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Abstract
In this contribution, we measured the third-order nonlinear optical response of bismuth oxychloride (BiOCl) nanosheets with the open-aperture (OA) and the closed-aperture (CA) Z-scan techniques with a variable excitation intensity at 1.34 µm. The effective nonlinear absorption coefficient βeff and the nonlinear refractive index n2 of the prepared BiOCl nanosheets with abundant oxygen vacancies were obtained under the excitation intensity. The third-order nonlinear optical susceptibility |χ(3)| was 1.64 × 10−9 esu. The nonlinear optical features of BiOCl enabled it as a superb saturable absorber for pulse laser generation. As a consequence, we demonstrated the first passively Q-switched Nd:GdVO4 laser with the BiOCl saturable absorber, producing a shortest pulse duration of 543 ns and a highest repetition rate of 227 kHz, leading to a maximum pulse energy of 74 nJ. Our findings show that BiOCl nanosheets with oxygen vacancies have large nonlinear optical sensitivities and can be exploited to generate optical pulses.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nonlinear optical (NLO) materials are of great importance in the optoelectronic and photonic industries [1]. Especially, as saturable absorbers (SAs), NLO media have an intense application in many fields such as electro-optic modulation, information processing and the ultrafast optical pulse generation [2]. Currently, various low-dimensional SAs have demonstrated outstanding nonlinear optical saturable absorption features such as their low saturation intensity, rapid recovery time, and other physicochemical properties [3]. In recent decades, low-dimensional metal oxides and metal halides have garnered a lot of interest in the nonlinear optical sector among low-dimensional materials [2,4].
As a typical metal oxyhalide, bismuth oxychloride (BiOCl) is a matlockite-type compound semiconductor. It has a layered structure composed of numerous [Cl-Bi-O-Bi-Cl] monolayers aligned along the (001) direction by van der Waals force interaction. As a non-toxic semiconductor, BiOCl has gained great attentions in photocatalysis, environmental pollution control, solar energy, and other domains [5–7]. Meanwhile, it has exceptional photophysical and chemical capabilities, and it is straightforward to inject doping or defect levels into its energy structure to enable it work in infrared photonic devices [7]. However, the NLO response in BiOCl is rarely investigated yet, which presents a significant obstacle for the optical applications especially in the light-mater interaction fields.
On the other hand, pulsed lasers operating in the near-infrared (NIR) region normally offer a wide range of applications in optical interconnections, medical treatment, and scientific research, etc. [8] Because it is both affordable and simple to implement, passive Q-switching is a popular method to obtain the stable optical pulses. A physicochemical stable saturable absorber is thus typically required in a Q-switching laser. BiOCl exhibits the good chemical stabilization, non-toxicity, and corrosion resistance, therefore it has been predicted as a saturable absorption material which can be employed in the passively Q-switching operations.
In this paper, we prepared BiOCl with numerous oxygen vacancies (BiOCl-OVs) via a facile solvothermal method. Subsequently, the structural properties of BiOCl nanosheets were confirmed with the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman shift spectra and UV-vis-NIR absorption spectra. The traditional open-aperture (OA) and the closed-aperture (CA) Z-scan measurements were then carried out to determine the nonlinear optical properties at 1.34 µm. The third-order optical susceptibility was as large as 1.64 × 10−9 esu. Finally, the BiOCl nanosheets were employed as a saturable absorber in the passively Q-switched Nd:GdVO4 laser at 1.34 µm to generate the Q-switching pulses. The shortest pulse duration of 543 ns was obtained with a largest pulse repetition rate of 227 kHz, leading to a highest pulse energy of 74 nJ. Our findings indicate that a BiOCl saturable absorber might be ideal for producing laser pulses in near-infrared (NIR) spectral region.
2. Fabrication and characterization of BiOCl nanosheets
BiOCl nanosheets with a large amount of oxygen vacancies (BiOCl-OVs) were produced using a facile solvothermal process, as previously described [9,10]. Numerous characterization techniques were utilized to determine the morphological, physical, and chemical features before investigating its optical nonlinear properties.
The morphologic features of the as-prepared BiOCl material were investigated using the SEM technique. As shown in Fig. 1(a), the produced tetragonal BiOCl nanosheets were square-like with side lengths less than 200 nm. The structural and phase characterization of the BiOCl nanosheets were studied by the TEM and XRD. Figure 1(b) clearly shows that the square layered BiOCl nanosheets possessed a diameter of ∼ 100 nm. Furthermore, because the nanosheet was translucent, its thickness could be deduced to be very thin, which was consistent with the previous literature [9]. The spacing lattices of the BiOCl nanosheets were 0.195 nm and 0.275 nm, respectively, as shown in Fig. 1(c), which agreed well with the (200) and (110) planes of tetragonal BiOCl (PDF card: 06-0249).
Fig. 1. Characterizations of BiOCl. (a) SEM; (b) and (c) TEM; (d) XRD pattern; (e) Raman spectra; (f) UV–vis–NIR absorption spectrum (Inset: Tauc plot).
XRD spectra obtained by a parallel beam Bruker D8 Advance X-ray diffractometer (Cu Kα) in a range of 2θ from 10° to 80° (Fig. 1(d)) revealed a (110) plane diffraction peak at 2θ=32.5° and a (200) plane diffraction peak at 2θ=46.6°, as well as (101), (102), (112), (113), (211), (212), and (220) plane peaks. The lattice constants of the as-prepared tetragonal BiOCl nanosheet could be determined as 3.891 Å, 3.891 Å, and 7.389 Å, respectively, along with all three included angles of crystal axes as 90°, and cell volume as 111.88 Å3, conforming well with the standard PDF card # 06-0249 of tetragonal BiOCl nanosheets.
The Raman spectrum was obtained by a HORIBA LabRAM HR Evolution Raman spectrometer to analyze the crystalline features of the as-prepared BiOCl nanosheets. As shown in Fig. 1(e), two peaks clearly locate at 143.6 cm-1 and 199.2 cm-1. Both peaks can be attributed to the intrinsic Bi-Cl stretching modes A1g and Eg in BiOCl, whereas the peak at 395.5 cm-1 originating from oxygen atom motion vanishes when compared to prior results [11], revealing oxygen vacancies in BiOCl nanosheets.
Figure 1(f) depicts the absorbance of BiOCl nanosheets from 200 to 1600 nm. The tetragonal BiOCl nanomaterial's broadband absorption in the near-infrared region is clearly visible. And the inset of Fig. 1(f) shows the Tauc plot of the indirect bandgap BiOCl [12]. In the inset, (αhν)1/2 is plotted as a function of the photon energy hν. By extrapolating the line to the horizontal axis, we can obtain the bandgap of the BiOCl nanosheets with abundant oxygen vacancies [13]. The bandgaps can be determined as 0.53, 0.94, 1.16 and 2.77 eV, which can be attributed to the oxygen vacancies, indicating that the BiOCl material can absorb the infrared photons. With the illumination of the light, the photoexcited carriers will quickly transfer to the new defect levels and separate. The fast carrier transfer and separation can efficiently enhance the nonlinear optical properties of BiOCl nanosheets.
3. Nonlinear optical response of BiOCl nanosheets
The photon energy at 1.34 µm is ∼0.93 eV, which is greater than the bandgap energy (∼0.53 eV), hence a variety of exciting optical manifestations of BiOCl can be expected. We used the conventional OA/CA Z-scan measurements to determine the NLO properties of the BiOCl sample at 1.34 µm. The excitation source was an actively Q-switched Nd:GdVO4 laser at 1340 nm at 3 kHz, producing the stable laser pulses with a duration of 100 ns. With the OA Z-scan method, the spot diameter varied with the relative position, leading to the different incident intensity on the BiOCl nanosheets. Figure 2 displays the nonlinear transmittance curve with the incident intensity. By fitting the experimental data with T = 1-ΔTexp(-I/Is)-Tns, the modulation depth ΔT was determined as ∼8.9%, the saturable intensity Is was ∼13.9 MW/cm2, and the non-saturated loss Tns was ∼4.2%.
To thoroughly investigate the NLO properties of BiOCl nanosheets, various excitation intensities were used to excite the material at different relative position on axis. Figure 3(a) and 3(b) depict the normalized nonlinear transmittance as a function of relative position under different excitation intensities. BiOCl possesses significant intensity-dependent nonlinear saturable absorption capabilities, as seen in Fig. 3(a). The nonlinear transmittance can be fitted by [14]:
Fig. 3. Measurements using the OA-Zscan. (a) and (b) BiOCl nonlinear transmittance curves at 1.34 µm. (c) The absolute value of the effective nonlinear absorption coefficient βeff.
Further increasing the excitation intensity, we observed the transmittance curve splitting owing to the two-photon absorption (TPA) effect in the saturable absorption progress. The transmittance curve of BiOCl develops a dip as the pump intensities increase, as shown in Fig. 3(b). When the on-axis intensity on the sample was so strong, the sample can simultaneously absorb two photons to make the electrons transition, leading to the transmittance of the sample decreased. In this case, the reverse saturable absorption occurred. The TPA coefficient βTPA can be calculated with:
In addition, the CA Z-scan technique was also carried out. Therefore, the nonlinear refractive index n2 can be calculated. Figure 4 depicts the normalized transmittances ratio of CA/OA Z-scan measurements at 1.34 µm, and the curve can be fitted with [17,18].
Furthermore, the third-order nonlinear optical susceptibility χ(3) can be determined with the nonlinear refractive index n2 and the nonlinear absorption coefficient βeff. The real component Reχ(3), imaginary part Imχ(3)$\textrm{Im}{\mathrm{\chi}^{(3 )}}$ and the absolute value of third-order NLO optical susceptibility |χ(3)| can be calculated with the following expressions [17,19]:
Table 1. Nonlinear optical parameters of BiOCl and other materials
4. Passively Q-switched Nd:GdVO4 laser with BiOCl saturable absorber
Figure 5 depicts a compact 60-mm-long concave-plane cavity with the BiOCl nanosheets as the saturable absorber. The pump source was an 808 nm fiber laser diode with a core diameter of 400 µm and a numerical aperture (NA) of 0.22. By using an imaging module with a focal length of 46.6 mm and 1:1 imaging ratio (DHC GC0-2901), the pump beam was focused into the Nd:GdVO4 crystal. By putting the thin indium foil wrapped crystal in a copper sink, the heat load of the Nd:GdVO4 crystal was removed by the water cooling system which was kept at 15 °C. The input concave mirror M1 possessed a radius of curvature (ROC) of 150 mm with an anti-reflective (AR) coating at 808 nm and a high-reflective (HR) coating at 1340 nm. The plane output coupler M2 had a transmittance of 3.8% at 1.34 µm. The BiOCl saturable absorber was put as much as close to the output coupler M2 in the laser resonator. A long-pass filter (FEL1050, cut-on: 1050 nm, Thorlabs Inc., USA) was placed behind M2 to get rid of the residual pump light. The output power was measured using a laser power meter (MAX 500AD, Coherent Inc., USA). A fast InGaAs pin photodetector and a digital oscilloscope (DMO4104C Tektronix Inc., USA) were used to capture the temporal pulse profiles. An optical spectrum analyzer (APE GmbH, Germany) was used to resolve the Q-switching pulse laser spectrum.
Fig. 5. Schematic setup of the passively Q-switched Nd:GdVO4 laser with BiOCl-OVs saturable absorber.
We initially explored the continuous-wave (CW) Nd:GdVO4 laser. For the CW operation, the threshold pump power was 0.1 W. At the maximum pump level of 1.67 W, the highest CW output power was 244 mW with an optical-optical efficiency of 14.6%, corresponding to a slope efficiency of 15.2%. Then the BiOCl saturable absorber was inserted into the laser cavity, after carefully adjusting the BiOCl saturable absorber, we were able to achieve the stable Q-switching operation when the incident pump power reached 0.99 W. The slope efficiency for the Q-switched Nd:GdVO4 laser was 2.3%. At the maximum incident pump power of 1.67 W, the highest passive Q-switching output power was 16.9 mW. The average CW output power as a function of the incident pump power is illustrated in Fig. 6(a), as well as the passive Q-switching output power.
Fig. 6. The experimental results of the BiOCl-OVs passively Q-switched (QS) lasers at 1.34 µm. (a) CW and QS average output power; (b) Single pulse energy and peak power versus incident pump power, the spectrum is shown inset; (c) The pulse width and repetition rate against the incident pump power; (d) The shortest PQS temporal pulse profile, with the accompanying pulse train shown inset.
The single pulse energy and the peak power can be calculated using the average output power, pulse width, and pulse repetition frequency. Figure 6(b) depicts the single pulse energy as the function of the increased incident pump power as well as the peak power. At the highest incident pump power of 1.67 W, the highest pulse energy and the largest peak power were 74 nJ and 137 mW, respectively. The low pulse energy came from the low output power, which can be attributed to the large insertion loss of the substrate. The operating spectrum of the passively Q-switched Nd:GdVO4 laser is centered at 1342 nm, shown in the inset of Fig. 6(b). With incident pump power augmentation, the FWHM pulse width reduced from 1009 to 543 ns, whilst the pulse repetition rate increased from 71 to 227 kHz, as shown in Fig. 6(c). This is a common characteristic of passively Q-switched lasers. Figure 6(d) represents a time domain wave pattern with pulse duration of 543 ns coming from a pulse train with a repetition rate of 227 kHz in the inset captured by an oscilloscope. It is worth noting that the pulse train is quite steady, with nearly little variation in the peak value. This also demonstrates that BiOCl may be employed as a NLO saturable absorber in the pulse laser field.
5. Conclusion
In conclusion, we fabricated bismuth oxychloride with oxygen vacancies through a facile solvothermal approach. A series of characterizations demonstrated that the material preparation was successful. The material's nonlinear absorption coefficient, nonlinear refractive index coefficient, and third-order susceptibility were then investigated using the open-aperture and closed-aperture Z-scan methods at 1.34 µm for the first time. The findings demonstrated that the material's nonlinear parameters were large enough when compared to other materials, implying that it has prospective applications in nonlinear optical regimes. Therefore, we transferred the BiOCl on the sapphire substrate to produce a thin layer as a saturable absorber and employed the 1.34 µm solid bulk laser for the Q-switching operation. The experimental results revealed that the material has an excellent Q-switching effect for 1.34 µm laser.
Funding
National Natural Science Foundation of China (12004213, 12174223, 12274263, 21872084, 52072351, 62175128).
Acknowledgments
H. Chu and Z. Pan thank the financial support from Shandong University.
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.
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