1. Introduction

Laser sources with emitting wavelengths in the 2.5-5 µm mid-infrared (mid-IR) region have emerged as ideal tools for various important applications such as spectroscopy, laser surgery in human tissue, countermeasures and remote sensing, etc [1–4]. Rare-earth doped optical fibers are outstanding waveguide platforms for the generation of guided laser emission with well-known advantages such as excellent beam quality, compactness, great heat dissipation capability, high single-pass gain and high flexibility. Pulsed fiber lasers that are capable of producing high-energy/peak power laser pulses are also in great demand for specific applications, such as biomedical surgery, laser processing, and mid-IR nonlinear wavelength conversion, where high-energy/peak power are essential. Mid-IR pulsed fiber lasers have been demonstrated with various Q-switching and mode-locking techniques. Q-switched fiber lasers, which are capable of producing microsecond and/or nanosecond pulses, are highly desired for numerous applications requiring high-energy pulses [5,6]. Q-switched operation of a fiber laser can be realized by modulating the Q-factor of the laser resonator actively with a modulator (e.g. acousto-optic and electric-optic devices) or passively with a saturable absorber (SA). In contrast to complicated and bulky active Q-switching methods, passive Q-switching is more attractive for compact and low-cost laser systems due to its simple and compact system structure and robustness. The choice of SA, which is used to periodically modulate the laser intracavity losses by introducing a strong intensity-dependent absorption in the laser cavity, is one of the most crucial aspects for passive Q-switching.

The development of mid-IR short-pulsed fiber lasers has historically been hindered by the poor availability of gain fibers because that the commonly used silica fibers are strongly absorbing for optical wavelengths above ∼2.4 µm due to vibrational resonances [1]. The rapid development of rare-earth doped soft-glass fibers in the last decade has led to tremendous progress in mid-IR fiber laser technology and is calling for SAs with performance levels on par with their near-infrared counterparts [7]. Being a powerful optical switch, semiconductor saturable absorber mirror (SESAM) has been considered to be the most mature SA due to its highly developed fabrication procedure, at-will parameter design, and great long-term stability. However, SESAM suffers from inherent drawbacks such as complex fabrication procedure, bulky structure, as well as narrow operating wavelength range (typically several tens of nm [8]) due to the limited energy bandgap of semiconductors and narrow bandwidth of distributed Bragg gratings. As another mature mid-IR SA, Fe²⁺: ZnSe crystal shows a broad absorption range of 2.5–4.2 µm and a high damage threshold. With Fe²⁺: ZnSe crystal as SA, a Q-switched fiber laser operating in the mid-IR waveband with output average power of 5 W has been demonstrated [9]. However, the bulk nature hampered its integration with fiber for laser sources with all-fiber configuration.

In the last decade, two-dimensional (2D) layered materials have been considered for substitutes for bulk SAs as mid-IR SAs because of their intriguing properties such as broadband operation, easy fabrication and integration into various fiber configurations, mechanical flexibility, excellent nonlinear optical response, and ultrafast dynamic processing. Since the first demonstration of 3 µm fiber lasers passively Q-switched by the revolutionary 2D material-graphene [10], other novel graphene-like 2D materials have been extensively studied as potential mid-IR SAs. In the past few years, mid-IR passively Q-switched fiber lasers have been reported using 2D material SAs such as topological insulators (TIs) [11,12], transition metal dichalcogenides (TMDs) [13,14], black phosphorus (BP) [15–19], and MXene [20–22], Sb [23], antimonene [24]. Although these 2D materials have demonstrated promising saturable absorption properties in the mid-IR spectral region, they exhibit shortcomings which limit their practical applications to a certain extent. For example, the modulation depth of graphene is a bit low (typically ∼1.3% per layer) for mid-IR pulsed fiber lasers [25]. TMDs generally exhibit large bandgaps and typically have resonant absorption in the visible. Though sub-bandgap absorption from crystallographic defects and edge states may present [26,27], precise control of the sub-bandgap absorption effect of TMDs is still a challenge. BP is easily oxidized and can be degraded within a few hours under ambient condition [28]. And this oxidation process of BP tends to be accelerated under thermal effects [15]. In addition, well-controlled and robust preparation methods for most of these 2D materials are still lacking. Therefore, the optimum choice of SAs for the mid-IR waveband is still an open research question and the identification of novel materials, especially those with 2D structures, as promising mid-IR nonlinear SAs, remains a longstanding goal.

As an emerging and new class of material family with periodic structure, metal-organic frameworks (MOFs) have attracted increasing attention in recent years. As a class of porous crystalline materials, MOFs are consisted of metal ions linked together by organic bridging ligands. MOFs exhibit remarkably large surface areas, ordered reticular structures, nanometer-sized void/pore spaces, tunable porosities and functionalities, tunable bandgap, high optical transparency, good conductivity, high elastic moduli, and high electric capacity, which render them versatile platforms for applications in sensing, gas storage, drug delivery, catalysis, nonlinear optics, as well as molecular separations [29–33]. Moreover, to leverage the enticing properties of MOFs, the fabrication techniques of MOFs have been significantly advanced, and both chemical vapor deposition and bulk synthesis are approaching commercial feasibility [34–36]. Recently, direct synthesis of MOF films with controlled crystalline orientation has also been demonstrated [37], further promoting the development of fabrication methods and represent an important step towards fully realizing the potential of MOFs in advanced technologies. To date, the linear and nonlinear optical performances of MOFs, such as remarkable second-harmonic generation intensity, multiphoton harvesting, microcavity two-photon lasers, and luminescence, have been demonstrated [38–41]. Nevertheless, the saturable absorption properties of MOFs and their use in fiber laser pulse generation have rarely been explored. Recently, nickel-p-benzenedicarboxylic acid MOFs (Ni-MOFs) have been demonstrated to be excellent broadband SAs for generating mode-locked laser pulses in the near-infrared regime (1.06 and 1.56 µm, 1.88 µm) [42,43]. Very recently, using zeolitic imidazolate framework-8 (ZIF-8), a typical kind of MOF, as the SA, L. Dong et al have demonstrated passively Q-switched fiber lasers at 1.06, 1.34, and 1.91 µm, and mode-locked fiber laser at 1.04 µm with good long-term stability and repeatability [44].

As a well-known subclass of MOFs, MIL-68(M) (MIL stands for Material Institute Lavoisier) materials, and M represents trivalent metal cations including Fe³⁺, Al³⁺, In³⁺ etc.) are assembled from the infinite chains of corner-sharing metal-centered octahedral MO₄(OH)₂, which are connected to each other via terephthalate ligands [45,46]. MIL-68(M) exhibit 2D crystalline structure, where the organic ligands (terephthalate) and metal atoms were orderly assembled in a layer-by-layer manner. MIL-68(M), such as MIL-68(Al) and MIL-68(Fe), exhibits superior chemical and thermal stability and a very high Brunauer-Emmett­-Teller (BET) surface area [46–48]. All these appealing features of MOFs-MIL-68(M) and the important progress made in recent linear and nonlinear optical studies of MOFs inspired us to explore the saturable absorption property of MIL-68(M) and its potential as a SA for the important fiber laser sources emitting light in the mid-IR spectral range.

Herein, we fabricated MIL-68(Al) and MIL-68(Fe) via hydrothermal method and demonstrated their uses as excellent saturable absorbers for Q-switched fiber lasers at 3 µm mid-IR regime. The Q-switched fiber lasers deliver nanosecond pulses of 567 ns and 696 ns with maximum average powers of 809.1 mW and 581.6 mW, respectively. The corresponding pulse energies are 4.49 µJ and 3.09 µJ, respectively. Our experimental results revealed that MIL-68(M) materials are promising optical switches for generating high-power short pulses in the mid-IR spectral range. Considering the chemical compatibility and rich variability of the MOFs, this work may open a new avenue for the development of compact 2D MOFs-based all-optical modulators for applications in the mid-IR region.

2. Preparation and characterization of MIL-68(Al) and MIL-68(Fe)

2.1 Synthesis of MIL-68(Al) and MIL-68(Fe)

All the chemicals and reagents were commercially obtained without further purification. N, N’-dimethylformamide (DMF, CAS No. 68-12-2), fluorhydric acid (HF, CAS No. 7664-39-3), hydrochloric acid (HCl, CAS No. 7647-01-0) and acetone (CH₃COCH₃, CAS No. 67-64-1) were purchased from Aladdin. Terephthalic acid (H₂BDC, CAS No. 100-21-0) was purchased from Zhengzhou Acmechem Co., Ltd. Iron (III) chloride hexahydrate (FeCl₃·6H₂O, CAS No. 10025-77-1) was purchased from Shanghai Titan Scientific Co., Ltd. Aluminum chloride hexahydrate (AlCl₃·6H₂O, CAS No. 7784-13-6) was purchased from Adamas-beta. Deionized water (resistivity: 18.2 MΩ×cm) was used throughout the work.

The synthesis of MIL-68(Al) was based on the BASF patent with slight modification [49]. Terephthalic acid (5 g, 30 mmol) and AlCl₃·6H₂O (4.88 g, 20.0 mmol) were dissolved into 100 mL of DMF respectively, followed by mixing in a 500 mL round bottom flask. After completely dissolving, the mixture was continuously stirred under refluxing at 135 °C for 18 h. The mixture was then cooled naturally to room temperature and filtered under vacuum. The solid was then washed three times with 50 mL of DMF at room temperature. Each washing step last 3 h. To remove the residual organics inside the nanocages, the solid was then washed four times with 50 mL MeOH. After drying at 80 °C overnight, the white powdery product (2.19 g) was obtained.

The synthesis of MIL-68(Fe) was based on the previous report with slight modification [47,50]. FeCl₃·6H₂O (0.81 g, 3 mmol) and terephthalic acid (1 g, 6 mmol) were dissolved into 50 mL DMF, separately. After stirring for 30 min, the solutions were mixed in a 250 mL teflon-lined autoclave. Mixture of HF (360 µL, 5 M) and HCl (360 µL, 1 M) was then added to the above solution. After completely dissolving, the reactor was sealed and heated at 100 °C for 120 h. After naturally cooling to room temperature, the brown participate was collected by centrifugation. The solid was washed with deionized water and acetone three times (6 h for each time) to eliminate the redundant reactant and DMF inside the pores. Yellow solid (0.98 g) was collected and activated under 100 °C in a vacuum oven for 12 h before using.

To study the saturable absorption properties in the mid-IR waveband of the MIL-68(Al) and MIL-68(Fe), we fabricated reflection-type MIL-68(M) saturable absorber mirrors (SAMs). The MIL-68(M) SAMs were fabricated by transferring the synthesized MIL-68(M) powders onto gold-coated mirrors. 10 mg MIL-68(M) powders were dispersed in 3 mL dichloromethane (DCM) solvent. Then the mixed solutions were sonicated for 1 hour and centrifuged at 2000 rpm for 5 min. The top 1 mL homogeneous mixed solution was collected and dipped on the upper surface of highly reflective gold-coated mirrors. DCM was gently evaporated at a 30 °C vacuum oven. The MIL-68(M) coated mirrors were placed to dry at room temperature for 12 hours for further characterizations and applications.

2.2 Characterization of MIL-68(Al) and MIL-68(Fe)

Figure 1 shows the crystal structure of the MIL-68 (M). It shows a layered structure and the layers are connected by benzene rings. The M clusters are connected to each other by oxygen and carbon. Figure 2(a) shows the morphology of the MIL-68(Al) crystals examined by a transmission electron microscope (TEM). As displayed in Fig. 2(a), the fabricated MIL-68(Al) contains disordered clusters of needle-like crystals with a broad size distribution. Figure 2(b) shows the X-Ray Diffraction (XRD) pattern of the MIL-68(Al) crystals, both peak positions and relative intensity are in good accordance with previous literature [51].

 

Fig. 1. (a) Schematic representation of the 2D MIL-68 (M) crystal structure. (b) and (c) the left and vertical view of the crystal structure. (d) monolayer structure.

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Fig. 2. (a) TEM image of MIL-68(Al) with a 100 nm scale ; (b) XRD pattern of as-synthesized MIL-68(Al); (c) nonlinear absorption spectrum of the MIL-68(Al); (d) TEM image of MIL-68(Fe) with a 100 nm scale; (e) XRD pattern of as-synthesized MIL-68(Fe); (f) nonlinear absorption spectrum of the MIL-68(Fe).

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Figure 2(c) shows the nonlinear transmission of the MIL-68(Al) measured by a typical balanced twin detector system elaborated in Ref. [14]. The source is a homemade mode-locked fiber laser at 2.87 µm with a repetition rate of 18.39 MHz and a pulse duration of ∼20 ps. The parameters of the MIL-68(Al) SA were fitted with the formula $R(I) = 1 - \mathrm{\Delta }R \cdot \exp ( - \frac{I}{{{I_{sat}}}}) - {R_{ns}}$, where $R(I)$ indicates the reflectivity, $\mathrm{\Delta }R$ is the modulation depth, I is the incident peak intensity, ${I_{sat}}$ is the saturation peak intensity and ${R_{ns}}$ represents the non-saturable loss. According to the experimental results, the modulation depth, non-saturable loss, and saturation peak intensity were fitted to be 23.89%, 47.15%, and 0.0635 GW/cm², respectively.

Similarly, we carried out the characterization of the as-prepared MIL-68(Fe) sample. As is shown in Fig. 2(d), the bare MIL-68(Fe) particles had a cuboidlike morphology with sizes of approximately 2–3 µm. Figure 2(e) shows the XRD pattern of the as-prepared MIL-68(Fe) crystals. We marked up the main peak positions and all the peak positions corresponded well to the previous works, which indicates the same crystal lattice parameters (cell lengths and angles) but an effect of preferred orientation (differences in relative intensity) [52]. Figure 2(f) shows the nonlinear transmission of the MIL-68(Fe). The modulation depth, non-saturable loss, and saturation peak intensity were fitted to be 17.01%, 47.44%, and 0.0486 GW/cm², respectively.

3. Experimental setup

The excellent nonlinear response at 2.8 µm of MIL-68(M) indicates its potential use as an efficient optical modulator in the mid-IR spectral region. To study its optical performance in the 2.8 µm waveband, we proposed the MIL-68(M) SAs enabled passively Q-switched mid-IR fiber lasers. The schematic diagram of the experimental setup is shown in Fig. 3. The pump source is a 30 W commercial laser diode (LD) operating at 976 nm with a fiber pigtail [core/numerical aperture (NA): 105/0.22]. A 3.8 m double cladding Er³⁺-doped ZBLAN fiber (Le Verre Fluoré, France) with an erbium doping concentration of 70,000 parts per million (ppm) was used as the gain medium, which has a core diameter of 15.6 µm ($NA = 0.12$) and an inner cladding diameter of 240×260 µm ($NA = 0.4$). The front end of fiber was perpendicularly cleaved to the axis of the fiber to provide a 4% feedback and the fiber was pumped through a coupling system which was composed of lens group L1 (plano-convex lens, $f = 16$ mm) and L2 (CaF₂, $f = 20$ mm). The other end was angle-cleaved at 10° to suppress parasitic feedback and a pair of ZnSe objective lens (L3:$f = 12$ mm, L4:$f = 6$ mm) was used to collimate and focus the output light beam onto the SAM. A specifically designed dichroic mirror (M1) (∼96% transmission at ∼976 nm, ∼95% reflection at ∼2.8 µm) was 45° placed between L1 and L2 for the separation of pump beam and laser. A bandpass filter (FB2750-500, Thorlabs), which has a bandwidth of ∼500 nm from 2550 to 3050 nm, is used to purify the ∼2.79 µm with a transmittance of ∼84%. Beam splitters are used to monitor the laser output power, temporal behavior and optical spectrum simultaneously. The average output power was recorded with a high-resolution thermal power sensor (Thorlabs, S405C). The temporal pulses were captured by a 2 ns response time indium arsenide (InAs) detector (Judson J12D) connected with a 350 MHz bandwidth digital oscilloscope. The optical spectrum was measured by a monochromator with a scanning resolution of ∼0.1 nm around 3 µm (Princeton Instruments Acton SP2300). Note that the laser powers mentioned in this Letter are all the corrected values according to the transmittance of the used splitters and filter.

 

Fig. 3. The schematic of the passively Q-switched Er³⁺-doped ZBLAN fiber laser using MOFs as the SA. LD, laser diode; L1, plano-convex lens ($f = 16$ mm); L2, CaF₂ plano-convex lens ($f = 20$ mm); L3 and L4, ZnSe objective lenses ($f = 12$ mm and $f = 6$ mm); M1, dichroic mirror; BS, beam splitter.

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4. Results and discussion

4.1. Performance of the Q-switching operation based on MIL-68(Al)-SAM

When we placed the as-prepared MIL-68(Al)-SAM at the focal point of the confocal setup, self-staring Q-switching was observed at the launched pump power of 2.59 W with a repetition rate of 151.79 kHz and a pulse duration of 707 ns, as shown in Fig. 4(a) and 4(c). Note that there is no sign of a stable Q-switched pulse generation when the gold mirror with no material coating was utilized instead. With the launched pump power gradually increased to 4.52 W, the stable Q-switched operation was sustained. The repetition rate of 180.26 kHz and the shortest pulse duration of 567 ns were recorded, as shown in Fig. 4(b) and 4(c). The optical spectrum measured under the maximum launched pump power of 4.52 W is shown in Fig. 4(d). The center wavelength of 2793.8 nm and the full width at half maximum (FWHM) of 1.4 nm were achieved. Figure 4(e) shows the radio frequency (RF) spectra. The signal-to-noise ratio (SNR) was measured to be 38.5 dB with the resolution bandwidth of 100 Hz at the frequency of 180.26 kHz, indicating a stable Q-switched operation.

 

Fig. 4. Q-switched pulse trains at the launched pump power of (a) 2.59 W and (b) 4.52 W, (c) Q-switched single-pulse waveforms at the pump power of 2.59 W and 4.52 W, (d) optical and (e) RF spectra of the Q-switched pulses at the pump power of 4.52 W, (f) repetition rate and pulse width as functions of the pump power, (g) output power and single-pulse energy as functions of the pump power.

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Figure 4(f) shows the variation of Q-switched pulses as a function of the launched pump power. Specifically, the repetition rate increased from 151.79 kHz to 180.26 kHz and the pulse duration decreased from 707.0 ns to 567 ns with the launched pump power rising from 2.59 W to 4.52 W. Figure 4(g) shows the evolution of the output power and pulse energy over the same pump range. When the pump power was improved from 2.59 W to 4.52 W, the Q-switched output power increased from 464.5 mW to 809.1 mW and the pulse energy raised from 3.05 µJ to 4.49 µJ. The slope efficiency is 18.39%. At the maximum launched pump power of 4.52 W, the highest peak power was calculated to be 7.91 W. Once the pump power exceeded 4.52 W, the pulse train became unstable, showing amplitude fluctuation and timing jitter and then reverted to continuous-wave (CW) regime. When the pump power was decreased back, stable Q-switched operation could be reproduced, indicating that the SA was not damaged due to the photothermal effect and verifying the high optical damage threshold of MIL-68(Al).

4.2. Performance of the Q-switching operation based on MIL-68(Fe)-SAM

In order to study the saturable absorption property of MIL-68(Fe) in the mid-IR waveband, we replaced the MIL-68(Al) SAM with MIL-68(Fe) SAM in the laser resonator. By increasing the pump power to 0.66 W, stable Q-switched pulses were obtained with a repetition rate of 122.08 kHz and a pulse duration of 2.43 µs, as displayed in Fig. 5(a) and 5(c). This stable Q-switching regime could be maintained until the maximum launched pump power of 4.52 W. When the pump power was increased to 3.63 W, the shortest pulse width of the Q-switched pulses was 696 ns at a repetition rate of 150.75 kHz, as shown in Fig. 5(b) and 5(c). The optical and radio frequency (RF) spectra of the Q-switched pulses at the launched pump power of 3.63 W were measured and shown in Fig. 5(d) and Fig. 5(e), respectively. The center wavelength is 2785.6 nm and the FWHM is 1.3 nm. The resolution bandwidth was 100 Hz and the SNR of 38.9 dB indicated a stable Q-switched operation. With further increasing the launched pump power beyond 4.52 W, the Q-switching became unstable and then disappeared. However, stable Q-switched operation could also be reproduced when we reduced the pump power.

 

Fig. 5. Q-switched pulse trains at the launched pump power of (a) 0.66 W and (b) 3.63 W, (c) Q-switched single-pulse waveforms at the pump power of 0.66 W and 3.63 W, (d) optical and (e) RF spectra of the Q-switched pulses at the pump power of 3.63 W, (f) repetition rate and pulse width as functions of the pump power, (g) output power and single-pulse energy as functions of the pump power.

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The evolutions of the pulse properties were also recorded with the increasing launched pump power. Figure 5(f) shows the pulse duration narrowed from 2.43 µs to 696 ns and the repetition rate increased from 122.08 kHz to 204.03 kHz when we increased the launched pump power from 0.66 W to 4.52 W. At the launched pump power of 3.63 W, the shortest pulse duration of 696 ns with a repetition rate of 150.75 kHz was obtained. When the pump power was increased further, the pulse duration increased a bit from 696 ns to 729 ns, which could be attributed to the excessive heat-induced performance changes of the MOFs sample at high incident intensities, as has previously been reported with other nanomaterials [14,17,24]. In addition, we measured the output power and calculated the single pulse energy at varied launched pump powers, as shown in Fig. 5(g). The output power increased almost linearly from 51.76 mW to 581.62 mW with a slope efficiency of 14.0%. At the launched pump power of 3.63 W, the highest pulse energy of 3.09 µJ and peak power of 4.44 W were calculated.

To evaluate the performances of the Q-switched fiber lasers, we compared the laser performances, including the output powers and pulse widths, of the 2D materials Q-switched single-mode fiber lasers operating in the 3-µm mid-IR waveband, as summarized in Fig. 6. We can clearly see that the output power achieved from this work is the highest while the pulse duration is the shortest.

 

Fig. 6. Comparison of laser performance of mid-IR passively Q-switched fiber lasers with different 2D nanomaterials.

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5. Conclusions

In conclusion, we have synthesized MIL-68(Al) and MIL-68(Fe) via hydrothermal method and fabricated the MIL-68(Al)- and MIL-68(Fe)- SAMs using the drop coating method. The nonlinear optical absorptions of the MIL-68(Al) and MIL-68(Fe) at 2.8 µm were characterized and the modulation depths were found to be 23.89% and 17.01%, respectively. With the as-prepared MIL-68(Al)- and MIL-68(Fe)- SAMs, we achieved Q-switched pulses from Er³⁺-doped ZBLAN fiber lasers at 2.8 µm, which is the first demonstration of MIL-68(M)-based laser pulse generation in the 3-µm mid-IR spectral region, to the best of our knowledge. The Q-switched fiber lasers deliver nanosecond pulses with high average output powers. Particularly, when using the MIL-68(Al)-SAM, we achieved stable Q-switched pulses with an average output power of 809.1 mW and a pulse duration of 567 ns at ∼3 µm. Our research results indicate that the MIL-68(M) are promising SA materials for high power nanosecond laser pulse generation at 3-µm waveband.