Cl, Br, I), have recently attracted much attention as optoelectronic semiconductors in high-performance photovoltaic devices [4–8]. This kind of materials also exhibit outstanding optical and electrical properties, such as high carrier mobility, tunable optoelectronic property, high fluorescence yield, etc [9–11]. All these excellent properties make the HOIPs promising candidates for other optoelectronic devices, such as lasers, photodetectors, light-emitting diodes and so on [12–15]. Beyond the applications mentioned above, interest has grown in the research of nonlinear optics regime. Recently, Zhang et al. investigated the nonlinear optical response of CH3NH3PbI3 and CH3NH3PbI3-xClx thin film at wavelength of 532 nm and 1.06 µm, and realized a pulsed laser with the perovskite as a pulse modulator [16]. Li et al. studied the saturable absorption properties of two-dimension (2D) CH3NH3PbI3 nanosheets at 800 nm and 1030 nm, and achieved the mode-locking fiber laser generating picosecond pulses at 1064 nm [17]. Mirershadi et al. measured the third-order susceptibilities of CH3NH3PbBr3 film by Z-scan technique at 532 nm [18]. Lu et al. studied the nonlinear optical properties of colloidal CH3NH3PbBr3 quantum dots at 800 nm [19]. Walters et al. investigated the two-photon absorption process of CH3NH3PbBr3 perovskites single crystals and reported the nonlinear absorption coefficient of it at 800 nm [20]. The unique nonlinear optical responses of HOIPs from visible to NIR regime have been reported successively [16–21], but the nonlinear optical properties of HIOPs in mid-IR regime have not been reported by far.
In this work, the nonlinear optical response in optical communication C-band and mid-IR region for the high-quality CH3NH3PbI3 film, which were prepared by a solution method, has been studied using Z-scan technique. We experimentally demonstrated CH3NH3PbI3 film shows broadband saturable absorption and self-focusing properties from NIR to mid-IR band, and distinguished the real and imaginary parts of third-order susceptibility. Our results confirm that the CH3NH3PbI3 thin film own excellent nonlinear optical properties in the mid-infrared regime, and indicate the application potentials of HOIPs based nonlinear photonics devices.
2. Material characterizations and experimental setup
The high-quality CH3NH3PbI3 films were prepared with the precursor solutions. Firstly, PbI2 films (~85 nm) were prepared on quartz glass substrates using thermally physical vapor deposition in a vacuum chamber at 10−4 Pa pressure. Secondly, a CH3NH3I solution (in isopropanol, 15 mg/mL) was spin-coated on the as-prepared PbI2 films, followed immediately by 10 minutes annealing at 100 °C at ambient environment. A cross-sectional scanning electron microscopy (SEM, Hitachi S-4800, Japan) image is shown in Fig. 1(a). It shows a dense-grained uniform morphology and the entire film is composed of a pinhole-free and well-crystallized compact CH3NH3PbI3 crystals. The uniform and smooth perovskite layer can cover the top of the substrate with a thickness ~180 nm. The X-ray diffraction (XRD, Rigaku D Max 2500, Japan) pattern of CH3NH3PbI3 thin film is shown in Fig. 1(b). It indicates that the perovskite thin film is highly crystallized and the diffraction peaks match well with the previously reported results [22, 23]. The linear transmittance spectrum of the perovskite from 1000 nm to 2500 nm was characterized by a spectrometer (Perkins Elmer Lambda 750), as shown in Fig. 1(c). Figure 1(d) shows the strong photoluminescence (PL) emission at 760 nm, which is in good agreement with previous reports [24, 25].
Fig. 1 (a) The cross-sectional SEM of CH3NH3PbI3 perovskite film. (b) The XRD pattern of CH3NH3PbI3 perovskite film. (c) The linear absorption spectrum of CH3NH3PbI3 perovskite film. (d) The photoluminescence spectrum of CH3NH3PbI3 perovskite film.
Single beam Z-scan technique was employed to investigate the broadband nonlinear optical response of bulk CH3NH3PbI3 film [26]. The schematic diagram of the experimental set up is shown in Fig. 2. We employed the mode-locked pulse laser with the central wavelength 1930 nm, pulse duration 2.8 ps, repetition rate 32.3 MHz (AdValue Photonics AP-ML 1). The incident optical power can be detected by a pyroelectric detector after the beam through a beam splitter. Then the laser beam through a CaF2 lens (focal length: 150 mm) lead to a 35 μm waist radius at the focus. The HOIPs sample and its quartz substrate were put on a motorized translation stage and irradiated by a focused beam. Another pyroelectric detector was placed behind the aperture. The transmission power was detected by another computer-controlled pyroelectric detector power meter. By adjusting the aperture, we can proceed open-aperture (OA) and closed-aperture (CA) Z-scan measurement. The transmittance S of the aperture is about 0.1 by controlling the radius of the aperture, where S = 1-exp(-2ra/wa), wa is the laser beam radius on the aperture and ra is the aperture radius. During the OA measurement, we can investigate the nonlinear absorption process while all of the transmission beam detected by the pyroelectric detector. During the CA measurement, only a part of on-axis beam can be detected, so we can investigate the combination of nonlinear absorption and nonlinear phase shift induced by nonlinear refraction process.
3. Experimental results and discussions
Figure 3(a) and Fig. 3(b) show both OA and CA dividing by OA Z-scan measurement traces at 1930 nm. From Fig. 3(a), we observed an upward peak at the focus, manifesting the characteristics of saturable absorption (SA). The OA Z-scan trace can be fitted by the following equation:
In Eq. (1), q0 = βI0Leff, where β is the nonlinear absorption coefficient, I0 means the optical intensity at the focus. Leff = [1-e-αL]/α is the sample’s effective length, L is the sample length and α is the linear absorption coefficient. By fitting the OA curve, the value of the nonlinear absorption coefficient is about –4.6 × 10−6 m/W when the incident optical intensity is 9.1 MW/cm2.Fig. 3 Nonlinear optical response of CH3NH3PbI3 perovskite film. (a) Open-aperture and (b) close-aperture dividing by open-aperture Z-scan trace with a peak intensity of 9.1 MW/cm2 at 1930 nm; (c) Open-aperture and (d) close-aperture dividing by open-aperture Z-scan trace with a peak intensity of 43.6 MW/cm2 at 1560 nm.
With dividing the CA signal by OA signal, we obtained the nonlinear refraction signals, as shown in Fig. 3(b). The pre-focal valley followed by post-focal peak indicates the positive sign of nonlinear refractive index n2 which means the CH3NH3PbI3 perovskite is self-focusing medium. The CA Z-scan curves can be fitted by the formula [27]:
In Eq. (2), T is the normalized transmittance, x = z/z0, where z0 = πω02/λ is the Rayleigh range and ΔΦ = kn2I0Leff means the focal nonlinear phase shift, k = 2π/λ is the wavenumber. The value of nonlinear phase shift, which can be obtained by fitting the nonlinear refractive signal, is about 0.11 rad, and the nonlinear refractive index of CH3NH3PbI3 perovskite can be calculated to be ~1.6 × 10−12 m2/W at the peak intensity of 9.1 MW/cm2.To investigate the broadband nonlinear optical properties of CH3NH3PbI3 perovskite, we also performed Z-scan measurement at 1560 nm. The pulse duration of the self-built Er-doped mode-locked fiber laser is 1.5 ps, and the repetition rate is 20.1 MHz. The optical power was controlled by an Er-doped fiber amplifier (EDFA), while the amplified laser beam was collimated by a collimator. The focusing lens is N-BK7 lens (focal length: 125 mm). The nonlinear optical response of CH3NH3PbI3 perovskite is shown in Fig. 3(c) and Fig. 3(d). The similar nonlinear optical response can be observed at 1560 nm which indicated the SA and self-focusing properties. By fitting the Z-scan curves, we obtained the nonlinear absorption coefficient and nonlinear refractive index are about –3.6 × 10−6 m/W and 1.4 × 10−12 m2/W, respectively. The organic-inorganic hybrids exhibit enhanced nonlinear optical response, as has also been observed in other metal-organic systems [28, 29]. The comparable large n2 manifesting the enormous potential of HOIPs in the NIR and mid-IR photonics.
In order to clarify the broadband nonlinear SA properties of CH3NH3PbI3 perovskite, we also investigated the relationship between the optical transmittance and the optical intensity. Figure 4(a) and Fig. 4(b) show the dependence of peak intensity on the optical transmittance of CH3NH3PbI3 perovskite. The curves in Fig. 4 can be fitted by the formula [30]:
where T is the optical transmittance, αs is the saturable loss or normalized modulation depth and αns is the nonsaturable loss, I is the incident peak intensity and Isat is the saturable optical intensity. By fitting the curves, we can obtain the saturable intensity of 0.10 MW/cm2 and 1.19 MW/cm2, and the modulation depth of 2.51% and 8.04%, respectively. The saturable absorption response of CH3NH3PbI3 perovskite with low-threshold saturable absorption may provide some novel nonlinear optical application in mid-IR regime.Fig. 4 The optical transmittance of CH3NH3PbI3 perovskite film with different peak intensity at (a)1930 nm and (b)1560 nm, respectively.
The absorption of light in semiconductors is a light-matter interaction process, in which the electrons in the interacted materials resonate with the electric field of light and induce polarization. As we know, the perovskite is a direct bandgap semiconductor with a bandgap of about 1.51 eV and the absorption decreased gradually with the red-shifting wavelength toward the NIR region [31, 32]. Thus, there should be no stimulated electron transition between the valence band and conduction band when the irradiation wavelength is longer than around 800 nm. However, some recent research discovered the HOIPs possess a contribution of sub-gap trap states under the conduction band from both the experimental and theoretical sides, which resulted in the most fraction of electron-hole recombination and broaden the absorption range [33]. In addition, the orientation of the organic molecules and the spin-orbit coupling can also contribute to decreasing the band gap [34, 35]. Our results manifest the saturable absorption of HOIPs in the mid-IR region, which may be explained by the transition terminated to the sub-gap trap states. The existence of sub-gap trap states makes HOIPs can operate as a broadband saturable absorber up to at least 2000 nm. With the excitation intensity increased, the photo-generated carrier concentration also increases, but the further absorption is blocked due to the Pauli blocking.
Compared our results with the previously reported, the similar saturable absorption behavior can be observed, but the nonlinear refractive index reported here is larger than the value of n2 of perovskites films with femtosecond pulse excitation and close to the value of n2 of perovskites film with nanosecond excitation in the visible or NIR region [16, 18, 36]. As we know, the effective nonlinear refractive index of materials measured by Z-scan technique will include all the contribution to n2. For the conventional semiconductor, the main mechanism of nonlinear refraction process is electronic nonlinearity induced by the distortion of electron cloud under intense light. But some non-instantaneous nonlinear effect will appear when the pulse duration increases, such as free charge carrier nonlinearity, thermal nonlinearity, etc. In addition, the high repetition rate of laser source could also lead to the accumulation of thermal, which will add the thermal nonlinearity. Hence, we consider the instantaneous and non-instantaneous nonlinear effect jointly act on the n2 of CH3NH3PbI3 perovskite film.
During the experimental measurement, we considered some deviations which would lead to the asymmetry of experimental result, such as the beam shapes, the laser’s jitter, the slow response of pyroelectric detector, etc. The possible experimental errors of 25% has been considered arising predominantly from the irradiance calibration. We also changed the irradiated position of perovskite film, the repeatable results ensure the effective of our results. The n2 of quartz glass substrate is typically of the order of 10−20 m2/W to 10−18 m2/W in the NIR region, which will have no contributions to the nonlinear optical response under the low optical intensity [37]. In addition, the CH3NH3PbI3 perovskite film is uniform, and its grain size is about 200 nm, which has no observable effect on the nonlinear optical properties here.
The development of mid-IR photonics provides strong motivations on searching suitable mid-IR optical materials. Our research about the nonlinear optical properties of HOIPs could promote the development of mid-IR photonics at least in two aspects. One concerns the ultrafast pulse mode-locking or Q-switching laser for its saturable absorption properties and another is correlated with the mid-IR nonlinear optical devices based on its Kerr nonlinearity. The mid-IR ultrafast pulse lasers remain a big challenge for the lack of practical saturable absorber with easy fabrication methods and small intrinsic loss. Based on our experimental results, we can deduce that HOIPs is a promising nonlinear optical material in the mid-IR regime for its broadband saturable absorption and Kerr nonlinearity properties. In association with its low-cost and intriguing charge-transport properties [9], HOIPs is therefore suggested to be highly favorable for integrated mid-IR photonics.
4. Conclusions
Here, we have investigated the third-order nonlinear optical response of CH3NH3PbI3 perovskite thin film by Z-scan technique from NIR to mid-IR region. We experimental distinguished the real and imaginary part of third-order susceptibility of the perovskite layer, which indicates broadband SA and Kerr nonlinearity up to 1930 nm. The low-threshold saturable absorption, and large n2 (~10−12 m2/W) makes the perovskite thin film a potential broadband nonlinear material for mid-IR photonics. Furthermore, the experimental results may provide more attention for other HOIPs in the field of nonlinear optical materials, and the large mid-IR Kerr nonlinearity may provide some new insights for mid-IR nonlinear optical devices.
Funding
National Natural Science Fund Foundation of China (NSF) (61475102, 11574079, and 61775056); the Joint Equipment Pre-Research Foundation of the Ministry of Education of China (6141A02033404); Natural Science Foundation of Hunan Province (2016JJ2028, 2017JJ1013); Open Fund of the State Key Laboratory on Integrated Optoelectronics, China (No. IOSKL2013KF13).
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