1 Introduction

The 2 μm pulsed laser has been widely studied for using in laser surgery, remote sensing, material processing, and scientific research [1,2,3,4], because it is not only in the eye-safe band but also in the range of the atmospheric window. The Q-switched and mode-locked laser technology are the primary technical means to generate pulse laser. Compared with active modulation devices, which require high voltage or RF drivers, passively Q-switched and mode-locked laser techniques are more attractive because of the compactness and variety of saturable absorbers used for modulation effects [5, 6]. Among them, semiconductor saturable absorber mirrors (SESAMs) have a high commercial application value due to their stable modulation performance, but the application is limited by the complex preparation process and narrow modulation width [7].

With further exploration, two-dimensional (2D) materials have become the most popular saturable absorber due to simple manufacturing techniques, broadband optical response, and tunable band-gap, such as graphene [8], black phosphorus (BP) [9], topological insulators (TIs) [10], transition metal dichalcogenides (TMDs) [11, 12], MXenes [13,14,15], and so on [16, 17]. However, the above 2D materials each have different aspects of disadvantages. For example, although graphene has a narrow band-gap and a large modulation bandwidth, it has lower damage threshold; BP is easily oxidized and, therefore, have poor stability in the air; the preparation process of topological insulators is still complicated; TMDs have longer exciton recovery time; MXenes exhibit poor stability and the preparation technology still needs to be improved. In recent years, layered double hydroxides (LDHs) have been reported as saturable absorbers for passively Q-switched and passively mode-locked lasers, due to their excellent controllability, broadband absorption properties, good modulation properties, and easy fabrication techniques [18, 19]. In 2021, using a NiCo-LDH saturable absorber, Xu et al. obtained a passively Q-switched 2 μm laser with the narrowest pulse width of 322.6 ns [20]. Cai et al. realized a passively mode-locked 2 μm laser based on NiV-LDH with the narrowest pulse width of 320 ps [21].

In this paper, we demonstrate a passively mode-locked Tm:YAG ceramic laser with a NiCo-LDH saturable absorber. A maximum Q-switched mode-locked laser output power of 278 mW is achieved at 12.8 W absorption pump power, corresponding to a slope efficiency of 2.21% and an optical conversion efficiency of 2.18%, respectively. The Q-switched mode-locked pulse repetition rate is 146 MHz, and the pulse width is calculated to be 221 ps.

2 Materials and characterization

NiCo-LDH nanosheets are prepared from NiCo-LDH powder by ultrasonic liquid phase-assisted exfoliation method. A small amount of NiCo-LDH powder is put into a centrifuge tube, and the centrifuge tube is filled with CH3OH2OH alcohol solution. The centrifuge tube is then placed in an ultrasonic cleaner for sonication for 12 h. To remove non-dispersed large particles and obtain pure NiCo-LDH nanosheet solution, the ultrasonic dispersion solution is centrifuged at 8000 rpm for 20 min. Finally, the NiCo-LDH nanosheet suspension is obtained by collecting the supernatant. In the passively mode-locked laser experiment, 20 μL of the supernatant is drop-coated onto a flat mirror coated with a 2 μm high reflectivity film, and dried at room temperature for 12 h to fabricate a NiCo-LDH saturable absorption mirror (SAM).

The morphological characteristics and microstructure of NiCo-LDH is characterized by inVia Raman microscope (Renishaw, UK), X-ray diffraction (XRD, Rigaku, Japan), scanning electron microscope (SEM, JSM-6700F, Japan), transmission electron microscopy (TEM, HT7800, Japan) and atomic force microscopy (AFM, MULTIMODE8, German), respectively. The Raman spectrum of NiCo-LDH powder is shown in Fig. 1a, and the characteristic peaks are located in 154 cm−1, 526 cm−1, 1068 cm−1 and 3611 cm−1 separately, which corresponds to the standard NiCo-LDH Raman spectrum, and the XRD pattern analysis results of the NiCo-LDH powder are displayed in Fig. 1b, the main peaks are located at the 2θ angle of 12.44°, 33.62°, 39.62°, 59.92°, which is consistent with the (003), (009), (015), (110) planes, respectively [22]. The SEM characterization results demonstrate the multilayer stack structure of NiCo-LDH powder, as shown in Fig. 1c. The NiCo-LDH nanosheets after sonication are exhibited in Fig. 1d. The NiCo-LDH nanosheets exhibit a ribbon shape with a length of about 1 μm and a width of about 100 nm. The NiCo-LDH nanosheets after sonication are stacked due to van der Waals forces, but the stacked nanosheets still maintain a thin morphology.

Fig. 1
figure 1

a Raman spectroscopy and b XRD pattern of NiCo-LDH powder; c SEM image of NiCo-LDH powder; (d) TEM image of NiCo-LDH nanosheet

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The AFM is used to detect the thickness of the NiCo-LDH nanosheets. The AFM image (Fig. 2) shows the height of the NiCo-LDH nanosheet in the scale of 5.5 µm × 5.5 µm. The heights along lines a and b are displayed in the right panel of Fig. 2, and the thickness of the NiCo-LDH nanosheets is about 12 nm.

Fig. 2
figure 2

AFM image and corresponding height distribution of the NiCo-LDH nanosheet

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The light absorption characteristics of NiCo-LDH powder in the infrared band is measured by a UV/VIS/NIR spectrophotometer (CARY500, Varian, American), and the results are revealed in Fig. 3a. The linear light absorption characteristics prove that NiCo-LDH powder has broadband absorption in the infrared band, and the absorption rate at 2 μm wavelength is 29.4%. The nonlinear absorption properties of NiCo-LDH nanosheets are measured by I-scan [23], and the results are displayed in Fig. 3b. The light source for the I-scan is a self-made acousto-optic Q-switched laser at 2 μm with a pulse width of 400 ns and a repetition rate of 1 kHz. The experimental data of the I-scan can be fitted using:

$$T = 1 - \Delta R \cdot \exp \left( - \frac{I}{{I_{s} }}\right) - \alpha_{ns} ,$$
(1)

where \(T\) is the transmission of the NiCo-LDH nanosheets, \(\Delta R\) is the modulation depth, \(I\) is the incident power intensity, \(I_{s}\) is the saturable fluence and \(\alpha_{ns}\) is the unsaturated loss. According to the fitting results, the modulation depth, saturable fluence and unsaturated loss of NiCo-LDH nanosheets at 2 μm are calculated to be 4.9%, 1.8 mJ/cm2 and 5.7%, respectively.

Fig. 3
figure 3

a The linear absorption of NiCo-LDH powder; b I-scan results of the NiCo-LDH nanosheet at 2 µm

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3 Experimental setup

The experimental setup of the Tm:YAG ceramic passively mode-locked laser is depicted in Fig. 4. A YAG ceramic with a Tm3+ doping concentration of 6 at.% and a dimension of 1 mm × 6 mm × 9 mm, is used as the laser medium. To dissipate heat loading, the Tm:YAG ceramic is wrapped with indium foil and tightly mounted in water-cooled heat sinks with a circulating water temperature of 10 °C. An X-type resonator, whose optical length is around 1.03 m, is employed to realize the passively mode-locked laser output in the 2 μm band. Through a 2:1 coupling system, a 793 nm laser diode with a core diameter of 200 μm and a numerical aperture of 0.22 is used to pump the Tm:YAG ceramic. Two concave mirrors (M1, M2) with a curvature radius of − 100 mm are used on both sides of the Tm:YAG ceramic to form the X-shaped upper-end folding mirror and are high-reflection-coated (> 99%) at 1820–2150 nm. Besides, mirror M1, which is used as an input mirror, is antireflection(AR)-coated at 790–810 nm. Correspondingly, two concave mirrors (M3, M6) with a radius of curvature of − 500 mm are used as the folding mirrors of the X-type cavity, and they are also high-reflection-coated at 1980–2150 nm. Finally, a mirror with transmission of 1% at 2 μm (M5) is used as the output coupler in the end of the right arm of the X-type cavity, and a flat high-reflection mirror (M4) is adopted as the end mirror for the left arm. With the ABCD matrix method, the resonator mode size on mirror M4 is calculated to be around 40 μm.

Fig. 4
figure 4

Experimental setup of the Tm:YAG ceramic passively mode-locked laser

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4 Results and discussion

The Tm:YAG ceramic continuous-wave (CW) laser output is obtained without the saturable absorber in the X-type resonator, as shown in Fig. 5a. When the absorbed pump power achieves 12.8 W, the output power is 321 mW. By a linear fit to the absorbed and output power, the slope efficiency is calculated as 2.54%, corresponding to an optical-to-optical conversion power of 2.51%. Through a spectrometer (APE-wave-scan, Germany), the central wavelength of the Tm:YAG ceramic laser is obtained at 2014.2 nm with a narrow spectral width of 0.6 nm.

Fig. 5
figure 5

The output power versus absorbed pump power and the corresponding output spectra (Inset) of the lasers. a CW laser. b QML laser

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To achieve 2 μm passively mode-locked laser output, the high-reflection mirror (M4) is drop-coated with the NiCo-LDH saturable absorber. In our experiment, the 2 µm QML laser with maximum output power of 278 mW under an absorbed pump power of 12.8 W was realized. As depicted in Fig. 5b, a slope efficiency of 2.21% is obtained while an optical-to-optical conversion efficiency is 2.18%. The central wavelength of the 2 μm passively mode-locked laser is located at 2012.1 nm with a narrow spectral width of 0.9 nm.

The temporal pulse train of the 2 μm QML laser is detected using an InGaAs PIN detector (EOT, ET-5000) and a digital oscilloscope (Tektronix, DPO 4102B-L 1 GHz 5G samples/s). The repetition frequency of the train is 146 MHz, corresponding to a round trip time of 6.8 ns in the X-type resonator, and the pulse laser train records with different time scales are depicted in Fig. 6.

Fig. 6
figure 6

The temporal pulse train records of the 2 μm QML laser

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The pulse width of the 2 μm QML laser can be further estimated as 1.25 times \(t_{re}\), which is the real rising time of the pulse laser and can be calculated by formula (2):

$$t_{me} = \sqrt {t_{re}^{2} + t_{p}^{2} + t_{osc}^{2} } ,$$
(2)

where \(t_{me}\) represents the measured rising time of the mode-locked pulse laser detected by the digital oscilloscope; \(t_{p}\) and \(t_{osc}\) are the rising time of the photoelectric probe and the digital oscilloscope, respectively [24]. Thus, the pulse width of the Tm:YAG ceramic QML laser can be estimated to be around 221 ps. Future work will focus on the optimization and dispersion compensation of the 2 μm QML resonator for shorter pulse width.

According to the analysis of the mode-locking theory [25], the realization of CW mode-locked laser output requires that the pulse energy in the cavity satisfies the Eq. (3):

$$E_{p,c} = \sqrt {F_{sat,L} A_{eff,L} F_{sat,A} A_{eff,A} \Delta R} ,$$
(3)

where \(E_{p,c}\) is the minimum intracavity pulse energy required to obtain CW mode-locking results; \(F_{sat,L}\) is the saturation flux of the gain medium and can be calculated with \(F_{sat,L} = {{h\upsilon } \mathord{\left/ {\vphantom {{h\upsilon } {2\sigma_{L} }}} \right. \kern-0pt} {2\sigma_{L} }}\) (\(\sigma_{L}\), which is equal to 3.04 × 10–21 cm2, is the emission cross-section of the Tm:YAG ceramic); \(A_{eff,L}\) and \(A_{eff,A}\) is the area of the fundamental laser mode on the Tm:YAG ceramic and the NiCo-LDH saturable absorber mirror, respectively; \(F_{sat,A}\) is the saturation energy density of the saturable absorber mirror; and \(\Delta R\) is the modulation depth of the NiCo-LDH saturable absorber mirror. Unfortunately, the calculated threshold pulse energy only can reach 3.8 µJ, which is far greater than the maximum pulse energy of 2.06 nJ inside the laser cavity in our experiment. The CWML laser performance of the NiCo-LDH SA still need to be explored further.

Table 1 summarizes reports on 2 μm all-solid-state QML lasers based on 2D materials saturable absorbers. As far as we know, this is the first time that NiCo-LDH has been used as the saturable absorber of 2 μm QML laser, and the estimated pulse width of the minimum 221 ps is achieved.

Table 1 2 μm QML laser based on various 2D material saturable absorbers
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5 Summary

In conclusion, the optical characteristics of NiCo-LDH and its performance as a saturated absorber for 2 μm ultrafast laser are in-depth investigated. By building an X-type Tm:YAG ceramic all-solid-state laser, the highest QML output power of 278 mW is finally realized at the absorbed pump power of 12.8 W. And a 2 μm QML pulse width of 221 ps at 2012.1 nm is achieved with a repetition frequency of 146 MHz. The results prove that Nico-LDH is a promising candidate as the mid-infrared ultrafast laser modulator.