Compared with homogeneous two-dimensional materials, two-dimensional nanomaterial heterostructures which consist of different kinds of two-dimensional materials exhibit stronger light-matter interaction. The topological insulator of Bi2Se3-Bi2Te3 heterostructures were synthesised by two-step reaction proceeding. The reaction arised in two steps: at first, Bi2Se3 nanosheets were prepared in the solvent of ethylene glycol (EG); secondly, Bi2Te3 was epitaxial grown on the Bi2Se3 nanosheets to form heterostructure materials. The spatial self-phase modulation (SSPM) effect of 457nm, 532nm and 671nm was achieved by dispersing the as-prepared Bi2Se3-Bi2Te3 heterostructure materials into ethanol as an optical medium. Furthermore, based on the effect of SSPM, a device called all-optical switching was also realized. As an interesting nonlinear optical materials, topological insulator Bi2Se3-Bi2Te3 heterostructures might be an effective proposal for photonics devices such as optical switchings, optical modulators, photodetectors, etc.
Topological insulator materials represented by Bi2Te3, Bi2Se3 and Sb2Te3 is a new type of two-dimensional nanomaterials with special energy band structure.1–3 There is a certain energy band gap between the conduction band and the valence band inside the topological insulators. However, at the surface or edge of the topological insulators, there is a topologically protected surface state or edge state without energy gap. This special energy band structure of the topological insulator materials therefore leads to an ultra-broadband spectral response range ranging from the visible to the microwave frequency. Furthermore, it has been demonstrated by Z-scan method that the topological insulators have saturable absorption properties.4–6 It is precisely because of these unique optical properties that topological insulators have important application value in non-linear optics, light modulation, fiber laser and so on.5,7–9 Heterostructure materials have combined different excellent properties of different two-dimensional nanomaterials, which overcome the inherent defects of homogeneous two-dimensional nanomaterials and shows distinctive optical properties.10,11 As a new kind of interesting optical materials, the optical properties and especially nonlinear optical properties of the heterostructures formed by topological insulator materials deserves our explorations and researches.
The heterostructures of topological insulator materials can be prepared by transfer technology or chemical growth method.12–16 The so-called transfer technology is, after the preparation of two-dimensional nanomaterials, different kinds of nanomaterials were transferred to the desired position and stacked to form the required heterostructure materials according to the requirements.11 Different kinds of two-dimensional materials are stacked together to obtain the required two-dimensional heterostructure materials through the transfer method, which are not bind to the growth conditions of the typical lattice-matching constraints between two-dimensional materials. However, the operation of this method is too complex to prepare in large quantities. Another method of synthesizing the heterostructures of topological insulator materials is the growth method, including chemical vapor deposition and solution method. A large amount of two-dimensional nanomaterials were grown by chemical vapor deposition (CVD).17 The heterostructures were obtained by epitaxy growth of the second two-dimensional nano-material on the first two-dimensional nanomaterial. In the continuous growth process of CVD, the reaction conditions, such as temperature, reaction atmosphere and pressure, must be controlled accurately and complicatedly to avoid the decomposition of the two-dimensional nanomaterials used as substrates and the homogeneous nucleation of epitaxial layer two-dimensional nanomaterials.
Compared with the rigorous requirements for experimental conditions of CVD, the preparation of two-dimensional heterostructure nanomaterials by solution-based synthesis method is more convenient, with large yield and inherent advantages.16,18 In addition, the heterostructure materials prepared by solution-based synthesis method can be well dispersed into different solution as optical media for optical properties measurement.19 The nonlinear optical properties of two-dimensional nanomaterials have attracted great attentions and have been applied in many aspects.20–22 The spatial self-phase modulation effect has been successfully observed in two-dimensional nanomaterials including graphene, transition metal dichalcogenide, black phosphorus and topological insulator materials.17,23–27 As a result, the third-order nonlinear coefficient and nonlinear refractive index of two-dimensional materials have been derived. Using 532nm continuous wave laser beams as light source, Wang et al. observed an obvious spatial self-phase modulation effect in the graphene dispersion, and thus deduced the nonlinear refractive index of the graphene to be n2≈10-5cm2W-1, and the third-order nonlinear susceptibility to be χ(3) ≈ 10-3 (e.s.u.).28 Under the continuous wave laser beam of 488nm, the SSPM effect was observed in the WS2 and MoSe2 dispersions.29 The nonlinear refractive indexes of the three transition metal dichalcogenides were deduced to be about 10-7 cm2W-1, and the third-order nonlinear susceptibility to be about 10-3esu. He et al. observed the spatial self-phase modulation effect in black phosphorous using femtosecond pulsed laser and deduced that the nonlinear refractive index is about 10-5 cm2W-1 and the third-order nonlinear coefficient is about 10-8 esu.24 The spatial self-phase modulation effect in Bi2Se3 and Bi2Te3 topological insulators have been confirmed,23,30 but so far, the spatial self-phase modulation effect in Bi2Se3-Bi2Te3 topological insulators heterostructure materials have not been experimentally verified.12–16
All-optical switching is a device to turn on and off the optical signals, which plays a key role in optical communication and all-optical signal processing.31 With the development of photonic information technology, all-optical switching devices with low power consumption, high speed and small size are urgently needed. Nonlinear optical materials are critical to the performance of optical switches. Compared with the traditional nonlinear optical materials, heterostructure materials have the advantages of ultrathin thickness, ultrafast optical response, ultra-strong light-matter interaction, and so on.3 Zhang et al. experimentally demonstrated the all-optical switching using two-dimensional material SnS as an optical medium, based on spatial cross-phase modulation.32,33 Two different wavelengths of light were selected as signal beam and control beam respectively. The signal beam with weak light intensity can be modulated by adjusting the control beam with strong light intensity. He et al. achieved tri-phase all-optical switching by using the topological insulator materials Bi2Se3 dispersion as an optical medium.23 By adjusting the intensity of the control beam, it is convenient to control the signal beam, which can ultimately present three different states including invariance, focusing and diffraction. In contrast to homogeneous topological insulator materials, heterostructure materials have stronger light-matter interaction so that they could be more effective as optical media for optical switching.
In this paper, topological insulator heterostructure nanomaterials were prepared through solvothermal method by two-step method. Bi2Se3 nanosheets were prepared using Bismuth triacetate and Sodium selenite as raw materials, PVP as surfactants and ethylene glycol as solvent. Subsequently, heterostructure materials were synthesized by the epitaxial growth of Bi2Te3 on the seed crystals of Bi2Se3 nanosheets. Furthermore, the spatial self-phase modulation effect of 457nm, 532nm and 671nm was achieved by dispersing the as-synthesized Bi2Se3-Bi2Te3 heterostructure materials into ethanol solution as an optical medium. Based on Bi2Se3-Bi2Te3 heterostructure materials, the all-optical switching was achieved innovatively employing the spatial self-phase modulation effect. By adjusting the light intensity of the control beam, the spatial light intensity distribution of the signal beam can be modulated conveniently.
II. SYNTHESIS AND CHARACTERIZATION OF Bi2Se3-Bi2Te3 HETEROSTRUCTURE NANOSHEETS
Bismuth triacetate (Bi(CH3CO2)3), Hydroxylamine solution (NH2OH) were purchased from Sigma-Aldrich. Sodium tellurite (Na2TeO3, ⩾97%), Ethylene glycol (EG), Acetic acid glacial (analytical reagent), Ethanol (analytical reagent) and Poly(vinyl pyrrolidone) (PVP, MW≈40,000) were purchased from Aladdin. Sodium selenite (Na2SeO3, ⩾99%) was purchased from Xiya Reagent. Acetone was purchased from Xilong Reagent. DI water was obtained using an 18MΩ DI water system of Aike DZG-303A.
In a typical synthesis, 0.5 g PVP was dissolved in 15 mL EG, at room temperature. 1mL acetic acid glacial, 0.3 mmol Bi(CH3CO2)3 and 0.45 mmol Na2SeO3 were gradually added into the mixture solution. Keeping stirring until Bi(CH3CO2)3 and Na2SeO3 were fully dissolved and the mixture solution was transparent. Then, the mixture solution was heated while stirring. When the temperature reaches 180 °C, 1.5 mL ethylene glycol and 1.5 mL hydroxylamine were mixed evenly and injected into the solution rapidly. The solution can be observed to turn black immediately. In order to promote the full growth of the Bi2Se3 grains, the reaction continued for 15 minutes at the temperature above 180 °C, and then the prepared solution was cooled naturally to room temperature. Bi2Se3-Bi2Te3 heterostructure nanosheets were synthesized by epitaxial growth of Bi2Te3 on the synthesized Bi2Se3 nanosheets. 0.6 mmol Bi(CH3CO2)3, 0.9 mmol Na2TeO3, 0.4g PVP and 1.5 mL hydroxylamine solution were added into the as-synthesized Bi2Se3 solution which was not centrifuged and washed. The reaction continued for about 5h at the temperature of 180 °C to complete. The reaction product was dispersed into 20 mL acetone and sonicated for 10 minutes, then centrifuged at a speed of 90 000 rpm for 10 minutes. Remove the upper suspension and disperse the precipitates into ethanol. Repeat the above steps of ultrasonic and centrifugal twice. The end precipitates were Bi2Se3-Bi2Te3 heterostructure nanosheets, which will later be dispersed in ethanol solution as an optical medium.
Figure 1(a) shows the morphology of two-dimensional Bi2Se3 nanosheets observed by Field emission scanning electron microscope. The obtained Bi2Se3 nanosheets are mainly hexagonal, with regular shapes and sizes. Moreover, it can also be seen that the yield of the Bi2Se3 nanosheets prepared by solution method was very high. The atomic force microscopy (AFM) images further determine the width and thickness of the Bi2Se3 nanosheets. Figure 1(b) shows the hexagonal morphology of Bi2Se3 nanosheets, while Figure 1(c) depicts the thickness distribution along the straight line in Figure 1(b).
Obviously, from Figure 1(c), the width is about 150 nm. The thickness is about 10 nm. Furthermore, the TEM image in Figure 1(d) reveals the perfect hexagonal morphology of a single Bi2Se3 nanosheet with a transverse width of about 150 nm. The energy dispersive spectroscopy (EDS) element mapping images of Bi2Se3 nanosheets are showed in Figure 1(e) and 1(f), which can more effectively describe the composition of the samples. The images clearly reveal the uniform distribution of elements Bi and Se. The high-resolution lattice fringes of Bi2Se3 nanosheets are showed in Figure 1(g) by a high-resolution Transmission Electron Microscopy (HRTEM) image. These fringes can be indexed to the facets and facets of the samples, respectively. Figure 1(h) shows the selected area electron diffraction (SAED) pattern, which is attributed to a six-fold symmetric  zone axis. The diffraction spots in Figure 1(h) individually correspond to the , and facets of the Bi2Se3 nanosheets.
Bi2Se3 and Bi2Te3 have very similar crystal structures and both belong to the layer-structured materials. In the crystal structure, each planar quintuple layer that consists of five consecutive atomic layers interacted by covalent bonds combines with the adjacent units by weak van der Waals interactions. These properties make the epitaxial growth of Bi2Te3 on Bi2Se3 nanosheets feasible. Bi2Te3 nanosheets were grown on the seed crystal of the as-prepared Bi2Se3 nanosheets to synthesize Bi2Se3-Bi2Te3 heterostructure nanosheets. During the reaction, the precursors of Bi and Te were added to the original solution of the as-prepared Bi2Se3 nanosheet solution without centrifugation and washing. Then the mixture was heated to react. As the reaction proceeds, Bi and Se combined to form Bi2Te3 nanoparticles, which attached to the surface and edge of Bi2Se3 nanosheets. Bi2Se3-Bi2Te3 heterostructure nanosheets were finally formed after slow growth for a long time.
Figure 2(a) shows the TEM images of the as-prepared Bi2Se3-Bi2Te3 heterostructure nanosheets. Compared to the Bi2Se3 nanosheets with regular hexagonal, the shape of the Bi2Se3-Bi2Te3 heterostructure nanosheets has been changed, which is not a perfect hexagonal shape, but basically remains the original shape. The transverse size of the Bi2Se3-Bi2Te3 heterostructure nanosheets is about 200 nm, which is slightly larger than that of the Bi2Se3 nanosheets. In addition, we have scanned the element distribution of the heterostructure materials, as shown in Figures 2(c), (d) and (e). It can be seen that Bi, Te and Se elements are uniformly distributed, which indicates that Bi2Te3 has uniformly enclosed the Bi2Se3 nanosheets, forming Bi2Se3-Bi2Te3 heterostructure nanosheets.
Figure 3 show the Raman spectra of the Bi2Se3 nanosheets and Bi2Se3-Bi2Te3 heterostructure nanosheets. The Raman spectrum of Bi2Se3 nanosheets contains three main peaks A11g=68 cm−1, E2g=128 cm−1 and A21g=171cm−1. The intensity of the A11g peak is the largest due to the ultrathin property of the synthesized Bi2Se3 nanosheets. The Raman spectrum of Bi2Se3-Bi2Te3 heterostructure nanosheets is consistent with the Raman spectrum of Bi2Te3 nanosheets, because the Bi2Te3 covers the surface of the Bi2Se3 nanosheets and forms a cladding layer. The Raman spectrum of the Bi2Se3-Bi2Te3 heterostructure nanosheets contains four main peaks A11g=63 cm−1, E2g=103 cm−1, A1u=121cm−1 and A21g=141cm−1. The intensity of the A1u peak is the largest and does not exist in the bulk materials of Bi2Te3, which indicates the cladding layer of Bi2Te3 in the Bi2Se3-Bi2Te3 heterostructure nanosheets is very thin.
III. BROADBAND NONLINEAR OPTICAL PROPERTIES AND OPTICAL SWITCHING
Figure 4 is the experimental setup of SSPM. In this setup, continuous wave lasers with different wavelengths of 457nm, 532nm and 671nm were used as light sources. The Bi2Se3-Bi2Te3 heterostructure was filled in a 10mm thick quartz cuvette and used as a nonlinear optical medium. In order to focus the incident laser beam, a f = 200mm lens is employed. L1 and L2 are the distances from the left and right surface of the quartz cuvette to the focus of lens, respectively, while D is the distance from the white screen to the quartz cuvette. After passing through the lens, the laser beam becomes a focused beam and then enters the Bi2Se3-Bi2Te3 heterostructure dispersion.
The spatial self-phase modulation effect can be observed when the laser beam passes through the quartz cuvette filled with few-layer Bi2Se3-Bi2Te3 heterostructure dispersion, and eventually the light spot diverges into diffraction rings which is detected by a laser beam profiling digital camera.
Figure 5(a) illustrates the SSPM process as time increases at (①-③) λ1=457nm, (④-⑥) λ2=532nm and (⑤-⑦) λ3=671nm. As the time increases, the shape of the diffraction ring changes rapidly. Within 0.3s, the number of rings of the diffraction ring increases, and the radius of the diffraction ring increases. The radius and number of diffraction rings reached a maximum at 0.2s, 0.28s, 0.32s, respectively for the wavelength of λ1=457nm, λ2=532nm, and λ3=671nm. After that, the diffraction rings begin to distort rapidly.
When the time are 0.84s, 1.04s, and 1.48s, respectively for λ1=457nm, λ2=532nm, and λ3=671nm, the diffraction rings distort, forming semi-circular diffraction rings.
The distortion is originated mainly from the non-axisymmetric thermal convections of the few-layer Bi2Se3-Bi2Te3 heterostructure induced by laser thermal effects. The refractive index of the Bi2Se3-Bi2Te3 heterostructure can be defined as: n = n0 + n2I. n0 = 1.37 is the linear refractive index. n2 is the nonlinear refractive index. I is the intensity of the incident light.
As long as the laser beam irradiates the Bi2Se3-Bi2Te3 heterostructure dispersion, the phenomenon of SSPM will emerge. The phase shift (Δψ) is given as:
λ is the excitation wavelength. Leff is the effective optical thickness of the Bi2Se3-Bi2Te3 heterostructure dispersion.
r is the radial coordinate. I(r, z) is the intensity distribution.
The number of rings N is
Thus, the nonlinear refractive index n2 is
Figure 5(b) shows the variation of the diffraction rings under different incident intensity for different wavelengths of λ1=457nm, λ1=532nm and λ1=671nm respectively. It can be seen that the number of diffraction rings is proportional to the incident light intensity, and the ratios are respectively dN/dI=0.9433 cm2/W, 0.4037 cm2/W, and 0.1244 cm2/W for different wavelengths of λ1=457nm, λ2=532nm, and λ3=671nm. Accordingly, it can be calculated that the nonlinear refractive indexes are n2=1.96×10-5 cm2/W, n2=1.39×10-5 cm2/W, and n2=3.2×10-6 cm2/W for λ1=457 nm, λ2=532 nm, and λ3=671 nm, respectively.
In the SSPM effect, after the diffraction rings are formed, the upper half of the diffraction ring collapses inward in a short time. Collapse may be caused by non-axisymmetric thermal effects caused by the laser beam and is closely related to the change of nonlinear refractive index of Bi2Se3-Bi2Te3 heterostructure.
The change of nonlinear refractive index can be given as following.
Figure 6(a) shows the schematic diagram of the distortion for the Bi2Se3-Bi2Te3 heterostructure. The spatial self-phase modulation effect can be observed when the laser beam passes through the quartz cuvette filled with few-layer Bi2Se3-Bi2Te3 heterostructure dispersion, and eventually the light spot diverges into diffraction rings. RH is the maximum diffraction radius. θH is the half-cone angle. Within a short time, the upper half of the diffraction ring collapses inward due to the thermal effect by laser irradiation. The upper half of the diffraction ring collapse radius is RD. The corresponding collapse angle is θD.
Figure 6(b) has shown the change of nonlinear refractive index of Bi2Se3-Bi2Te3 heterostructure when the distortion occurs. The change of nonlinear refractive index for Bi2Se3-Bi2Te3 heterostructure dispersion at 457nm is strongly dependent on the power of laser beam. Even a slight change in the incident intensity will lead to a dramatic change of the nonlinear refractive index.
Using Bi2Se3-Bi2Te3 heterostructure as a nonlinear optical medium, spatial self-phase modulation effects are observed at multiple different wavelengths. As shown in Figure 7(a), the experimental scheme of the all-optical switching setup based on SSPM effect is proposed.
A laser beam of λ=671nm is used as the signal light beam, and the light intensity is adjusted to 4.334 W/cm2. When the signal light beam irradiates the Bi2Se3-Bi2Te3 heterostructure dispersion solution, the spatial self-phase modulation diffraction ring is not excited due to the weak light intensity of signal beam. Another laser beam of λ=457nm is used as the control light beam, and passes through the Bi2Se3-Bi2Te3 heterostructure dispersion solution as well. When the intensity of control beam is 1.45 W/cm2 and very weak, the signal light beam and the control light beam keep their initial spatial intensity distribution as Gaussian spots as shown in Figure 6(b)①. When the intensity of the control light beam increases up to 2.58W/cm2 and the signal light beam is fixed to 4.334 W/cm2, the signal light beam is converted into diffraction rings from Gaussian spot. Meanwhile, the control light beam is also converted into diffraction rings. As the intensity of the control light beam increases continuously, the number of diffraction rings for the control light and the signal light increase correspondingly, as shown in Figure 7(b)③. Figure 7(c) demonstrates the relationship between the diffraction rings number of the signal light beam and the intensity of the control light beam. As the intensity of the control light beam increases, the number of the diffraction rings of the signal light beam also increases. The number of the diffraction rings of the signal light beam is proportional to the intensity of the control light beam, which can be attributed to the spatial cross-phase modulation effect.
Overall, by taking two-dimensional Bi2Se3-Bi2Te3 heterostructure materials as a nonlinear optical medium, all-optical switching is successfully realized. Bi2Se3-Bi2Te3 heterostructure nanosheets were successfully synthesized by solution method. Bi2Se3-Bi2Te3 nanosheets were mainly hexagonal. The Bi2Se3-Bi2Te3 heterostructure material was used as a nonlinear optical medium to achieve the spatial self-phase modulation effect of 457nm, 532nm and 671nm. Furthermore, based on the effect of SSPM, a device called all-optical switching was also realized. It is conceivable that Bi2Se3-Bi2Te3 heterostructure materials can be used as excellent all-optical processing media for practical applications, which will lead to the development of new optoelectronic devices.
This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61875133, 11874269, 61505111 and 11604216), the Guangdong Natural Science Foundation (Grant No. 2018A030313198), the Jiangxi Natural Science Foundation (Grant Nos. 20171BAB201017, 20161BAB201002 and 20151BAB207056), the Science and Technology Project of Jiangxi Provincial Education Department (Grant No. GJJ161066), the Project for Distinguished Young Scholars of Jiangxi Province under Grant 20171BCB23098.