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All optical switching of two continuous waves in few layer bismuthene based on spatial cross-phase modulation Lu Lu, Wenhui Wang, Leiming Wu, Xiantao Jiang, Yuanjiang Xiang, Jianqing Li, Dianyuan Fan, and Han Zhang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00849 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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All optical switching of two continuous waves in few layer bismuthene based on spatial cross-phase modulation Lu Lu1, Wenhui Wang1, Leiming Wu2, Xiantao Jiang1, Yuanjiang Xiang1, Jianqing Li2, Dianyuan Fan1and Han Zhang1* 1Shenzhen

Engineering Laboratory of Phosphorene and Optoelectronics, Collaborative Innovation Center for

Optoelectronic Science and Technology, Shenzhen 518060, China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 2Faculty

of Information Technology, Macau University of Science and Technology, Macao519020, SAR P. R. China

Abstract: Bismuthene, the last and heaviest group-VA elemental two-dimensional material, received tremendous interests owing to its advantages in electronic-transport, semi-metallic bonding, and intrinsic spin-orbit coupling. However, light-bismuthene interaction is relatively less investigated. Herein, sonochemical exfoliation approach had been employed to deliver a successful synthesis of few-layer bismuthene with an average thickness of ~3 nm and a lateral size of ~0.2 µm. The corresponding band structure from mono- to sextuple-layer had been therotically calculated and it was found that bismuthene possesses a thickness dependent energy gap from almost zero to 0.55 eV, suggesting that bismuthene may also find unique applications from Terahertz, mid-infrared towards infrared regime. The nonlinear optical absorption and refraction parameters had been well characterized by laser Z-scan and spatial phase modulation measurement techniques, respectively. By taking advantage of its strong nonlinear refraction effect, all optical switching of two different laser beams based on spatial cross-phase modulation had been eventually realized. It is further found that a modulated signal light clearly observed as switch light is turned on. The achievement of all optical switching suggests that the bismuthene-based 2D material is indeed an excellent candidate for an all optical switcher. Particularly, the semi-metallic and long term stable property in few layer bismuthene make it as a promising nonlinear optical material for infrared and mid-infrared optoelectronics. Our work

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demonstrates a large potential of this new material for nonlinear photonics and this contribution may provide new photonics avenue towards bismuthene-based devices (such as broadband detector, nonlinear optical switcher, phase modulator, etc). Keywords: Bismuthene, Sonochemical exfoliation, Cross-phase modulation, All optical switching, Modulation depth

In the past decade, two dimensional (2D) layered materials have attracted growing interest since the discovery of graphene1. Various types of 2D materials, ranging from Group III to Group VA elements2-10 have been intensively investigated. Very recently, Group-VA (nitrogen group) elements had been widely investigated because their mono-layers possess a broad range of band-gaps from 0.36 eV to 2.62 eV aiding in the light emitting wavelength from near-infrared to visible band with advantages in broadband photo-response6. Group-VA mono-layers including puckered phosphorene, arsenene, and buckled bismuthene possess carrier motilities as high as several thousands of cm2V-1s-1 6. A rising interest in Group-VA 2D materials emerges after the first demonstration of high performance transistor device based on few-layer phosphorene at 20147. Phosphorene recently emerges as a novel 2D material with a layer-dependent direct band gap, strong light-matter interaction at particular resonant wavelength, high carrier mobility and anisotropic electronic transportation, holding great promises in various applications from field-effect transistors7, photo-detectors11, batteries12, polymer composites13, to bio-sensing14. However, limited by relatively weak stability and ease of oxidation in phosphorene15-17, researchers started to explore new type of Group-VA 2D materials beyond phosphorene that possess distinguished optoelectronic response and enhanced stability18. Antimonene, another type of group-VA elemental 2D material, had been theoretically predicted with improved stability and remarkable electronic properties6, 8-10. Accordingly, antimonene presents high carrier mobility6, excellent ultraviolet response19 and high nonlinear refractive index20. Even though antimonene has excellent stability, its indirect band gap might give rise to low photoelectric response in contrast with direct band-gap semiconductor8. The lack of direct energy gap in antimonene may restrict its potentials in optoelectronics. Bismuth is one of the main elements in group VA column, and exhibits a series of peculiarities that have driven the subject of experimental and theoretical interest for decades. Due to a very


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long mean free path and low carrier density, it may show unique applications in electron transportation21. With a lattice structure similar as graphene and a large spin-orbit interaction, bismuth has all the potential to be a topological semimetal or semiconductor21. If the size and shape of layered bulky bismuth are downsized to nanometer scale, they often reveal anomalous atomic structures as well as exotic functional properties that they may not possess in the bulk counterpart. Group VA elements are known to show rich allotropic transformation because their semi-metallic bonding character can be readily shifted towards either metallic or covalent side 22-28

.

In nonlinear optics, cross phase modulation (XPM) occurs because signal light experiences an additional phase accumulation or change induced by another co-propagating switching light beam. This effect becomes more evident or stronger on condition that the signal and switching light encounter a temporal overlap during the propagation through nonlinear optical medium29. In the past decade, XPM has been widely researched thanks to its potentials in light-control-light technology30-33. XPM is correlated with the coupling strength between two waves at different frequencies which had been applied to XPM-induced compression30 and optical switching31. XPM also depends on the coupling strength between orthogonally polarized components in Kerr shutter32 and intensity discriminator33. XPM refers to a phase accumulation of a probe beam modulated by another signal pulse beam. It can also be employed in applications of quantum non-demolition measurements and quantum phase gates, and manipulation of quantum information34, 35. Frequency modulation controlled by XPM in optical fibers had been investigated since 199736. The all-optical modulation format converter by using XPM in optical fiber has been researched37. A study of XPM degradation of differential-phase-shift-keyed (DPSK) signals due to amplitude-shift-keyed signal has been performed by using pump-probe simulation38. In addition, XPM is a kind of nonlinear optical effect that limits system performance in Wavelength Division Multiplexing (WDM) systems39. In this context, this term typically refers to a kind of cross-phase modulation not only based on the Kerr effect, but also on changes in the refractive index in optical communications40, 41. Recently, two-color all-optical switching in MoS2 has been investigated42, while it can be achieved based on the spatial self-phase modulation as authors claimed. In our previous work, the strong nonlinear refraction effect has been investigated43. In this contribution, XPM refers to a change in the optical phase of light beam caused by mutual interactions with


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another beam in a nonlinear medium, that is, few layer bismuthene. Herein, we demonstrate the synthesis, layer dependent band structure, nonlinear absorption, refraction and cross-phase modulation in few-layer bismuthene. We theoretically investigated the band structure and found that electronic gap can be tuned between zero and mid-infrared, evidencing that few-layer bismuthene might be a promising broadband advanced optical material ranging from visible, infrared towards mid-infrared and even THz band. Verified by laser Z-scan technique, few-layer bismuthene film is found to exhibit a saturation of optical absorption with low modulation depth (less than 3%). Based on the XPM in bismuthene dispersion, mutual modulation of two laser beams has been observed with enhanced modulation depth. We also found that the switch light can transfer the signal light from ‘off’ state to ‘on’ state, i.e., all optical switching. Due to the cross-phase modulation effect in few layer bismuthene, all optical switching induced by two-color continuous waves was eventually realized. The experimental achievement of all optical switching indicates the bismuthene-based material is indeed an excellent candidate for an all optical switcher. These observations are likely to motivate further exploration of bismuthene-based photonics devices.

Results and Discussions Characterizations In this section, we investigate few layer bismuthene with rhombohedral A7 structure43-45. Sonochemical exfoliation method is employed to synthesize large-sized few layer bismuthene (see Experimental Section). Transmission electron microscopy (TEM) image and high resolution TEM (HRTEM) image (Figure 1a) depict that nanoflakes has a lateral size of ~0.2 µm with a rigid arrangement of lattice planes. The structure of β-phase bismuthene viewing along [001] zone axis, the observed inter-distance of the lattice fringes is 0.237 nm in accordance with the (111) inter-planar distance of the rhombohedral A7 structure43. The atomic force microscopy (AFM) image of the sonochemically exfoliated few-layer bismuthene (see Figure 1b) presents that the nanoflake has a height of ~3 nm with a smooth surface and an irregular profile. The crystalline structure of the as-prepared bismuthene layer is confirmed by scanning electron microscope (SEM) as shown in Fig. 1(c). Figure 1(d) illustrates the linear absorption spectrum of few layer bismuthene. X-ray photoelectron spectroscopy (XPS) spectrum (see Figure 1e) demonstrates three


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sharp photoelectron peaks at 23.6 eV, 25.9 eV, 26.9 eV, 96.1 eV, 123.1 eV, 159.1 eV, 182.1 eV, 442.1 eV, 680.1 eV and 943.0 eV corresponding to the Bi-Bi 5d5/2, 5d, 5d3/2, 5p3/2,5p1/2, 4f7/2, 4f5/2, 4d5/2, 4p3/2 and 4s orbital bonding, respectively. Raman spectrum of few layer bismuthene nanoflake shown in Figure 1(f) is further characterized. The disappearance of Eg Raman band at 70 cm−1 and A1g Raman band at 97 cm−1 of bismuth46 in these spectra is attributed to the Rayleigh rejection filter cutoff at ∼100 cm−1.

Figure 1 Atomic structure of synthesized bismuthene: (a) TEM image, inset: HRTEM image and scale bar is 2 nm; (b) AFM image; (c) SEM image; (d) Absorption spectrum; (e) XPS spectrum; (f) Raman spectra.

Structural and Electronic properties of bismuthene Concerning the aforementioned AFM image, the measured average height is about 3 nm, almost corresponding to 6 layers of bismuthene. Therefore, we can conclude that few-layer bismuthene had been successfully fabricated in this work. Figure 2(a) represents the top view of monolayer structure bismuth and Figure 2(b) is the side view of bulk phase bismuth. In previous literature47-49, the electronic properties of ultrathin bi-layer bismuth has been investigated by first-principles. In this section, calculations are performed by the density functional theory (DFT) with the exchange correlation contributions treated by Perdew-Burke-Ernzerhof (PBE) function. The generalized gradient approximation (GGA) exchange-correlation function is used in the Vienna Ab initio Simulation Package (VASP) code. In all the calculations, we use projector augmented wave


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(PAW) potential and with the plane-wave cutoff energy of 500 eV. The positions of atoms and the cell parameters of the monolayer and multi-layers are fully optimized. In the structural optimizations, the force acting on each atom is less than 1 meV/Å, and the total energies of the optimized structures are well converged with criteria of 10-5 eV per cell. A well-converged Monkhorst-Pack k-point set (12×12×1) is utilized for the calculations for monolayer and multi-layer structures. In order to avoid any interaction between monolayer and multi-layers, a vacuum space of 20 Å is used. The relaxation of the crystalline structure is performed using a Gaussian-smearing approach with a width of 0.1 eV to speed up convergence. Due to the layer structures of material Bi, we have taken considering of the interlayer interactions vander Waals (vdW) forces. The in-plane lattice constant are respectively 4.34 Å and 4.37 Å and the in-plane Bi-Bi bond are 3.05 and 3.08 Å without and with vdW forces for the monolayer Bi. According to our results, we have found that with the vdW interactions, the in-plane lattice constant a little larger as well as the in-plane bond length than without the interactions. The detail structural information are shown in the (SI Text, section S1). We have also calculated the binding energy of the monolayer and multilayer bismuthene with and without vdW interactions. Here, we define the binding energy of few-layer bismuthene as: Ebinding= -(Emulti-M*Emono)/M, where Emlti and Emono denotes the total energy of multi-layer and monolayer, and M denotes the number of the layers. By vdW interactions, we observe thatthe binding energy is larger than without considering vdW force, for the bilayer bismuthene, the binding energy are 67.8 eV and 178.8 eV with and without vdW interactions, respectively. With the increasing number of the layer, the multilayer system tends to be more stable. The total energy of bismuthene (i.e., monolayer and multilayer) and the interlayer binding energy are shown in the supporting information (SI Text, section S2) and (SI Text, section S3), respectively. In our work, based on the first-principle calculation, we investigate the relation between band-gap and layers of bismuthene. Due to the heavy atoms of bismuth, it needs to consider spin orbital coupling (SOC) and the figure 2 (c) shows the band structures of bismuth films along [001] axis with one to six layer thickness with (i.e., red solid line) and without (i.e., black solid line) SOC. When SOC is included, it is found that the bands at the top of the valence band at the G high symmetry point degenerate and the electronic band structures reduce from a direct value of 0.55 eV to indirect value of 0.50 eV for the monolayer bismuthene, which is consistent with Aktürk E.49 and Freitas R. R.50 As the number increased, with and without SOC,


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these systems are all metallic phase. We theoretically show that the electronic gap between the valence and conduction bands can be tuned between terahertz and mid-infrared regime. The band-gap of monolayer of Bi (111) on the [001] axis is about 0.55 eV, in good agreement with previous calculations51, and double- to sextuple-layers close to zero band-gap. This renders few-layer bismuthene with a tunable energy gap and may open the way for developing photodetectors and lasers tunable by the electric field effect.

Figure 2 Schematics of (a) top view and (b) side view of the (111) phase based on the A7 structure of the bulk Bi 22

. The yellow arrow is the [001] axis. Red and blue arrows are [111] and [010] axes, respectively. (c) Band

structures of Bi (111) films with one to six layer thickness.

Nonlinear Optical Saturable Absorption Properties of Bismuthene In the following, we focus on investigating nonlinear optical (NLO) absorption by using the open aperture Z-scan technique. The experimental setup is shown in Fig. 3 (Experimental section). In order to understand the ultrafast nonlinear optical response of few-layer bismuthene, Z-scan


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technique was used in the experiments52.

Figure 3 The experimental setup of open aperture Z-scan technique.

The open aperture Z-scan measurements of bismuthene samples at wavelength of 800 nm and 400 nm are shown in Figs. 4(a) and 4(c), respectively. The normalized transmittance gradually increases with the approaching of the bismuthene sample with respect to the focus point (z = 0), indicating that the absorption of few-layer bismuthene becomes saturated with the increase of the incident pump intensity. This process is also well-known as a typical saturable absorption (SA) behavior. In Figure 4(b) and (d), we fitted the data with the input intensity I based on the saturable absoption model for two level saturable absorber model53 with an intensity dependent transmission according to   α0 T ( I ) = exp  −  α NS + 1+ I / IS  

  ,  

(1)

Where, αNS and α0 (1+I / IS ) are the nonsaturable and saturable components, I S is the saturable intensity, and I in the incident light intensity. The modulation depth of bismuthene measured in our experiments is defined as the normalized transmittance difference between high and low irradiation intensities according to

{

}

AS = exp ( −α NS ) − exp − (α NS + α0 ) / exp ( −α NS ) = 1 − exp ( −α0 )

(2)

By fitting the experimental data, the modulation depths are found to be about 2.68% at 800 nm and 2.51% at 400 nm, respectively. We note that the modulation depth is relatively low compared with black phosphorus in literature54.


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Figure 4 The experimental results measured by Z-scan technique for bismuthene. The open Z-scan curves for (a) 800 nm and (c) 400 nm. The normalized transmittance and input peak intensity for (b) 800 nm and (d) 400 nm.

Cross-phase Modulation of Bismuthene As shown in the Z-scan experiments, the optical modulation depth of bismuthene is relatively low (less than 3%) at the visible band, however, a high modulation depth plays a critical role in all optical switching55. In this section, we explore how to obtain a relatively high modulation depth and all optical switching of bismuthene based on cross-phase modulation (XPM). The experimental operation of XPM is demonstrated in Experimental Section. Figure 5 is the experimental setup of XPM, where both output power of 532 nm and 633 nm continuous waves are intensity-tunable. Two different laser beams at 532 nm and 633 nm are employed as the switching and signal light, respectively. When the power of 532 nm is higher than that of 633 nm, and vice versa. In supporting information, the propagation direction behind sample of switch and signal laser is shown (SI Text, section S4) and the propagation can be stochastic (SI Text, section S5). Only one beam (signal light or switch light) incidents the bismuthene sample, the XPM experiment can be simplified to the spatial self-phase modulation (SSPM) experiment, and the results of strong nonlinear refraction effect based on SSPM shown in supporting information (SI


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Text, section S6). In XPM experiment, we investigated the diffraction ring formation process, number of diffraction rings, and modulation depth of nonlinear refractive index, sequentially.

Figure 5 Schematic experimental setup of XPM

In our experiment, we investigate the process of diffraction ring formation between switch light (532 nm) and signal light (633 nm). Figure 6 (a)-(c) is the diffraction ring formation process of 532 nm with an intensity of 5.48 W/cm2. Figure 6 (d)-(f) is the formation process of 633 nm excited by switch light at 532 nm with an intensity of 5.48 W/cm2, and the initial intensity of 633 nm is equal to 1.08 W/cm2 (i.e., below threshold intensity without diffraction ring). In each image, the time marked at the corner is the time delay after the beginning of the formation process detected by charge coupled devices (CCDs). From Figure 6, the formation process of switch light and signal light were synchronously generated, and number of rings are equal. It is further shown that the changes of rings of signal light almost depends on that of switching light. Particularly, signal light (i.e., initial intensity blow threshold intensity) keeps the Gaussian shape at the ‘off’ state without switch light, while transfer the 633 nm laser to the ‘on’ state excited by switch light. To some extent, all optical switching can be achieved based on the cross-phase modulation experiment.


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Figure 6 Formation process of all-optical switching based on XPM: (a), (b) and(c) the formation process of 532 nm laser with intensity I532=5.48 W/cm2; (d), (e) and (f) the formation process of 633 nm laser corresponding with the excitation of 532 nm laser I532=5.48 W/cm2.

Figure 7 Ring number of signal light 633 nm based on XPM: (a) I633=1.08 W/cm2,(b) I633=2.26 W/cm2,(c) I633=2.96 W/cm2without the excitation of 532 nm laser beam; (d) I633=1.08 W/cm2,(e) I633=2.26 W/cm2,(f) I633=2.96 W/cm2with the excitation of 532 nm laser beam I532=5.48 W/cm2.

Figure 7 mainly shows the number of rings of signal light based on XPM. Figure 7 (a)-(c) show the maximum ring number of 633 nm at an intensity of 1.08 W/cm2, 2.26 W/cm2 and 2.96 W/cm2, respectively. Figure 7 (d)-(f) show the maximum ring number of 633 nm excited by 532 nm


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switch light with the intensity of 5.48 W/cm2. As the switch light is introduced, it is found that the final number of rings of 633 nm is based on superposition principle of the initial ring number of 633 nm and excited ring number of 533 nm. Besides, the formation time of 633 nm laser is shorten by excited switch light (i.e., the time marked at the corner of each image). Concerning the formation of XPM, we further demonstrated that free carriers generated by photons at one light can diffract under the impact of another light, as if they were generated by the latter photons. In our experiment, we fix the power of one beam and increase that of the other, the ring number increase simultaneously. In Fig. 8(a), by fixing the intensity of laser beam at 633 nm while boosting the intensity of laser beam at 532 nm, the ring number of the 633 nm pattern exhibits linear dependence on the sum of the two beam excitations. Once the coherence of switch and signal light occurs, the superposition principle becomes valid. The same phenomena also appears for the 532 nm ring in Fig. 8(b). This phenomenon can explain why in XPM one can still have linear dependence and still obtain a relatively large nonlinear refractive index.

Figure 8 Dependence of the 633 nm and 532 nm beam ring number on the sum intensity.

We further investigate the modulation depth of nonlinear refractive index excited by switch light, defined as the ratio of variance of nonlinear refractive index ∆n2 to the sum of nonlinear refractive index ∑n2 in this experiment (SI Text, section S7). It is clear that the modulation depth of nonlinear refractive index is equivalent to the aforementioned modulation depth in Z-scan experiment. Figure 9(a) shows that the modulation of nonlinear refractive index excited by 532 nm laser beam. By fixing the intensity of the 633 nm beam, the modulation depth increases as the


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switch light increase. When the intensity of 633 nm is below threshold intensity without diffraction ring, the modulation depth ∆n2/ ∑n2 achieve the maximum. Similar result appears for the excitation of 633 nm in Fig. 9 (b). From Figure 9, it is shown that the modulation depth excited by 532 nm is larger than that of 633 nm, which suggests that the optical modulation depth is related to the incident energy of switch light. Moreover, when the switch light was turned off, no modulated signal is detected. When the switch light is turned on, a modulated signal light is clearly observed, in which cross-phase modulation of two-color laser beam could function as an all-optical switcher. According to the XPM experiment, we have achieved all optical switch based on few-layer bismuthene, and the modulation depth obviously increased with the increase of the incident laser beam, which is benefit to be selected as one good candidate for all optical switcher under high power regime.

Figure 9 Dependence of the 633 nm and 532 nm beam modulation depth on the total incident laser intensity.

To evaluate the switching performance during switching period, the contrast ratio should be investigated. In the previous literatures, the definition of contrast ratio can be described as the power ratio between the switched signal and non-switched signal at output port56. While for an amplitude-modulated spatial light modulator (SLM)57, defined as the ratio of the maximum intensity transmitted through (or reflected off) the SLM when turned on (ION) to the maximum intensity transmitted through (or reflected off) the SLM when turned off (IOFF). It is clear that two definitions are equivalence. In our work, we detected the power ratio transmitted through the sample between the switched signal and non-switched signal (i.e., PON /POFF), respectively.


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Taking the 532 nm wavelength as switching laser for example, in the following Table, the contrast ratio of 633 nm from Gaussian beam to two diffraction rings are slightly larger than 1.3. Table 1 Contrast ratio (CR) of all optical switching N

0

1

2

PON (mW )

1.44

2.23

2.89

POFF(mW)

1.04

1.66

2.18

CR

1.38

1.34

1.33

Note: N is short for diffraction rings number

Discussions and Analysis The optical Kerr effect manifests itself temporally as self-phase modulation, a self-induced phase-shift and frequency-shift of a laser beam as it travels through a nonlinear optical medium. Spatially, an intense beam of light in a medium will produce a change in the medium's refractive index that mimics the transverse intensity pattern of the beam. Through Pauli blocking of interband transitions, it shifts the absorption threshold of bismuthene towards higher frequency, resulting in a much lower attenuation of the signal wave. The switch light leads to modulation of the signal output from the bismuthene dispersion. This accounts for the mechanism of all optical switching modulation in this experiment shown in Fig. 10(a). Based on the all-optical switching, there is another important principle in this work, i.e., Superposition Principle. The applicability of superposition principle is based on coherence. Once the laser beam propagates through bismuthene medium at the same position, the superposition principle is satisfied shown in Fig. 10(b).

Figure 10 (a) Schematic describes switch and signal of carriers in the linearly dispersive valence and conduction


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bands of bismuthene; (b) Superposition principle of switch and signal light. ψ1 and ψ2 represent the electron wave functions of switch light and signal light.

Conclusion In conclusion, high quality few-layer bismuthene had been successfully synthesized and the band structure has been theoretically calculated. In the meanwhile, its nonlinear optical response at the visible band had been characterized by Z-scan and cross-phase modulation measurement technique, respectively. We found that its nonlinear saturable absorption exhibits low modulation depth. We utilize the XPM effect in bismuthene in order to achieve high modulation depth and all optical switching in spatial domain. Through Pauli blocking of inter-band transition, as it shifts the absorption threshold of bismuthene towards higher frequency, and it is therefore found that the switch light leads to modulation of the signal output from the bismuthene dispersion. Results show that formation process, number of diffraction rings of signal light sensitively depend on those of switch light. Once the coherence of switch and signal light occurs, the superposition principle becomes valid. Furthermore, we also investigate the modulation depth of nonlinear refractive index in bismuthene, the results show that the excellent filtering of the signal light when the switch light is turned off and a modulated signal light clearly observed when the switch light is turned on. Our systematic study evidences that bismuthene could be considered as a new kind of promising all optical switching material. Our research work may provide an inroad for developing bismuthene-based photonics that can fill the blank space where black phosphorus or antimonene could not fulfill in another aspect. In the future, it is further anticipated that bismuthene-based photonic devices such as passive Q-switcher, nonlinear switcher, detector or light modulator might emerge, thanks to the considerably high light-matter interaction strength and enhanced stability in bismuthene.

Experimental Section Few-layer Bismuthene: few-layer bismuthene was produced by the sonochemical exfoliation method. Bulk bismuth (99.999%, Aladdin) was firstly grinded into bismuth powder and added into a bottle with ethanol solution. Subsequently, 0.3275 mL of bismuth ethanol solution, 9.6725 mL of ethanol were added in a spiral glass bottle and kept under ice-bath sonication (Thermostatic


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Ultrasonic Cleaning Machine SBL-22DT, China) and probe sonication (Ultrasonic Homogenizer JY00-IIDN, China) for 15 hours, respectively. The resulting suspension was centrifuged for 20 min at 7000 rpm to separate the centrifuge and supernatant. Characterizations: TEM and HRTEM images were obtained from the transmission electron microscope (JEM-2100, Japan) at an acceleration voltage of 200 kV. The height and phase of the few-layer bismuthene were characterized by AFM observation using an ICON Bruker system in a tapping mode with samples dispersed on Si/SiO2 substrates by a drop-casting method. SEM analysis was carried out on the scanning electron microscope (TM3000, Japan) at a voltage of 20 kV. The absorption spectra were studied by means of UV-visible-NIR spectrophotometer (Lambda 750, USA). XPS was performed on the PHI 5000 VersaProbe II using an Al Kα (λ = 0.83 nm, hυ = 1486.7 eV) X-ray source operated at 2 kV and 20 mA. Raman spectrum was obtained by the Reishaw Via confocal Raman microscope equipped with a 514nm Ar ion laser as the excitation light. Z-scan techniques: the incident laser was obtained from a Coherent femtosecond laser (center wavelength: 800 nm, pulse duration: 100 fs, 3 dB spectral width: 15 nm, and repetition rate: 1 kHz). The focal length of the lens is 500 mm. The experimental setup was calibrated with graphene film. According to the graphene measurement, the incident beam waist was fitted to be about 30 µm.

XPM Measurement: the 633 nm (HNL210L) and 532 nm (SPROUT-H-5W) continuous wave laser were regard as signal light and switch light, respectively. One mirror (M) was used to change the path of switch light, and the beam splitter (BS) was used to compound switch and signal light. The continuous waves were focused onto the few-layer bismuthene suspension with the aid of a lens with a focal length of300 mm. A 10 mm thickness quartz cuvette was used to hold the dispersion solution. The distance between the lens and the front surface was 245 mm. After exiting the cuvette, the laser beam started to diverge into conical diffraction rings which were then projected on to the charge coupled device (WinCamD-FIR2-16-HR) placed at a distance of 60 mm behind the sample.

Acknowledgements L. L. and L. W. contribute equally to this work. The research is partially supported by the National Natural Science Fund (Grant Nos. 61435010 and 61575089), China Postdoctoral Science Fund


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(2017M612721),

Science

and

Technology

Innovation

Commission

of

Shenzhen

(KQTD2015032416270385 and JCYJ20150625103619275), the Science and Technology Planning Project of Guangdong Province (Grant No. 2016B050501005), the Educational Commission of Guangdong Province (Grant No. 2016KCXTD006) and the Science and Technology Development Fund (No. 007/2017/A1) , Macao SAR, China.

Supporting Information: This material is available free of charge via the Internet at http://pubs.acs.org. In the Supporting Information, we provide additional information regarding the structure parameters of multilayer bismuthene, total energy of bismuthene, binding energy of bismuthene, stochastic propagation direction of two laser beam, analysis of XPM of bismuthene, strong nonlinear effect of bismuthene and modulation depth of nonlinear refractive index.

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Switch based on Tri-arm Mach-Zehnder employing All-optical Flip-flop, IEEE International Conference on Communications, 2007, 65: 2257-2262

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For Table of Contents Use Only

All optical switching of two continuous waves in few layer bismuthene based on spatial cross-phase modulation Lu Lu, Wenhui Wang, Leiming Wu, Xiantao Jiang, Yuanjiang Xiang, Jianqing Li, Dianyuan Fanand Han Zhang

TOC:

The TOC figure displays a modulated signal light clearly observed as switch light is turned on, all optical switching induced by two-color continuous waves is realized based on the cross-phase modulation effect.


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Figures

Figure 1 Atomic structure of synthesized bismuthene: (a) TEM image, inset: HRTEM image and scale bar is 2 nm; (b) AFM image; (c) SEM image; (d) Absorption spectrum; (e) XPS spectrum; (f) Raman spectra.


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Figure 2 Schematics of (a) top view and (b) side view of the (111) phase based on the A7 structure of the bulk Bi

22

. The yellow arrow is the [001] axis. Red and blue arrows are [111] and [010] axes,

respectively. (c) Band structures of Bi (111) films with one to six layer thickness.

Figure 3 The experimental setup of open aperture Z-scan technique.


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Figure 4 The experimental results measured by Z-scan technique for bismuthene. The open Z-scan curves for (a) 800 nm and (c) 400 nm. The normalized transmittance and input peak intensity for (b) 800 nm and (d) 400 nm.

Figure 5 Schematic experimental setup of XPM


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Figure 6 Formation process of all-optical switching based on XPM: (a), (b) and(c) the formation process of 532 nm laser with intensity I532=5.48 W/cm2; (d), (e) and (f) the formation process of 633 nm laser corresponding with the excitation of 532 nm laser I532=5.48 W/cm2.

Figure 7 Ring number of signal light633 nm based on XPM: (a) I633=1.08 W/cm2,(b) I633=2.26 W/cm2,(c) I633=2.96 W/cm2without the excitation of 532 nm laser beam; (d) I633=1.08 W/cm2,(e) I633=2.26 W/cm2,(f) I633=2.96 W/cm2with the excitation of 532 nm laser beam I532=5.48 W/cm2.


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Figure 8 Dependence of the 633 nm and 532 nm beam ring number on the sum intensity.

Figure 9 Dependence of the 633 nm and 532 nm beam modulation depth on the total incident laser intensity.


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Figure 10 (a) Schematic describes switch and signal of carriers in the linearly dispersive valence and conduction bands of bismuthene; (b) Superposition principle of switch and signal light. ψ1 and ψ2 represent the electron wave functions of switch light and signal light.

Figure S1 The different propagation direction of switch and signal laser, (a) and (d) are the schematic experimental setup of XPM with different direction; (b) and (e) are the snapshots of the stable pattern; (c) and (e) are the angles of switch and signal laser behind sample.

Figure S2 (a) the schematic experimental setup of SSPM; (b) the parallel propagation direction of two laser beams; (c) the snapshot of the stable pattern.


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Figure S3 The stochastic propagation direction of switch and signal laser

Figure S4 Snapshots of the pattern formation at (a)-(c) 532 nm and (d)-(f) 633 nm laser beam excitations.

Figure S5 Number of rings of bismuthene solution increases with the intensity at different wavelengths: (a) λ=532 nm and (b) λ=633 nm. The amount-of-substance concentration of each figure is C=2.5×10-3mol/L.


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0.40

Normalized absorption (a.u.)

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Bismuthene

0.32

0.24

0.16

1.0

1.5

2.0 2.5 3.0 Photon energy (eV)

3.5

4.0

Figure S Changes of normalized absorption versus photon energy


All-Optical Switching of Two Continuous Waves ... - ACS Publications

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