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Abstract
We report a passively mode-locked Pr:LiYF4 (Pr:YLF) visible laser using a palladium diselenide (PdSe2) as a saturable absorber (SA) for the first time, to the best of our knowledge. The nonlinear optical properties of two-dimensional (2D) PdSe2 nanosheets in the visible band were studied by the open-aperture Z-scan technique. The results indicate the significant saturable absorption properties of PdSe2 nanosheets in the visible region. Furthermore, the continuous wave mode-locked (CWML) visible laser based on PdSe2 SA was successfully realized. Ultrashort pulses as short as 35 ps were obtained at 639.5 nm with a repetition rate of 80.3 MHz and a maximum output power of 116 mW. The corresponding pulse energy was 1.44 nJ and peak power was 41.3 W. These results indicate that 2D PdSe2 SA is a promising high stability passively mode-locked device for ultrafast solid-state visible lasers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Visible lasers have attracted much attention due to their numerous applications in a wide range of topical areas, including microscopy, medicine, material processing, display technology, and scientific research, and our daily life [1–3]. In the last decade, plenty of visible laser transitions based on trivalent rare-earth ions doped crystals have been reported [4]. Among them, praseodymium ion (Pr3+) doped fluoride is one of the most successful crystals for achieving efficient visible lasers benefitting from its various transition lines in the visible band and large absorption cross-section. The continuous wave (CW) output power of direct pumped Pr3+ lasers has promoted remarkable benefitting from the development of GaN-based blue laser diodes (LDs) and frequency-doubled optically pumped semiconductor lasers (2ω-OPSLs). Many groups have applied their efforts in high power Pr3+ visible laser pumped by blue LDs or 2ω-OPSLs. To the best of our knowledge, the highest CW Pr3+ laser pumped by blue LDs are 6.7 W at 640 nm [5], 3.7 W at 607 nm [5], and 1.7 W at 523 nm [6]. For 2ω-OPSLs pumped lasers, the highest CW output powers are 2.8 W at 640 nm, 1.8 W at 607 nm, and 2.9 W at 523 nm [7]. Compared with 2ω-OPSLs, there is no nonlinear optical conversion in the GaN-based blue LDs that serve as convenient, efficient, and low-cost as pumping sources. Visible ultrafast lasers play a unique role in material processing and medical treatment [8,9], which are not available in CW or Q-switched operation. Passively mode-locked is considered to be one of the most effective methods to obtain ultrafast pulse laser. In 2014, Gaponenko et al. employed a GaInP-quantum well-based semiconductor saturable absorber mirror (SESAM) to realize passively CW mode-locked laser pumped by 2ω-OPSLs [10]. In 2016, Iijima et al. reported the first SESAM mode-locked Pr:YLF laser that was directly pumped by InGaN LDs [11]. The highest averaged mode-locked output power was 65 mW and the pulse width of the mode-locked output was 45 ps at the absorbed pump power of 3.8 W. In 2017, Zhang et al. reported mode-locked lasers with wavelengths ranging from 522 to 639 nm based on two-dimensional transition metal dichalcogenide (TMDC) MoS2 optical modulator [12]. In their contribution, ultrafast output was achieved at 639 nm with the output power of 22 mW and the pulse width of 55 ps using an eight-layer MoS2 sample. Moreover, with a 1-3 layer MoS2 sample, the 639 nm mode-locked output was achieved with the output power of 46 mW and pulse width of 25 ps. Later, passively Pr-doped fluoride crystal mode-locked visible lasers have been reported with NiO, graphene SAs [13–15]. At present, the output power of mode-locked laser pumped by GaN-based LD in the visible band was limited to dozens of milliwatts, and the pulse width remained in dozens of picoseconds. Compared with the development of infrared mode-locked laser [16–18], the status of directly pumped Pr3+ visible mode-locked lasers have lagged behind. Exploring high efficiency, low cost, and high stability passively mode-locked devices are necessarily important to solve this problem.
Recently, a novel TMDC, PdSe2, has been drawn much attention benefitting from its excellent physical properties. In comparison with commonly reported large-gap TMDCs, PdSe2 has a modifiable thickness dependent bandgap from 0.03 to 1.37 eV [19]. With such a wide tunable bandgap, PdSe2 could be applied as a promising broadband optical modulator. Besides, PdSe2 has been proved to be quite air-stable [20]. The great stability property makes the PdSe2 be potential in practical applications as an optoelectronic device. In 2020, Zhang et al. first reported Er-doped and Yb-doped mode-locked fiber laser based on the PdSe2 SA. For Er-doped mode-locked operation, the central wavelength, pulse width, and pulse repetition rate were 1561.77 nm, 323.7 fs, and 20.37 MHz, respectively. For Yb-doped mode-locked operation, the pulse width of 767.7 ps and output power of 15.6 mW at 1067.37 nm were obtained [21]. In 2021, Yu et al. reported comprehensive study on the nonlinear optical processes in PdSe2. In their contribution, large two-photon absorption coefficients and giant modulation depths were obtained in 1-3 layer PdSe2 [22]. However, the ultrafast visible lasers based on the PdSe2 SAs have not yet been explored.
In this paper, we studied the nonlinear optical properties of 2D PdSe2 nanosheets in the visible band by the open-aperture Z-scan technique. Furthermore, a stable CWML operation based PdSe2 SA was successfully realized in a solid-state Pr:YLF laser at 639.5 nm for the first time. A pulse duration of 35 ps was produced with a maximum average output power of 116 mW and a repetition rate of 80.3 MHz. The corresponding pulse energy was 1.44 nJ and peak power was 41.3 W. These results indicate the promising potential of 2D PdSe2 SA for ultrafast pulses generation in the visible band.
2. Preparation and characterization of 2D PdSe2 nanosheets
In our experiment, the 2D PdSe2 nanosheets were prepared by the liquid-phase exfoliation (LPE) method which has been demonstrated to be a simple, high efficiency, and low cost way to fabricate 2D materials. The fabrication process was briefly described as follows. First, a commercially available high purity PdSe2 powder was first mixed with an alcohol solution and then sonicated for 4 h to fabricate thin nanosheets. To avoid a possible change of physical properties of PdSe2 under high temperature, the ultrasonic process was operated at a suitable interval. After that, the as-prepared suspension was centrifuged at 4000 rpm for 15 minutes to remove the undissolved flakes, and only the supernatant was collected. Finally, the extracted PdSe2 supernatant was spin-coated upon a sapphire substrate and dried under an infrared oven lamp.
The PdSe2 has a stratiform structure that is identical to graphene and black phosphorene (BP), and the van der Waals force dominates the interactions between the layers of PdSe2. The lattice structure of PdSe2 is given in Fig. 1, which exhibits a unique plicate crystal structure. Raman spectrum measurement was performed to analyze the atomic structural arrangement of the PdSe2. Figure 2(a) shows the Raman spectrum in the 100-400 cm−1 wavenumber range (with a 532 nm laser as an exciting source) of the PdSe2 sample. Two pronounced Raman peaks of the PdSe2 were A1g mode and A3g mode, which are identified at ∼143.8 cm-1 and ∼257.4 cm-1, respectively. Among them, A1g mode was dominated by the vibration of Se atoms, and A3g mode involved the relative vibration between Pd and Se atoms [23]. The typical transmission electron microscopy (TEM) image of PdSe2 nanosheets with an optical resolution of 100 nm is presented in Fig. 2(b). It can be expected that few-layer PdSe2 nanosheets are exfoliated from the bulk form. The distribution and surface morphology thickness of 2D PdSe2 nanosheets were obtained by using an atomic force microscope (AFM). The AFM image for a square region with dimensions of 2.5 µm × 2.5 µm, and the corresponding height profile of the marked areas, are given in Figs. 2(c) and (d), respectively. As illustrated in the height profile, the thicknesses of the PdSe2 nanosheets were in the range of 5.4-5.7 nm, corresponding the nanosheets with about 8 PdSe2 layers (the average thickness of a single layer PdSe2 is about 0.7 nm) [22].
Fig. 2. (a) The Raman spectra of PdSe2. (b) TEM image of PdSe2 nanosheet. (c) AFM image of PdSe2 nanosheets. (d) The corresponding height profiles of PdSe2 nanosheets.
The nonlinear absorption performance of the PdSe2 SA at 640 nm was investigated by the open-aperture Z-scan measurement technique. The experimental setup of open-aperture Z-scan is shown in Fig. 3. The laser source was a femtosecond laser at 640 nm with a pulse width of 120 fs and a repetition rate of 5 kHz. The near-parallel light was separated at the ratio of 50:50 by a beam splitter and two power meters were used to measure the output power. The PdSe2 sample was fixed at the stage, and the open-aperture Z-scan system measures the total transmittance through a sample as a function of incident laser intensity when moving the computer-controlled stage along the z-axis. Figure 4(a) presents the measured open-aperture Z-scan curves of the PdSe2 SA. When the sample position z approaches the focal point of the focusing lens (z = 0 mm), the normalized transmittance gradually increases, indicating strong saturable absorption of PdSe2 nanosheets in the visible region. To further study the optical saturable absorption behavior of PdSe2 SA, the Z-scan data were fitted using the following formula:
where T(I), I, Tns, ΔR, and Is are the transmittance rate, input optical intensity, nonsaturable loss, modulation depth, and saturation intensity, respectively. The nonlinear transmittance curve versus the energy intensity is shown in Fig. 4(b). The modulation depth, saturable intensity and nonsaturable loss were calculated to be 18.2%, 0.91 mJ/cm2, and 22.3%, respectively. The excellent characteristics of high modulation depth and large saturable intensity are promising for the application of ultrafast lasers.Fig. 4. (a) Open aperture Z-scan curve of the PdSe2 SA. (b) Transmittance versus the incident optical intensity.
3. Experimental setup
To further investigate the ability of PdSe2 SA to generate visible ultrafast pulses lasers, its application in a continuous-wave mode-locked (CWML) solid-state laser at 639 nm was studied. The experimental setup of the PdSe2-based mode-locked Pr:YLF laser is shown in Fig. 5. To obtain a stable and efficient mode-locked laser output, a 1.87 m long folded resonator was designed. A dimension of 3×3×8 mm3, a-cut Pr:YLF crystal with 0.5 at.% Pr3+-doped was selected as the gain medium. The absorption of the anisotropic Pr:YLF crystal is polarisation dependent, with peak wavelengths for σ and π absorption at 442 nm and 444 nm, respectively. To increase the absorbed pump power of Pr:YLF crystal, two commercially InGaN blue LDs with horizontal emission around 442 nm and 444 nm were selected as the pump sources. The 442 nm pump light was converted into vertically polarized light by using a half-wave plate. The pump lights of two LDs were combined by a polarizing beam splitter (PBS) and focused into the Pr:YLF crystal through a lens with a focal length of 75 mm. To increase the heat dissipation effect and reduce the heat effect, the Pr:YLF crystal was wrapped in indium foil and mounted in a copper block connected to a water-cooling system at a constant temperature of 17°. The M1, M3, M4, M5 and M7 were flat mirrors, while M2 and M6 were concave mirrors with curvature radii of 500 mm and 200 mm, respectively. All the mirrors were highly reflective (HR) coated at 639 nm and antireflective (AR) coated at the pump wavelength. The output coupler (OC) was a flat mirror with a transmission of 1% at 639 nm. The distance between the PdSe2 SA and M7 was about 1 mm. Based on the ABCD matrix theory, radius of the oscillating beams at Pr:YLF crystal and PdSe2 SA were determined to be about 54 µm and 26 µm, respectively.
4. Results and discussion
For the CWML laser experiment, the prepared PdSe2 SA was inserted into the cavity with carefully adjusting. The relationship of the average output power with respect to the absorbed pump power of the laser is plotted in Fig. 6(a). The threshold absorbed pump power of the PdSe2-based laser was 1.34 W. When the absorbed pump power reached 1.84 W, the laser ran into metastable Q-switched mode-locking (QML) regime. The QML operation was gradually suppressed when the pump power increased, and the laser turned into the stable CWML operation when the absorbed pump power reached 2.42 W. The maximum average output power was about 116 mW under the absorbed pump power of 2.97 W with a slope efficiency of 6.9%. The instability of average output power (output power, rms) at the maximum absorbed pump power was measured to be about 1.8% for 1 h, indicating the CWML laser based on PdSe2 SA has high stability. Figure 6(b) shows the typical CWML pulse trains at the time scales of 20 ns and 20 µs, which were recorded by an oscilloscope (WAVESURFER 10, Teledyne LeCroy) and a fast photodetector (New Focus 1601). From Fig. 6(b), it can be seen that the pulse shape was quite symmetrical and smooth, which reflects the excellent performance of PdSe2-based CWML laser output. Figure 6(c) displays the autocorrelation trace of the laser pulse which was measured by a commercial intensity autocorrelator (Pulsecheck 150 USB, A·P·E GmbH). The pulse duration was calculated to be 35 ps by sech2 pulse shape fitting. Figure 6(d) shows the optical spectrum of CWML laser which was measured by an optical spectrum analyzer (MS 3504i). Under the maximum output power, the spectrum of CWML centered at 639.5 nm with a corresponding full-width at half-maximum (FWHM) of 0.29 nm was recorded.
Fig. 6. (a) The average output power versus absorbed pump power of the mode-locked Pr:YLF laser. (b) The typical CWML pulse trains recorded on different time scales. (c) Autocorrelation trace for 35 ps duration. (d) Mode-locked Pr:YLF laser spectrum.
The radio-frequency (RF) spectra at different spans were recorded by a spectrum analyzer (FSC 3, Rohde&Schwarz) to further verify the stability of the CWML operation. From Fig. 7(a), a sharp peak located at 80.3 MHz with a signal-to-noise ratio reaching about 63 dB was observed. The corresponding cavity round-trip time was 12.5 ns, which was well matched with the cavity length. No spurious frequency components or modulations have been found over the entire 700 MHz span, as shown in Fig. 7(b), demonstrating the clean and stable CWML operation. To study the beam quality of the mode-locked Pr:YLF laser, a CCD camera (LaserCam HR II - 2/3”, COHERENT) and a lens with a focal length of 75 mm were used to measure spatial beam profile and beam quality. The detector was moved in a straight line on the platform, and the radius of the beam in the tangential and sagittal directions was recorded. The far-field 2D beam profile was nearly circular in outline, and the tangential direction Mx2 and sagittal direction My2 were calculated as 1.41 and 1.17, as shown in Figs. 7(c) and (d). The maximum output pulse energy and peak power of CWML laser were calculated to be 1.44 nJ and 41.3 W, respectively. For comparison, the performance of InGaN laser-diode-pumped Pr3+-doped mode-locked lasers based on different SAs is summarized in Table 1, and it is obvious that the mode-locked Pr:YLF laser with PdSe2 SA has higher pulse energy and peak power compared with mode-locked lasers based on other SAs. The results indicate that 2D PdSe2 is an excellent optical modulator for high power visible mode-locked lasers. It is worth noting that the intense radiation of picoseconds laser brings heat deposition in the lattice, which finally causes thermal damage. Therefore, evaluating the thermal damage threshold of the optical modulator is crucial. The maximum intracavity optical intensity Imax on the PdSe2 SA under the maximum absorbed pump power can be computed by the following formula:
Fig. 7. (a)RF spectrum of the mode-locked Pr:YLF laser. (b) Wide-span RF spectrum within 700 MHz. (c) Spatial beam profile of the CWML laser. (d) M2 factor of the CWML laser.
Table 1. Comparison of InGaN laser-diode-pumped Pr3+-doped mode-locked lasers characteristics based on different SAs.
5. Conclusions
In summary, the 2D PdSe2 nanosheets were successfully synthesized by using the LPE method. Characteristics of nonlinear optical absorption were investigated by applying the open-aperture Z-scan technique. By using 2D PdSe2 nanosheets as SA, the mode-locked Pr:YLF laser at 639.5 nm was realized for the first time, which delivered a pulse width of 35 ps, pulse energy of 1.44 nJ, and peak power of 41.3 W at a corresponding repetition rate of 80.3 MHz. Our research work indicates that 2D PdSe2 nanosheets could be an excellent passively mode-locked device, and may be beneficial for designing the 2D optoelectronic devices in the visible band.
Funding
National Natural Science Foundation of China (12004208); Qilu University of Technology (Shandong Academy of Sciences) International Cooperation Project of Science, Education and Industry Integration and Innovation Pilot (2020KJC-GH12); “University 20” Innovation Team Project of Jinan (2020GXRC004).
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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