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We report a passively Q-switched Yb-doped vectorial fiber laser based on hybrid organic-inorganic perovskites (HOIPs) CH3NH3PbI3. The nonlinear absorption properties of CH3NH3PbI3 have been investigated based on the open-aperture Z-scan technique. With the HOIPs pulse modulator, the passively Q-switched cylindrical vector beam can be generated with the shortest pulse width 919 ns, pulse repetition frequency 36.4 kHz, and pulse energy 0.77 μJ, respectively. Furthermore, the radial and azimuthal polarization pulse beam can be obtained and switched by controlling the intra-cavity polarization.
© 2017 Optical Society of America
1. Introduction
All-optical light modulation based on novel materials plays an increasingly important role for it can allow the signal processing to be implemented fully in the photonic domain [1,2]. Among the various optical modulators, the nonlinear pulse modulators have been extensively studied for they can provide a cost-effective solution to modulate the laser system to deliver various pulsed laser, which are very important for environmental monitoring, material processing, medicine and security applications, etc [1–5].
Driven by the requirements, it is necessary to seek low cost and high performance nonlinear optical materials. Moreover, it will be more favorable to exhibit controlled potential by external fields. Remarkable results have been achieved with different materials or devices, such as the semiconductor saturable absorption mirror (SESAM) [6,7], ion-doped crystals [8–10], carbon nanotube [11,12], graphene and graphene-like materials [13–29] in recent years. With the emerging novel functional materials and evolving technology, the optical community is still trying to find low-cost nonlinear functional materials. Recently, the light absorption characteristics of hybrid organic−inorganic perovskites (HOIPs) materials, with direct bandgap nature of the charge carrier transition, have been studied and applied in different optoelectronic devices, such as photodetectors, light-emitting diodes and laser devices [30–37]. Especially, the applications for photovoltaics (PVs), HOIPs solar cells with energy conversion efficiency already exceeds 20% [38]. The interest in the nonlinear optics investigation has been paralleled by the development of linear optical properties. However, there is always lack of the nonlinear optical research of perovskites until recently few research groups report the relevant works [39–42]. Based on open- and closed-aperture Z-scan technique, the nonlinear refractive index n2 and saturable absorption properties of metal halide perovskites have been investigated by using lasers with different pulse width and different wavelength [39–42]. They have shown that the hybrid organic−inorganic perovskites can exhibit a large third-nonlinear refractive index and possess saturable absorption effects at 532 nm or 1 μm [39,40]. Moreover, a passively Q-switched Nd:YGG solid-state laser has been demonstrated based on CH3NH3PbI3 as saturable absorber (SA) [40].
On the other hand, the nonlinear SA was mainly use to modulate the polarization light with spatial homogenous polarizations, such as linear polarization light, circular polarization light and so on [43,44]. However, the pulsed laser with spatially inhomogeneous polarizations, like radial or azimuthal polarization light, which has some distinct advantages and important applications in high resolution imaging, materials microfabrication, optical particle trapping and so on, has been paid less attention [45–47].
Here, we investigated the nonlinear absorption properties of hybrid organic−inorganic perovskites at 1 μm by using open-aperture Z-scan technique. Furthermore, the hybrid organic−inorganic perovskites were used to act as a Q-switcher to modulate the pulsed fiber lasers with spatially inhomogeneous polarizations. The maximum output power of 28 mW, corresponding to the shortest pulse width, maximum pulse repetition frequency, maximum pulse energy of 919 ns, 36.4 kHz, 0.77 μJ, has been obtained. Moreover, the vectorial fiber laser can generate switchable radially or azimuthally polarized pulse output.
2. Material characterizations and experimental setup
The high-quality CH3NH3PbI3 films were fabricated by precursor solutions, which were made of methylammonium iodide (CH3NH3I) and lead iodide (PbI2) dissolved in anhydrous N, N-dimethylformamide (DMF) with a molar ratio of 1:1 (concentrations of 550 mg/mL). Then the solutions were vigorously stirred for 12 h at 60 °C. The precursor was dropped onto the glass substrate, then the perovskite SA was made after annealing for 1 hour in atmospheres. A scanning electron microscope (SEM) image of self-crystalized perovskite was shown in Fig. 1(a), which suggests that the perovskite was well crystallized, and the thickness of the perovskite layer is about 280 nm. The X-ray diffraction (XRD, Rigaku D/Max 2500, Japan) pattern of CH3NH3PbI3 thin film is shown in Fig. 1(b), which indicates that the perovskite thin film is highly crystallized and the diffraction peaks match well with the previously reported results [48,49]. Figure 2(a) has shown the linear transmittance of CH3NH3PbI3 films from 200 to 1500 nm and the linear transmittance is about 68% at 1 μm. In order to confirm the nonlinear saturable absorption role of CH3NH3PbI3 films, the open-aperture Z-scan technique has been employed (with a homemade picosecond fiber laser centered wavelength: 1072 nm, pulse width: 125 ps, repetition rate: 20 MHz). Figure 2(b) has shown the typical open-aperture Z-scan curve, the modulation depth and saturation intensity of CH3NH3PbI3 film is 22.7% and 1.42 MW/cm2, respectively.
Fig. 2 (a) Linear transmittance curve of perovskite SA. (b) The open-aperture Z-scan trace for perovskite SA at 1072 nm wavelength.
To demonstrate the perovskites Q-switched vectorial fiber laser, a laser resonant cavity has been designed carefully, as shown in Fig. 3. The pump source is a fiber-coupled wavelength-locked 976 nm laser diode from Top Photonics with a core diameter of 105 µm and numerical aperture of 0.22. The gain fiber from Liekki is a 1.7-m-long non-polarization maintaining Yb-doped double clad fiber, which has a core diameter of 10 µm with a numerical aperture of 0.08 and the inner clad diameter of 125 µm with 0.46 numerical aperture. The V number of this fiber is calculated to be around 2.4 for 1040 nm, which can guarantee the fundamental mode propagating along the fiber. The absorption coefficient of this fiber is ~4 dB for the pump light. Both the front end and the rear end of the fiber are cleaved at an inclined angle of 8° to suppress any reflection back into the fiber. The pump light was collimated by convex mirror L1 (Thorlabs, LA1805-B, f = 30 mm) and focused by convex mirror L2 (Thorlabs, LA1951-B, f = 25.4 mm). The cavity of fiber laser consists of the optical round trip between the laser mirror M2 and M3. M2 is a HR mirror (reflectivity > 99%) at the wavelength range of 1000 nm~1200 nm. M3 is the output coupler mirror and has a partial reflection (85%) at 1040 nm, and M1 (Thorlabs, DMSP 1000) is a 45° dichroic mirror having 98% reflectivity at the lasing wavelength of 1040 nm and 86% transmission at 976 nm. The laser emerging from the front end was collimated by L2 and reflected to M2 through dichroic mirror M1, and the rear end light was collimated by L3 (Thorlabs, LA1951-B, f = 25.4 mm). A Glan laser polarizer (GLP) (Thorlabs, GT19-C) and S-waveplate (Altechna R&D, topological charge is 0.5) were placed in the internal cavity between the collimating lens L3 and the lens L4. The total length of the cavity is about 2.2 m. When the Gaussian beam passing sequentially through the GLP and S-waveplate, it will be converted to linear polarized light and radially or azimuthally polarized beam. When reasonable rotated the direction of GLP, the radially or azimuthally polarized beam could be efficiently switched. From the theoretical calculation, the radially or azimuthally polarized beam can be partial reflected by M2 and sequentially passed through the S-waveplate and GLP, which will be converted back to the previous incident polarization state [50]. The perovskite SA component was first fixed in a mount and connected with a three-dimension manual translation platform, and then placed in the confocal arrangement (L4, Thorlabs, LA1951-B, f = 25.4 mm; L5, Thorlabs, LA1805-B, f = 30 mm) with an optimized position.
3. Experimental results and discussions
The vectorial fiber laser began to operate in passively Q-switched regime once the incident pump power exceeded 2.55 W. The passively Q-switched output spectrum based on HOIPs SA has been depicted in Fig. 4. The pulse output characteristics of the passively Q-switched vectorial fiber laser based on HOIPs SA have been shown in Fig. 5.
Fig. 5 The output characteristics of HOIPs SA passively Q-switched vectorial fiber laser. (a) Output power and pulse energy as a function of incident pump power. (b) Pulse width and pulse repetition frequency as a function of incident pump power.
As can be seen from Fig. 5, when further increased the incident pump power to 2.86 W, the output power increased from 2 mW to 28 mW. At the same time, like typical passively Q-switched laser, the pulse width decreased from 4.2 μs to 919 ns and the pulse repetition frequency increased from 15.2 kHz to 36.4 kHz. The output pulse energy has been calculated and the maximum pulse energy of 0.77 μJ can be obtained. Figure 6(a) shows the measured oscilloscope traces under different incident pump power. As depicted in Fig. 6(a), the pulse repetition frequency increased and the pulse width become narrow gradually. Figure 6(b) has shown the single pulse profile, which has the full width at the half maximum of 919 ns. Table 1 summarizes the results ever obtained from passively Q-switched 1 μm laser with different SAs. As shown in the table, one can find that the performance of perovskites CH3NH3PbI3 Q-switcher can be superior or at least comparable to other SAs, especially the maximum pulse energy of the output laser.
Fig. 6 (a) The oscilloscope traces of HOIPs SA passively Q-switched pulse with different incident pump power. (b) The single pulse trace of HOIPs SA passively Q-switched vectorial fiber laser at the maximum incident pump power.
Table 1. Typical 1 μm passively Q-switched fiber lasers by different SAs
In order to ensure the vectorial properties of output beam, the intensity profiles of the output beam and the polarization properties have been measured. Figure 7(a) shows the intensity pattern of the radial polarization beam. We can see that the beam shows the typical donut-shaped profile. Figure 7(b)-7(e) illustrate corresponding intensity pattern of radial polarization beam transmitted through a polarization analyzer when the polarization analyzer was rotated at different angles. As shown in Fig. 7(b)-7(e), the double-lobe intensity patterns of transmitted radial polarization laser beam were always parallel to the corresponding optical axis direction of polarization analyzer, which indicates that the laser beam output from this fiber laser is radial polarization beam. Figure 8(b)-8(e) illustrate corresponding intensity pattern of azimuthal polarization beam transmitted through a polarization analyzer when the polarization analyzer was rotated at different angles. As shown in Fig. 8(b)-8(e), the double-lobe intensity patterns of transmitted azimuthal polarization laser beam were always perpendicular to the corresponding optical axis direction of polarization analyzer, which indicates that the laser beam output from this fiber laser is azimuthal polarization beam.
Fig. 7 (a) The experimental far-field intensity distributions of radially polarized beam at the maximum output power. (b)-(e) The radially polarized beam pattern after the passage through the polarization analyzer. The white arrows represent the optical axis direction of polarization analyzer.
Fig. 8 (a) The experimental far-field intensity distributions of azimuthally polarized beam at the maximum output power. (b)-(e) The azimuthally polarized beam pattern after the passage through the polarization analyzer. The white arrows represent the optical axis direction of polarization analyzer.
4. Conclusion
The high-quality CH3NH3PbI3 films have been prepared by precursor solutions, and its saturable absorption ability around 1 μm was confirmed. Furthermore, we have demonstrated a passively Q-switched Yb-doped vectorial fiber laser based on the CH3NH3PbI3 SA. The Q-switched vectorial fiber laser can deliver pulsed laser with shortest pulse width 919 ns, pulse repetition frequency 36.4 kHz, pulse energy of 0.77 μJ, respectively. In comparison with the other typical nonlinear optical materials around 1 μm, the laser with comparable pulse width and relatively high pulse energy can be delivered from the CH3NH3PbI3 Q-switched fiber laser. This work may pave the way for HOIP as an optical modulator to modulate pulse fiber lasers with spatially inhomogeneous polarizations. On the other hand, the nonlinear optical properties of HOIPs represent a new paradigm which will provides some deep physical insight of HOIPs and promote further development of devices based on HOIPs.
Funding
This work is partially supported by National Natural Science Fund Foundation of China (NSF) (61475102 and 11574079), the Natural Science Foundation of Hunan Province, China (No. 2016JJ2028), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics, China (No. IOSKL2013KF13).
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