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Abstract
High quality monolayer WS2 was successfully fabricated by chemical vapor deposition method. The nonlinear optical response of monolayer WS2 is demonstrated for the first time. Due to the relatively low modulation depth of 1.4% and saturable intensity of 68.6 kW/cm2 of monolayer WS2, a robust continuous-wave mode-locked (CWML) nanosecond laser with a repetition rate of 93.1 MHz is obtained. To the best of our knowledge, this is the highest repetition rate of nanosecond pulses generated from CWML lasers. This work provides an effective approach to obtaining nanosecond pulsed lasers with repetition rates of hundred-megahertz.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The wide demand of pulsed lasers in medical, communication and manufacturing fields promotes the development and upgrading of pulsed laser technology [1–4]. Q-switching and mode-locking techniques are effective means to obtain pulsed lasers [5–7]. The pulsed laser with repetition rate of kilohertz and pulse width of nanosecond can be obtained by Q-switching technique. To obtain higher repetition rates, such as megahertz (MHz) to hundred-MHz, mode-locking technology is required. In the research of continuous-wave mode-locked (CWML) lasers, more attention is focused on the pursuit of narrower pulse width, such as picosecond and femtosecond [8, 9], but few nanosecond pulse laser is realized. Therefore, how to obtain nanosecond pulsed lasers with repetition rate of hundred-MHz is a topic worthy of attention.
The discovery of graphene [10] quickly aroused the interest of scientists, and two-dimensional (2D) materials has become a hotspot in the fields of physics, chemistry, biology and information engineering [11–13]. Transitionmetal dichalcogenides (TMDCs) come into the view of researchers because of its excellent performance. The rise of TMDCs not only provides materials for basic research, but also provides broad prospects for practical applications in semiconductor industry, energy storage industry, satellite remote sensing and logic devices [14–16]. Tungsten disulfide (WS2) is one of the most representative materials in TMDCs. For bulk WS2, the band gap is 1.3 eV, and its electronic structure is affected by the d orbitals of tungsten atoms and the Pz hybrid orbitals of sulfur atoms [17]. As the number of layers decreases, the band gap increases, the electron transition of monolayer WS2 changes from indirect to direct, and band splitting disappears [18], which makes it suitable for direct application in optoelectronic devices. Advances in materials drive the development of devices. A great deal of enthusiasm is being devoted to the research of the nonlinear saturation absorption characteristics of 2D WS2. Mao et al. [19] demonstrated the first mode-locking operation with few-layer WS2 saturable absorber (SA) in fiber laser. Hou et al. [20] applied WS2 nanosheets into a solid-state laser oscillator and obtained 736 fs pulses. These two works open the door to the application of WS2 in fiber and solid-state pulsed lasers, respectively. Up to now, few-layer WS2 SAs have been efficiently applied to the generation of pulsed lasers from visible to mid-infrared wavelengths [21–24]. However, as far as we know, there are few reports on the nonlinear optical response of monolayer WS2 and its application as laser modulator, especially as a mode-locker to generate pulsed lasers. This is interesting and worthy of study in both material properties and laser applications.
The continuous improvement of low dimensional material growth and preparation technology brings more choices and new opportunities for the progress of laser technology. The research of laser with better performance, simpler structure and lower cost is the eternal theme of laser technology development. The SA is one of the crucial factors to obtain pulsed lasers with specific performance, and the modulation depth and saturable intensity are its key parameters. As mentioned above, CWML technology is needed to obtain hundred-MHz pulsed laser. According to the principle of mode-locking, nanosecond pulse widths require a smaller modulation depth of SA. Therefore, the monolayer WS2 with excellent photoelectric properties provides an idea for obtaining hundred-MHz nanosecond pulsed lasers.
In this work, the monolayer WS2 SA was successfully fabricated by chemical vapor deposition (CVD) method. Through a variety of characterization, the sample is confirmed to have high-quality and uniform monolayer WS2 film. The low modulation depth and saturable intensity of the monolayer WS2 sample were measured, which provided the premise for the generation of nanosecond CWML pulses with high repetition rates. The monolayer WS2 SA was applied to a solid-state laser, and the mode-locking operation with a repetition rate of 93.1 MHz and a pulse width of 3.7 ns was achieved experimentally.
2. Preparation and characterization of monolayer WS2
The preparation of WS2 by CVD method has the advantages of controllable layer number, low cost, simple operation, high efficiency and the potential for large-scale production. Figure 1(a) is the schematic diagram of a self-assembled multi-temperature zones CVD system. Monolayer WS2 was prepared by strictly controlling the temperature of sulfur source and tungsten source, growth temperature and time. Then the monolayer WS2 film was transferred to a sapphire substrate. More detailed growth and transfer processes have been reported in previous work [25].
Fig. 1. (a) Schematic representation of growth of WS2 monolayers. (b) Low magnification optical microscope (OM) image and (c) high magnification OM image, (d) fluorescence microscope (FL) image, (e) atomic force microscope (AFM) image, (f) thickness curve of monolayer WS2 film.
Firstly, the large area uniform and continuous morphology of the sample was observed by low magnification optical microscope (OM, DM 4000 M, Leica), as shown in Fig. 1(b). Figure 1(c) shows a high magnification OM with a scale length of 20 µm. Figure 1(d) is a fluorescence microscope (FL) image of WS2 corresponding to the OM image. WS2 is a direct bandgap semiconductor only when it is monolayer. In general, only the monolayer WS2 can be observed with strong fluorescence under fluorescence microscope. Thus, it can be used as an important basis to judge whether WS2 is a single layer. It is worth noting that the obvious fluorescence enhancement phenomenon can be seen in the FL image, which is caused by the grain boundary at the junction of monolayer WS2 [18]. The atomic force microscope (AFM, Multi Mode 8, Veeco Instruments Inc) image is the most direct evidence to judge the thickness of WS2. The exact thickness of monolayer WS2 film was determined by AFM image and its thickness spectrum line as shown in Figs. 1(e) and (f). The thickness of the sample is 0.73 nm, which is highly consistent with that reported in the literature [26, 27]. It can be seen from the AFM image that the surface of the monolayer WS2 grown by CVD is clean and flat, which is very suitable for the optical modulator of solid-state lasers.
The Raman spectrum (LabRAM HR Evolution, Horiba JY) shown in Fig. 2(a) marks two characteristic peaks at 356 cm-1 and 419 cm-1, corresponding to the $\textrm{E}_{\textrm{2g}}^\textrm{1}$ and ${\textrm{A}_{\textrm{1g}}}$ mode, respectively, which is consistent with the reported results [28, 29]. Figure 2(b) shows that the monolayer WS2 has a single intense photoluminescence (PL) emission peak. Since PL emission of low-dimensional materials is usually determined by excitons, the direct band gap of monolayer WS2 should be less than the emission peak of 1.96 eV [30]. Figure 2(c) is a high-resolution spherical aberration-corrected transmission electron microscope (STEM, Titan ETEM G2, FEI) image. The atoms are clearly visible with complete arrangement and no obvious defects, which can be well matched with the atomic structure of monolayer WS2.
Fig. 2. (a) Raman spectrum, (b) photoluminescence spectrum, (c) spherical aberration correction transmission electron microscope (STEM) image of monolayer WS2 film.
The geometric optimization and electronic structures calculations of monolayer WS2 were executed in QuantumATK [31]. Generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional was adopted in all calculations. To avoid the interaction with the adjacent periodic structures, a vacuum at least 15 Å was set in calculations. The well-conserved k-point sampling of 31×31×1 was used for the structural relaxation, and mesh cut-off with 100 Hartree was employed for the density of real space grids. The atoms were relaxed until the force of the per atom was less than 0.01 eV/Å during the optimization. Figure 3(a) gives the geometric structure of monolayer WS2. Each unit cell consists of one W atom and two S atoms, and the optimized lattice constant of the monolayer WS2 is 3.16 Å. The monolayer WS2 is a direct semiconductor with the band gap about 1.9 eV plotted in Fig. 3(b), which is consistent with the previous results [32, 33], and in good agreement with the PL spectrum.
Fig. 3. (a) The top and side views of monolayer WS2. The black dashed lines show the WS2 unit cell. (b) The band structure of monolayer WS2.
The saturable absorption capability of the monolayer WS2 SA was confirmed using a self-constructed CWML laser with the repetition rate and pulse width of 91 MHz and 15 ps, respectively. The experimental schematic diagram is shown in Fig. 4(a). Figure 4(b) depicts the nonlinear transmission rate of the sample as a function of the laser energy intensity. The experiment data was fitted by the following equation:
where $T$, $\Delta R$, $I$, ${I_\textrm{s}}$are the transmission, modulation depth, incident intensity, saturable intensity, respectively. The modulation depth and saturable intensity were obtained to be 1.4% and 68.6 kW/cm2, respectively. In the CWML laser, a low modulation depth benefits robust CWML operation without passively Q-switched effect.Fig. 4. (a) Measurement schematic diagram of the nonlinear optical property. (b) The nonlinear transmission curve of WS2 film.
The Haus principal equation for mode-locking of a fast saturated absorber under ideal conditions is shown as follows [34]:
Pulse width is expressed as:
Therefore, the non-chirped pulse width that can be obtained from a perfectly saturated ideal fast saturated absorber can be expressed as [35]:
3. Experimental setup
To further investigate characteristics of CWML pulsed laser with monolayer WS2 as SA, a mature Nd:YVO4 crystal (a-cut, 0.3 at. % Nd3+, 4×4×8 mm3) with narrow gain bandwidth was selected as the gain medium, based on the above mode-locking theory. The stability conditions that need to be satisfied to realize CW mode-locking can be expressed as [36]:
Taking the above parameters into consideration, a typical X-shaped five-mirror resonator was designed, with a total optical length of 1.61 m. Schematic of the monolayer WS2 passively CWML laser is shown in Fig. 5(a). The X-shaped cavity corresponds to a small laser oscillating spot, which greatly reduces the threshold for starting mode-locking and makes it easy to obtain stable CWML laser. The pumping source is a fiber-coupled laser diode (LD) with a central wavelength of 803 nm. The percentage of the absorbed pump power within the launched pump power is about 58%, as shown in Fig. 5(b). In order to match the core diameter of 200 µm and numerical aperture of 0.22 of LD to obtain the appropriate pump spot, a 1:1 coupling collimation system is used to focus the pump light onto the gain medium. In order to alleviate the thermal lens effect, the laser crystal was wrapped with indium foil and then clamped in the copper block. During the experiment, the copper crystal clamp was cooled by a constant temperature circulating water cooling system, and the water temperature was maintained at about 13 °C. The plan-concave lenses M1, M2, and M3 have curvatures of 500 mm, 500 mm, and 300 mm, respectively, and are coated with a high-transmission film for the pumping wavelength and a high-reflection film for the laser wavelength. M4 is a planar mirror with a reflectivity greater than 99.9% at 1064 nm. Flat mirror M5 is an output coupler (OC) with a transmission of 2%. Using the ABCD matrix to simulate the laser oscillating spots in the resonator, the minimum optical waist radius in the crystal is calculated to be 105 µm, which is matched well with the focusing spot of pump light on the crystal. The WS2 SA is placed in the resonator 10 cm away from the output mirror, where the optical waist radius is about 80 µm.
Fig. 5. (a) Schematic of the WS2 mode-locked laser. (b) Absorption of 803 nm pump by Nd:YVO4 crystal at different launched powers.
4. Results and discussion
Using the above-mentioned cavity design and gradually increasing the pumping power, the Q-switched mode-locked pulses are observed first. When the absorbed pump power is 0.882 W, a stable CWML trains appear. As the pumping power continues to be increased, the laser maintains a stable and CWML state within a considerable range of pumping power, as shown in Fig. 6(a). When the absorbed pump power reaches to 3 W, the corresponding maximum output power is 406 mW. Through linear fitting, the slope efficiency is 14.8%. When the absorbed pump power exceeded 3 W, the laser operation state changes from continuous mode-locking to Q-switched mode-locking, the average output power decreases slightly. With the continuous increase of pump power, the output power continues to increase, but at a slower rate, which may be due to the instability of the cavity caused by thermal lensing effect. As a result, the loss in the resonant cavity increases, and the laser operation changes from continuous-wave mode-locking to Q-switched mode-locking. In terms of power, the average output power and slope efficiency of the laser are reduced. The average output power and slope efficiency can be further improved by optimizing and improving the crystal cooling technology to achieve efficient and uniform thermal management under high power pumping. In addition, transferring the monolayer WS2 film to a reflection mirror to make a WS2 SA mirror is also an effective solution, which can avoid introducing additional insertion loss in the resonant cavity, thus improving the output power and the slope efficiency of the CWML laser.
Fig. 6. (a) Average output power of the mode-locking operation versus absorbed pump power. (b) CWML pulse trains in different timescales.
When we reduced the pump power, stable continuous-wave mode-locked operation was reproduced, indicating that there was no optical damage of the monolayer WS2 SA. The energy density $\Phi$ of the monolayer WS2 SA in the resonant cavity of mode-locking operation can be calculated by the following formula:
where Pout, T, f, r are the maximum average output power, transmission of output coupler, repetition rate, and oscillating laser radius, respectively. The value of $\Phi$ is calculated to be 1.1 mJ/cm2. Experimentally, when the energy density is less than or equal to 1.1 mJ/cm2, the SA can work normally, in other words, the damage threshold of the absorber device is greater than 1.1 mJ/cm2.The temporal profiles of the WS2 CWML Nd:YVO4 laser pulse trains were recorded by an oscilloscope (DPO 4104, Tektronix) and a fast photodetector (InGaAs ET-3000) with time scales from 40 ns/div to 10 ms/div, as shown in Fig. 6(b). The pulse width of 3.7 ns can be read directly from the oscilloscope. Smooth pulses in the short time range and orderly sequences in the long time scale indicate the stable operation of the CWML mode. The CWML laser can operate steadily for several hours at the maximum pump power under laboratory conditions, with a temperature of 25 °C and a humidity of ∼30%. The sable CWML operation could be maintained during continue observation for several hours in laboratory environment, which with a temperature of 25 ℃ and a humidity of about 30%. And as long as the pump power is added to the corresponding mode-locking threshold, the stable mode-locking can start automatically.
To further determine the stability of CWML pulses, the radio frequency (RF) spectrum was traced with a spectrum analyzer (Rohde & Schwarz-FSC). Figure 7 displays a clean and sharp single pulse with the signal-to-noise ratio of ∼30 dB without spurious frequency components. The RF spectrum illustrates the basic repetition rate of 93.1 MHz, which corresponds to the cavity length of 1.61 m. The spectrum of the CWML laser was recorded with a spectrum analyzer (Avaspec-3648-USB2). The central wavelength was located at 1064 nm, with a corresponding full-width at half-maximum (FWHM) of 0.8 nm.
It is worth mentioning that the obtained CWML pulses were measured by a oscilloscope with band-width of 1 GHz and a fast photodetector with rising edge time of 175 ps, which is adequate to get nanosecond pulse width. Nanosecond pulses based on WS2 as an SA have been realized in fiber lasers [37]. However, due to the long resonator of fiber ML lasers, it is difficult to improve the pulse repetition rate, which is only 23.8 MHz. In addition, the average output power is limited to 10.55 mW. Compared with the above fiber ML laser, solid-state laser has its unique advantages: First of all, solid-state lasers allow a large mode volume, usually a few millimeters long gain medium can provide sufficient gain, with little effect on the length of the cavity. Meanwhile, the structure of solid-state mode-locked laser is simple. Usually, a stable mode-locked resonator can be established by using four or five mirrors with appropriate parameters. In addition, the compression of cavity length, that is, the increase of repetition frequency, can be achieved by directly reducing the distance between cavity mirrors. Thus, we apply the monolayer WS2 to the solid-state Nd:YVO4 laser, which increases the repetition rate and average output power of nanosecond pulses by nearly 4 times and 40 times, respectively. Experiments show that the monolayer WS2 with low modulation depth is more conducive to achieve robust solid-state CWML operation, and pulses of hundred-MHz magnitude can be obtained by combining with good resonator design. In addition, it is expected to obtain GHz nanosecond pulses by applying the monolayer WS2 as a mode-locker in bow-tie ring cavity with lengths from several centimetres to several tens of centimetres [6]. In 2002, Krainer et al. [38] report the highest repetition rate of Nd:YVO4 picosecond laser, pumped by Ti:sapphire laser, experimentally push the repetition rate up to 157 GHz by using semiconductor saturable absorber mirrors, reaching the fundamental limit to the repetition rate. Drawing on the quasi-monolithic resonator design applied in the above work, combined with optimized monolayer WS2 saturable absorber mirror, the LD pumped nanosecond pulse laser at hundred-GHz is worthy of expectation.
In conclusion, monolayer WS2 SA has been successfully prepared by CVD method and its nonlinear optical response is characterized for the first time. Thanks to the relatively low modulation depth of monolayer WS2 SA, a stable CWML laser with a pulse width of 3.7 ns and a repetition rate of 93.1 MHz is obtained. This provides a new way to obtain nanosecond pulsed lasers at hundred-MHz or even GHz.
Funding
National Natural Science Foundation of China (11974220, 51732010).
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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