The mid-infrared (MIR) spectrum ranging from 2 to 20 µm has diverse applications such as bio-photonics and medical photonics, absorption spectroscopy, environmental monitoring and security, and free-space communications . Among these applications, owing to the important atmospheric windows and strong fundamental absorptions of molecular species, the MIR absorption spectroscopy is of great interest in civilian and military domains . In such a spectroscopy, because the fundamental absorption and vibration bands of most molecules appear in the MIR region, detecting with the MIR sources is of more sensitivity and precision . For real applications, MIR lasers with high power and high energy are in urgent need to achieve a sensitive and precise detection. Thus, the advent of the MIR optoelectronic and electronic devices is highly desirable for next-generation photonic applications.
It is well known that the nonlinear optical materials play an essential role in the advanced photonics as the fundamental components , , , , , , , . In recent years, the two-dimensional (2D) materials have emerged as promising candidates for the applications in the MIR region , , , ,  because of their excellent electronic and optoelectronic properties and broadband nonlinear optical absorption. Among these 2D materials, transition metal dichalcogenides have drawn plenty of attention , , , , as they are stable and have a direct band gap in the visible light range in a monolayer form. In particular, monolayer molybdenum disulfide (MoS2) has demonstrated promising application in transistors, photocatalysis, solar cells, and other photonic applications , , , . For example, MoS2 2D layers have been successfully used as the nonlinear photonic devices to Q-switch and mode-lock the MIR lasers at 2–3 µm , , , , , , , , . In these photonic applications, the device performance is strongly dependent on the quality and size of the MoS2 layers , [, 34]. The large-scale MoS2 monolayer with good quality is highly desired as the nonlinear saturable absorber to integrate with the optical devices, which, however, is still a challenging issue.
In the present work, we report nonlinear properties and modulation characteristics in the MIR spectral band by using the large-scale monolayer MoS2. The nonlinear linear optical characteristics were studied by exploiting the open aperture Z-scan techniques, which show a modulation depth of 26%, a low saturable intensity of 271 kW/cm2, and a high effective nonlinear absorption coefficient βeff of −16 cm/MW. All these results reveal that the monolayer MoS2 is an excellent saturable absorber for the MIR pulse generation and an appealing nonlinear linear optical photonic device to modulate the optical pulses. Using the monolayer MoS2 as the nonlinear optical saturable absorber, a passively Q-switched Tm,Ho:CLGA disordered crystal laser was realized operating at 2.1 µm in the MIR region with a recorded shortest pulse duration (765 ns) for the first time.
2 Monolayer MoS2 preparations, characterizations, and simulation
The good-quality monolayer MoS2 thin film was grown by physical vapor deposition (PVD) on the c-plane sapphire substrate (2-inch in diameter) using the sputtering process as documented in our previous works , , , . Figure 1 displays the characterization of the sputtering deposited 2-inch monolayer MoS2. From the atomic force microscopy image (Figure 1(A)), we can see that the MoS2 film is continuous and smooth in a large scale with a small root mean square roughness of ∼0.21 nm. In addition, the thickness of the as-grown MoS2 films was also measured. To expose the substrate, we used a knife to scrape off the MoS2 film. The raised parts marked by two dashed lines in Figure 1(B) are caused by the MoS2 stacked on the edge during the scratching process. Figure 1(C) shows the height profile of the MoS2 film on the sapphire substrate. The unusual height bump is caused by the stacking of materials on the scratched edge. It can be seen that the thickness of the as-grown MoS2 film was about 0.8 nm, suggesting the single layer of MoS2. The X-ray photoelectron spectroscopy in Figure 1(D) shows the core-level peaks of Mo 3d3/2 (∼233 eV), Mo 3d5/2 (∼229.6 eV), and S 2s (∼227 eV), which are the typical peaks of tetravalent Mo4+ and divalent S2− in 2H-MoS2 semiconductor . We also do not observe the Mo suboxidation states from the Mo 3d core-level spectra. The thickness of the sputtered MoS2 film can be accessed by using Raman spectra. As shown in Figure 1(E), the relative shift between A1g and E2g is about 20.1 cm−1, indicating the dominant monolayer MoS2 on the sapphire substrate . The sputtered monolayer MoS2 shows strong photoluminescence (PL) emission, as shown in Figure 1(F). As the PL emission is usually determined by exciton for the low-dimensional materials, the direct band gap of monolayer MoS2 should be smaller than the dominant emission peak of 1.87 eV. The SEM images of the monolayer MoS2 thin film is also shown in Figure 1. The white straight line in Figure 1(G) is the substrate scratched with a knife, which is beneficial to observing the morphology of the MoS2 film more clearly through comparison. Figure 1(H) is the SEM image after further increasing the magnification. It can be seen from both SEM images that the as-grown monolayer MoS2 film features the excellent uniformity. All these characterizations confirm the good quality of the deposited large-scale monolayer MoS2 on the sapphire substrate.
To further investigate the band gap of monolayer MoS2, we also simulated the band structure of monolayer MoS2 with density functional theory calculation based on Vienna ab initio Simulation Package (VASP) code. The projected augmented wave pseudopotentials are used, as implemented in the VASP. In the calculations, Perdew–Burke–Ernzerhof of generalized gradient approximation function is used for exchange–correlation interaction. The cut-off energy of planewave basis for calculation was set as 400 eV. Optimization of geometric structure was relaxed until the total energy and maximum residual force are 10−6 eV and 0.02 eV/A. K-point meshes of 14 × 14 × 1 have been used. Figure 2(A) displays the model of monolayer MoS2. The result is shown in Figure 2(B), which indicates that the direct band gap of monolayer MoS2 is about 1.8 eV. The simulated result is in good agreement with the PL spectrum.
An open-aperture Z-san technology was exploited to measure the nonlinear saturable absorption (SA) features of the monolayer MoS2 at 2 µm. The laser source used in nonlinear absorption measurements is a home-made Q-switched laser with a pulse width of 50 ns at a repetition rate of 3 kHz. A lens with a focal length of 200 mm is used as a focusing device. Then the sample is put behind the lens, moving along the z-axis. With the different location of the sample on the z-axis, the pump laser intensity on the sample will change accordingly. To avoid the thermal effect on the sample, a pulsed laser with a low frequency and a low power is used in our case, which can efficiently reduce the heat accumulation on the MoS2 film , and the low power density incident on the MoS2 also decreases the influence of thermal-induced nonlinearity. In addition, the focal length of the used lens is large, which can cause the slow beam caustic to reduce the heat accumulation. Moreover, the large size of the uniform monolayer MoS2 film benefits the thermal diffusion.
Comparisons of saturable absorption properties of molybdenum disulfide (MoS2) saturable absorber in the mid-infrared (MIR) band.
|λ (nm)||Saturable absorber||Modulation depth (%)||Saturable absorption intensity (MW/cm2)||Ref.|
|2090||Large-sized monolayer MoS2||26||0.27||This work|
3 MIR photonic device: saturable absorber
In this section, we would like to address the MIR photonic application of the monolayer MoS2 as a saturable absorber to produce short MIR laser pulses. The laser cavity was a compact plane-concave resonator with a length of 15 mm, as shown in Figure 5. The input mirror M1 (radius of curvature: 200 mm) was coated for high reflectance at 2.1 µm and antireflectively coated for the pump wavelength at 794 nm. The gain medium was a polished c-cut Tm,Ho:CaLu0.1Gd0.9AlO4 (Tm,Ho:CLGA) disordered crystal. In the experiment, the laser crystal was mounted into a water-cooled copper sink cooled at 15 °C. The large-scale monolayer MoS2 was placed near the output coupler (OC) as the saturable absorber to generate the short MIR pulses. The OC was a plane mirror with a T = 1% at 2.1 µm. The pump source was a commercial fiber-coupled diode laser emitting the radiation at 794 nm (Coherent Inc., USA). The diameter and the numerical aperture of the coupled fiber were 400 μm and 0.22, respectively. The pump beam was focused into the gain crystal with a waist radius of 100 μm by a 1:1 optical imaging system. A longpass filter was put behind the OC to block the residual pump beam. The pulse temporal behavior was recorded by a DPO 7104C digital phosphor oscilloscope with a high-speed photodetector. A laser power meter was used to measure the average output power (Figure 5).
First, we investigated the free running of the Tm,Ho:CLGA disordered crystal laser without the monolayer MoS2 as the saturable absorber. The threshold absorbed pump power for the lasing was 2.38 W. The maximum output power was 86 mW, with a slope efficiency of 4.3%, as shown in Figure 6(A). When the monolayer MoS2 was inserted into the laser resonator working as the saturable absorber, the stable passive Q-switching operation can be achieved by slightly aligning the mirrors. In this case, the threshold power for the laser operation is slightly increased to 2.7 W, while the stable Q-switching operation starts at 2.98 W. Under the highest absorbed pump power of 4.26 W, the maximum output power for the passive Q-switching operation is 56 mW, corresponding to a slope efficiency of 3.3%. As Figure 6(B) shows, the pulse duration decreases monotonically from 4 µs to 765 ns, while the pulse repetition rate increases from 15 to 36 kHz. Once the output power, the pulse duration, and the pulse repetition rate were given, the pulse energy and the peak power can be estimated. The corresponding highest pulse energy and the maximum peak power are 1.55 µJ and 2.03 W at an absorbed pump power of 4.26 W, respectively. The temporal pulse profile and stable pulse train with the pulse width of 765 ns at the repetition rate of 36 kHz are displayed in Figure 6(C) and (D), respectively. From the recorded pulse train, the pulse peak-to-peak fluctuation is estimated as 4.6% in the RMS error method, demonstrating a stable Q-switching operation.
The comparisons of the Q-switched laser performance at around 2 μm between our work and previous reported works have also been summarized in Table 2. The good laser performance is mainly attributed to the strong SA properties of monolayer MoS2, such as large modulation depth and low saturable intensity. The resonator and saturable absorber can be further optimized to have a better performance, such as a shorter pulse duration and higher pulse energy. In addition, during the laser experiments, the monolayer MoS2–based SA worked stably always even at a higher pump power, which indicated its relative high damage threshold.
Comparisons of the Q-switched laser performance at 2 μm with previous reported works.
|Crystal||λ (nm)||Saturable absorber||Pulse width (ns)||Single pulse energy (μJ)||Ref.|
|Tm,Ho:CLGA||2090||Large-sized monolayer MoS2||765||1.55||This work|
To make sure the laser is working in the MIR region, the oscillating wavelength of the monolayer MoS2 saturable absorber Q-switched Tm,Ho:CLGA disordered crystal laser was recorded by an IR spectroscope meter (Avantes, the Netherlands). As Figure 7(A) shows, the peak of oscillating wavelength locates at ∼2090 nm with a full-width at half-maximum (FWHM) of 20 nm. Compared with the broad spontaneous emission spectrum of Tm,Ho:CLGA disordered crystal (∼300 nm) , the linewidth of MoS2-based Q-switched laser is much narrower, which indicates that the stable laser operates in the constructed solid-state laser device . Moreover, the beam caustic versus the position is shown in Figure 7(B). From the experimental data, the beam quality M2 from the monolayer MoS2 saturable absorber Q-switched Tm,Ho:CLGA laser at 2.1 µm can be estimated as <1.1 under the highest absorbed pump power of 4.26 W, indicating good optical properties of the large-size monolayer MoS2–based saturable absorber.
It is worth mentioning that the monolayer MoS2 possesses higher thermal conductivity, which benefits the thermal diffusion . Meanwhile, owing to the horizontal extension of materials in large-sized MoS2, the MoS2 around the beam will help reduce the heat accumulation of the MoS2 illuminated by the beam. Both factors make the large-sized monolayer MoS2 good thermal stability, which ensures good modulation performance. On the other hand, the excellent uniformity of large-sized MoS2 thin-films ensures the continuity and repeatability of laser experiment. What’s more, under the premise of ensuring uniformity, large-size MoS2 thin films can break through the limitations of the manufacturing process to achieve large-scale preparation and tunable thickness, which is also very important for promoting the application of MoS2 nanofilms in optoelectronic fields.
In conclusion, we have demonstrated the application of large-scale monolayer MoS2 for the MIR photonic applications. The PVD monolayer MoS2 shows a large modulation depth of 26% and a low saturable intensity of 271 kW/cm2, indicating that the monolayer MoS2 is an excellent saturable absorber for the MIR pulse generation. The effective nonlinear absorption coefficient βeff is measured to be −16 cm/MW. Furthermore, the TPA coefficient and the nonlinear refractive index of monolayer MoS2 are also determined as −31 ± 0.8 cm/MW and −25 ± 0.6 × 10−4 cm2 MW−1, respectively. Based on the large nonlinear absorption coefficient and modulation depth, we demonstrate a passively Q-switched Tm,Ho:CaLu0.1Gd0.9AlO4 (Tm,Ho:CLGA) disordered crystal laser at 2.1 μm by using the monolayer MoS2 as the saturable absorber for the first time, which shows remarkable performance such as a short pulse width of 765 ns and pulse repetition rate of 36 kHz. Our results unravel that sputtered large-scale monolayer MoS2 is a promising candidate for the MIR photonic devices.
The authors would like to thank Dr. Na Qi and Mr. Daozhi Li from Shandong University for their help in the morphology characterization and the DFT simulation.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: This work is partially supported by National Natural Science Foundation of China (NSFC) (61575109, 12004213), Fundamental Research Fund of Shandong University (2018TB044), and Foundation of President of China Academy of Engineering Physics (YZJJLX2018005). D.C., S.W., J.C., and M. Y. acknowledge the funding support from A*STAR Science and Engineering Research Council PHAROS 2D Program (SERC Grant No. 152-70-00012).
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