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Abstract
Metal-organic frameworks (MOFs) materials with sufficient charge mobility and promising structural flexibility are appealing for compact optoelectronic nano devices. In this work, the nonlinear optical features of the MOFs-CuBTC were comprehensively investigated for the first time. The CuBTC saturable absorbers were synthesized via liquid-phase exfoliation (LPE) method and characterized for the morphology and structure. The nonlinear optical response of CuBTC was characterized by employing an open-aperture Z-scan technique, which showed the great nonlinear optical modulation potential for the pulsed laser. Thereby, the CuBTC saturable absorber was implemented in the stable passively Q-switched lasers with the operating wavelengths at ∼1 and ∼2 µm to verify its broadband modulation properties, which produced the shortest pulse durations of 131 and 346 ns, respectively. These investigations confirmed that CuBTC can be a promising modulation device candidate for generating short laser pulses in wide spectral regions.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) nanomaterials have triggered great interests due to their simple fabrication, compact structure, low cost, and unique electro-optical properties. Since 2009 when graphene was successfully utilized as the saturable absorbers (SAs) for the ultrashort laser pulses [1], ultrafast optoelectronics applications based on 2D materials have been extensively studied. Wide-spectral saturable absorption has been experimentally demonstrated in 2D nanomaterials with outstanding performance, such as low saturation intensity, deep modulation depth, and fast recovery time of excited carriers [2]. However, the current 2D nanomaterials more or less have some limitations when they are applied as wideband saturable absorbers for short pulse generation. For instance the graphene, the inherent zero bandgap, weak electronic on/off ratio, and low monolayer modulation depth (<2.3%) restrict its application in optical modulation [3]. Transition metal disulfides (TMDs) materials have large bandgap and poor thermal conductivity [4], limiting their applications in the mid-infrared region. Black phosphorus (BP) is unstable and easily oxidized in the air, which limits its long-term stability applications [5]. Therefore, the exploration of novel nanomaterials with excellent nonlinear optical performances will make a significant contribution to the field of ultrafast optoelectronics applications.
Recently, metal-organic frameworks (MOFs) as novel types of nanoporous materials have captured widespread attention during the past two decades. MOFs are a typical organic and inorganic hybrid, composed of metallic ions or clusters and organic ligands [6]. Due to their remarkable merits, including high porosity, high surface area, strong thermal stability, adjustable structure, and band-gap tenability [7], MOFs have shown broad application potential in fields of the chemical reaction (catalysis [8], ion exchange [9], molecular separation [10]), new energy sources (gas storage, adsorption separation [11], supercapacitors [12]), microelectronics (optical materials, magnetic materials [13], sensors [14]), medical science (photodynamic therapy [15], drug delivery [16], and biomedical imaging [17]). Compared with conventional nanomaterials, MOFs have the advantages of larger specific surface area and easier interaction of active sites with the reactants, which demonstrates the application foreground in the area of compact and small size optoelectronic nanodevices [18]. Besides, the coordination of organic linkers to metal nodes often results in enhanced photo-physical behavior [19], showing the uniqueness of MOFs with promising optical properties.
CuBTC, also known as HKUST-1, is one typical MOFs extensively researched in the field of gas adsorption separation and storage [20]. Compared with other MOFs materials, CuBTC has a suitable channel window of 9×9 Å2 and a specific surface area of nearly 1500 m2/g [21,22]. The stable skeleton structure and chemical properties make it non-decomposable even when calcined at 280 °C and can be preserved for a long time without deterioration. So far, the nonlinear optical performance of CuBTC has rarely been reported and the optical modulation performance in the pulse generation remains blank in the broad spectra band.
In this paper, the characteristics of copper benzene-1,3,5-tricarboxylate (CuBTC) MOFs material were investigated. The CuBTC saturable absorber was fabricated by liquid-phase exfoliation method, following the detailed characterization of its morphology and structure. The broadband nonlinear optical response of CuBTC in the near-infrared (NIR) region was characterized via the open-aperture Z-scan technique. Finally, we exemplified its application as a broadband saturable absorber for passively Q-switching (PQS) operation at ∼1 µm and ∼2 µm for the first time. The obtained shortest pulse durations were 131 and 346 ns, respectively. The experimental results validated the broadband nonlinear optical modulation characteristics of CuBTC as SAs in the NIR region, indicating the promising modulation ability for generating short laser pulses. Thus, this study may provide new insights into novel MOFs-based nonlinear and ultrafast photonics devices.
2. Preparation and characterization of CuBTC
2.1 Synthesis of the CuBTC powder
The CuBTC powder was synthesized by the traditional solvothermal route as this method gives higher purity, surface area, and yield of MOFs than other methods [22]. Solutions of Cu(NO3)2·3H2O and benzene-1,3,5-tricarboxylic acid were dissolved in the mixture of N, N-dimethylformamide (DMF), ethanol, and water in a vial. The vial was heated in the vacuum oven at 100 °C for 24 h. After the crystallization process, the solution was cooled to room temperature. The blue CuBTC precipitates were collected and washed with DMF and ethanol several times to remove impurities.
2.2 Fabrication of CuBTC SAs
By the conventional ultrasound-assisted liquid-phase exfoliation (LPE) method, we fabricated the CuBTC saturable absorbers. In the preparation process, 20 mg CuBTC powder was firstly dispersed in 10 ml of isopropyl alcohol (IPA) solution and ultrasonically shaken for 4 h. Then, the obtained CuBTC dispersion was centrifuged with a rotating speed of 6000 rpm for 15 min. Subsequently, the supernatant liquid was collected and dropped onto a quartz glass substrate of 20×20×0.5 mm3, which was placed on a spin coater and spin-coated at a speed of 500 rpm. Finally, CuBTC SA was successfully fabricated after air-dried at room temperature.
2.3 Characterization and analysis
Prior to investigating the nonlinear optical properties of the as-prepared CuBTC SA, a series of characterization studies were carried out to investigate its structure and morphology. The outstanding physical and chemical properties of CuBTC are strictly related to its structural features. As reported in Fig. 1(a), the cubic, wide-open framework of this material, whose chemical composition is Cu3(BTC)2(H2O)3, consists of two Cu2+ ions linked together by four carboxylate groups to form a paddle-wheel unit. Each carboxylate bridge is part of 1,3,5-benzene tricarboxylate (BTC) connect to the molecule [23].
Fig. 1. Results of the CuBTC characterization. (a) The crystal structure of CuBTC. (b) SEM images. (c) TEM images. (d) XRD pattern of the as-synthesized CuBTC. (e) Raman spectra of the as-prepared samples. (f) Linear optical transmittance spectrum.
The surface morphology studies were conducted on Scanning Electron Microscope (SEM) (JSM-5610LV, 0.5-35 kV, JEOL), along with Transmission Electron Microscope (TEM) (FEI Tecnai G20) analysis to further determine the structural information of the as-prepared sample. As shown in Fig. 1(b) from the high power microscope, the crystal structure of CuBTC included unsmoothed surfaces with obvious cracks and porous structures. From the TEM micrograph in Fig. 1(c), we could observe the as-prepared layered CuBTC SA retains some large pores and defective structures. Figure 1(d) depicted the X-ray diffraction (XRD) pattern of the as-synthesized CuBTC showing the intensive peaks at 2 theta angles (5∼50°) in agreement with the standard CuBTC structure data (ICDD database no. PDF 00-065-1028) [24]. These clear and sharp peaks indicated that the synthesized CuBTC powder possessed a high purity of crystalline phase.
To further identify the detailed information about the chemical structure and molecular vibration modes of the sample, a Raman spectrometer (HORIBA LabRAM HR Evolution) was employed for the analysis under the excitation of a 532 nm laser and 1mW nominal laser power. Figure 1(e) depicted the Raman spectrum at a high resolution of 5 cm−1. Typical bands that caused by the C = C modes of the benzene ring (1611 cm−1 and 1004 cm−1), the stretching vibrations of sym C-O2 (1460 cm−1) and asym C-O2 (1540 cm−1), as well as the C-H band (742 cm−1 and 824 cm−1), could be clearly identified in Fig. 1(e). In addition, the Cu-Cu stretch and Cu-O stretch were observed in the low frequency region below 500 cm−1. These characteristic peaks were in good agreement with previous reports and indicated the successful fabrication of CuBTC SA [23].
The linear transmission spectra of the as-prepared CuBTC SA in the range of 500-2500 nm were measured with a UV/VIS/NIR spectrophotometer (Hitachi UH4150). As shown in Fig. 1(f), the transmittance of the CuBTC SA was higher than 75% in the scope of 500-2500 nm spectral region. Considering the wavelength of the laser, there were certain absorptions at 1 and 2 µm, indicating that it would be possible for the prepared CuBTC to act as an optical switch utilized in laser pulse generation. It should be noted that to focus on the properties of CuBTC nanomaterials, a calibration was done to eliminate the influence of the quartz substrate.
3. Nonlinear absorption properties
To characterize the nonlinear absorption characteristics of CuBTC SA at ∼1µm and ∼2 µm, open-aperture Z-scan measurement was employed as shown in Fig. 2, which is one of the most commonly used techniques [2]. Homemade pulsed lasers with the pulse widths of 100 ns operating at 1µm and 70 ns at 2 µm were employed as the pump sources, respectively. The maximum intensities on the axis employed for Z-scan measurements were roughly 8.67 Mw/cm2 at 1 µm and 8.12 Mw/cm2 at 2 µm, respectively. In this scheme, the CuBTC sample was put on the guide rail between a focus lens and the power meter, so we could change the incident laser intensity on the SA by moving the sample position on the guide rail.
The Z-scan measurement was fitted by the formula [25]:
Fig. 3. The measured saturable absorption data and their corresponding fitting curves of CuBTC at (a and b) 1 µm, and (c and d) 2 µm spectral bands.
The nonlinear transmittance curves of the CuBTC SA were shown in Fig. 3(b) and (d). The experimental data were fitted by the following nonlinear transmission formula [26]:
where T is the transmission, $\Delta T$ is the modulation depth, I and ${I_S}$ are the incident intensity and the saturation intensity (${I_S} = \textrm{h}\nu /\sigma \tau$, where $\textrm{h}\nu $ is the photon energy, σ is the absorption cross-section, and τ in the decay time of an upper-level electron), respectively. And ${T_{ns}}$ is the non-saturable loss. It can be seen that the transmittance increased along with the increasing incident intensity until saturation. The modulation depth, the nonlinear saturation loss, the saturation intensity, and the nonlinear absorption coefficient were obtained by fitting the experimental data as shown in Table 1. Irradiated on the pulsed laser at 2 µm wavelength region with the pulse widths of 70 ns and spot radius of 50 µm, the sample was not damaged, therefore the laser-induced damage threshold should be estimated above ∼250 MW/cm2. The relatively low saturation intensity and large modulation depth at both spectral wavebands revealed the potential of CuBTC SA for laser pulse modulation.
Table 1. Nonlinear Absorption Properties of the CuBTC SA at ∼1 and 2 µm
4. Application for Q-switching laser
To further confirm the broadband saturable absorption properties of the prepared CuBTC SA, passively Q-switching (PQS) solid-state lasers were demonstrated with two different gain mediums. The schematic diagram of the setup is depicted in Fig. 4. The utilized setup configuration for both lasers was a 30 mm long plane-parallel straight cavity, including an input flat mirror (M1) coated with high-reflectivity (HR) at the laser wavelength and anti-reflectivity (AR) at the pump wavelength, and a planar output coupler (OC). The setup details were listed in Table 2.
Table 2. Setup Details for the CuBTC Q-Switching Lasers
By means of an optical refocus module (1:1), the pump beam was focused on the gain crystals surface. To minimize the effects of thermal load, the gain mediums were both wrapped with thin indium foil and mounted in a copper heat sink, which was cooled at a constant temperature of 15 °C. A filter was placed behind the OC to block the residual pump laser. The laser output power was measured using a laser power meter (Thorlabs PM100D).
In our case, the pump energy was not absorbed completely by the gain medium, leading to the extra thermal load on the SA. To prevent thermally induced instability of the CuBTC SA, the pump power was not further enhanced.
4.1 Q-switching at ∼1 µm band
Firstly, we set up the continuous wave (CW) operation without the SA in the cavity. The CW Nd:GdVO4 laser was generated when the absorbed pump power increased to 0.49 W. As can be seen in Fig. 5(b), the CW average output power kept increasing almost linearly with the increase of pump power, corresponding to a slope efficiency of 34%. Under the absorbed pump power of 1.17 W, the maximum output power was 236 mW. After inserting the CuBTC sample into the cavity as SA, align the mirrors to realize the passively Q-switching operation. The average output powers were measured and shown in Fig. 5(b). Due to the insert of SA, the threshold pump power for PQS laser operation increased from 0.49 W to 0.68 W. When the absorbed pump power reached 1.17 W, the maximum average output power of 103 mW was achieved, corresponding to a slope efficiency of 20.4%. The output laser spectra of the Nd:GdVO4 laser was measured by a laser spectrometer (APE WaveScan, APE Inc.) with a resolution of 0.5 nm, as depicted in Fig. 5(a). For the Nd:GdVO4 laser, the emission wavelengths were both stabilized at 1063.8 nm in CW and Q-switching operations. The detailed pulse temporal behavior was recorded by a digital phosphor oscilloscope (Tektronix DPO42102B-L) in combination with an InGaAs PIN photodetector. The pulse duration, repetition rate, single pulse energy, and peak power versus the incident pump power were depicted in Fig. 5(c) and (d). The pulse duration decreased from 945 ns with increasing pump power and the shortest pulse duration was ultimately measured to be 131 ns, while the corresponding repetition rate augmented from 70 to 476 kHz. As a result, the highest pulse energy and maximum peak power were calculated to be 0.22 µJ and 1.65 W. Figure 7 illustrated the typical pulse train with the shortest pulse duration. Also, under the maximum absorbed pump power, the pulse instability was measured to be less than 5% over 60 minutes, indicating the chemical stability of CuBTC SA.
Fig. 5. Output from the CuBTC Q-switched Nd:GdVO4 laser at 1 µm. (a) Output spectra of Nd:GdVO4 lasers in the CW and Q-switched regime. (b) CW and Q-switched average output power, (c) Pulse duration and repetition rate, (d) Single pulse energy and peak power versus the absorbed pump power.
4.2 Q-switching at ∼2 µm band
In this section, we also demonstrated the CW and PQS operation in this wavelength band, which was demonstrated in Fig. 6(b). The emission spectra of the Tm:YAP laser was displayed in Fig. 6(a), attributed to the insertion of SA, the PQS emission central wavelength blueshifted from 1993 to 1978 nm. The shift of wavelength was attributed to the laser mode competition induced by the variation of intracavity loss. For the Tm:YAP laser, under an absorbed pump power of 5.1 W, the maximum CW average output power of 1305 mW was obtained, corresponding to a slope efficiency of 38.6%. The maximum average output power of 604 mW was achieved for the PQS laser at the same pump power, corresponding to a slope efficiency of 19.5%.
Fig. 6. Output from the CuBTC Q-switched Tm:YAP laser at 2 µm. (a) Output spectra of Tm:YAP lasers in the CW and Q-switched regime. (b) CW and Q-switched average output power, (c) Pulse duration and repetition rate, (d) Single pulse energy and peak power versus the absorbed pump power.
As demonstrated in Fig. 6(c) and (d), the pulse duration almost linearly decreased from 1890 to 346 ns, and the corresponding repetition rate rose from 30 to 112 kHz. When the pump power continued to increase, the Q-switched operation became unstable, probably because of the thermal effects and oversaturation of the SA. The shortest pulse duration of 346 ns was obtained at the previous pump level, the maximum repetition rate, single pulse energy, and peak power were 112 kHz, 5.39 µJ, and 15.58 W, respectively. At that moment, the power intensity in the cavity was calculated to be ∼30 MW/cm2, lower than the damage threshold abovementioned and no damage to the SA was observed. The temporal pulse profile at a repetition rate of 112 kHz and the shortest pulse shape are exhibited in Fig. 7.
Fig. 7. Temporal profiles of typical Q-switched pulse trains and the shortest pulse shapes. (a) Nd:GdVO4 laser. (b) Tm:YAP laser
Table 3 summarizes a few nanomaterials employed as SAs in the passively Q-switched solid-state lasers in the last few years. As shown in Table 3, in comparison with other 2D materials SAs at 1 and 2 µm, the nonlinear optical modulation performance of CuBTC saturable absorber proved to be a promising modulator candidate for generating short-pulse lasers in broad spectral regions, showing the promising optical modulation characteristics of MOFs materials.
Table 3. Performance Comparison of PQS Solid-State Lasers with Different SAs at 1 and 2 µm
5. Conclusion
In summary, the optical nonlinear properties of CuBTC MOFs materials were investigated in the 1 µm and 2 µm spectral region for the first time, to the best of our knowledge. CuBTC saturable absorber was fabricated by the LPE method and employed in all-solid-state Nd:GdVO4 and Tm:YAP PQS lasers. The experimental work presented here provided the first demonstration of CuBTC SA utilized in realizing stable passively Q-switched operation in a broad spectra band. In addition, the minimum pulse duration of 131 ns and 346 ns were obtained from Q-switched Nd:GdVO4 and Tm:YAP laser operating at 1 and 2 µm, respectively. These results verified the excellent nonlinear optical modulation properties of CuBTC MOFs materials as the saturable absorber and the potential in generating ultrashort laser pulses in the NIR region, paving the way towards advanced photonics based on novel MOFs materials.
Funding
National Natural Science Foundation of China (61605100); National Key Research and Development Program of China (2016YFB1102201, 2018YFB1107403); Taishan Scholar Foundation of Shandong Province (tsqn201812010); Young Scholars Program of Shandong University; Qi Lu Young Scholars Program of Shandong University.
Acknowledgment
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (NSFC) (Grant No. 61605100), the National Key Research and Development Program of China (Grant Nos. 2016YFB1102201 and 2018YFB1107403) and the Taishan Young Scholar Program of Shandong Province (Grant No. tsqn201812010). Wenchao Qiao acknowledges financial support from the Young Scholars Program of Shandong University; Tao Li and Tianli Feng acknowledge financial support from the Qi Lu Young Scholars Program of Shandong University.
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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