Ultra-broadband Nonlinear Saturable Absorption for Two-dimensional Bi2TexSe3−x Nanosheets

We report the ultra-broadband nonlinear optical (NLO) response of Bi2TexSe3−x nanosheets produced by a facile solvothermal method. Our result show that the extracted basic optical nonlinearity parameters of Bi2TexSe3−x nanosheets, αNL, Imχ(3), and FOM reach ~104 cm/GW, ~10−8 esu and ~10−13 esu cm, respectively, which are several orders of magnitude larger than those of bulk dielectrics. We further observed the excitation intensity dependence of the NLO absorption coefficient and the NLO response sensitivity. The mechanisms of those phenomena were proposed based on physical model. The wavelength dependence of the NLO response of Bi2TexSe3−x nanosheets was investigated, and we determined that the Bi2TexSe3−x nanosheets possess an ultra-broadband nonlinear saturable absorption property covering a range from the visible to the near-infrared band, with the NLO absorption insensitive to the excitation wavelength. This work provide fundamental and systematic insight into the NLO response of Bi2TexSe3−x nanosheets and support their application in photonic devices in the future.


Results and Discussion
The morphology of the as-prepared Bi 2 Te x Se 3−x nanosheets was characterized by SEM, AFM and TEM. As shown in Fig. 1(a-d), we find that the hexagonal nanosheets are largely distributed on the substrate in high yield. The excellent crystallinity can be well defined by the regular shape and sharp edges. The average size of the platelets is from 400 nm to 5 μ m. Figure 1(e-h) depict the TEM images of the Bi 2 Te x Se 3−x nanosheets. It can be seen that the thickness is symmetrical, with a relatively uniform contrast. The selected area electron diffraction (SAED) patterns shown as insets indicate the single-crystalline nature of these nanosheets. Figure 1(i-l) depict high-resolution transmission electron microscopy (HRTEM) images. Figure 1(i) clearly presents that the lattice space of Bi 2 Te 3 is approximately 0.22 nm, corresponding to the spacing of the (110) planes of the rhombohedral phase. In Fig. 1(j-l), the spacings of 0.22 nm, 0.23 nm, and 0.22 nm are consistent with the lattice spaces of Bi 2 Te 2 Se, Bi 2 TeSe 2 , and Bi 2 Se 3 , respectively. It is implied that the nanosheets are well-crystallized with few atomic defects. The AFM is used to determine the sample thickness, as shown in Fig. 1(m-p). By the analysis of the height profile (across the dotted line inset of the AFM image), the average thickness of the sample is determined to be 35 nm, 80 nm, 45 nm and 30 nm for Bi 2 Te 3 , Bi 2 Te 2 Se, Bi 2 TeSe 2 and Bi 2 Se 3 , respectively. The material characterization of the Bi 2 Te x Se 3−x nanosheets indicates that all our samples possess a similar submicron-scale morphology, nanoscale thickness and excellent crystallinity.
The linear absorption spectrum of the Bi 2 Te x Se 3−x nanosheets is shown in Fig. 2(a). An ultra-broadband linear optical absorption is observed, which is ascribed to the gapless surface state and narrow bulk state gap. Furthermore, we extract the linear absorption coefficient α 0 from the absorption spectrum of the sample. The absorption coefficient of the Bi 2 Te x Se 3−x nanosheets is determined to be ~10 4 cm −1 .
The Raman spectrum of the Bi 2 Te x Se 3−x nanosheets is shown in Fig. 2 35 . Comparing the lattice parameters of the four compounds, it is found that the lattice parameters of Bi 2 Te 3 (a = 4.390 Å, c = 30.460 Å) are slightly larger than those of Bi 2 Te 2 Se (a = 4.303 Å, c = 30.010 Å), Bi 2 TeSe 2 (a = 4.218 Å, c = 29.240 Å) and Bi 2 Se 3 (a = 4.140 Å, c = 28.636 Å). This result verifies that the Te atom is really replaced byan Se atom, which leads to the unit cell shrinkage of the Bi 2 Te x Se 3−x (x = 0, 1, 2) nanosheets.
The NLO properties of the 2D Bi 2 Te x Se 3−x nanosheets were investigated using an open aperture (OA) Z-scan system with femtosecond laser pulses at 532 nm, 800 nm, 1050 nm, and 1550 nm. As the sample was scanned Scientific RepoRts | 6:33070 | DOI: 10.1038/srep33070 across the focus along the optical axis, the transmitted pulse energies in the presence of the far-field aperture were probed by a detector. Figure 3(a-d) show the Z-scan traces of the Bi 2 Te x Se 3−x nanosheets that were obtained by a 532-nm laser pulse at different excitation intensities. All of the curves exhibit a "bell shape", which is induced by the saturable absorption effect (negative NLO absorption). Bi 2 Te x Se 3−x nanosheets normally have a narrow band gap (0.15 ~ 0.3 eV) 36 , so it is reasonable to assume that the one-photon induced absorption dominates the NLO process with a photon energy of 0.8 ~ 2.33 eV (λ = 532 nm ~ 1550 nm). The saturable absorption mechanism can be explained as follows: under weak excited light with a photon energy larger than the bulk state bandgap, the electrons in the valence band can be excited to the conduction band and then occupy states in the conduction band, whereas under a high enough intensity of excited light, all the available states in the conduction band are occupied by photo-generated carriers; owing to the Pauli blocking principle, an optical bleaching effect occurs (i.e., saturable absorption). A schematic diagram is shown in Fig. 4.
According to previous reports 27 , for TI nanosheets, the larger the subsurface bulk region, the more sensitive the absorption is to the excitation intensity. For our results in Fig. 3(a,b) on Bi 2 Se 3 and Bi 2 Te 3 , respectively, the NLO absorption is insensitive to the excitation intensity, which is consistent with the model above. Interestingly, the saturable absorptions of Bi 2 TeSe 2 and Bi 2 Te 2 Se, in Fig. 3(c,d), respectively, show a greater sensitivity to the excitation intensity. We attribute this to the increased contribution of the bulk conduction state in the nonlinear response. We favour this explanation for two reasons. First, the Bi 2 TeSe 2 and Bi 2 Te 2 Se samples are tens of nanometres thicker than those of Bi 2 Te 3 and Bi 2 Se 3 , as shown in Fig. 1(m-p). That means they would possess a much larger bulk region and thereby show much stronger bulk physical properties. Second, the Se atom replacing the Te in Bi 2 Te 3 can cause disorder (impurities) in the structure, which leads to the symmetry breaking and intrinsic doping. Such a structural change will result in changes in the insulation state and conduction state 13 , which are attributed to the saturable absorption being sensitive to the excitation intensity of the Bi 2 Te x Se 3−x (x = 1, 2) nanosheets. With the aim to demonstrate the role of surface effects in the nonlinear response of Bi 2 Te x Se 3−x nanosheets, as a representative, we performed an OA Z-scan to measure the nonlinear optical properties of different thickness Bi 2 TeSe 2 nanosheets. Figure S3(a,b) displays the OA Z-scan results for the two different thickness samples. We can see that all the samples possess a typical saturable absorption property. Different saturable absorption sensitivities to the excitation intensity are observed. The thinner Bi 2 TeSe 2 nanosheet shows a greater sensitivity to the excitation intensity, which is consistent with the result above. However, it should be noted that this result cannot definitively confirm the role of the surface state in the nonlinear optical properties of the Bi 2 TeSe 2 nanosheet because it is still challenging to precisely control the crystal morphology in terms of geometry and thickness 5 . Although the thickness of the nanosheets is roughly controlled, its lateral size still varies from several hundred nanometres to several micrometres. This means that the surface-to-volume ratio of the Bi 2 Te x Se 3−x nanosheets cannot be precisely controlled. To unambiguously determining the role of the surface state in the large nonlinear  coefficients of the Bi 2 Te x Se 3−x nanosheets, the exact control of the synthesis is essential, and work is currently underway.
To quantitatively determine the NLO absorption coefficients and identify the corresponding physical mechanisms, we fitted the OA Z-scan data by the NLO absorption model. Based on a spatially and temporally Gaussian pulse, the normalized energy transmittance, T OA (z), is given by [37][38][39] (1) to the OA Z-scan curves, the NLO absorption coefficient can be extracted. In Fig. 3(a-d), the solid curves are the fitting results based on the NLO absorption model and agree well with the experimental data. The insets of Fig. 3(a-d) delineate the dependence of the NLO absorption coefficient on the excitation intensity. For the Bi 2 Se 3 nanosheets, the NLO absorption coefficient increases from − 2.1 × 10 4 cm/GW to − 0.34 × 10 4 cm/GW as the excitation intensity increases from 7.3 GW/cm 2 to 43.6 GW/cm 2 . It is found that the NLO absorption initially exhibited sustained growth and then reached a plateau as the excitation intensity continued to increase. For the three other Bi 2 Te x Se 3−x nanosheets, a similar trend is observed. The mechanism of this trend is unclear. Based on our results, it is reasonable to deduce that there is continuous competition between ground state bleaching and free-carrier absorption (FCA) 37 . We use the energy level diagram shown in Fig. S2 to interpret the evolution of the Bi 2 Te x Se 3−x nanosheets'NLO absorption coefficient. As illustrated in Fig. 4, with a high enough excitation intensity, Pauli blocking leads to a saturable absorption in the Bi 2 Te x Se 3−x nanosheets. This process corresponds to grounding state bleaching. As the excitation intensity increases, the FCA contributes to the reserve saturable absorption. This FCA process provides a new channel for excited carrier absorption that effectively increases the NLO absorption. The NLO absorption tends to be unchanged owing to the balance between the ground state bleaching and FCA.
To investigate the saturation intensity (I s ) of the Bi 2 Te x Se 3−x nanosheets, a hyperbolic approach saturation numerically model for semiconductors is used, which is expressed as 40,41 and α NL 0 are the intensity-dependent and low-intensity coefficients, respectively. The best fit is shown in the inset of Fig. 3, which indicates that I S ~ 10 9 W/cm 2 . Table 1 summarizes the saturation intensity I S for Bi 2 Te x Se 3−x nanosheets at different wavelengths. The saturation intensity of the as-prepared Bi 2 Te x Se 3−x nanosheets sample is consistent with those of the recently reported Bi 2 Se 3 42 and Bi 2 TeSe 2 28 . With the aim of demonstrating the dependence of the NLO absorption response on the excitation wavelength for Bi 2 Te x Se 3−x nanosheets, we performed an OA Z-scan experiment at 800 nm, 1050 nm and 1550 nm. Figure 5(a-d) depicts the Z-scan result of the Bi 2 Te x Se 3−x nanosheets at different wavelengths. The saturable absorption process dominates the NLO response induced by a one photon absorption in the Bi 2 Te x Se 3−x nanosheets. As noted above, at the considered excitation wavelengths (532 nm, 800 nm, 1050 nm, and 1550 nm), the smallest photon energy 0.8 eV (λ = 1550 nm) may be larger than the band gap of the Bi 2 Te x Se 3−x nanosheets. Thus, two-photon and multiphoton absorption may be extremely suppressed during the NLO process at low excitation intensity. The NLO response of the three other Bi 2 Te x Se 3−x nanosheets confirmed this phenomenon. In our experiment, for every type of Bi 2 Te x Se 3−x nanosheet, the excitation light maintains a similar intensity for all wavelengths.
We extracted the NLO absorption coefficients at different wavelengths, as shown in Fig. 5(e,f). It can be found that the NLO absorption response has no obvious dependence on the excitation wavelength, which means that the NLO absorption is insensitive to the excitation wavelength. In a conventional optical nonlinear enhancement  hetero-nanostructure system 43,44 , the nonlinear response is sensitive to the excitation wavelength. When the excitation wavelength varies near the surface plasmon resonance band, the field enhancement caused by the surface plasmon resonance increases the incident irradiance. This makes it possible to observe the nonlinear response sensitivity to the excitation wavelength. In the UV-Vis-NIR absorption spectra of the Bi 2 Te x Se 3−x nanosheets, in Fig. 2(a), there is no significant surface plasmon resonance absorption in the entire visible and near-infrared range. This indicates that no significant local field enhancement appears as the excitation wavelength changes, which may be attributed to the oscillator strength of the dipole transition being nearly the same under different excitation wavelengths. Additionally, the result reveals that the Bi 2 Te x Se 3−x nanosheets present ultra-broadband saturable absorption properties (from the visible-band to the telecommunication C-band), which plays an important role in ultrashort pulsed laser generation.
To understand the other basic NLO properties of the Bi 2 Te x Se 3−x nanosheets, we obtained the third-order NLO susceptibility Imχ (3) and the figure of merit FOM of Bi 2 Te x Se 3−x nanosheets at different wavelengths. The imaginar y part of the third-order NLO susceptibility Imχ (3) can be expressed as 20 , where c is the speed of light in vacuum, λ is the wavelength of the excitation laser, n is the linear refractive index, and α NL is the NLO absorption coefficient. The refractive index n can be calculated from the reflectivity 45 where the reflection spectrum of the Bi 2 Te x Se 3−x nanosheets is depicted in Figure S4). The figure of merit was defined to eliminate the discrepancy caused by the linear absorption α 0 :  Table 1. Compared to previous works, the values of the FOM are approximately two orders of magnitude larger than that of graphene ~5 × 10 −15 esu cm, graphene oxide ~4.2 × 10 −15 esu cm 46 , reduced graphene oxide ~0.36 × 10 −15 esu cm 47 , and MoS 2 /NMP dispersions ~1.06 × 10 −15 esu cm 20 , one order of magnitude larger than that of multilayer MoS 2 (25-27 layers) ~1.1 × 10 −14 esu cm and WS 2 (18-20 layers) ~2.16 × 10 −14 esu cm 19 , and close to that of monolayer WS 2 ~ 1.1 × 10 −13 esu cm 19 . Our results indicate that the Bi 2 Te x Se 3−x nanosheets possess a fascinating NLO response in an ultra-broadband. They can therefore be an excellent potential alternative material for an ON/OFF resonance, saturable absorber.

Conclusions
In summary, we have performed fundamental and systematic measurements of the NLO response of Bi 2 Te x Se 3−x nanosheets using the OA Z-scan technique. Our results demonstrate that the Bi 2 Te x Se 3−x nanosheets exhibit ultra-broadband saturable absorption properties and possess fascinating NLO parameters, i.e., α NL ~ − 10 4 cm/GW, Imχ (3) ~ 10 −8 esu, and FOM ~ 10 −13 esu cm. Under a ~GW/cm 2 excitation light intensity, there is insensitive saturable absorption for Bi 2 Se 3 and Bi 2 Te 3 . Intriguingly, significantly sensitive saturable absorption was observed for Bi 2 Te 2 Se and Bi 2 TeSe 2 , which was ascribed to the change in the bulk region and the atom replacement induced increase of the conduction state. Furthermore, we find that the NLO absorption responses have a weak dependence on the excitation wavelength. This work demonstrates that Bi 2 Te x Se 3−x nanosheets are a promising alternative material for saturable absorbers and other nanophotonic devices.