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Abstract
Two-dimensional transition metal carbides/nitrides (MXenes) have attracted a lot of attention as optical materials owing to their unique electronic and optical features. In this paper, we demonstrated the nonlinear optical response of Ti3C2 MXene in the near infrared (NIR) regime at 1 and 1.3 µm. The nonlinear optical absorption behavior was systematically studied by the open/closed aperture (OA/CA) Z-scan techniques. Under a low excitation intensity, the saturable absorption dominated the optical nonlinear process, and the maximum effective nonlinear absorption coefficients βeff were −0.59 cm/MW and −0.68 cm/MW at 1 and 1.3 µm, respectively. The nonlinear refractive index n2 and the third-order susceptibility of Ti3C2 Mxene at 1 µm were −2.18 ×10−2 cm2/GW and 3.65×10−3 esu, respectively. While with the excitation laser at 1.3 µm, the nonlinear refractive index n2 was −1.64×10−2 cm2/GW and the third-order susceptibility was 2.75×10−3 esu. With the increase of incident optical intensity, the two-photon absorption (TPA) process was observed, leading to the reverse absorption phenomenon. Our results confirmed that Ti3C2 MXene possessed intensity-depended nonlinear optical properties, which could be applied in various nonlinear optical devices.
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1. Introduction
Nowadays, optical materials are of great importance for the applications in optical communications [1], data processing and storage [2], photoelectrochemistry [3], and photoharvesting, etc. Thus an agent with large nonlinear optical (NLO) response could be of interest for the optical pulse modulation and manipulation. In these NLO media, saturable absorption, reverse saturable absorption (RSA) and multi-photon absorption (MPA) are usually performed with the different-intensity light interaction [4,5].
Up to date, a plenty of two-dimensional materials have emerged in the nonlinear optical material family, such as graphene [6–9], black phosphorus [10–13], topological materials [14], and transition metal compounds [15,16]. Recently, transition metal compound MXenes have drawn an intense of attention owing to the special optical and electronic properties. As a branch of MXenes, Ti3C2 MXene has been widely used in the fields of chemical batteries [17–19], photocatalysis [20–23] and microwave absorption [24–26] due to the unique layered structure and superior electrochemical properties. Besides, Ti3C2 MXene exhibits excellent nonlinear optical performance. So far, Ti3C2 MXene has been employed as a saturable absorber to realize wide-band Q-switching and mode-locking operations [27–33]. Moreover, monolayer Ti3C2 MXene has the strong two-photon absorption (TPA) effect at 780 and 1100 nm, which is of great importance for the possible photonics applications [34]. However, the systematic study on the NLO properties of Ti3C2 MXene is still a blank, and more details need to be done in order to assist with the practical applications.
In this paper, the NLO properties of Ti3C2 MXene were investigated at 1 and 1.3 µm for the first time using open and closed aperture (OA/CA) Z-scan techniques. The NLO response was revealed to change with the variation of excitation pump energy. Through fitting experimental data, we obtained the effective nonlinear absorption coefficient βeff, nonlinear refractive index n2 and third-order susceptibility χ(3) of the synthesized sample. The results confirmed the large NLO response of Ti3C2 MXene, which can be applied as a potential saturable absorber.
2. Characterization
We used the same preparation method reported in the previous study [35] to obtain Ti3C2 MXene sample. Before analyzing the NLO properties of Ti3C2 MXene, we made a series of characterization to show its surface morphology and structure. Figure 1(a) shows the scanning electron microscopy (SEM) results of Ti3C2 MXene. We can clearly see the layered structure of MXene, which is consistent with previous report [36]. The material was further observed by the transmission electron microscopy (TEM). According to Fig. 1(b), the monolayer thickness of Ti3C2 MXene is 0.89 nm. Then we determined the microstructure of the sample by HRTEM images. A very regular and well-defined hexagonal lattice can be observed in Fig. 1(c), showing the excellent crystallinity of Ti3C2 MXene. The inset of Fig. 1(c) demonstrates the selected area electron diffraction (SAED) map of the corresponding region with obvious diffraction points, which is consistent with the observed results in Fig. 1(c). We also measured the atomic composition and corresponding proportion of the prepared Ti3C2 MXene. As shown in Fig. 1(d), we can get that the weight ratios of Ti, C, O and F in the sample are 48.8%, 20.5%, 17.7% and 12.9%, respectively. Figure 1(e) shows the detailed element distribution of Ti3C2 MXene, demonstrating the homogeneity of our prepared material. To understand the internal structure of Ti3C2 MXene in more detail, we measured the XRD and Raman spectra. By analyzing the XRD pattern of Ti3C2 MXene in Fig. 1(f), we can see that the peak at 2θ = 39° is weakened compared with the standard peak of Ti3AlC2 (JCPDS No.52-0875), indicating that the Al layer is selectively etched by HF acid solution. Figure 1(g) shows the Raman spectrum of Ti3C2 MXene, whose characteristic peaks are located at 390 cm−1 and 619 cm−1, respectively. All characterization data confirmed that the Ti3C2 MXene was successfully prepared. The MXene was transferred on a quartz substrate via the spin-coating method. Figure 1(h) clearly displays the linear absorption spectra of transferred Ti3C2 MXene from 900 to 1500 nm, which also indicates the possible optical application in the near infrared region. The inset of Fig. 1(h) gives the morphology and thickness of the MXene on the quartz with a maximum thickness of ∼ 118 nm. After the laser beam excitation, we observed no light-induced degradation, showing the quite stability of the as-prepared MXene.
Fig. 1. Ti3C2 MXene characterization: (a) SEM; (b) TEM; (c) HRTEM and SAED pattern (inset); (d) Elemental content ratio; (e) Detailed element distribution; (f) XRD pattern and (g) Raman spectrum; (h) UV-vis-NIR absorption spectrum and morphology and thickness diagram of experimental sample (inset).
3. Nonlinear optical properties
3.1 Nonlinear saturable absorption
The detailed NLO response of Ti3C2 MXene was studied by a conventional open/closed aperture (OA/CA) Z-scan system with different excitation intensity at 1 and 1.3 µm in the nanosecond level. Therefore, in our case, we mainly investigated the nanosecond nonlinearities of the as-prepared Ti3C2 MXene. In order to comprehensively study the NLO characteristics of Ti3C2 MXene, different excitation intensities were implemented to excite Ti3C2 MXene at both wavelengths. The nonlinear transmissivities with the excitation intensities are given in Fig. 2. It can be seen that Ti3C2 Mxene exhibits strong intensity-dependent nonlinear saturable absorption properties. The nonlinear transmission can be fitted with the following formulas:
Fig. 2. The OA Z-scan results of Ti3C2 Mxene at (a) 1.06 µm and (b) 1.34 µm. Solid lines: fittings, dots: experimental data. Modulation depth (c) at 1.06 µm and 1.34 µm versus incident intensity. Effective nonlinear absorption coefficient βeff versus incident intensity (d) at 1.06 µm and 1.34 µm.
As can be seen from the normalized transmittance curves given in Fig. 2(a) and (b), the transmittance at different incidence intensities change symmetrically with the position of the Z-axis, which belongs to typically saturated absorption. This proved that the prepared Ti3C2 MXene material had good nonlinear absorption characteristics. Meanwhile, as shown in Fig. 2(c), the modulation depths at both wavelengths increase with the incident intensity. The maximum modulation depths were 10.9% and 9.3% at 1.06 and 1.34 µm, respectively. Figure 2(d) shows the variation of effective nonlinear absorption coefficient βeff with incident intensity. Obviously, |βeff| decreased with the increase of incident intensities. At a lower excitation intensity, the single-photon saturable absorption played a dominant role in the nonlinear absorption. With the increase of incident intensity, the two-photon absorption (TPA) and reverse saturable absorption (RSA) would occur, resulting in the decrease of |βeff|. The maximum βeff at 1 and 1.3 µm were −0.59 and −0.68 cm/MW, respectively.
3.2 Third-order NLO susceptibility
In order to obtain nonlinear refractive index n2 and third-order nonlinear susceptibility, the CA Z-scan technology was implemented. Figure 3 shows the transmission differences of CA/OA Z-scan experiments at 1 and 1.3 µm, respectively, and the transmission can be fitted with the following expression [37]:
In addition, the third-order susceptibility is an essential parameter of the NLO materials. We can obtain the third-order NLO susceptibility χ(3) of the sample from the nonlinear absorption coefficient βeff and nonlinear refractive index n2, and the real part $\mathrm{\Re }$χ(3) and imaginary part $\mathrm{\Im }$χ(3) can be calculated by the following formula [37]:
The typical NLO parameters are listed in Table 1.
Table 1. NLO parameters summary of Ti3C2 MXene in the NIR spectral band.
3.3 Two-photon absorption
To confirm the TPA effect in the saturable absorption progress, we further increased the excitation intensity. As shown in Fig. 4, with the increase of the pump intensities, the transmission curve of Ti3C2 MXene exhibits the valley at each wavelength. When the incidence range was strong enough, the evident desaturation absorption phenomenon appeared, which can be attributed to the TPA effect. The TPA coefficient can be fitted by the following formula [37]:
4. Conclusion
In summary, we synthesized Ti3C2 MXene and carried out a series of characterization. The nonlinear optical responses at 1 and 1.3 µm were studied by the open/closed aperture Z-scan technique for the first time. The open-aperture Z-scan technique was used to measure the saturation absorption of Ti3C2 MXene at different incidence intensities. It was found that at low incidence intensities, the saturable absorption played a dominant role in the nonlinear absorption. With the increase of incidence intensities, the proportion of two-photon absorption and reverse saturable absorption gradually increased. The third-order optical susceptibility of the sample was measured by the closed-aperture Z-scan technique. χ(3) of the Ti3C2 MXene sample were 3.65×10−3 and 2.75×10−3 esu at 1 and 1.3 µm, respectively. Our work confirmed Ti3C2 Mxene can be used as an alternate material for nonlinear optical devices with large NLO response in the near-infrared region.
Funding
National Natural Science Foundation of China (12004213, 12174223, 21872084).
Acknowledgments
H. C. would like to thank the financial support from the Young Scholar 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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