Application of Pr-MOFs as saturable absorbers in ultrafast photonics

Abstract

As a newly synthesized metal–organic framework (MOF) material, Pr-MOFs have been theoretically predicted by researchers to have excellent optical response and nonlinear absorption properties. However, using Pr-MOFs for ultrafast photonics has yet to be reported. In this study, a Pr-MOFs-based saturable absorber (SA) was applied as a mode-locked device in an erbium-doped fiber laser, and mode-locked pulses, harmonic mode-locked pulses, and nanosecond pulses were obtained. The Pr-MOFs-based SA was successfully prepared using the direct coupling technique, and the balanced twin-detector technique acquired a modulation depth of 2.41%. Mode-locked pulses with a pulse width of 1.261 ps and harmonic mode-locked pulses with a pulse width of 1.538 ps, in addition to nanosecond pulses with a pulse width of 37.17 ns, were generated using Pr-MOFs-based SAs. Our work demonstrates the nonlinear optical properties of Pr-MOFs and broadens their application in the field of ultrafast photonics.

---

1. Introduction

Ultrashort optical pulses are highly favored for their high resolution and intensity and have wide applications in fields such as optical communication,¹ optical sensing,^2,3 optical surgery,⁴ optical imaging,⁵ data storage,⁶ and industrial processing.⁷ Mode-locked is the primary technique used for generating picosecond or femtosecond ultrashort pulses. It achieves high-quality single-frequency output by effectively controlling the laser's output and eliminating the effect of the laser's natural linewidth on pulse performance (stability and purity). In addition, mode-locked allows precise control of the laser's output frequency and frequency stability, making it a reliable choice in high-precision measurement and frequency comparison. Therefore, research on mode-locking technology is significant for improving laser performance. Mode-locked technology includes active mode-locked and passive mode-locked. However, active mode-locked technology requires additional optical components, increasing the cavity's complexity and loss.⁸ Passive mode-locked technology has advantages like narrow-band output, high stability, and simple structure. Saturable absorbers (SAs) play a vital role in passive mode-locked lasers.^9–12 A semiconductor saturable absorber mirror (SESAM) is a common SA, which is widely used in passive mode-locked operations. The SESAM process is mature and stable, but the disadvantages are the inability to achieve broadband response, long response time, and unique structures that only allow for some fiber structure design.¹³ Two-dimensional (2D) materials have attracted much attention recently because of their advantages as saturable absorbers, such as their particular layered structure, excellent nonlinear optical properties, and diverse preparation methods.

Until now, some 2D materials such as graphene,^14–16 carbon nanotubes (CNTs),¹⁷ transition metal dichalcogenides (TMDs),^18–20 topological insulators (TIs),²¹ Mxene,^22–24 2D transition metal chalcogenides (TMCs),²⁵ group III metal monochalcogenide MX (M = In and Ga; X = Se, Te, and S),²⁶ 2D bismuth-based compound,²⁷ 2D quantum dots (QDs),²⁸ transition metal oxides (TMOs),²⁹ and black phosphorus (BP),³⁰ have shown good saturable absorption properties and unique physical properties, demonstrating great potential for application in passive mode-locked fiber lasers. However, commonly used materials need address practical issues, such as optical and thermal stability, which limit their applications.^31–34 Graphene's zero band gap and weak absorption (only ∼2.3% light absorption) are reported to severely limit its light modulation capability and applications in light-matter interaction.³⁵ The intrinsic band gap of TMDs is in the visible/near-infrared region, limiting their practical application.^36–38 BP is easily oxidized in air, which is unsuitable for long-term stability applications.^39,40 Therefore, finding new SAs with high performance, low cost, and increased stability is necessary. Several studies have shown that metal–organic frameworks (MOFs), as a solid-state hybrid material, have been proven to overcome many difficulties due to their unique porous structure, known as a research hotspot and frontier in the field of materials.⁴¹ MOFs are three-dimensional (3D) networks of metal ions immobilized by multi-isotope organic molecules,^42,43 with a very stable crosslinked mesh structure. Therefore, the materials are not susceptible to aging. In 1995, the Yaghi group in the United States synthesized the first MOF material, and in 1999 developed MOF-5, a MOF material with permanent pores, ushering in the “golden age” of MOFs.^44,45 MOFs have advantages such as large surface area, nanoscale structure and functional diversity, presence of unsaturated metal sites, tunability of band gap, compatibility with water and organic solvents, and they also have excellent electrical conductivity, high elastic modulus, good optical transparency, and capacitance.⁴⁶ More than 2000 MOFs have been investigated for specific applications, including gas storage, supercapacitors, chemical sensing, nano-catalysis, and nonlinear optics.^47–51

In recent years, due to the unique combination of metal ions and organic ligands in MOF materials, MOFs have unique and diverse optical properties and also exhibit good nonlinear optical properties, such as second-order nonlinear optical effects (optical nanosecond switching, second harmonic generation, etc.), third-order nonlinear optical effects (optical phase conjugation, self-focusing, etc.), and nonlinear absorption, etc.⁵² The nonlinear optical properties of MOFs have potential applications, such as in optical communication, optical storage, optical computing, and optical medicine.^53–55 They can be used to prepare efficient optical devices and optical materials.⁵⁶ However, the research on the application of MOFs in ultrafast photonics has just started. Jiang et al. fabricated Ni-MOF-SA based on D-fiber, which was applied in erbium-doped and ytterbium-doped fiber lasers and recorded pulse widths of 0.749 ps and 240 ps separately.⁵⁷ Zhang et al. reported the generation of ultrashort pulses in erbium- and thulium-doped fiber lasers using microfiber-based Ni-MOF SAs, with ultrashort pulse widths of 0.384 ps and 1.3 ps generated at the operating wavelengths of 1563 nm and 1882 nm, respectively.⁵⁸ ZIF-67 was operated as an SA for passive Q-switch in the near-infrared (NIR) band, and stabilized Q-switched laser pulses of 120 ns and 108 ns were obtained by Pan et al.⁵⁹ Porous MOF-derived CuO octahedra as SAs were applied to an erbium-doped fiber laser by Zhao et al. to obtain harmonic (40th order with a repetition frequency of 238 MHz) soliton molecular outputs and mode-locked pulses with a signal-to-noise ratio (SNR) of 55 dB.⁶⁰ Chu et al. obtained Q-switched pulses at 2271.7 nm using ZIF-67-based SAs (with an output power of 220 mW, a pulse width of 220 ns).⁶¹ Prepared ZnO/Co₃O₄ nanocomposites by calcining core–shell ZIF-8@ZIF-67 as SAs were successfully applied to an erbium-doped fiber laser (EDFL) for the first time by Kong et al. Fundamental and harmonic (242nd and 181st order) mode-locked pulses were obtained.⁶² Dong et al. introduced ZIF-8 into a ytterbium-doped fiber laser (YDFL) and obtained stable dissipative soliton mode-locked operation with a pulse width of 21.3 ps.⁶³ The generation of femtosecond-scale pulses in the 1.5 μm region using SAs based on rGO-Co₃O₄ and rGO-ZIF-67 was reported by An et al.⁶⁴ ZIF-8 and ZIF-67 are the MOF materials that are more concentrated in the current research. There are many kinds of MOF materials, and most of the MOF materials are still in the initial stage of research on the application of nonlinear photonics and femtosecond lasers, so it is of great significance and urgency to expand the application of MOFs in these fields.

Here, we investigate the properties of Pr-MOFs and report its use as a SA to generate ultrafast pulses in an EDFL for the first time. The sandwich-structured SA device is prepared by transferring the synthesized Pr-MOFs crystal to the fiber end face and its nonlinear saturable absorption properties is explored. Mode-locked pulses, harmonic mode-locked (second order) pulses, and nanosecond pulses were obtained by adjusting the polarization controller (PC) as the pump power was changed from 90 mW to 846 mW. Our work shows that MOFs, especially Pr-MOFs, have great potential for building mode-locked and harmonic mode-locked fiber lasers.

2. Synthesis and characterization of Pr-MOFs

2.1. Synthesis of Pr-MOFs

We prepared Pr-MOFs crystals using hydrothermal synthesis.^65,66 First, Pr(NO₃)₃·6H₂O and ligand were dissolved in 10 mL of water (Pr(NO₃)₃·6H₂O concentration of 10⁻³ mol L⁻¹) according to the substance mass ratio of 4 : 1. Then placed in a stainless steel reactor (tetrafluoroethylene lined) and adjusted to a PH value of 6–7 by addition of NaHCO₃. The reaction was hydrothermal at 140° for 4 days and then cooled to room temperature after a one-day. Red Pr-MOFs crystals were finally obtained, washed with water, and used for subsequent tests.

2.2. Characterization of Pr-MOFs

X-ray single crystal diffraction analyses suggested that the compound was crystalized in orthorhombic crystal system and Pccn space group. The crystallographic unique unit consisted of one Pr(III) cation, one L²⁻ anion ligand (H₂L = 5-(pyridin-3-ylmethoxy) isophthalic acid), half of a carbonate anion and one coordination water molecule, the formula of the compound should be defined as [Pr(L)(CO₃)_0.5(H₂O)]_n (2308657†). As shown in Fig. 1, the Pr(III) cation was eight coordinated by four carboxylate oxygen atoms from four L²⁻ ligands, three oxygen atoms of two carbonate anions and one water molecule. The carbonate and L²⁻ anion both coordinated with four Pr(III) cations through bridging or chelating modes (shown in Fig. 1(b) and (c)). Thus, Pr(III) cations were linked by the carbonate anions to form one dimensional Pr(III)–CO₃²⁻ chain along c direction (shown in Fig. 1(d) and (e)). L²⁻ anion ligands using carboxylate groups connected those Pr(III)–CO₃²⁻ chains to form three-dimensional network (shown in Fig. 1(f)).

Fig. 1 (a) The coordination environment of Pr(III) cation. (b) Carbonate anion. (c) L²⁻ anion ligand. (d) The structure of Pr(III)–CO₃²⁻ chain view along b direction. (e) The structure of Pr(III)–CO₃²⁻ chain view along c direction. (f) The three-dimensional structure of the compound. (g)–(i): Morphological characteristics of Pr-MOFs crystals. (g) SEM images of morphological features of Pr-MOFs displayed at 100 μm resolution. (h) SEM images of morphological features of Pr-MOFs displayed at 50 μm resolution. (i) SEM images of morphological features of Pr-MOFs displayed at 10 μm resolution. (j) X-ray diffraction of the Pr-MOFs crystal. (k) The absorption spectrum of Pr-MOFs crystal.

Scanning electron microscopy (SEM: JSM7800F) was used to characterize the surface morphology of Pr-MOFs crystals and the images recorded are shown in Fig. 1(g–i). A clear lamellar structure can be observed at a scale of 100 μm (shown in Fig. 1(g)). Sharper lamellar Pr-MOFs crystals were recorded in Fig. 1(h) and (i) at scales of 50 μm and 10 μm, respectively.

The crystalline structure of the free-standing Pr-MOFs was confirmed by X-ray diffraction (XRD) pattern, as shown in Fig. 1(j). The three main diffraction peaks at 2θ = 16.28°, 18.201°, and 25.049° correspond to the (−301), (−3−12), and (−2−13) lattice faces in single-crystalline [Pr(L)(CO₃)_0.5(H₂O)]. As shown in Fig. 1(k), the absorption spectra of Pr-MOFs recorded by UV-VIS-IR spectrophotometer with a resolution of 0.1 nm showed that Pr-MOFs crystals had strong light absorption characteristics in the 1515–1565 nm band.

In the experiments, we prepared Pr-MOFs-based SAs using the direct-coupling technique, as shown in Fig. 2(a). First, Pr-MOFs crystals suitable for the fiber core size were selected at 20× resolution in an optical microscope (DVE3630, Otter Optics). Then, we transferred the Pr-MOFs crystals to the end face of the fiber tip using the needle. Finally, the Pr-MOFs-containing optical fiber was fixed and connected to another optical fiber with a fiber optic adapter. The Pr-MOFs-based sandwich structure of the SA was successfully prepared.

Fig. 2 (a) Process for the preparation of Pr-MOFs-based SA. (b) Schematic of the balanced twin-detector measurement setup. (c) The nonlinear transmittance of Pr-MOFs-based SA.

To demonstrate the saturable absorption properties of SAs based on Pr-MOFs, we investigated the nonlinear optical response of the devices using a balanced twin-detector setup fabricated in our laboratory, as shown in Fig. 2(b). A commercial 1.5 μm femtosecond fiber laser (the center wavelength is 1564.656 nm, the pulse width is 452 fs, and the repetition frequency is 23.6 MHz) was used for the experiments. An adjustable fiber optic attenuator (pVOA-1550-1-B-30-SM-B, Max-ray photonics) is used for power regulation and separated in a ratio of 50 : 50 by an optical coupler (OC). Then, one side was passed directly through a single-mode fiber and the other through a sandwich-structured SA. Finally, the output power was tested simultaneously using two power meters. The nonlinear optical transmission curve of Pr-MOFs-based SA is illustrated in Fig. 2(c). The experimental results show that the transmittance increases with the increase of incident light intensity, which is well fitted to the SA model, The data recorded by adjusting the attenuator were fitted by the following:

where T(I), ΔT, I, I_sat, and T_ns are linear parameters of transmission, modulation depth, input intensity, saturated power intensity, and non-saturated loss, respectively. As shown in Fig. 2(c), the ΔT is 2.41% and the I_sat is 26.11 MW cm⁻². The results confirm that the Pr-MOFs-based SA is fabricated successfully. The modulation depth facilitates the generation of ultrafast pulses by the fiber laser.

3. Experimental setup

Fig. 3 depicts the schematic representation of the mode-locked fiber laser utilizing the SA based on Pr-MOFs. To obtain higher power mode-locked pulses, An erbium-doped fiber (Er80-8-125, Liekki) with a core diameter, erbium ion concentration, and dispersion parameters of 8 μm, 3150 ppm, and 15.7 ps km⁻¹ nm⁻¹, respectively, was used as the laser gain medium, which length was optimized to about 0.46 m. A 42 m section of single-mode fiber (G652D, SMF-28) with a dispersion parameter of 17 ps km⁻¹ nm⁻¹ was added to the resonant cavity for dispersion management, resulting in the cavity length of the entire resonant cavity was 51.5 m. Therefore, the net dispersion of the EDFL is calculated as −1.0938 ps². An optical device known as a laser diode (PUMPL-976-1550-FA-B-YP) serves as the pump source by injecting light into the laser cavity using a wavelength-division multiplexer (WDM:980/1550). A PC was employed to manipulate the cavity polarization and intracavity birefringence to stabilize the mode-locked operation. It is worth mentioning that, to investigate the high-power mode-locked laser's performance, a polarization-independent isolator (PI-ISO) was employed to ensure the laser's unidirectional operation. In addition, monitoring the fiber laser's operating status was achieved by the 10% output portion of the OC and the other 90% was connected back to the loop. The performance of the output laser was recorded by an optical spectrum analyzer (AQ6370D, Yokogawa), a radio frequency (RF) spectrum analyzer (FPC1000, Rohde & Schwarz), an optical power meter (JW3208C, Joinwit), a digital oscilloscope (MDO3102, Tektronix) and autocorrelator (PulseCheck50, APE) and a 3 GHz photodetector (PD-03-FA-D, Max-ray photonics).

Fig. 3 Experimental setup of the mode-locked fiber laser.

4. Results and discussion

Firstly, when the Pr-MOFs-based SA was not added to the cavity, we fully adjusted the PC and pump power, but no pulse was generated. The mode-locked operation can be accomplished by inserting a Pr-MOFs-based SA in the cavity and regulating the PC and pump power. In the following experiments, we obtained mode-locked pulses, harmonic mode-locked (second-order) pulses, and nanosecond pulses, respectively.

4.1. Characteristics of mode-locked pulses at the fundamental frequency

As the pump power was slowly increased to 90 mW, a weak continuous wave was first recorded by carefully adjusting the polarization state in the cavity. At this point the laser power was not high enough for the SA to be bleached. Further increasing the pump power to 212 mW allows for the observation of a stable mode-locked pulse through adjustment of the PC. At this juncture, the laser output presented an average power of approximately 4.125 mW. We recorded the mode-locked operating characteristics at a pump power of 212 mW, as shown in Fig. 4. The optical spectrum analyzer was employed to record the output optical spectrum of the mode-locked operation, with a resolution of 0.02 nm, as illustrated in Fig. 4(a). The figure shows that the center wavelength of the mode-locked operation is 1561.5 nm, and the 3 dB spectrum bandwidth is 2.564 nm. Fig. 4(b) depicts a typical set of mode-locked pulse trains where the time interval between pulses is 250.6 ns, which is congruent with the overall length of the 51.5-m laser ring cavity. Fig. 4(c) illustrates a linear correlation between the average output power and pulse energy of the mode-locked laser with the pump power. As the pump power is raised from 90 mW to 500 mW, the output power rises from 1.141 mW to 12.225 mW, the single pulse energy increases from 0.286 nJ to 3.064 nJ, and the corresponding maximum optical conversion efficiency is 2.445%. The autocorrelation trace is shown in Fig. 4(d). The recorded pulse traces were well fitted to the sech² curve with a full width at half maximum (FWHM) of 1.261 ps. As a result, the time-bandwidth product (TBP) was computed to be 0.398, which is slightly higher than the theoretical limit (0.315), suggesting that there is a smaller chirp of the pulse at the fundamental frequency.

Fig. 4 Characteristics of mode-locked pulses at 212 mW pump power. (a) Optical spectrum width at 3 dB. Inset: Output optical spectrum. (b) Pulse trains, inset: individual pulse shapes. (c) Average output power and pulse energy as a function of pump power. (d) Autocorrelation trace. (e) RF spectrum with a fundamental frequency of 3.99 MHz and a resolution of 1 KHz. (f) RF spectrum.

In addition, higher stability (SNR > 45 dB) is one of the critical parameters to demonstrate excellent mode-locked pulse characteristics. The RF spectrum of the mode-locked laser based on Pr-MOFs is presented in Fig. 4(e and f). Fig. 4(e) illustrates the SNR at a fundamental frequency repetition rate of 3.99 MHz, a span of 5 MHz, and a resolution of 1 KHz. Additionally, Fig. 4(f) displays the RF spectrum with a frequency range of 100 MHz. The analysis of all the experimental data shows that the work achieves a high stability of the mode-locked pulse.

4.2. Characterization of harmonic mode-locked pulses

A stable mode-locked operation at the harmonic frequency (second-order) was observed when the pump power was raised to 288 mW. Simultaneously, the mode-locked operation at the fundamental frequency persists following the adjustment of the PC. By slowly increasing the pump power and fine-tuning the PC, the system achieved stable harmonic mode-locked (second-order) operation at a pump power of 444 mW. The pulse characteristics of the harmonic mode-locked are recorded in Fig. 5. The shape of the output optical spectrum is essentially the same as that of the mode-locked pulse, which is a typical Kelly sideband peaked soliton shape. The harmonic mode-locked pulse is characterized by a center wavelength of 1558.3 nm and a 3 dB bandwidth of 2.6 nm, as depicted in Fig. 5(a). The temporal separation between consecutive pulses amounts to 125.3 ns, precisely half of the interval observed for the fundamental frequency mode-locked pulses, as illustrated in Fig. 5(b). Furthermore, Fig. 5(c) represents the RF spectrum of the 100 MHz wideband. The SNR of the harmonic mode-locked pulse at the 7.98 MHz repetition frequency is about 60.3 dB, which indicates that the amplitude fluctuation is insignificant during the laser operation and shows reasonable stability. However, the side-mode suppression ratio (SMSR) is only about 25 dB due to the competition between the harmonic and fundamental mode-locking mechanisms.⁶⁷ The pulse traces recorded by the autocorrelator and the sech² fitted spectra indicate (displayed in Fig. 5(d)) that the FWHM is calculated to be 1.538 ps by sech² curve fitting. The calculated TBP is about 0.494, which is also more significant than the theoretical conversion limit value of 0.315, suggesting a slight chirp in the harmonic mode-locked fiber laser. According to the analysis of the experimental phenomena, the pulse width of harmonic mode-locked operation increases mainly due to the following two aspects: first, after the pulse is split into harmonic mode-locked pulses, the number of pulses increases and the interval between adjacent pulses becomes smaller. In this process, the single pulse energy and the peak intensity of the pulse exhibit a decrease, leading to a corresponding reduction in the nonlinear effect and the broadening of the mode-locked pulse. Second, the greater chirp in the cavity during harmonic mode-locked pulse generation also causes pulse broadening. After 48 hours of shutdown, we repeated the experiment, and the fiber laser still had stable mode-locked output. We conjectured that the Pr-MOF-based SAs have long-term stability and can withstand long-term illumination operation.

Fig. 5 Characterization of harmonic mode-locked (second-order) pulses at pump power of 444 mW. (a) The output optical spectrum (b) pulse trains. (c) RF spectrum in the spectral range of 100 MHz with a resolution of 1 KHz. (d) Measured and fitted autocorrelation traces of the spike.

When the pump power exceeds 500 mW, the saturation absorption effect leads to an unstable harmonic mode-locked (second-order) pulse. In addition, when the pump power is higher than 548 mW, we can also obtain harmonic mode-locked pulses of different orders (third and fourth) by adjusting the PC. However, the phenomenon is not stable and therefore not recorded.

4.3. Characterization of nanosecond pulses

Several experiments have demonstrated that the laser enters a mode-locked state, generating nanosecond pulses when the pump power exceeds 600 mW. The specific features of these nanosecond pulses are illustrated in Fig. 6. By precisely adjusting the PC, the stability of the nanosecond pulse phenomenon is achieved when the pump power is increased to 618 mW. Fig. 6(a) displays the output optical spectrum with a central wavelength of 1556.216 nm. A typical pulse sequence recorded with a digital oscilloscope is shown in Fig. 6(b), in which the pulse width reached 42.97 ns, and some spikes at the top of the soliton pulse result in a less smooth pulse envelope. The inset shows a pulse train in the 5 μs range, which offers a small amount of jitter phenomenon, so we infer there may be some external interference or dispersion effects. With a pump power of 618 mW, the average output power recorded by the optical power meter was 30.5625 mW. Calculation of the single-pulse energy resulted in 7.66 nJ, corresponding to an optical conversion efficiency of 4.95%. By further increasing the pump power, the laser will continue to operate in a mode-locked state with nanosecond pulses at the fundamental frequency, and the output single-pulse energy will also increase. Fig. 6(c) captures the optical spectrum of the nanosecond pulse at a pump power of 846 mW, which is in general agreement with the shape recorded at 618 mW. The digital oscilloscope recorded a pulse width for a single pulse to be 37.17 ns (shown in Fig. 6(d)). Meanwhile, the average output power was 43.925 mW, and the single-pulse energy was calculated to be 11.009 nJ with an optical conversion efficiency of 5.19%.

Fig. 6 Characterization of nanosecond pulses. (a) Output optical spectrum at pump power of 618 mW. (b) Pulse sequence at pump power of 618 mW. (c) Output optical spectrum at pump power of 846 mW. (d) Pulse sequence at pump power of 846 mW.

There are two main reasons for a nanosecond pulse mode-locked state in a laser cavity. On the one hand, the formation of nanosecond pulses is usually due to the presence of large chirps in the laser cavity. In the fabricated 1.5 μm EDFL with a cavity length of 51.5-m, the single-mode fiber exhibits anomalous dispersion, resulting in a dispersion of −1.0938 ps² for the entire laser cavity. When a pulse is formed in a laser cavity, the sizeable anomalous dispersion causes the pulse to accumulate a negative chirp, increasing the pulse width and generating mode-locked pulses on the nanosecond scale.^70,71 On the other hand, by increasing the pump power appropriately, the nonlinear effect is further enhanced, which destroys the soliton structure and also changes the output to a nanosecond pulse. In addition, the increase in pump power leads to an increase in the internal temperature of the fiber, which also affects the nonlinear characteristics. From the analysis of the experimental data, the burr exists at the top of the corresponding single pulse at different pump powers, and the smoothness of the whole pulse envelope is thus affected to a certain extent. With the increase of the pump power, the burr at the top of the single pulse is significantly reduced, and the whole pulse envelope becomes smoother. Nevertheless, if the pump power is further increased, it may lead to the phenomenon of the thermal effect of the SA.

We have investigated some experimental results of MOF materials as modulation devices to generate pulses in several bands. As shown in Table 1, compared with other MOF materials applied to fiber lasers, we fabricated the SA based on the sandwich structure of Pr-MOFs with higher experimental reproducibility. We obtained mode-locked pulses with narrower pulse widths in the 1.5 μm band, the most limited of which can be up to 1.261 ps, and we also received mode-locked pulses with higher stability. However, we can only obtain a maximum repetition frequency of 7.98 MHz, and the modulation depth of the Pr-MOFs-based SA needs to be improved, which will be a crucial breakthrough direction for our future work.

Table 1 Summary of the performance of fiber lasers based on SAs of MOF materials

Saturable absorption materials	Integration method	Modulation depth (%)	Repetition rate (MHz)	Output pulse width (ps)	SNR (dB)	Ref.
rGO-Co3O4	Microfiber	10.41	5.55	0.7934	60.47	64
rGO-ZIF-67	Microfiber	6.61	5.53	2.44	46.2
Ni-MOF	Microfiber	6.57/14.25	17/13.9	0.384/1.3	>64/>56	58
Ni-MOF	D-shape fiber	—	9.57/6.47	240/0.749	52/58	57
ZnO/Co3O4-A	Microfiber	0.48	6.17	1.72	36.5	62
ZnO/Co3O4-N	Microfiber	4.5	6.06	1.71	35
ZIF-67	D-shape fiber	0.26	9.02	1.12	48.4	68
ZIF-67	Sandwiched	4.17	32.6 KHz	—	47	69
Pr-MOFs	Sandwiched	2.41	3.99/7.98	1.261/1.538	59.4/60.3	ours

---

5. Conclusions

In summary, we performed SEM, XRD, and UV-Vis-NIR examinations on the Pr-MOFs crystals and analyzed the basic information of the crystal surface morphology, molecular structure, and elemental composition. The results show that the sample has a high degree of crystallinity and a clear lamellar structure. In addition, our fabricated Pr-MOFs-based SA has excellent nonlinear properties, with a modulation depth of 2.41% measured by the balanced twin-detector technique. Since the prepared SAs have high damage thresholds, Pr-MOFs-based mode-locked and harmonic mode-locked fiber lasers have been realized for the first time. The central wavelength, spectrum 3 dB bandwidth, RPR, and pulse duration of the mode-locked pulses are 1561.5 nm, 2.564 nm, 3.99 MHz, and 1.261 ps, respectively, and those of the harmonic mode-locked (second-order) pulses are 1558.3 nm, 2.6 nm, 7.98 MHz, and 1.538 ps, respectively. This demonstrates the superior nonlinear optical properties of Pr-MOFs. Therefore, the excellent optical properties of Pr-MOFs will be a direction for the future optoelectronic industry to explore in-depth and will be fully applied to fiber lasers, photodetectors, nonlinear optics, and infrared optoelectronic devices.

Author contributions

Xiaohui Du: writing – original draft, data curation, formal analysis. Houting Liu: writing – review & editing, supervision, methodology. Shaokai Li: data curation, investigation. Zefei Ding: investigation. Chenyue Liu: investigation. Cunguang Zhu: writing – review & editing, supervision, formal analysis. Pengpeng Wang: writing – review & editing, conceptualization, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported in part by the Natural Science Foundation of China under Grant 61705080.

Notes and references

1. Z. Liu, X. Liu, B. Sun, X. Tan, H. Ye, J. Zhou and G. Liao, A Cu-Doping Strategy to Enhance Photoelectric Performance of Self-Powered Hole-Conductor-Free Perovskite Photodetector for Optical Communication Applications, Adv. Mater. Technol., 2020, 5(8), 2000260.
2. Z. Li, Y. Tong, X. Fu, J. Wang, Q. Guo, H. Yu and X. Bao, Simultaneous distributed static and dynamic sensing based on ultra-short fiber Bragg gratings, Opt. Express, 2018, 26(13), 17437–17446.
3. L. Liu, D. Peng, S. Fu, Y. Wang and Y. Qin, Multi-band microwave signals generation based on a photonic sampling with a flexible ultra-short pulse source, Opt. Express, 2022, 30(18), 32151–32161.
4. A. J. Zhang, J. L. Duan and Y. B. Xing, Application of thulium-doped laser in the biomedicine field, Laser Optoelectron. Prog., 2022, 59(1), 0100004.
5. K. Wang, D. Zhang, K. Charan, M. N. Slipchenko, P. Wang, C. Xu and J. X. Cheng, Time-lens based hyperspectral stimulated Raman scattering imaging and quantitative spectral analysis, J. Biophotonics, 2013, 6(10), 815–820.
6. W. Li, X. Zeng, Y. Dong, Z. Feng, H. Wen, Q. Chen and Y. Cao, Laser nanoprinting of floating three-dimensional plasmonic color in pH-responsive hydrogel, Nanotechnology, 2021, 33(6), 065302.
7. Z. Wu, H. Bao, Y. Xing and L. Liu, Tribological characteristics and advanced processing methods of textured surfaces: A review, Int. J. Adv. Des. Manuf. Technol., 2021, 114, 1241–1277.
8. J. Qin, R. Dai, Y. Li, Y. Meng, Y. Xu, S. Zhu and F. Wang, 20 GHz actively mode-locked thulium fiber laser, Opt. Express, 2018, 26(20), 25769–25777.
9. J. He, L. Tao, H. Zhang, B. Zhou and J. Li, Emerging 2D materials beyond graphene for ultrashort pulse generation in fiber lasers, Nanoscale, 2019, 11(6), 2577–2593.
10. J. S. Liu, X. H. Li, Y. X. Guo, A. Qyyum, Z. J. Shi, T. C. Feng and X. F. Liu, SnSe₂ nanosheets for subpicosecond harmonic mode-locked pulse generation, Small, 2019, 15(38), 1902811.
11. T. Chai, X. Li, T. Feng, P. Guo, Y. Song, Y. Chen and H. Zhang, Few-layer bismuthene for ultrashort pulse generation in a dissipative system based on an evanescent field, Nanoscale, 2018, 10(37), 17617–17622.
12. J. Feng, X. Li, G. Zhu and Q. J. Wang, Emerging high-performance SnS/CdS nanoflower heterojunction for ultrafast photonics, ACS Appl. Mater. Interfaces, 2020, 12(38), 43098–43105.
13. G. Wang, A. A. Baker-Murray and W. J. Blau, Saturable absorption in 2D nanomaterials and related photonic devices, Laser Photonics Rev., 2019, 13(7), 1800282.
14. B. Fu, Y. Hua, X. Xiao, H. Zhu, Z. Sun and C. Yang, Broadband graphene saturable absorber for pulsed fiber lasers at 1, 1.5, and 2 μm, IEEE J. Sel. Top. Quantum Electron., 2014, 20(5), 411–415.
15. J. He, L. Tao, H. Zhang, B. Zhou and J. Li, Emerging 2D materials beyond graphene for ultrashort pulse generation in fiber lasers, Nanoscale, 2019, 11(6), 2577–2593.
16. X. Li, X. Yu, Z. Sun, Z. Yan, B. Sun, Y. Cheng and Q. J. Wang, High-power graphene mode-locked Tm/Ho co-doped fiber laser with evanescent field interaction, Sci. Rep., 2015, 5(1), 16624.
17. S. Xu, F. Wang, C. Zhu, Y. Meng, Y. Liu, W. Liu and Y. Xu, Ultrafast nonlinear photoresponse of single-wall carbon nanotubes: a broadband degenerate investigation, Nanoscale, 2016, 8(17), 9304–9309.
18. N. Dong, Y. Li, S. Zhang, N. McEvoy, R. Gatensby, G. S. Duesberg and J. Wang, Saturation of two-photon absorption in layered transition metal dichalcogenides: experiment and theory, ACS Photonics, 2018, 5(4), 1558–1565.
19. L. N. Duan, Y. L. Su, Y. G. Wang, L. Li, X. Wang and Y. S. Wang, Passively mode-locked erbium-doped fiber laser via a D-shape-fiber-based MoS₂ saturable absorber with a very low nonsaturable loss, Chin. Phys. B, 2016, 25(2), 024206.
20. T. Feng, D. Zhang, X. Li, Q. Abdul, Z. Shi, J. Lu and Q. J. Wang, SnS₂ nanosheets for Er-doped fiber lasers, ACS Appl. Nano Mater., 2019, 3(1), 674–681.
21. H. Haris, H. Arof, A. R. Muhammad, C. L. Anyi, S. J. Tan, N. Kasim and S. W. Harun, Passively Q-switched and mode-locked Erbium-doped fiber laser with topological insulator Bismuth Selenide (Bi₂Se₃) as saturable absorber at C-band region, Opt. Fiber Technol., 2019, 48, 117–122.
22. L. Dong, H. Chu, Y. Li, X. Ma, H. Pan, S. Zhao and D. Li, Surface functionalization of Ta₄C₃ MXene for broadband ultrafast photonics in the near-infrared region, Appl. Mater. Today, 2022, 26, 101341.
23. X. Jiang, S. Liu, W. Liang, S. Luo, Z. He, Y. Ge and H. Zhang, Broadband nonlinear photonics in few-layer MXene Ti₃C₂Tx (T = F, O, or OH), Laser Photonics Rev., 2018, 12(2), 1700229.
24. H. Pan, Y. Hu, X. Ma, H. Chu, Y. Li, Z. Pan and D. Li, Tunable optical nonlinearities and surface plasmon resonance in Fe₃O₄ nanoparticles-decorated Ti₃C₂ MXene composite for ultrafast fiber lasers, Mater. Today Nano, 2023, 100356.
25. J. Qiao, S. Ahmed, J. Yu, R. Fan, G. Liu, Y. H. Tsang and S. Feng, CrTe₂ as a new saturable absorber for a passive mode-locking Er-doped laser, J. Mater. Chem. C, 2023, 11(39), 13438–13445.
26. S. Ahmed, J. Qiao, P. K. Cheng, A. M. Saleque, M. N. A. S. Ivan, T. I. Alam and Y. H. Tsang, Two-dimensional gallium sulfide as a novel saturable absorber for broadband ultrafast photonics applications, ACS Appl. Mater. Interfaces, 2021, 13(51), 61518–61527.
27. C. Zhang, H. Chu, Z. Pan, H. Pan, S. Zhao and D. Li, Single crystalline BiOCl nanosheets with oxygen vacancies for ultrafast mode-locking operation, Opt. Laser Technol., 2023, 159, 108945.
28. S. Ahmed, J. Qiao, P. K. Cheng, A. M. Saleque, M. I. Hossain, L. H. Zeng and Y. H. Tsang, Tin Telluride Quantum Dots as a Novel Saturable Absorber for Q-Switching and Mode Locking in Fiber Lasers, Adv. Opt. Mater., 2021, 9(6), 2001821.
29. L. Dong, H. Chu, Y. Li, S. Zhao, G. Li and D. Li, Nonlinear optical responses of α-Fe₂O₃ nanosheets and application as a saturable absorber in the wide near-infrared region, Opt. Laser Technol., 2021, 136, 106812.
30. X. Chen, J. S. Ponraj, D. Fan and H. Zhang, An overview of the optical properties and applications of black phosphorus, Nanoscale, 2020, 12(6), 3513–3534.
31. A. J. Kuehne and M. C. Gather, Organic lasers: recent developments on materials, device geometries, and fabrication techniques, Chem. Rev., 2016, 116(21), 12823–12864.
32. S. A. Veldhuis, P. P. Boix, N. Yantara, M. Li, T. C. Sum, N. Mathews and S. G. Mhaisalkar, Perovskite materials for light-emitting diodes and lasers, Adv. Mater., 2016, 28(32), 6804–6834.
33. J. M. Pietryga, Y. S. Park, J. Lim, A. F. Fidler, W. K. Bae, S. Brovelli and V. I. Klimov, Spectroscopic and device aspects of nanocrystal quantum dots, Chem. Rev., 2016, 116(18), 10513–10622.
34. G. S. He, L. S. Tan, Q. Zheng and P. N. Prasad, Multiphoton absorbing materials: molecular designs, characterizations, and applications, Chem. Rev., 2008, 108(4), 1245–1330.
35. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber and A. K. Geim, Fine structure constant defines visual transparency of graphene, Science, 2008, 320(5881), 1308.
36. D. Mao, S. Zhang, Y. Wang, X. Gan, W. Zhang, T. Mei and J. Zhao, WS₂ saturable absorber for dissipative soliton mode locking at 1.06 and 1.55 μm, Opt. Express, 2015, 23(21), 27509–27519.
37. M. Zhang, G. Hu, G. Hu, R. C. T. Howe, L. Chen, Z. Zheng and T. Hasan, Yb-and Er-doped fiber laser Q-switched with an optically uniform, broadband WS₂ saturable absorber, Sci. Rep., 2015, 5(1), 17482.
38. Y. Ge, Z. Zhu, Y. Xu, Y. Chen, S. Chen, Z. Liang and D. Fan, Broadband nonlinear photoresponse of 2D TiS₂ for ultrashort pulse generation and all-optical thresholding devices, Adv. Opt. Mater., 2018, 6(4), 1701166.
39. A. Favron, E. Gaufrès, F. Fossard, A. L. Phaneuf-L’Heureux, N. Y. Tang, P. L. Lévesque and R. Martel, Photooxidation and quantum confinement effects in exfoliated black phosphorus, Nat. Mater., 2015, 14(8), 826–832.
40. M. Zhang, Q. Wu, F. Zhang, L. Chen, X. Jin, Y. Hu and H. Zhang, 2D black phosphorus saturable absorbers for ultrafast photonics, Adv. Opt. Mater., 2019, 7(1), 1800224.
41. B. Lebeau and P. Innocenzi, Hybrid materials for optics and photonics, Chem. Soc. Rev., 2011, 40(2), 886–906.
42. S. L. James, Metal-organic frameworks, Chem. Soc. Rev., 2003, 32(5), 276–288.
43. F. X. L. i Xamena, A. Abad, A. Corma and H. Garcia, MOFs as catalysts: Activity, reusability and shape-selectivity of a Pd-containing MOF, J. Catal., 2007, 250(2), 294–298.
44. O. M. Yaghi, G. Li and H. Li, Selective binding and removal of guests in a microporous metal–organic framework, Nature, 1995, 378(6558), 703–706.
45. H. Li, M. Eddaoudi, M. O'Keeffe and O. M. Yaghi, Design and synthesis of an exceptionally stable and highly porous metal-organic framework, Nature, 1999, 402(6759), 276–279.
46. O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Reticular synthesis and the design of new materials, Nature, 2003, 423(6941), 705–714.
47. H. Sung Cho, H. Deng, K. Miyasaka, Z. Dong, M. Cho, A. V. Neimark and O. Terasaki, Extra adsorption and adsorbate superlattice formation in metal-organic frameworks, Nature, 2015, 527(7579), 503–507.
48. D. Maspoch, D. Ruiz-Molina, K. Wurst, N. Domingo, M. Cavallini, F. Biscarini and J. Veciana, A nanoporous molecular magnet with reversible solvent-induced mechanical and magnetic properties, Nat. Mater., 2003, 2(3), 190–195.
49. F. Qu, H. Jiang and M. Yang, Designed formation through a metal organic framework route of ZnO/ZnCo₂O₄ hollow core–shell nanocages with enhanced gas sensing properties, Nanoscale, 2016, 8(36), 16349–16356.
50. Q. Wang and D. Astruc, State of the art and prospects in metal–organic framework (MOF)-based and MOF-derived nanocatalysis, Chem. Rev., 2019, 120(2), 1438–1511.
51. O. R. Evans and W. Lin, Crystal engineering of NLO materials based on metal–organic coordination networks, Acc. Chem. Res., 2002, 35(7), 511–522.
52. R. Medishetty, J. K. Zaręba, D. Mayer, M. Samoć and R. A. Fischer, Nonlinear optical properties, upconversion and lasing in metal–organic frameworks, Chem. Soc. Rev., 2017, 46(16), 4976–5004.
53. T. N. Nguyen, F. M. Ebrahim and K. C. Stylianou, Photoluminescent, upconversion luminescent and nonlinear optical metal-organic frameworks: From fundamental photophysics to potential applications, Coor. Chem. Rev., 2018, 377, 259–306.
54. X. Yang, X. Lin, Y. S. Zhao and D. Yan, Recent advances in micro-/nanostructured metal–organic frameworks towards photonic and electronic applications, Chem. – Eur. J., 2018, 24(25), 6484–6493.
55. B. Li, H. M. Wen, Y. Cui, W. Zhou, G. Qian and B. Chen, Emerging multifunctional metal–organic framework materials, Adv. Mater., 2016, 28(40), 8819–8860.
56. V. Stavila, A. A. Talin and M. D. Allendorf, MOF-based electronic and opto-electronic devices, Chem. Soc. Rev., 2014, 43(16), 5994–6010.
57. X. Jiang, L. Zhang, S. Liu, Y. Zhang, Z. He, W. Li and H. Zhang, Ultrathin metal–organic framework: an emerging broadband nonlinear optical material for ultrafast photonics, Adv. Opt. Mater., 2018, 6(16), 1800561.
58. Q. Zhang, X. Jiang, M. Zhang, X. Jin, H. Zhang and Z. Zheng, Wideband saturable absorption in metal–organic frameworks (MOFs) for mode-locking Er-and Tm-doped fiber lasers, Nanoscale, 2020, 12(7), 4586–4590.
59. H. Pan, X. Wang, H. Chu, Y. Li, S. Zhao, G. Li and D. Li, Optical modulation characteristics of zeolitic imidazolate framework-67 (ZIF-67) in the near infrared regime, Opt. Lett., 2019, 44, 5892.
60. Y. Zhao, W. Wang, X. Li, H. Lu, Z. Shi, Y. Wang and G. Shan, Functional porous MOF-derived CuO octahedra for harmonic soliton molecule pulses generation, ACS Photonics, 2020, 7(9), 2440–2447.
61. H. Chu, L. Dong, Z. Pan, X. Ma, S. Zhao and D. Li, Passively Q-switched Tm: YAP laser with a zeolitic imidalate framework-67 saturable absorber operating at 3H4 → 3H5 transition, Opt. Laser Technol., 2022, 147, 107679.
62. L. Kong, H. Chu, M. Xu, S. Xu, Z. Pan, S. Zhao and D. Li, Oxygen vacancy engineering of MOF-derived ZnO/Co₃O₄ nanocomposites for high harmonic mode-locking operation, J. Mater. Chem. C, 2022, 10(43), 16564–16572.
63. L. Dong, H. Chu, Y. Li, S. Zhao and D. Li, Broadband optical nonlinearity of zeolitic imidazolate framework-8 (ZIF-8) for ultrafast photonics, J. Mater. Chem. C, 2021, 9(28), 8912–8919.
64. M. An, Z. Pan, X. Li, W. Wang, C. Jiang, G. Li and Z. Zhang, Co-MOFs as Emerging Pulse Modulators for Femtosecond Ultrafast Fiber Laser, ACS Appl. Mater. Interfaces, 2022, 14(48), 53971–53980.
65. C. Yu, Y. Wang, J. Cui, D. Yu, X. Zhang, X. Shu and Y. Wu, MOF-74 derived porous hybrid metal oxide hollow nanowires for high-performance electrochemical energy storage, J. Mater. Chem. A, 2018, 6(18), 8396–8404.
66. B. Bohigues, S. Rojas-Buzo, M. Moliner and A. Corma, Coordinatively unsaturated Hf-MOF-808 prepared via hydrothermal synthesis as a bifunctional catalyst for the tandem N-Alkylation of amines with benzyl alcohol, ACS Sustainable Chem. Eng., 2021, 9(47), 15793–15806.
67. X. Ma, W. Chen, L. Tong, S. Liu, W. Dai, S. Ye and W. Gao, Experimental demonstration of harmonic mode-locking in Sb₂Se₃-based thulium-doped fiber laser, Opt. Laser Technol., 2021, 143, 107286.
68. M. Xu, H. Chu, Y. Hu, H. Pan, Z. Pan, S. Zhao and D. Li, Polyhedron ZIF-67 nanoparticles deposited on a D-shape fibre for stable soliton operation in an ultrashort fibre laser, Mater. Chem. Front., 2022, 6(18), 2729–2734.
69. M. Xu, H. Chu, H. Pan, S. Zhao and D. Li, Highly stable passively Q-switched erbium-doped fiber laser with zeolitic imidazolate framework-67 saturable absorber, Infrared Phys. Technol., 2022, 125, 104274.
70. X. Li, X. Liu, X. Hu, L. Wang, H. Lu, Y. Wang and W. Zhao, Long-cavity passively mode-locked fiber ring laser with high-energy rectangular-shape pulses in anomalous dispersion regime, Opt. Lett., 2010, 35(19), 3249–3251.
71. X. Zhang, C. Gu, G. Chen, B. Sun, L. Xu, A. Wang and H. Ming, Square-wave pulse with ultra-wide tuning range in a passively mode-locked fiber laser, Opt. Lett., 2012, 37(8), 1334–1336.

---

Footnote

† CCDC 2308657. For crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc00340c

---