Method for femtosecond laser regulation of few-layer molybdenum disulfide thin film sulfur vacancy defects

Abstract

The application discloses a method for femtosecond laser regulation and control of a few-layer MoS2 film sulfur vacancy defect, and the method comprises the following steps: S1, taking a few-layer MoS2 film and placing it on a displacement platform; S2, placing an attenuation sheet with a 380-1800 nm wavelength transmittance of 0.01%-1% above the few-layer MoS2 film; S3, moving the displacement platform to focus a femtosecond laser focus on the surface of the few-layer MoS2 film, starting the femtosecond laser for preheating, adjusting the parameters of the femtosecond laser, setting a galvanometer to perform line scanning on the few-layer MoS2 film, setting a certain interval for the line scanning to realize surface scanning on the MoS2 film, and preparing a MoS2 film with sulfur vacancy defects on the surface of the few-layer MoS2 film. The method can obtain a few-layer MoS2 film with surface flatness and specific concentration of sulfur vacancy defects on an intrinsic few-layer MoS2 film, and the concentration of the sulfur vacancy defects can be up to 9.5%, which can meet the requirements of passive mode locking of a 1-micron-band optical fiber laser and a 1.5-micron-band optical fiber laser.

Description

Technical Field

[0001] This invention relates to the fields of ultrafast laser technology, two-dimensional semiconductor materials, and laser micro / nano processing technology, specifically to a method for controlling sulfur vacancy defects in few-layer MoS2 thin films using femtosecond lasers, which can be applied to passively mode-locked lasers. Background Technology

[0002] Ultrafast lasers, due to their advantages such as high peak power, narrow pulse width, and low processing thermal effects, are widely used in ultra-precision machining, ultrafast spectroscopy, laser communication, and medical fields. Saturable absorbers (SAs), with their nonlinear optical absorption characteristics, are the core devices for generating ultrafast laser output through passive mode-locking technology. Their performance parameters, such as carrier recovery time, modulation depth, and unsaturated absorption loss, directly affect the laser's mode-locking self-starting capability, output pulse width, output power, and stability. Semiconductor saturable absorber mirrors (SESAMs) are currently the most commercially mature SAs. Their main advantage lies in the fact that their performance parameters can be precisely optimized through process control, enabling them to meet the specific needs of different types of ultrafast lasers and effectively improve the overall performance of the laser. However, SESAMs also have inherent limitations, including complex fabrication processes, high costs, and operating wavelengths limited by the material's band gap.

[0003] Since graphene was first used as a mode-locked laser (SA) for mode-locked laser output, a large number of two-dimensional materials, including black phosphorus, transition metal chalcogenides (MoS2, WTe2, etc.), topological insulators, MXenes, and perovskites, have been shown to possess saturable absorption properties and have demonstrated good potential in mode-locked lasers. However, saturable absorption (SA) of two-dimensional materials remains in the laboratory verification stage, with the core bottleneck being the lack of performance tuning methods. Thickness control is one of the main ways to adjust the saturable absorption performance of two-dimensional materials. Taking molybdenum sulfide (MoS2) as a typical example, monolayer MoS2 has a fast carrier recovery time but a limited modulation depth, while multilayer MoS2 has a large modulation depth but a slow recovery. This inherent contradiction makes it difficult to achieve on-demand design of saturable absorption (SA) of two-dimensional materials to achieve performance similar to SESAM. Existing research shows that introducing vacancy defects into two-dimensional materials can significantly affect their nonlinear optical absorption properties. Therefore, vacancy defect concentration is expected to become an important dimension for tuning the nonlinear absorption performance of saturable absorption (SA) of two-dimensional materials. Summary of the Invention

[0004] To address the issues of fast carrier recovery time but limited modulation depth in existing monolayer MoS2 and large modulation depth but slow recovery in multilayer MoS2, which fails to meet SESAM performance requirements, this invention provides a method for femtosecond laser-induced sulfur vacancy defects in few-layer MoS2 films. This method uses a femtosecond laser to induce sulfur vacancy defects of varying concentrations in MoS2, thereby achieving nonlinear optical absorption properties of the material under different defect concentrations, such as modulation depth, saturated absorption intensity, and unsaturated absorption loss. This enables on-demand design of the SESAM performance of two-dimensional materials.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The first aspect of this invention provides a method for femtosecond laser manipulation of sulfur vacancy defects in few-layer MoS2 thin films, the method comprising the following steps:

[0007] S1. Take a few layers of MoS2 film and place them on a displacement platform;

[0008] S2. Place an attenuator with a transmittance of 0.01%-1% for wavelengths of 380-1800 nm on top of the few-layer MoS2 film;

[0009] S3. Move the displacement platform to focus the femtosecond laser onto the surface of the few-layer MoS2 film, turn on the femtosecond laser for preheating, adjust the femtosecond laser parameters, set the galvanometer to perform line scanning on the few-layer MoS2 film, set the line scanning interval to achieve surface scanning of the MoS2 film, and prepare a MoS2 film with sulfur vacancy defects on the surface of the few-layer MoS2 film.

[0010] The method of this invention utilizes the light field distribution characteristics of a Gaussian beam, which is strong in the center and weak at the edges, under the action of a femtosecond laser and after the action of an attenuator, so that only the energy of the center of the spot reaches the modification threshold of MoS2, thereby reducing the linewidth of the scan that was originally focused on the surface of the MoS2 thin film from about 20 μm to about 1.5 μm.

[0011] In this invention, the femtosecond laser beam, after being expanded, is guided through a series of mirrors into an optical system consisting of a galvanometer and a field mirror. By setting the pulse repetition frequency f and the scanning speed v of the galvanometer, the distance d between pulses can be controlled. The relationship between these three parameters is d = v / f. During laser scanning, when d is relatively small, the high pulse overlap rate will lead to energy superposition and cause ablation damage to the material. Therefore, it is necessary to reasonably adjust the distance between pulses to ultimately obtain a MoS2 thin film with sulfur vacancy defects.

[0012] This invention achieves controllable adjustment of sulfur vacancy defects in few-layer molybdenum disulfide thin films by adjusting the femtosecond laser irradiation power. Characterization shows that the S / Mo atomic ratio of the few-layer molybdenum disulfide thin film can reach as low as 1.81; when the S / Mo atomic ratio is lower than the ideal stoichiometry of 2.00, sulfur vacancy defects are formed, with a sulfur vacancy defect concentration of up to 9.5%.

[0013] Preferably, the attenuator has a transmittance of 0.01%-1% for a wavelength of 1030 nm. Using an attenuator with a transmittance greater than 1% will cause the MoS2 film to be removed, while using an attenuator with a lower transmittance (below 0.01%) will have no processing effect.

[0014] Preferably, the few-layer MoS2 film has 4-6 layers; more preferably, the few-layer MoS2 film has 5 layers.

[0015] Preferably, in step S3, the pulse width of the femtosecond laser is 500-700 fs; more preferably, the pulse width of the femtosecond laser is 500-600 fs.

[0016] Preferably, in step S3, the output power of the femtosecond laser is 11-16 W; more preferably, the output power of the femtosecond laser is 12-15 W.

[0017] Preferably, in step S3, the repetition frequency of the femtosecond laser is 50-200 kHz; more preferably, the repetition frequency of the femtosecond laser is 50-100 kHz.

[0018] Preferably, in step S3, the scanning speed of the line scan is 50-200 mm / s; more preferably, the scanning speed of the line scan is 50-100 mm / s.

[0019] Preferably, in step S3, the adjacent lines of the line scan are spaced 1-3 μm apart to achieve surface scanning of the MoS2 film; more preferably, the adjacent lines of the line scan are spaced 1-2 μm apart.

[0020] A second aspect of the present invention provides a MoS2 thin film with sulfur vacancy defects, wherein the MoS2 thin film with sulfur vacancy defects is prepared by the method described above for controlling sulfur vacancy defects in few-layer MoS2 thin films using femtosecond laser.

[0021] Preferably, the S / Mo atomic ratio of the MoS2 thin film is 1.81-1.97.

[0022] The third aspect of the present invention provides the application of the MoS2 thin film with sulfur vacancy defects as a saturable absorber in an ultrafast fiber laser.

[0023] Preferably, the ultrafast fiber laser is a 1-micron wavelength fiber laser or a 1.5-micron wavelength fiber laser.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films of the present invention employs a femtosecond laser galvanometer scanning method with an attenuator. By setting appropriate laser power, pulse width, wavelength, repetition frequency, and an attenuator with suitable transmittance, as well as appropriate galvanometer scanning speed and scanning interval, a few-layer MoS2 thin film with a certain concentration of sulfur vacancy defects can be obtained on intrinsic few-layer MoS2 thin films. The sulfur vacancy defect concentration can reach up to 9.5%, which can meet the passive mode-locking requirements of fiber lasers, including those in the 1-micron and 1.5-micron wavelength ranges. Specifically, the present invention has the following advantages:

[0026] (1) Compared with traditional methods such as objective lens focusing and continuous light processing, this invention utilizes the high single pulse energy of femtosecond pulses and the filtering effect of attenuator to achieve the effect of narrow linewidth and high speed modification. Under the premise of comparable modification effect, the speed of processing sulfur vacancy defects is increased from micrometers per minute to 200 mm / s, so that the processing speed meets the needs of industrial applications.

[0027] (2) Compared with existing femtosecond laser modulation, this invention, based on the characteristics of intrinsic few-layer MoS2 thin films, adjusts the energy of the laser spot reaching the surface of the MoS2 thin film to an appropriate range, so as to achieve reasonable sulfur vacancy defect modulation without thinning the MoS2 thin film, so as to meet the passive mode-locking requirements of fiber lasers in the 1-micron band and the 1.5-micron band.

[0028] (3) The method of the present invention can be used to prepare few-layer MoS2 films with sulfur vacancy defects. The repeatability is good and the controllability is high. As needed, the parameters can be adjusted to prepare few-layer MoS2 films with sulfur vacancy defects. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of using a femtosecond laser to modulate a few-layer MoS2 thin film.

[0030] Figure 2 This is a schematic diagram of galvanometer line scanning and area scanning.

[0031] Figure 3Characterization images of MoS2 thin films with sulfur vacancy defects prepared by femtosecond laser at a laser power of 12.11 W are shown, including (a) peak fitting of the fine spectrum of Mo 3d in X-ray energy dispersive X-ray spectroscopy, (b) peak fitting of the fine spectrum of S 2p in X-ray energy dispersive X-ray spectroscopy, and (c) electron microscope image.

[0032] Figure 4 To obtain Raman characteristic peak spectra of few-layer MoS2 films with sulfur vacancy defects using femtosecond laser control at different laser powers.

[0033] Figure 5 The graph shows the variation of the S / Mo atomic ratio of MoS2 thin films with sulfur vacancy defects obtained by femtosecond laser control under different laser powers.

[0034] Figure 6 This is a schematic diagram of the optical path for using a few-layer MoS2 thin film with sulfur vacancy defects controlled by a femtosecond laser in an ultrafast fiber laser. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0037] Figure 1 This is a schematic diagram illustrating the use of femtosecond lasers to control sulfur vacancy defects in few-layer MoS2 thin films according to the present invention. The system for controlling sulfur vacancy defects in few-layer MoS2 thin films according to the present invention includes: a femtosecond laser, a beam expander, a reflector, a galvanometer, an F-Theta field mirror, an attenuator, and a displacement platform. The number of reflectors is not limited and can be used according to actual conditions. The specific implementation is as follows: The laser emitted by the femtosecond laser passes through the beam expander and then through the reflector into the optical system composed of the galvanometer and the F-Theta field mirror. The parameters of the femtosecond laser are adjusted, and the scanning path and scanning speed of the galvanometer are set. The femtosecond laser scans the MoS2 thin film on the displacement platform by passing through the galvanometer, the field mirror, and the attenuator.

[0038] Example 1

[0039] like Figure 1 As shown, the method for preparing the few-layer MoS2 thin film containing sulfur vacancies includes the following steps:

[0040] (1) A five-layer MoS2 thin film grown on a large area of ​​double-sided polished sapphire is placed on a high-precision displacement platform, and an attenuator with a transmittance of 0.1% at 1030 nm wavelength is placed on top of the MoS2 thin film.

[0041] (2) Start the XYZ high-precision displacement platform to focus the laser onto the MoS2 thin film surface, turn on the femtosecond laser for preheating, and adjust the pulse width of the femtosecond laser to 585 fs, wavelength to 1030 nm, repetition frequency to 50 kHz, and power to 12.11 W; Figure 2 As shown, by setting the galvanometer to perform line scanning at a scanning speed of 100 mm / s and setting the interval of each line to 1 μm to perform surface scanning of the MoS2 film, a MoS2 saturable absorber with sulfur vacancy defects is prepared on the surface of the few-layer intrinsic MoS2 film.

[0042] (3) Remove the MoS2 film and perform tests and analyses on it. For example... Figure 3 As shown in (a) and (b), peak fitting was performed on the fine spectra of Mo 3d and S 2p in X-ray energy dispersive spectroscopy (XPS), respectively. The peak area of ​​the S 2p peak was 21984.91 CPS·eV, and the peak area of ​​the Mo 3d peak was 55497.66 CPS·eV. Using sensitivity factors Mo 3d = 3.321 and S 2p = 0.668, the calculated S / Mo atomic ratio was 1.97, and the sulfur vacancy defect concentration was approximately 1.5%. As shown in Figure (c), after 100,000x magnification under an electron microscope, no ablation marks were found on the material surface, and the film surface was intact and smooth. Many small particles on the surface were gold powder sprayed to increase conductivity and improve electron microscope resolution, while the larger particles were dust impurities adsorbed on the surface. Figure 4 As shown, the Raman characteristic peaks of the thin film showed no obvious shift, the peak intensity did not decrease significantly, and there was no obvious ablation, removal or thinning on the film surface.

[0043] Example 2

[0044] The method for preparing the few-layer MoS2 thin film containing sulfur vacancies in this embodiment is based on Example 1, but with the power of the femtosecond laser changed to 12.72 W. The other steps are the same as in Example 1. The S / Mo atomic ratio was found to be 1.95 and the sulfur vacancy defect concentration was about 2.5%.

[0045] Example 3

[0046] The method for preparing the few-layer MoS2 thin film containing sulfur vacancies in this embodiment is based on Example 1, but with the power of the femtosecond laser changed to 13.86 W. The other steps are the same as in Example 1. The S / Mo atomic ratio was found to be 1.91 and the sulfur vacancy defect concentration was about 4.5%.

[0047] Example 4

[0048] The method for preparing the few-layer MoS2 thin film containing sulfur vacancies in this embodiment is based on Example 1, but with the power of the femtosecond laser changed to 14.37 W. The other steps are the same as in Example 1. The S / Mo atomic ratio was found to be 1.84 and the sulfur vacancy defect concentration was about 8.0%.

[0049] Example 5

[0050] The method for preparing the few-layer MoS2 thin film containing sulfur vacancies in this embodiment is based on Example 1, but with the power of the femtosecond laser changed to 14.87 W. The other steps are the same as in Example 1. The S / Mo atomic ratio was found to be 1.81 and the sulfur vacancy defect concentration was about 9.5%.

[0051] Comparative Example 1

[0052] like Figure 1 As shown, the method for preparing the few-layer MoS2 thin film containing sulfur vacancies includes the following steps:

[0053] (1) A five-layer MoS2 thin film grown on a large area of ​​double-sided polished sapphire is placed on a high-precision displacement platform, and an attenuator with a transmittance of 0.1% at 1030 nm wavelength is placed on top of the MoS2 thin film.

[0054] (2) Start the XYZ high-precision displacement platform to focus the laser onto the MoS2 thin film surface, turn on the femtosecond laser for preheating, and adjust the pulse width of the femtosecond laser to 585 fs, wavelength to 1030 nm, repetition frequency to 50 kHz, and power to 16.13 W; Figure 2 As shown, by setting the galvanometer to perform line scanning at a scanning speed of 100 mm / s and setting the interval of each line to 1 μm to perform surface scanning of the MoS2 film, a MoS2 saturable absorber with sulfur vacancy defects is prepared on the surface of the few-layer intrinsic MoS2 film.

[0055] (3) Remove the MoS2 film and perform tests and analyses on it. For example... Figure 5 As shown, the S / Mo atomic ratio is 2.07, indicating that both S and Mo atoms are removed from intrinsic MoS2 under higher laser power. Figure 4 As shown, the Raman characteristic peak of the film did not shift significantly, but the peak intensity decreased significantly, indicating that the surface of the film material was removed and thinned, which is not the desired induced sulfur vacancies.

[0056] Comparative Example 2

[0057] The preparation method of the few-layer MoS2 thin film containing sulfur vacancies in this comparative example is based on Comparative Example 1, but the power of the femtosecond laser is changed to 17.51 ​​W. The other steps are the same as those in Comparative Example 1. The S / Mo atomic ratio was measured to be 2.26, indicating that under higher laser power, both S and Mo atoms in intrinsic MoS2 are removed.

[0058] In summary, this invention specifically optimizes the femtosecond laser modulation method for sulfur vacancy defects in few-layer MoS2 films to control sulfur vacancy concentration. The few-layer MoS2 films prepared using this method achieve a sulfur vacancy defect concentration of up to 9.5%, meeting the high performance requirements of saturable absorbers. Compared to other methods with higher laser powers, this invention achieves this by selecting an optimal laser power of 14.87 W (e.g., ...). Figure 5 As shown in the figure, compared with the intrinsic MoS2 film, the sulfur vacancy defect concentration of the few-layer MoS2 film prepared by the method of the present invention is significantly increased, and it fully possesses the passive mode-locking performance for use in fiber lasers, thus meeting the requirements for saturable absorber films for use in 1 / 1.5 micrometer band fiber lasers.

[0059] Figure 6 This diagram illustrates the optical path of a few-layer MoS2 film with sulfur vacancy defects, fabricated using femtosecond lasers at different laser powers, for use in an ultrafast fiber laser. The few-layer MoS2 film containing sulfur vacancies acts as a saturable absorber. Pump light from the pump source is coupled into the gain fiber via a wavelength division multiplexer. The gain fiber absorbs the pump light, generating stimulated emission gain, which provides the gain medium for laser oscillation. As the optical signal propagates cyclically through the fiber ring resonator, it sequentially passes through a polarization controller, the sulfur-vacancy-containing few-layer MoS2 film saturable absorber, an optical isolator, and an output coupler, forming a unidirectional closed resonant cavity.

[0060] Among them, the optical isolator is used to suppress back-propagating light in the cavity and ensure that the laser oscillates stably in a single direction; the polarization controller is used to adjust the polarization state in the cavity so that the polarization of the optical field is optimally matched with the nonlinear absorption characteristics of the saturable absorber, thereby improving the stability of the pulse output; the output coupler is used to couple part of the laser in the cavity to the output, while the remaining laser continues to be amplified in the resonant cavity.

[0061] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films, characterized in that, The method includes the following steps: S1. Take a few layers of MoS2 film and place them on a displacement platform; S2. Place an attenuator with a transmittance of 0.01%-1% for wavelengths of 380-1800 nm on top of the few-layer MoS2 film; S3. Move the displacement platform to focus the femtosecond laser onto the surface of the few-layer MoS2 film, turn on the femtosecond laser for preheating, adjust the femtosecond laser parameters, set the galvanometer to perform line scanning on the few-layer MoS2 film, set the line scanning interval to achieve surface scanning of the MoS2 film, and prepare a MoS2 film with sulfur vacancy defects on the surface of the few-layer MoS2 film.

2. The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to claim 1, characterized in that, The few-layer MoS2 thin film has 4-6 layers.

3. The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to claim 1, characterized in that, In step S3, the pulse width of the femtosecond laser is 500-700 fs.

4. The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to claim 1, characterized in that, In step S3, the output power of the femtosecond laser is 11-16 W.

5. The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to claim 1, characterized in that, In step S3, the repetition frequency of the femtosecond laser is 50-200 kHz.

6. The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to claim 1, characterized in that, In step S3, the scanning speed of the line scan is 50-200 mm / s.

7. The method for femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to claim 1, characterized in that, In step S3, the spacing between adjacent lines in the line scan is 1-3 μm.

8. A MoS2 thin film with sulfur vacancy defects, characterized in that, The MoS2 thin film with sulfur vacancy defects is prepared by the method of femtosecond laser modulation of sulfur vacancy defects in few-layer MoS2 thin films according to any one of claims 1-7.

9. The MoS2 thin film with sulfur vacancy defects according to claim 8, characterized in that, The S / Mo atomic ratio of the MoS2 thin film is 1.81-1.

97.

10. The application of the MoS2 thin film with sulfur vacancy defects as described in claim 8 or 9 as a saturable absorber in an ultrafast fiber laser.

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