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Abstract
Indium Tin Oxide nanowire arrays (ITO-NWAs), as epsilon-near-zero (ENZ) materials, exhibit a fast response time and a low saturable absorption intensity, which make them promising photoelectric materials. In this study, ITO-NWAs were successfully fabricated using a chemical vapor deposition (CVD) method, and the saturable absorption properties of this material were characterized in the near-infrared region. Further, passively Q-switched all-solid-state lasers were realized at wavelengths of 1.0, 1.3, and 2.0 µm using the as-prepared saturable absorber (SA). To the best of our knowledge, we present the first application of ITO-NWAs in all-solid-state lasers. The results reveal that ITO-NWAs may be applied as an SA while developing Q-switched lasers and that they exhibit a broad application prospect as broadband saturable absorption materials.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Diode-pumped lasers have extensive applications in several fields, including micromachining, nonlinear optics communication, and material processing [1–3]. Pulsed lasers can provide much higher peak power when compared with that provided by continuous-wave (CW) lasers, and passive Q-switching is an important technique to generate nanosecond pulsed lasers [4–7]. An efficient optical modulator is the key element in passively Q-switched lasers to obtain pulses with a short pulse width and high peak power. Therefore, the manufacture of new high-performance saturable absorbers (SAs) has become a popular field in laser research [8–17]. The evolution of SAs has considerably promoted the development of solid-state lasers.
In recent years, various low-dimensional materials have been developed as SA candidates. However, the currently available options are affected by the necessity for fine-controlled material fabrications, inefficient light-matter interaction, and large optical losses [18,19]. An epsilon-near-zero (ENZ) material with vanishing permittivity exhibits excellent optical features such as high reflection, ultra-strong optical nonlinearity, near-zero refractive index, and infinite phase velocity [18–21]. As a typical ENZ material, indium tin oxide (ITO) exhibits high chemical stability, high damage threshold, large optical nonlinearity, and efficient light-matter interaction because of the nature of its oxides [13,22–25]. Up to now, some reports have investigated the application of ITO as SAs in fiber lasers, but fewer reports have investigated this application in solid-state lasers [26–28]. These reports suggest that ITO exhibits the following advantages as an SA. First, the ITO has a broad bandwidth and a strong peak owing to the low carrier density of conductive oxides [22]. Second, ITO exhibits large optical nonlinearities and ultrafast recovery time [23]. Alam et al. observed that ITO has a short recovery time of 360 fs and can obtain ultrafast and large intensity-dependent refractive index [29]. Finally, ITO has a high damage threshold and low wavelength-dependent saturable absorption intensity and can serve as a broadband optical modulator [12]. ITO has attracted considerable attention owing to its excellent optical properties and widespread applications, especially as an SA material in pulsed lasers. However, several ITO nanostructures are randomly distributed with uncontrolled directions of crystallographic growth [30,31]. The nanowires with a controlled crystallographic direction have greater potential due to their interesting electrical, optical, and mechanical properties [32]. Indium tin oxide nanowire arrays (ITO-NWAs) were successfully fabricated using a chemical vapor deposition (CVD) method. The synthesizing materials exhibit a perfect single-crystalline nature [20]. ITO-NWAs can make the material of ITO great advantages and are expected to become a promising SA for developing passively Q-switched lasers. However, to the best of our knowledge, the application of ITO-NWAs as an SA in all-solid-state bulk lasers has not yet been reported.
In this work, a high-quality ITO-NWAs SA is successfully prepared using a CVD method, and the saturable absorption properties of it were investigated. Using the ITO-NWAs SA, broadband passively Q-switched lasers at 1.0, 1.3, and 2.0 µm were realized for the first time. This study identifies ITO-NWAs as a potential broadband SA for solid-state pulsed lasers and looking forward to develop into an effective optical modulator for ultrafast photonics.
2. Preparation and characterization of the ITO-NWAs SA
ITO-NWAs, a type of ITO nanostructures, were prepared by the CVD method [33]. First, the Au film was annealed to Au nanoparticles in a double temperature zone plasma-enhanced chemical vapor deposition (DT-PECVD) furnace (temperature 500°C). The Au film of 3.1 mm was formed by a mixed solution containing 2 mL gold chlorate solution, 3 mL sodium hydroxide solution, 0.5 mL sodium tartrate solution, and 0.4 mL glucose solution. Au nanoparticles with a relatively uniform gap on the mica sheet were obtained. Then, 0.004 g indium oxide and 0.036 g tin oxide mixed with graphite powder in the same quality. Finally, the ITO-NWAs were grown on the mica sheet (10×10×1 mm3) in the low temperature zone (500°C) through the reaction of the mixture from high temperature zone (840°C) with a special environment (Ar 160 sccm environment for 80 min). The final ITO-NWAs were obtained by double temperature zone DT-PECVD as catalyzed by Au nanoparticles. It has a controlled crystallographic direction and excellent optical properties. The surface topography of the exfoliated ITO-NWAs SA was studied by scanning electron microscopy (SEM), and these images are presented in Figs. 1(a)–1(b). As shown in the top-view SEM image (Fig. 1(a)) and side-view SEM image (Fig. 1(b)), all the ITO nanowires are straight and perpendicular to the substrate, forming an array of well-aligned nanowires. The broadband nonlinear saturable absorption properties of the ITO-NWAs SA were characterized at wavelengths of 1.0, 1.3, and 2.0 µm. As shown in Figs. 2(a)–2(c), the dots denote the experimental data, whereas the red line denotes the theoretical fitting obtained while using the following formula [34]:
where A is the normalized parameter, δα is the absolute modulation depth, I is the incident intensity, and Isat is the saturation intensity. The transmittance of the ITO-NWAs SA increased with the rise of input laser intensity, indicating its desirable saturated absorption behavior. Based on fitting curve, for 1.0, 1.3, and 2.0 µm lasers, the saturable intensities are 4.3, 10.7, and 4.8 µJ∕cm2, and the modulation depths are 13.4%, 20.3%, and 14.9%, respectively.
Fig. 1. SEM images of ITO-NWAs: (a) top-view SEM image and (b) a typical side-view SEM image (scale bar: 200 nm).
Fig. 2. Nonlinear transmission versus energy intensity of ITO-NWAs at (a) 1.0 µm, (b) 1.3 µm, and (c) 2.0 µm.
3. Broadband Q-switched lasers with the ITO-NWAs SA
Passively Q-switched lasers are achieved at 1.0, 1.3, and 2.0 µm based on the ITO-NWA SA using a compact two-mirror resonator (Fig. 3). The initial two wavelengths are realized by Nd:GdVO4 (3 × 3 × 4 mm3), and the final wavelength is realized using the Tm:GdVO4 crystal (3 × 3 × 5 mm3). Both sides of the Nd:GdVO4 crystal were anti-reflection (AR) coated for 1.0 and 1.3 µm, respectively. And both sides of the Tm:GdVO4 crystal were AR coated at 2.0 µm. To increase heat dissipation, the crystals were mounted on a copper block with 13°C cycling water cooling. The optimal transmittance of the three output couplers (OCs) were 10%, 10%, and 2% at 1.0, 1.3, and 2.0 µm, respectively.
For the 1.0 and 1.3 µm lasers, the resonators consisted of concave input mirrors (IMs) and plane OCs with a length of 2.5 cm. The IMs with a radius of curvature of 100 mm were AR coated for 808 nm and high-reflection (HR) coated for 1.0 and 1.3 µm, respectively. The OCs were two coated plane mirrors with a transmission of 10% for 1.0 µm and 1.3 µm. A 808 nm commercial fiber-coupled CW laser diode (LD) was used as a pump source for two experimental configurations, with a fiber core diameter of 200 µm and a numerical aperture (NA) of 0.22. For the 2.0 µm laser, the resonator comprised a plane IM and concave OC, with a physical length of approximately 9.8 cm. The plane IM was AR coated for 780-810 nm and HR coated for 1.9-2.0 µm. The OC was a concave mirror with a 100 mm radius and had a transmission of 2% at 1.9-2.0 µm. A LD with a wavelength of 792 nm was employed as the pump source, which had a NA of 0.22 and a core diameter of 105 µm.
By inserting the ITO-NWAs SA into the appropriate position in the resonator and increasing the pump power, passively Q-switched operations were achieved at different wavelengths. The average powers of the output laser beam were measured with a power meter (30A-SH-V1, Israel). The laser pulse trains were recorded by a fast InGaAs photodetector and a digital oscilloscope (Tektronix DPO4104, 1 GHz bandwidth, 5 G samples/s). The average output power, peak power, pulse duration, and repetition rate versus the absorbed pump power at different wavelengths are shown in Figs. 4(a)–4(f), respectively.
For the lasers at 1.0 µm and 1.3 µm, the maximum average output powers were 0.18 W and 0.32 W, respectively. At the maximum absorbed power of 3.14 W, the corresponding highest peak power of the 1.0 µm and 1.3 µm lasers were 2.07 W and 4.69 W, respectively (Figs. 4(a)–4(b)). As shown in Figs. 4(d)–4(e), the pulse width decreased with an increase in the absorbed pump power, while the pulse repetition rate increased. At the maximum absorbed pump power, the shortest pulse widths were 422 and 296 ns with the corresponding repetition rates of 200.5 and 230.2 kHz at 1.0 µm and 1.3 µm, respectively. For the laser at 2.0 µm, the maximum average output power was 0.08 W, corresponding to the highest peak power of 1.19 W at an absorbed pump power of 2.01 W (Fig. 4(c)). Figure 4(f) shows the variation trends of pulse width and repetition rate with the increasing of the absorbed pump power. The maximum repetition rate was 48.2 kHz, and the shortest pulse width was 1.36 µs at the wavelength of 2.0 µm. The typical Q-switched pulse trains at all the wavelengths are shown in the left-hand side of Figs. 5(a)–5(c), and look uniform and stable. As shown in the right-hand side of Figs. 5(a)–5(c), we measured the output spectra of the passively Q-switched lasers at different wavelengths using a spectrometer (Avaspoe-3648-USB2). A summary of the results for the lasers with ITO-NWA SA under the maximum absorbed pump power is presented in Table 1. According to these results, ITO-NWAs SA exhibits the obvious advantages of narrow pulse width at 1.3 µm, and high repetition rates at 1.0 and 1.3 µm.
Fig. 4. Average output power and peak powers versus the absorbed pump power of the passively Q-switched lasers at (a) 1.0 µm, (b) 1.3 µm, and (c) 2.0 µm. Pulse width and repetition rate versus the absorbed pump power at (d) 1.0 µm, (e) 1.3 µm, and (f) 2.0 µm.
Fig. 5. Typical pulse trains and output spectra of the passively Q-switched lasers at (a) 1.0 µm, (b) 1.3 µm, and (c) 2.0 µm.
Table 1. Results of the passively Q-switched lasers
4. Conclusions
In summary, an ENZ material, ITO-NWAs SA was fabricated in this study using a CVD method, and the broadband saturable absorption properties of the sample were investigated at wavelengths of 1.0, 1.3, and 2.0 µm. To the best of our knowledge, for the first time, we have experimentally demonstrated passively Q-switched all-solid-state lasers by using ITO-NWAs as the SA ranging from 1.0 to 2.0 µm. The results indicate that ITO-NWAs, exhibiting a high damage threshold, large optical nonlinearity, and broad range of saturable absorption spectrums, can be utilized as a promising broadband SA for pulsed lasers in the near-infrared region.
Funding
National Natural Science Foundation of China (NFSC) (Nos. 11974220, 11674199).
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