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Hexagonal nanosheets of Sb2Te3 single crystals with uniform morphology are successfully synthesized through a solvothermal method. We experimentally demonstrate that topological insulator Sb2Te3 as an optical media can be used for the generation of ring-shaped beams based on a giant optical nonlinearity and the effect of graded-index plasma lens. Good propagation properties in free space are exhibited by the generated ring-shaped beam and the dark spot size of ring-shaped beams can be conveniently controlled by adjusting the power of the pump beam. We also find that it is more easily to generate a ring-shaped beam in the high concentration Sb2Te3 dispersion solution and the divergence angle of the ring-shaped beam increases as the concentration of dispersion solution increase.
© 2014 Optical Society of America
1. Introduction
Since graphene was first isolated from graphite, its electronic and optical properties have been investigated extensively and applied in various fields [1–3]. The success of graphene applications highly motivates the exploration of other graphene-like materials [4,5]. Antimony telluride (Sb2Te3) as a p-type semiconductor with a narrow band gap is a layer material with a tetradymite structure [6,7]. Recently, this semiconductor material has attracted great attentions as a three-dimensional topological insulator (TI), which is characteristic of a combination of an insulating bulk and conducting surface states of massless Dirac fermions [8–10]. A similar band structure between graphene and TI suggests that TI may possess a lot of fantastic electromagnetic and photonics properties, such as broadband nonlinear absorption and giant optical nonlinear effect [11].
Previously, F. Bernard et al. firstly demonstrated that TIs exhibit saturable absorption through investigating the nonlinear optical property of TIs [12]. In the following, Zhao et al. further presented mode-locked picosecond pulses in fiber laser by using TI as an effective saturable absorber [13,14]. Meanwhile, TIs have been employed to generate pulsed laser at 1064 nm [15], 1550 nm [16,17], 1645 nm [18] and 2 [19]. Recently, J. Sotor et al. firstly reported a mode-locked Er-doped fiber laser based on TI: Sb2Te3 saturable absorber [20–22]. Besides that, TIs are also found to have a large nonlinear refractive index. Stemming from the thermal effect due to band-gap shrinking, the nonlinear refractive coefficient of TI: Sb2Te3 reaches up to [23]. Very recently, Lu et al. investigated the nonlinear optical property of TI: Bi2Se3, and found it shows giant nonlinear refractive index of 10−14 m2/W at 800 nm under femtosecond laser illumination [24]. Taking advantage of the large nonlinear refractive index of TI: Bi2Se3, high repetition rate pulses are obtained and it demonstrates that TIs can operate as a high-performance nonlinear photonic device in the laser system [17]. The potential photonic applications of TIs at microwave band are recently presented by Zhang et al. [25]. These experimental findings show that TIs can be a promising nonlinear optical material with applications in pulsed laser operation and optical devices, such as wavelength converter and optical switch. In the previous work, we demonstrated a new method for the generation of ring-shaped beams (RSBs) in carbon disulfide (CS2) based on a graded-index plasma lens [26]. Enlightened by the similarity of large nonlinear refractive index between TI and CS2, one may wonder whether TI: Sb2Te3 can be used for generating RSBs, which have attracted more and more attentions because of their wide applications in many areas, e.g., cold atom guidance [27], optical tweezers [28,29], and superresolution fluorescence microscopy [30]. Why choose TI: Sb2Te3 for the generation of RSBs? There are two reasons, on the one hand is TIs possess a giant optical nonlinearity, the nonlinear refractive index of TIs directly affects the partial intensity of the beam and it plays an indirect role in the generation of plasma channels; On the other hand, the more important thing is that the surface states in TIs enable the transport of spin-polarized electrons while preventing “scattering” typically associated with power consumption, in which electrons deviate from their trajectory. This is equivalent to reduce the ionization threshold of the dispersion solution and it is more easily to ionize the dispersion solution under a same intensity of the pump beam. They will play an essential role in the generation of plasma channels during laser irradiation in TI: Sb2Te3 dispersion solution. Corresponding to that, it is more sensitive and easily to generate an RSB in TI: Sb2Te3 dispersion solution based on the effect of graded-index plasma lens.
In this paper, we first demonstrate the preparation and characterization of Sb2Te3. Based on the effect of graded-index plasma lens, we then experimentally display TI: Sb2Te3 as an optical media can be used for the generation of RSBs, for the first time. The generated RSB has good propagation properties in the free space and we can easily control the dark spot size of the RSB by changing the power of pump pulses. It finds that the power required for the formation of a same RSB in the high concentration solution is less than that of in a low concentration solution. In addition, the divergence angle of the ring-shaped beam increases as the concentration of dispersion solution increase.
2. Material preparation and experimental setup
2.1 Material preparation and characterization
Several methods have been proposed for the synthesis the single crystals of Sb2Te3, such as mechanical exfoliations [31], peeling by an atomic force microscope tip [32], molecular beam epitaxial growth [33], chemical vapor transport [34], electrochemical deposition [35], solvothermal synthesis and other wet chemical methods [36]. In these preparation methods, the solvothermal process is an effective and convenient approach for preparing the nanostructured Sb2Te3-based materials with desired structures and morphologies. For example, Sb2Te3 nanostructures in a variety of controlled morphologies, including nanorods, nanobelts, nanosheets, nanoplates, and nanoforks have been obtained from solvothermal synthesis [37,38,39]. Here, we use a facile solvothermal method to synthesize Sb2Te3 hexagonal nanosheets. In a typical synthesis, a stoichiometric ratio of bismuth chloride (SbCl3), and sodium selenide (Na2TeO3) are dissolved in ethylene glycol with vigorous stirring. Then the mixture is transferred into the Teflon-lined stainless-steel autoclave and heated to 180°C. The autoclave is maintained at the reaction temperature for 8h and then cooled to room temperature naturally. The black powders are collected by filtering, washed with distilled water and ethanol, and finally dried at 60°C in vacuum overnight. The as-grown and washed powders are dispersed in an isopropanol (IPA) solution.
Field-emission scanning electron microscopy, transmission electron microscope, atomic force microscope, X-ray diffraction and Raman microscope are employed to characterize the products. The morphology and size of the as-prepared Sb2Te3 samples are characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) as shown in Fig. 1. FESEM images and TEM images are obtained with a JSM-6700F microscope and a JEOL 3010 microscope with an accelerating voltage of 300 kV, respectively. The lower magnification FESEM image [Fig. 1(a)] reveals that a large number of sheet-like structures are randomly dispersed on the surface of the substrate. The obtained products are predominantly hexagonal-based plates of uniform size and well-defined shape. A higher magnification FESEM image [Fig. 1(b)] shows that the edge length of plates is in the range of 1-1.5 μm, and that their thickness is about 120 nm. TEM provides further insight into the microstructural details of the Sb2Te3 nanostructures. Figure 1(c) is a typical TEM image of a single nanosheet, which clearly demonstrates the nanosheet has perfect hexagonal morphology.
Fig. 1 (a) Low-magnification FESEM image (b) High-magnification FESEM image of Sb2Te3 nanosheets. (c) TEM image of a single perfect hexagonal nanosheet.
To further confirm the thickness and the width of as-prepared Sb2Te3 nanosheets, the atomic force microscope (AFM) topography images of Sb2Te3 nanosheets are investigated. We carry out AFM measurements in a Multimode 8 system. As shown in Fig. 2, the Sb2Te3 nanosheet has very clean and flat surface with a uniform thickness about 120 nm across the lateral dimensions. The height profiles corresponding to the line-cut in Fig. 2(a) is shown in Fig. 2(b). The two dotted lines in Fig. 2(b) correspond to the two blue points of the line-cut in Fig. 2(a). The width of the Sb2Te3 nanosheet is about 1.5 μm, which is represented by the distance between the two dotted lines.
Fig. 2 (a) Topographic AFM images of the Sb2Te3 nanosheet. (b) The height profiles corresponding to (a). (c) Three-dimensional images corresponding to (a).
The X-ray diffraction (XRD) pattern of the products is performed on a D8-Advanee X-ray diffractometer with a Cu-Kα radiation. Figure 3 displays the XRD patterns of the freshly prepared Sb2Te3 nanosheets. All the diffraction peaks can be indexed to rhombohedral Sb2Te3 (space group: R-3m) with lattice constants a = 0.4262 nm, c = 3.0450 nm (JCPDS card No. 15-0874). This result indicates that Sb2Te3 products obtained via our synthetic method consist of a pure phase. Raman spectroscopy is a sensitive probe to the local atomic arrangements and vibrations of the materials. It has been widely used to investigate the microstructure of the nano-sized materials. The Raman scattering spectrum of the as-prepared Sb2Te3 nanosheets is obtained by a Labram-010 system and it is shown in Fig. 3(b). The spectrum contains four main peaks which correspond to Eg1, Ag1, Eg2 and A1g2, respectively.
Fig. 3 (a) XRD patterns of the as-prepared Sb2Te3 nanosheets. (b) Raman spectra of Sb2Te3 at 632nm laser excitation.
2.2 Experimental setup
Figure 4 shows our experimental setup for the RSB generation. In the experiment, a common He-Ne laser with a central wavelength of 632 nm, is used as a probe laser source. A Ti:sapphire amplified laser system generates 123 fs pump pulses with a central wavelength of 800 nm at a repetition rate of 1 kHz. Inset maps 4(a) and 4(b) show the initial spatial intensity distributions of pump and probe beams, respectively. Inset map 4(c) displays four kinds of nonlinear samples in the experiment, from left to right in the order: IPA solution, Sb2Te3 dispersion solutions in IPA with a concentration of 156, 312, and 625 ug/ml. Profiles of femtosecond and continuous beams are nearly Gaussian, with full width at half maximum (FWHM) values of 0.58 and 1.16 mm, respectively. The pump beam passes through an attenuator (A1) for regulating the input pulse power. Beam splitters (BS1 and BS2) are dichroic mirrors coated to have high reflectivity at 800 nm and high transmission at 632 nm. The pump beam and probe beam are spatially overlapped by BS1. In addition, they propagate collinearly through a nonlinear material (NM). In the experiment, Sb2Te3 dispersion solution in IPA is chosen as a NM and it is filled in a quartz cuvette with 2 cm path length. The pump beam is directed to a beam dump reflected by BS2 and the profile of probe beam is then imaged onto a high-resolution charge-coupled device (CCD) camera (Coherent Laser Cam-HRTM Beamview, pixels, pixel size of ). The adjustable attenuator A2 is customized to protect the CCD camera from damage caused by high-powered laser beam.
Fig. 4 Experimental scheme for the generation of ring-shaped beams. M1, silver-coated plane mirror; A1 and A2, attenuators; BS1 and BS2, beam splitters; NM, nonlinear material. Inset maps (a) and (b) show the initial spatial intensity distributions of pump and probe beams, respectively. Inset map (c) displays four kinds of nonlinear samples, from left to right in the order: IPA solution, Sb2Te3 dispersion solutions in IPA with a concentration of 156, 312, and 625 ug/ml.
3. Results and discussions
Figure 5 displays the formation of an RSB in the Sb2Te3 dispersion solution at concentration of 156 ug/ml. When the power of the femtosecond pulse (Pfs) is 0 mW, the probe beam keeps its initial spatial intensity profile. Increasing the power and we can see from Fig. 5(b) that a dark spot appears in the center of the probe beam. How does it happen? This is known as the effect of the graded-index plasma lens [26]. As femtosecond pulses propagate in the Sb2Te3 dispersion solution, the giant optical nonlinearity and the surface states of Sb2Te3 could play an important role in the formation of plasma channels. The electron density distribution in the plasma channel generated by ionization is close to Gaussian because of a Gaussian intensity profile of the femtosecond beam. As we know, laser beams can only propagates in the plasma where the electron density is below its critical value. The electron density in the center area of the plasma is higher than the critical value, so the probe beam cannot pass through this region. While the probe beam can propagate in the periphery of the channel because of the low electron density. The refractive index distribution in the plasma is also close to Gaussian, but different from the electron density profile in the channel, the refractive index in the center of the plasma is lower than its perihery. As a resut of a graded difference in refractive index exists between the center of the plasma and its periphery. Similar to a beam passing through a graded-index diverging lens, the probe beam will be deflected when it propagates in the periphery of the plasma, hence, a dark spot of the RSB is formed. From Fig. 5(c)-5(e), the dark spot size (DSS) is enlarged when Pfs is increased. This is because that the region of the plasma where the electron density higher than the critical value increases with the increase of Pfs, that is to say, the area where probe beam can pass through decreases as Pfs increase, resulting that a larger dark spot of RSB. So the DSS of an RSB can be controlled conveniently by adjusting Pfs. If Pfs is set to zero, the RSB produced by such graded-index plasma lens can be recovered its initial beam profile immediately.
Fig. 5 Spatial intensity distributions of RSB at Pfs of (a) 0, (b) 6, (c) 13, and (d) 27 mW and (e) corresponding cross line (y = 0) of RSB when Pfs is tuned.
Figure 6 shows relationships between Pfs and the DSS of RSBs generated in IPA and three Sb2Te3 dispersion solutions (156, 312, and 625 ug/ml). DSS is defined as the FWHM of the radial-intensity distribution inside the notch of the RSB. An RSB with DSS = 0.69 mm will be generated in IPA when Pfs is 27.5 mW, this power is far greater than the power required for the generation of a same RSB in Sb2Te3 dispersion solution, the infiuence of IPA is small and it decreases as DSS increases, so the infiuence of IPA for the formation of RSB can be neglected. Simlar to Fig. 5, Fig. 6 displays visually that the DSS of RSB increases as Pfs increase. The power required for the generation of a same RSB in the three kinds of Sb2Te3 dispersion solutions decreases with the increase of the concentration. For example, an RSB with DSS = 1 mm will be produced in the dispersion solution with the concentration of 156 ug/ml when Pfs is 12 mW, while the power is 4.6 mW when the concentration is increased to 625 ug/ml. From a simple physical standpoint, we understand the phenomenon as follows. As the pump beam propagates in the dispersion solution, the nonlinear effect is enhanced with the increase of the concentration and it is more sensitive and easily to generate a plasma channel. The power required for the generation of a same plasma channel in the high concentration solution is lower than that of in a low concentration solution. So, in order to generate a same RSB, the needed power is inversely proportional to the concentration of the dispersion solution. That is to say, it is more easily to generate an RSB in the high concentration Sb2Te3 dispersion solution.
Fig. 6 The DSS of RSBs generated in four types of medium when Pfs is tuned. Spatial intensity profiles of the probe beam after passed through four dispersion solutions at concentration of (a) 625 ug/ml (b) 1.25 mg/ml (c) 2.5 mg/ml, and (d) 5 mg/ml.
In order to obtain the upper limit of the concentration of TI can be used for the solution. Probe beams are directed into some Sb2Te3 dispersion solutions with different concentrations. Inset maps of Fig. 6 show the spatial intensity profiles of the probe beam after passed through four dispersion solutions at concentration of (a) 625 ug/ml (b) 1.25 mg/ml (c) 2.5 mg/ml, and (d) 5 mg/ml. From the inset maps, we can see that the intensity of the probe beam decreases with the increase of concentration. When the concentration of Sb2Te3 dispersion solution is 5 mg/ml, the intensity of the probe beam is very low. Continue to increase the concentration, the intensity of the probe beam captured by CCD will reduce to zero, that is to say, there is no light can pass through the dispersion solution. So the upper limit of the concentration of TI can be used for the solution is 5 mg/ml.
In order to investigate the linear propagation of the generated RSB in free space, the CCD camera is moved along the optical axis and then a serial of spatial intensity profile maps of RSBs will be recorded in the CCD. Figure 7 shows that the intensity distributions of the RSB generated in the Sb2Te3 dispersion solutions (156ug/ml) as a function of the propagation distance D when Pfs is 8 mW. The propagation distance D is defined as the distance between the CCD and the glass cuvette. Because the size of RSB is beyond the sensing area of CCD, so we use a digital camera to capture the image of RSB when the distance is 450 cm. We can see from Fig. 7 that the RSB formed in Sb2Te3 dispersion solutions can keep its initial beam profile almost invariant and has good propagation properties in free space.
Fig. 7 Intensity distribution of RSBs in free space propagation at different propagation distances of (a)20, (b) 25, (c) 30, (d) 35, (e) 40, (f) 45 cm, and (g) 450 cm when Pfs is 8 mW.
To further investigate the divergence of the RSB in the free space, we first let the probe beam pass through air, IPA and three types of Sb2Te3 dispersion solutions (156, 312, and 625 ug/ml). Their beam widths at different propagation distance are displayed in Fig. 8(a). Beam widths of the probe beam after passed through Sb2Te3 dispersion solutions are larger than that of IPA and air. Moreover, beam widths of the probe beam all increase with the increment of D. For air and IPA, when D is increased from 15 cm to 45 cm, the beam width of the probe beam increases by 14.9% and 15.9%, respectively. And for the Sb2Te3 dispersion solutions, the linear refractive index of dispersion solutions increases with the increase of the concentration, so we can see that degree of divergence of the beam width increases as the concentration increase. The probe beam will be transformed into an RSB under the affect of the pump beam, next, we will show the divergence of the RSB in the free space. Figure 8(b) displays relationships between the DSS of RSBs generated in Sb2Te3 dispersion solution with different concentrations and D when Pfs is 8 mW. As the concentration of Sb2Te3 dispersion solution is 156 ug/ml, the DSS of RSB increases by 47.2% when D is increased from 15 cm to 45 cm. The difference in refractive index between the center of the plasma and its periphery increases with the increasing concentration, causing the magnification effect of the graded-index plasma lens to become more prominent. So, the growth rate of DSS at concentration of 156 ug/ml is smaller than that at 312 ug/ml and 625 ug/ml.
Fig. 8 (a) Beam widths of probe beams after passed through different medium at different linear propagation distance D. (b) Relationships between the DSS of RSBs generated in Sb2Te3 dispersion solution with different concentrations and D when Pfs is 8 mW.
Apart from using IPA as a solvent in the experiment, we try to find other solvents can be used for further reduce the effect of solvent. For this reason, we measure the formation process of RSB in three other Sb2Te3 dispersion solutions [ethanol, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP)] and also investigate the propagation of generated RSB in the free space. Figure 9(a) shows Psubt varies with DSS in the four kinds of Sb2Te3 dispersion solution at a concentration of 625 ug/ml. Psubt is defined as the subtraction between the power required for Sb2Te3 dispersion solution and that of for solvent under the condition of generating a same RSB. We can assess the influence of the solvent for the generation of RSB by Psubt. More specifically, the effect of solvent is inverse proportion to Psubt. From Fig. 9(a), we can see that Psubt increases as DSS increase in the four dispersion solutions, it shows that the influence of solvent decreases with an increasing DSS. The influence of IPA and ethanol is smaller than that of DMF and NMP when DDS is larger than 0.85. Compared with three other solvents, the effect of ethanol for the formation of RSB is the smallest.
Fig. 9 (a) Psubt varies with DSS in the four kinds of Sb2Te3 dispersion solution. (b) The propagation of generated RSBs in the free space.
In the following, we will demonstrate the propagation of RSBs generated in four dispersion solutions and the results are displayed in Fig. 9(b). The DSS of RSBs generated in four dispersion solutions all increase with the increment of D, and the divergence of RSB generated in IPA is the smallest than that of in three other solvents. In the four types of solvents, comprehensive consideration of the generation and propagation of RSB, IPA is the best choice for the generation of RSB.
4. Conclusions
In conclusion, we successfully synthesize Sb2Te3 hexagonal nanosheets by a solvothermal method, the as-prepared Sb2Te3 nanosheets have uniform morphology and they are 1.5 μm in edge lengths and 120 nm thick. Then, we experimentally demonstrate the generation of RSB in Sb2Te3 dispersion solution. Good propagation properties in free space are exhibited by the generated RSB and we can regulate DSS of RSB by adjusting Pfs conveniently. Generating a same RSB, Pfs required for the Sb2Te3 dispersion solution at a concentration of 625 ug/ml is less than that at 156 ug/ml and 312 ug/ml. In additional, the DSS and degree of divergence of RSB increase with increasing propagation distance and concentration of dispersion solution, respectively.
Acknowledgments
This research is supported in part by the Specialized Research Fund for the Doctoral Program of Higher Education of China (20110161110012), the Department of Science and Technology of Hunan Province (2013FJ2018), and Hunan Provincial Innovation Foundation For Postgraduate (CX2014B151).
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