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Abstract
An experimental and theoretical investigation into the nonlinear absorption properties of MAX phase Ti2AlC was conducted at 1900 nm wavelength. First, the nonlinear absorption coefficient measurement of Ti2AlC was carried out using an open-aperture (OA) Z-scan technique. This measurement revealed that the nonlinear absorption coefficient of Ti2AlC was ∼(-24.13×103) cm2/GW at 1900 nm. Subsequently, the energy band structure of the Ti2AlC was calculated through density functional theory (DFT) calculation. This calculation confirmed that Ti2AlC had a metallic band structure implying an ultrawide absorption bandwidth. Finally, the feasibility of fabricating an all-fiberized device of a saturable absorber (SA) using Ti2AlC was conducted with a side-polished fiber platform. The SA was successfully used for the generation of femtosecond soliton pulses with features of 17.91 MHz repetition rate, 4.3-nm bandwidth, and ∼960 fs pulse width at 1922 nm. To the best of authors’ knowledge, this is the first demonstration of the use of a MAX phase-based SA for femtosecond mode-locking in the 1.9 μm spectral region.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optics has in recent decades become one of the most intensively studied scientific fields. Nonlinear optical responses of materials include second harmonic generation [1], the Kerr effect [2], saturable absorption [3], and two-photon absorption [4]. Those responses can be used for the implementation of a range of functional photonic devices, such as optical switches [5,6], frequency converters [7], optical limiters [8], and saturable absorbers [3]. Among the nonlinear optical responses mentioned above, saturable absorption, which occurs within semiconducting materials due to the Pauli’s Blocking principle [9], has been attracting huge technical attention in the field of ultrafast fiber lasers. It is well-known that a particular functional device called the “saturable absorber” (SA) is essentially required for passively inducing Q-switching or mode-locking in laser cavities. Ultrafast pulsed lasers have been used as light sources in many applications [10–13].
To date, various emerging nano-materials have been found to possess the desired saturable absorption properties that are efficient enough for practical SAs, even though most of the commercially available ultrafast lasers employ SAs based on III–V compound semiconductors, which are also commercially available [14]. Intensive investigations into a number of nanomaterials in terms of saturable absorption have been conducted so far, and the investigated materials include carbon nanotubes (CNTs) [3,15–17], graphene [18–23], topological insulators (TIs) [24–30], topological semimetals [31], transition metal dichalcogenides (TMDCs) [32–38], transition metal monochalcogenides (TMMCs) [39], black phosphorus (BPs) [40,41], gold nano-particles [42,43], and MXenes [44–50].
Among the aforementioned nano-materials, MXenes are a new family of 2D materials. Owing to their outstanding electronic and photonic properties [51–54], they have become a hot issue in the fields of materials science and engineering. The general formula of MXene is Mn+1XnTx (n=1–3; M:early transition metal; X:carbon and/or nitrogen; T:surface terminations). MXenes can be extracted from their ternary MAX phase precursors through chemical etching [55,56]. Even though MXenes are known to be highly useful in a variety of applications, such as electrochemical capacitors, water purification, chemical catalysts, and biosensors [53–56], the use of a chemical etching process based on hydrofluoric (HF) acid limits their large-scale fabrication due to the acute toxicity of HF acid.
Therefore, technical interests in precursor, MAX phases have been growing rapidly, since they still possess some photonic and electronic properties, that are comparable to those of MXenes. In particular, the saturable absorption properties of MAX phases are known to be comparable to those of MXenes [57–62]. For example, our group first demonstrated the use of MAX phase Ti2AlC as an SA [57]. Ahmad et al. demonstrated a Ti3AlC2 MAX phase-based SA for Q-switching [58,59]. Jafry et al. implemented a mode-locked erbium (Er)-doped fiber laser using a Ti3AlC2 MAX phase-based SA [60]. Huhammad et al. implemented a Q-switched ytterbium-doped fiber laser with a Ti3AlC2 MAX phase-based SA [61]. Recently, the use of MAX phase Ti2AlC for a mode-locked Er-doped fiber laser was reported by Sun et al. [62]. MAX phases are a group of ductile ceramic materials with the chemical formula Mn+1AXn (n = 1, 2, 3…), where M is an early transition metal, A is the IIIA or IVA group element, and X denotes either nitrogen or carbon [63–66]. MAX phases are known as robust materials that are useful for nuclear engineering, high-temperature applications and aerospace [65,67–69].
In this work, we studied the nonlinear absorption properties of MAX phase Ti2AlC at a wavelength of 1900 nm using an open-aperture (OA) Z-scan measurement. From this measurement, the nonlinear absorption coefficient of Ti2AlC was estimated as ∼(-24.13×103) cm2/GW at 1900 nm. Subsequently, the density functional theory (DFT) calculation was performed to better understand the energy band structure of the Ti2AlC. According to the calculation, Ti2AlC was observed to have a metallic band structure, indicating that it has a ultrawide absorption bandwidth. Finally, the practical fabrication of an SA using Ti2AlC on a side-polished fiber platform was conducted. The soliton pulses with a pulse width of 960 fs at 1922 nm were successfully generated using Ti2AlC-based SA incorporating a thulium/holmium (Tm-Ho) co-doped fiber laser cavity. As far as the authors are aware, these results first demonstrated the use of a MAX phase-based SA for mode-locking in the 1.9 μm spectral region.
2. Experimental and theoretical investigation of material properties
Ti2AlC bulk powder, which is commercially available (99.9% Carbon-Ukraine Ltd.), was used for this investigation. To obtain the information on the material properties of the Ti2AlC particles, various characterization techniques were employed. Figure 1(a) and 1(b) show scanning electron microscopy (SEM) images of the used Ti2AlC particles. The size of the Ti2AlC was a few micrometers and layered structures were clearly evident in the SEM images. Next, Fig. 1(c) shows the energy dispersive spectroscopy (EDS) measurement that was conducted. The spectrum shows strong peaks corresponding to titanium (Ti), aluminum (Al), and carbon (C). Figure 1(d) depicts the measured X-ray diffraction (XRD) pattern of the prepared Ti2AlC sample. Nine 211 MAX phase peaks were observed at 2θ = (13, 26.15, 33.9, 43.5, 53.1, 60.7, 71.75, and 74.95)° for the (002), (004), (100), (103), (106), (110), (109), and (116) crystalline planes, respectively [70]. The highly intense, sharp (002) peak represents the high degree of crystallinity of the Ti2AlC material [70]. Figure 2 shows the X-ray photoelectron spectroscopy (XPS) measurement that was subsequently performed. The XPS peaks from carbide phases can be seen in Ti 2p3/2, Al 2p3/2, and C 1s (453.7, 70.9, 280.7) eV, respectively [71]. The additional peaks at 457.8 eV and 463.6 eV correspond to titanium dioxide (TiO2) [71,72], while the peak at ∼73.4 eV corresponds to aluminum oxide (Al2O3) [71,72]. A strong peak, which is attributable to C-C/C-H can be identified in the C 1s (284.4 eV) spectrum [71]. The XPS measurement results clearly show that the Ti2AlC micrometer-sized particles became substantially oxidized.
Fig. 1. Measured (a) SEM image and (b) magnified SEM image of the prepared Ti2AlC particles. (c) EDS spectrum and (d) XRD pattern of the Ti2AlC particles.
Fig. 2. The X-ray photoelectron spectroscopy (XPS) spectra of (a) Ti 2p, (b) Al 2p, and (c) C 1s in the prepared Ti2AlC particles.
To obtain a deeper understanding of the electrical properties of Ti2AlC, DFT calculations were conducted to investigate the electric band structures of bulk Ti2AlC using a Quantum Express package [73]. Figure 3(a) illustrates the Ti2AlC crystal structure. It was reported that Ti2AlC crystallizes in a Cr2AlC-type structure with the space group P63/mmc [74]. Ti2AlC compounds have a hexagonal structure with lattice constants of a = b = 3.069 Å, and c = 13.736 Å. The volume and c/a ratio are 112.052 Å and 4.47572, respectively. These values are well-known for Ti2AlC [75,76]. Figures 3(b) and 3(c) show the calculated energy band structure of Ti2AlC and the corresponding density of states (DOS), respectively. The results show that there is no band gap at the Fermi level. This means that Ti2AlC is metallic.
Fig. 3. (a) Structure of the Ti2AlC crystal. (b) The calculated electron band structure and (c) the density of states of Ti2AlC.
An experimental study of the nonlinear optical properties of Ti2AlC was performed using the OA Z-scam measurement at 1900 nm, and Fig. 4 illustrates the measurement setup. We used a 1900 nm femtosecond fiber laser (36.93 MHz, 613 fs) as a light source. The incident laser was focused by a plano-convex lens onto the prepared Ti2AlC sample. The incident laser beam passed through the Ti2AlC sample, and the transmitted beam was directly collected by power meter to perform the OA Z-scan measurement. During the course of the Z-scan measurement, the output beam changes according to the sample position, and it was measured via the power meter reading of the change of the transmitted beam. The Z-scan measurement was conducted by moving the sample from (-20 to 20) mm.
The normalized transmittance was used to measure the nonlinear absorption coefficient β from the following fitting curve [77,78]:
3. Preparation of a Ti2AlC-based saturable absorber
Before fabricating a Ti2AlC-based SA, the linear absorption property of the Ti2AlC was characterized by spectrophotometer (Shimadzu UV-3600). For this measurement, composites of Ti2AlC and polyvinyl alcohol (PVA) was prepared. The prepared composite was dropped onto a clean glass slide, and subsequently dried for 24 h to form a solid film since it is hard to directly measure the linear absorption spectrum of TI2AlC particles without PVA-assisted solid film formation. Figure 6(a) shows the measured linear absorption spectrum of the Ti2AlC/PVA film with the spectrum of the clean slide glass as a background reference. The result clearly shows that the TI2AlC particles have a wide absorption band over a range from (1000 to 2000) nm.
Fig. 6. (a) Measured linear absorption spectrum of the Ti2AlC/PVA film. (b) Schematic of the Ti2AlC/PVA-deposited side-polished fiber.
An all-fiberized SA was fabricated by dropping and drying the prepared Ti2AlC/PVA solution onto the polished fiber platform, as illustrated in Fig. 6(b). The measured insertion loss and polarization-dependent loss (PDL) of the SA were ∼2.98 dB and ∼1.7 dB at 1900 nm, respectively. The prepared SA was then characterized from a perspective of nonlinear transmission by launching high peak power femtosecond pulses into the fiberized SA. A mode-locked, 1.9-μm fiber laser (repetition rate: 36.95 MHz, pulse width: ∼692 fs), which was built in our laboratory, was used as an input pulse source for this measurement. Figure 7 show the measured nonlinear transmission curve of the fabricated all-fiberized SA. The fitting curve for the transmission data (T(I)), which is based on the following equation, was also incorporated in the figure [79]:
where $\Delta T$, I, and ${I_{sat}}$ denote the modulation depth, pulse energy, and saturation energy, respectively, and ${T_{ns}}$ stands for the nonsaturable loss. From this measurement, the following SA parameters could be extracted: the saturation power and the modulation depth are ∼19.8 W and ∼13.7% at 1900 nm, respectively.Fig. 7. (a) Schematic of the nonlinear transmission measurement setup. (b) Nonlinear transmission graph of the Ti2AlC/PVA-based SA.
4. Fiber laser mode-locking experiment
The efficacy of the all-fiberized Ti2AlC/PVA-based SA was tested by incorporating it into a Tm-Ho co-doped fiber laser cavity. The gain fiber length was 1 m. The pump sourced for gain medium was a 1550 nm laser diode. An all-fiberized cavity was composed of a 1550/2000 nm wavelength division multiplexer (WDM), an optical isolator, and an optical coupler with a split ratio of 90:10. The output beam was obtained from the 10% port of the coupler. Figure 8(a) shows the laser cavity configuration.
Fig. 8. (a) Tm-Ho co-doped fiber laser configuration. Measured (b) optical spectrum, (b) oscilloscope trace, (c) autocorrelation trace and (e) electrical spectrum of the output pulses. Inset: electrical spectrum with a wide span of 1 GHz.
The pump power threshold for mode-locking was ∼211 mW, whereas the continuous-wave (CW) lasing was observed at 154 mW. Figure 8(b) shows the measured optical spectrum at a pump power of 297 mW, along with the fitting curve. From the spectrum, the center wavelength and 3-dB bandwidth were ∼1922 nm and ∼4.3 nm, respectively. Figure 8(c) shows the measured oscilloscope trace of the mode-locked pulses at the same pump power. The period of the output pulses was measured to be ∼55.83 ns, while the pulse repetition rate was 17.91 MHz. Next, we measured the autocorrelation trace of the mode-locked pulses (Fig. 8(d)). The pulse width was estimated to be ∼960 fs. The measured electrical spectrum in Fig. 8(e) illustrates a signal-to-background ratio of ∼70 dB at signal frequency of ∼17.91 MHz. Furthermore, the strong harmonic components, which was observed in the 1-GHz span electrical spectrum in the inset of Fig. 8(e), implies the stable status of the output pulses.
5. Conclusion
In summary, we have investigated the nonlinear optical property of the Ti2AlC MAX phase using both experimental and theoretical approaches. Using the OA Z-scan measurement, the nonlinear absorption coefficient, ∼(-24.13×103) cm/GW of Ti2AlC could be obtained at 1900 nm. The feasibility of using Ti2AlC for a practical SA operating in the 1.9 μm spectral region was tested by measuring its nonlinear transmission property and subsequently applying the prepared SA to a Tm-Ho co-doped fiber ring cavity. The Ti2AlC/PVA-based SA was shown to be capable of inducing a femtosecond mode-locking.
We believe that the results provide strong evidence that Ti2AlC MAX phase can be used as a useful saturable absorption material for ultrafast mode-locking in the wavelength band of 2 μm.
Funding
Institute of Information and Communications Technology Planning & Evaluation (2021-0-01810); National Research Foundation of Korea (2021R1A2C1004988).
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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