Introduction

Topological insulators (TIs) are electronic materials that exhibit an insulation state for their bulk but a conduction state on their edge and surface1,2,3,4. In recent years, significant efforts have been devoted to controlling the crystal morphology of TIs5,6,7, extending their applications in the electronics and optoelectronics fields8,9,10, and improving their performance in thermoelectric devices11,12.

Owing to the strong light-matter interactions in two-dimensional (2D) materials, the linear optical properties of the Bi2TexSe3−x family have been intensely studied in previous works13,14,15. Meanwhile, inspired by the progress of NLO applications of TI materials16,17,18,19,20, NLO research into TIs has attracted a great deal of interest. Zhang et al.21,22,23 demonstrated a series realization of ultra-short pulse generation based on a Bi2Se3 saturable absorber. Subsequently, similar work on other members of the Bi2TexSe3−x family, i.e., Bi2Te3, Bi2TeSe2, and Bi2Te2Se24,25,26,27,28, was reported. However, those works generally focused on the modulation depth, saturation intensity and nonsaturable loss. Although those are necessary indicators for the application of saturable absorbers, investigation into the basic NLO parameters, such as the NLO absorption coefficient αNL and third-order NLO susceptibility Imχ(3), has been limited. Those basic coefficients are important optical parameters for 2D TIs, which determine the performance of 2D TIs-based nanophotonic devices.

In this work, using a high-yield facile solvothermal method, we synthesized large-scale and high-quality Bi2TexSe3−x nanosheets and verified the process using material characterization. For experimental investigation of NLO response, self-phase modulation29,30,31,32 and Z-scan measurement20,33,34 are the most popular experimental methods, and we chose the latter. Significant saturable absorption was observed at all of the excitation wavelengths (532 nm, 800 nm, 1050 nm and 1550 nm), which indicated their ultra-broadband saturable absorption characteristics. Based on the NLO theory model, we extracted the NLO absorption coefficient αNL, third-order NLO susceptibility Imχ(3), figure of merit (FOM) for the third-order optical nonlinearity, and saturation intensity Is from the Z-scan result. The excellent NLO response, i.e., αNL ~ 104 cm/GW, Imχ(3) ~ 10−9 esu, FOM ~10−13 esu cm and Is ~ 109 W/cm2, implies that the Bi2TexSe3−x nanosheets have the potential for application in nanophotonic devices. The results for the different excitation wavelengths demonstrated that the NLO response is independent of the excitation wavelength. Additionally, it was found that atom-doped Bi2TexSe3−xnanosheets (Bi2TeSe2, Bi2Te2Se) showed a more sensitive nonlinear absorption response to photoexcitation.

Materials and Methods

Synthesis of Bi2TexSe3−xnanosheets

The precursor materials for the solvothermal synthesis of Bi2TexSe3−x nanosheets were bismuth nitrate (Bi(NO3)3), bismuth oxide (Bi2O3), selenium oxide (SeO2) and tellurium oxide (TeO2), all of which were purchased from Alfa Aesar. Polyvinylpyrrolidone (PVP, K30) was purchased from TCI, and sodium hydroxide (NaOH) and ethylene glycol (EG) were acquired from Shanghai Chemical Reagent Co. (Shanghai, China). All of the chemicals were used as received without further purification.

The Bi2TexSe3−x nanosheets, i.e., Bi2Se3, Bi2Te3, Bi2TeSe2, Bi2Te2Se, were synthesized by the same procedures as described in our previous work, ref. 5. Based on the solvothermal method, we produced high-quality Bi2TexSe3−xnanosheets with a regular hexagonal morphology and good crystallinity.

Material characterization

The morphology of the graphene-Bi2Te3 heterostructure was investigated by scanning electron microscopy (SEM) (FEI Quanta 200 FEG, acceleration voltage: 5–30 kV) and transmission electron microscopy (TEM) (HRTEM, FEI Tecnai G2 F20 STWIN, FEI). The topography and thickness of the as-produced samples were determined by atomic force microscopy (AFM) (Agilent 5500). The optical nonlinearity of the sample was investigated by a conventional Z-scan technique. A femtosecond laser pulse was produced by an optical parametric amplifier (TOPAS, USF-UV2) that was pumped by a Ti: Sapphire regenerative amplifier system (Spectra-Physics, Spitfire ACE-35F-2KXP, Maitai SP and Empower 30) with a pulse duration of 35 fs and a pulse repetition rate of 2 kHz. The laser beam was focused by a lens with a focus length of 150 mm. A computer-controlled translation stage was employed to move the sample along the propagation direction (z-axis) of the laser pulses, and the transmitted pulse energies were probed by a detector (OPHIR, PD300R-IR).

Results and Discussion

The morphology of the as-prepared Bi2TexSe3−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 μm.

Figure 1
figure 1

Characterization of Bi2TexSe3−x nanosheets: (a–d) Scanning electron microscopy (SEM), (e–h) transmission electron microscopy (TEM), (i–l) high-resolution transmission electron microscopy (HRTEM) and (m–n) atomic force microscopy (AFM) images of Bi2Te3, Bi2Te2Se, Bi2TeSe2 and Bi2Se3. The insets of (e–h) are the selected area electron diffraction (SAED) patterns of the nanosheets.

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Figure 1(e–h) depict the TEM images of the Bi2TexSe3−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 Bi2Te3 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 Bi2Te2Se, Bi2TeSe2, and Bi2Se3, 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 Bi2Te3, Bi2Te2Se, Bi2TeSe2 and Bi2Se3, respectively. The material characterization of the Bi2TexSe3−xnanosheets indicates that all our samples possess a similar submicron-scale morphology, nanoscale thickness and excellent crystallinity.

The linear absorption spectrum of the Bi2TexSe3−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 α0 from the absorption spectrum of the sample. The absorption coefficient of the Bi2TexSe3−xnanosheets is determined to be ~104 cm−1.

Figure 2
figure 2

(a) Absorption spectra and (b) Raman spectra of Bi2TexSe3−x nanosheets, taken with an excitation laser wavelength of 633 nm.

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The Raman spectrum of the Bi2TexSe3−x nanosheets is shown in Fig. 2(b) and was obtained by a laser excitation wavelength of 633 nm. It is observed that the typical three main Raman peaks corresponding to , and are located in low frequency range from 30 to 200 cm−1 for all compounds. For the Bi2Te3 nanosheets, the three Raman peaksappear at 62 cm−1, 102 cm−1 and 137 cm−1, respectively. With more Se incorporation and the replacement of Te, the vibrational modes of the Bi2Te2Se nanosheets are slightly red-shifted, i.e., 65 cm−1, 102 cm−1 and 138 cm−1, compared with those of the Bi2Te3 nanosheets. For Bi2TeSe2 and Bi2Se3, significant red-shifts exist for the three main peaks, i.e., the three peaks for Bi2TeSe2 occur at 70 cm−1, 129 cm−1 and 171 cm−1 and for Bi2Se3 they move to 71 cm−1, 132 cm−1 and 174 cm−1. This suggests that the stoichiometric ratio of the four compounds has an obvious effect on the vibrational modes. Further understandings about the Raman spectroscopy have been reported in our previous work, ref. 5. To further verify the crystal structure and phase of the Bi2TexSe3−x nanosheets, we performed an X-ray diffractometer (XRD) analysis on the four compounds, as shown in Fig. S1. The XRD pattern can be steadily indexed to the rhombohedral Bi2Te3 structure (refer to JCPDS Card Number 82–0358, space group: )35. Comparing the lattice parameters of the four compounds, it is found that the lattice parameters of Bi2Te3 (a = 4.390 Å, c = 30.460 Å) are slightly larger than those of Bi2Te2Se (a = 4.303 Å, c = 30.010 Å), Bi2TeSe2 (a = 4.218 Å, c = 29.240 Å) and Bi2Se3 (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 Bi2TexSe3−x (x = 0, 1, 2) nanosheets.

The NLO properties of the 2D Bi2TexSe3−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 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 Bi2TexSe3−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). Bi2TexSe3−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.

Figure 3
figure 3

OA Z-scan result of Bi2TexSe3−x for different intensities: (a) Bi2Se3, (b) Bi2Te3, (c) Bi2TeSe2, (d) Bi2Te2Se; Inset: NLO absorption coefficients vary with excitation intensity.

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Figure 4
figure 4

Schematic diagram of optical saturable absorption in Bi2TexSe3−x nanosheets.

CB: Conduction band; VB: Valence band.

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According to previous reports27, 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 Bi2Se3 and Bi2Te3, respectively, the NLO absorption is insensitive to the excitation intensity, which is consistent with the model above. Interestingly, the saturable absorptions of Bi2TeSe2 and Bi2Te2Se, 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 Bi2TeSe2 and Bi2Te2Se samples are tens of nanometres thicker than those of Bi2Te3 and Bi2Se3, 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 Bi2Te3 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 state13, which are attributed to the saturable absorption being sensitive to the excitation intensity of the Bi2TexSe3−x (x = 1, 2) nanosheets. With the aim to demonstrate the role of surface effects in the nonlinear response of Bi2TexSe3−x nanosheets, as a representative, we performed an OA Z-scan to measure the nonlinear optical properties of different thickness Bi2TeSe2 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 Bi2TeSe2 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 Bi2TeSe2 nanosheet because it is still challenging to precisely control the crystal morphology in terms of geometry and thickness5. 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 Bi2TexSe3−x nanosheets cannot be precisely controlled. To unambiguously determining the role of the surface state in the large nonlinear coefficients of the Bi2TexSe3−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, TOA (z), is given by37,38,39

where q0 = αNLI0Leff, αNL is the NLO absorption coefficient, Leff = [1−exp (−α0L)]/(−α0L), α0 is the linear optical absorption coefficient and L is the sample path length. By fitting equation (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 Bi2Se3 nanosheets, the NLO absorption coefficient increases from −2.1 × 104 cm/GW to −0.34 × 104 cm/GW as the excitation intensity increases from 7.3 GW/cm2 to 43.6 GW/cm2. 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 Bi2TexSe3−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 Bi2TexSe3−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 Bi2TexSe3−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 (Is) of the Bi2TexSe3−x nanosheets, a hyperbolic approach saturation numerically model for semiconductors is used, which is expressed as40,41 , where and are the intensity-dependent and low-intensity coefficients, respectively. The best fit is shown in the inset of Fig. 3, which indicates that IS ~ 109 W/cm2. Table 1 summarizes the saturation intensity IS for Bi2TexSe3−x nanosheets at different wavelengths. The saturation intensity of the as-prepared Bi2TexSe3−x nanosheets sample is consistent with those of the recently reported Bi2Se342 and Bi2TeSe228.

Table 1 Linear and NLO parameters of Bi2TexSe3−x nanosheets measured based on the Z-scan technique.
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With the aim of demonstrating the dependence of the NLO absorption response on the excitation wavelength for Bi2TexSe3−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 Bi2TexSe3−x nanosheets at different wavelengths. The saturable absorption process dominates the NLO response induced by a one photon absorption in the Bi2TexSe3−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 Bi2TexSe3−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 Bi2TexSe3−x nanosheets confirmed this phenomenon. In our experiment, for every type of Bi2TexSe3−x nanosheet, the excitation light maintains a similar intensity for all wavelengths.

Figure 5
figure 5

OA Z-scan result of Bi2TexSe3−x nanosheets at different wavelengths (a) Bi2Se3, (b) Bi2Te2Se, (c) Bi2TeSe2, (d) Bi2Te3; (e–h) NLO absorption coefficients are proportional to wavelength for Bi2Se3, Bi2Te2Se, Bi2TeSe2, and Bi2Te3, respectively.

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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 system43,44, 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 Bi2TexSe3−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 Bi2TexSe3−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 Bi2TexSe3−x nanosheets, we obtained the third-order NLO susceptibility Imχ(3) and the figure of merit FOM of Bi2TexSe3−x nanosheets at different wavelengths. The imaginary part of the third-order NLO susceptibility Imχ(3) can be expressed as20 , where c is the speed of light in vacuum, λ is the wavelength of the excitation laser, n is the linear refractive index, and αNL is the NLO absorption coefficient. The refractive index n can be calculated from the reflectivity45 (, where the reflection spectrum of the Bi2TexSe3−x nanosheets is depicted in Figure S4). The figure of merit was defined to eliminate the discrepancy caused by the linear absorption α0: . For the Bi2Te3 nanosheets, the results are demonstrated to be Imχ(3) = (−1.22 ± 0.24) × 10−8 esu and FOM = (3.70 ± 0.74) × 10−13 esu cm. The values obtained for different wavelengths are listed in 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 cm46, reduced graphene oxide ~0.36 × 10−15 esu cm47, and MoS2/NMP dispersions ~1.06 × 10−15 esu cm20, one order of magnitude larger than that of multilayer MoS2 (25–27 layers) ~1.1 × 10−14 esu cm and WS2 (18–20 layers) ~2.16 × 10−14 esu cm19, and close to that of monolayer WS2 ~ 1.1 × 10−13 esu cm19. Our results indicate that the Bi2TexSe3−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 Bi2TexSe3−x nanosheets using the OA Z-scan technique. Our results demonstrate that the Bi2TexSe3−x nanosheets exhibit ultra-broadband saturable absorption properties and possess fascinating NLO parameters, i.e., αNL ~ −104 cm/GW, Imχ(3) ~ 10−8 esu, and FOM ~ 10−13 esu cm. Under a ~GW/cm2 excitation light intensity, there is insensitive saturable absorption for Bi2Se3 and Bi2Te3. Intriguingly, significantly sensitive saturable absorption was observed for Bi2Te2Se and Bi2TeSe2, 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 Bi2TexSe3−x nanosheets are a promising alternative material for saturable absorbers and other nanophotonic devices.

Additional Information

How to cite this article: Wang, Y. et al. Ultra-broadband Nonlinear Saturable Absorption for Two-dimensional Bi2TexSe3-x Nanosheets. Sci. Rep. 6, 33070; doi: 10.1038/srep33070 (2016).