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Abstract
We report the feasibility of a MAX-phase material for implementation as a saturable absorber. Our saturable absorber was fabricated as a composite of Ti2AlC particles and polyvinyl alcohol (PVA) using a sandwich-structured fiber-ferrule platform. The saturation intensity and modulation depth of the prepared SA were measured at ∼31.5 MW/cm2 and ∼6.3%, respectively. Using the Ti2AlC/PVA composite-based SA within an erbium-doped fiber laser ring cavity, stable Q-switched pulses were readily obtained at a wavelength of 1.56 µm. This experimental demonstration unveils the potential of micrometer-sized MAX-phase particles for implementation as low-cost, practical, saturable absorbers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Saturable absorption is a nonlinear optical phenomenon of semiconducting materials in which absorption decreases as incident light intensity increases due to the Pauli’s blocking principle [1]. This is an important phenomenon, as “saturable absorbers (SA)” are key passive components of ultrafast pulsed lasers. Commercial-grade SAs have most commonly been fabricated with III-V compound semiconductors [2]. Despite wide-spread implementation of III-V semiconductor-based saturable absorbers, there remain quite a few technical and practical issues with these materials, such as their narrow operating bandwidths, and complicated and expensive fabrication processes. To overcome these drawbacks, alternative saturable absorption materials have been intensively investigated. To date, carbon nanotubes (CNTs) [3–5], graphene [6–8], graphene oxide (GO) [9,10], graphite [11,12], topological insulators (TIs) [13–20], transition metal dichalcogenides (TMDs) [21–33], gold nanoparticles [34–36], black phosphorus (BP) [37–39], skutterudites [40,41], and MXenes [42–45] have been identified as efficient saturable absorption materials.
Recently, a new family of 2D materials called MXenes has been extensively investigated for various applications, including electrochemical capacitors, catalysts, biosensors, and water purification, due to their superb photonic and/or electronic properties [46–49]. The chemical formula of MXenes is Mn+1XnTx (n = 1–3), where M is an early transition metal (i.e., Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc.), X is carbon and/or nitrogen and Tx represents surface terminations. In the typical structure of a MXene, n + 1 layers of M cover n layers of X in the form of [MX]nM. MXenes are produced by etching MAX-phases with strong etching solutions that contain fluoride ions [50,51]. For instance, a MXene of Ti3C2Tx can be obtained by selectively removing the aluminum (Al) atoms from the MAX-phase Ti3AlC2 using a HF solution [52]. Recently, the saturable absorption properties of MXenes, such as Ti3CTx and Ti3C2Tx, have been extensively investigated and their excellent performance comparable to those of other 2-dimensional (2-D) materials has been well demonstrated [42–45].
However, there remains a technically interesting question as to whether or not a MAX-phase, the seed material for MXene synthesis, by itself possesses saturable absorption properties suitable for implementation as practical passive Q-switches or mode-lockers. MAX-phases are polycrystalline materials of hexagonal ternary layered carbides and nitrides. The general formula of MAX-phases is Mn+1AXn, where M is an early transition metal, A is an A-group element, and X denotes either nitrogen or carbon [53–56]. MAX-phases can be divided into three categories based on integer number n and are referred to as 211, 312, and 413 phases [54, 57]. These nanolaminate materials possess the combined properties of metallic and ceramic materials since their energy band structure depends on crystal direction [58]. Bulk MAX phases are typically polycrystalline compounds with randomly-oriented, micrometer-sized grains, which therefore have very complicated energy band structures. MAX phases have been useful materials in the fields of aerospace, nuclear engineering, and high-temperature applications [55, 59–61] due to their low density, high modulus of elasticity at room temperature, good oxidation resistance, and high melting point [62–64], even if their crystalline purity is much lower than that of MXenes.
There have been a number of investigations on new satuarable absorption materials including graphene, transition metal dichalcogenides, topological insulators, and so on, as aforementioned. Note that most of the investigations have been focused on 2-D materials. In terms of nanoscience, 2-D materials are interesting media for the implementation of high performance electronic or photonic devices. However, the fundamental question is whether or not 2-D materials are really essential to saturable absorbers [30, 32]. The associated question is if it is possible to realize saturable absorbers based on low-cost, bulk materials, which exhibit performance comparable to the ones based on 2-D materials. It should be noticed that MXenes are 2-D materials, while MAX-phases are bulk.
Herein, we report the experimental investigation of the saturable absorption properties of the MAX-phase Ti2AlC. More specifically, we demonstrate the use of a composite of Ti2AlC particles and polyvinyl alcohol (PVA) for implementation of a fiberized SA that can operate at 1.5-µm wavelengths. A Ti2AlC/PVA composite-based saturable absorber was fabricated in a sandwich-structured fiber-ferrule platform through deposition of the Ti2AlC/PVA composite onto the flat surface at the end of a fiber ferrule. Using the Ti2AlC/PVA composite-based SA within an erbium (Er)-doped fiber laser ring cavity, stable Q-switched pulses were readily obtained at a wavelength of 1.56 µm. The temporal width of the output pulses was measured at 4.88 µs at the maximum pulse-repetition rate of 27.45 kHz.
2. Characterization and fabrication of Ti2AlC/PVA-based saturable absorber
To fabricate a saturable absorber for this experimental demonstration, commercially available Ti2AlC powder (99.9%, Carbon-Ukraine ltd.) was used with PVA to form a composite. Ti2AlC powder (50 mg) and 10 mg of PVA were added to 20 ml of distilled water in a vial, as shown in Fig. 1(a). After stirring this solution for 30 min, a small amount of the composite solution was directly dropped onto the surface of the flat-end of a FC/APC fiber ferrule using a pipette. Subsequently, the ferrule was heated in an oven at a temperature of 60 oC for 30 minutes to dry the dropped solution. Another FC/APC fiber ferrule was then connected to the ferrule with the deposited Ti2AlC/PVA to form a sandwich-structured saturable absorber. The insertion loss of our prepared SA was ∼4.4 dB.
Fig. 1. (a) Photograph of our prepared Ti2AlC/PVA solution. (b) Measured SEM image of the Ti2AlC particles. (c) Measured SEM image of the Ti2AlC/PVA film deposited onto the flat surface of a fiber ferrule. Inset: Magnified SEM image around the core region.
The material properties of the Ti2AlC particles, were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Figure 1(b) shows SEM image of the Ti2AlC particles. The sizes of the Ti2AlC particles range from a few micrometers to tens of micrometers. It is obvious from the SEM image that the Ti2AlC particles are bulk-structured grains as expected. An SEM image of the Ti2AlC/PVA film deposited onto the end surface of a fiber ferrule was also measured as shown in Fig. 1(c). Quite a few particles were observed to exist in the core region. The large particles could be the reason why our prepared SA exhibited such a high insertion loss (∼4.4 dB). The EDS spectrum of the Ti2AlC particles is shown in Fig. 2. Strong peaks that correspond to titanium (Ti), aluminum (Al), and carbon (C) were clearly observed [65].
The XPS measurements were subsequently conducted to discern the stoichiometry of the particles, as shown in Fig. 3. From the spectra, we determined that the energies of Ti 2p3/2, Al 2p3/2, and C 1s were 453.3 eV, 71.3 eV, and 281 eV, respectively [66]. The additional peaks at ∼458 and ∼464 eV in the Ti 2p region correspond to titanium dioxide (TiO2) [66,67], and the additional peak at ∼74 eV in the Al 2p region corresponds to aluminum oxide (Al2O3) [66,67]. Figure 3(c) shows additional peaks at 284.4 eV (C-C/C-H bond) and ∼288.4 eV (O = C-O bond) [66]. The XPS measurement clearly shows that the Ti2AlC particles were substantially oxidized during this fabrication process.
Fig. 3. X-ray photoelectron spectroscopy (XPS) profiles of (a) Ti 2p, (b) Al 2p, and (c) C 1s spectra from the Ti2AlC particles.
The linear absorption of a Ti2AlC/PVA film was measured using a spectrophotometer (UV-3600PLUS, Shimadzu). For this linear absorption measurement, a Ti2AlC/PVA film was deposited onto a glass slide. As shown in the linear absorption spectrum in Fig. 4(a), the Ti2AlC/PVA film can be readily used in implementations requiring 1.5 µm band saturable absorbers. Next, we measured the nonlinear transmission curve of the prepared Ti2AlC/PVA-based SA as a function of peak intensity of the incident beam. A 1560 nm mode-locked fiber laser with a temporal width of ∼780 fs at a repetition rate of ∼22.26 MHz was used as the input light source. Figure 4(b) shows the measured nonlinear transmission curve together with a fitted curve defined by [68]:
where $T(I)$ is the transmission, $\Delta I$ is the modulation depth, I is the input-pulse energy, ${I_{sat}}$ is the saturation energy, and ${T_{ns}}$ is the nonsaturable loss. The measured modulation depth and the saturation intensity were ∼6.3% and ∼31.2 MW/cm2, respectively.Fig. 4. (a) Linear optical absorption spectrum of the Ti2AlC/PVA composite and (b) nonlinear transmission curve of our prepared Ti2AlC/PVA-based SA.
3. Passively Q-switched fiber laser setup and results
Figure 5 shows the experimental schematic of our Q-switched fiber laser configuration. For the laser cavity, 2.3-m length of Er-doped fiber (EDF) was used as a gain medium (LIEKKI Er20-4/125, nLight). A 980-nm laser diode was used as a pumping source that was coupled into the gain medium through a 980/1550 nm wavelength division multiplexer (WDM). To obtain unidirectional light propagation, an optical isolator was used after the EDF. The laser output was extracted via the 10% port of a 90:10 optical coupler. The Ti2AlC/PVA-based SA was placed after the optical isolator.
Using the fiber laser setup, we measured the output pulse characteristics as a function of pump power. When the pump power was increased to ∼30 mW, stable Q-switched pulses were readily obtained and were maintained up to a maximum pump power of ∼74 mW. Figure 6(a) shows the measured optical spectrum of the output Q-switched pulses at a pump power of ∼74 mW. The center wavelength and 3-dB bandwidth were measured to be ∼1559.4 and ∼1.6 nm, respectively. Figure 6(b) shows the measured oscilloscope traces of the output pulses using various pump powers. Clear changes to pulse repetition rate were observed as a function of pump power. Above 74 mW pump power, Q-switched pulses became unstable and disappeared. Mode-locking did not occur in this experiment. With our prepared SA it was impossible to induce mode-locking. From our experience, it is very difficult to induce mode-locking at such a high insertion loss level of ∼4.4 dB when a sandwich-structured fiber-ferrule platform is employed for the implementation of an SA. Now we are making every effort to reduce the insertion loss of the Ti2AlC-based SA for mode-locking of a fiber laser. If we use an optimized laser cavity incorporating a Ti2AlC-based SA with a reduced insertion loss, it should be possible to produce mode-locked pulses.
Fig. 6. (a) Optical spectrum of the output pulse at a pump power of 74 mW. (b) Oscilloscope traces of the output pulses at a range of pump powers.
Next, we investigated the output characteristics of repetition rate, temporal width, pulse energy and average output power as functions of pump power, and the results are summarized in Fig. 7. Figure 7(a) shows the measured repetition rate and pulse width as a function of pump power. With increasing pump power, the repetition rate increased from 16.14 kHz to 27.45 kHz, whereas the pulse width decreased from 9.24 µs to 4.88 µs due to a strong pumping-induced gain compression effect [17,38]. Note that this phenomenon has been commonly reported in SA-based passively Q-switched fiber lasers [12,18,26]. Figure 7(b) shows that both pulse energy and average output power increased with increase to pump power. The maximum values of average output power and single pulse energy at maximum pump power were ∼0.62 mW and ∼22.58 nJ, respectively.
Fig. 7. (a) Repetition rate and temporal width of the output pulses as a function of pump power. (b) Pulse energy and average output power as a function of pump power.
In order to check the repeatability of the sample preaparation process, we fabricated around 20 samples and measured the insertion loss of each sample. We found that Ti2AlC-based SAs properly worked as a Q-switch in the case that their insertion losses were in a range from 4 to 7 dB. Each sample was found to have slightly different properties such as insertion loss and modulation depth due to non-uniform distribution of Ti2AlC particles.
We also investigated the long-term stability of the Q-switched fiber laser by measuring the optical spectrum every 10 minutes for 1 hour, as shown in Fig. 8. Stable operation of the laser over this duration was clearly observed. Furthermore, the oscilloscope traces of the output pulses, which correspond to the pulse spectra, were also measured to investigate the temporal stability. No temporal fluctuations of the output pulses were observed.
Lastly, the output performance of the passively Q-switched fiber laser proposed in this study was compared with that of recently demonstrated, passively Q-switched erbium-doped fiber lasers using other saturable absorption materials. The results are summarized in Table 1. The prepared Ti2AlC-based SA exhibited a modulation depth larger than those measured from Bi2Te3, WS2, WSe2, and In0.2Co4Sb12-based SAs but smaller than the superlative modulation depth of the BP-based SA (∼18.55%). However, our fiber laser had a maximum output power smaller than those achieved with graphene, Bi2Se3, Sb2Te3, TMDs, gold nanorods, and BP. We believe that further optimization of the fiber cavity parameters and a higher pump power are required to increase the output power performance of our fiber laser. It is evident from this comparison table that our prepared Ti2AlC-based SA exhibits performance comparable to other 2-D material-based ones.
Table 1. Performance comparison of the present work to previous Q-switched Er-doped fiber lasers incorporating other saturable absorption materials
4. Conclusion
In conclusion, the feasibility of implementing micrometer-sized particles of bulk MAX phase Ti2AlC as saturable absorber was investigated. This saturable absorber was fabricated by drop-casting a Ti2AlC/PVA solution onto the top surface of an FC/APC fiber ferrule and, drying in an oven. Using the prepared Ti2AlC/PVA-based SA, stable Q-switched pulses were obtained from an erbium-doped fiber ring cavity. The minimum pulse width of the output pulses was ∼4.88 µs at a pump power of ∼74 mW.
We believe that this experimental demonstration reveals the significant potential of micrometer-sized MAX-phase particles in the field of pulsed fiber laser technology. It must be noted that low-cost bulk materials could be a preferred option for implementation of practical saturable absorber as they do not require the complicated and sophisticated fabrication processes required for nano-structured 2-D materials. Since the laser performance of this demonstration does not outperform, those previously demonstrated with other nanomaterials, further optimization of the fiber laser parameters should be conducted. Furthermore, more detailed investigations into the recovery time of MAX phase materials are required to determine whether or not their saturable absorption performance is applicable to ultra-fast mode-locking.
Compared to MXenes, which demand a complicate fabrication process including dangerous fluoride ion-based chemical etching, MAX-phases have self-evident advantages in terms of fabrication and handling in the particular application of saturable absorbers. We believe that our work could be a useful database for material science since this is a first time report on saturable absorption properties of a family of MAX-phases.
Funding
National Research Foundation of Korea (NRF) (2018R1A2B6001641); Ministry of Trade, Industry and Energy (MOTIE) (10048726).
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