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Abstract
All-optical light amplitude tuning functionality is demonstrated in a layered tungsten disulfide (WS2) nanosheets coated microfiber (MF) structure. Due to the strong light-matter interactions between WS2 nanosheets and the evanescent field around the MF, a large variation in the transmitted power can be observed under both external and internal pump light excitations over a broadband spectrum (~100 nm). A power variation rate of ~0.3744 dB/mW is obtained under external violet pump light excitation, whereas the power variation rate of similar devices in the state of the art are usually less than 0.3 dB/mW. In terms of the response time, a moderate rise/fall time of ∼20.5/19.6 ms is achieved, which is mainly limited by the employed structure fabrication methods. These results indicate that the optical transmitted power of the WS2 coated MF can be modulated by different pump light with the power in the order of mW, thus the proposed device might have potential applications in all optical controllable devices and sensors, etc.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As the semiconductor fabrications enter sub-10 nm regime, the performance of the transistors, which is the workhorse of electronic circuit will be degraded [1–3]. This is due to the fact that direct source to drain tunneling effect through channel potential barrier which causes excessive power dissipation. One possible solution to this is to build all optical systems. Therefore, it is of significance to develop all optical active tunable components with different functionalities [4–10].
Here we present the all-optical light amplitude tuning functionality by coating two dimensional (2D) materials onto the microfiber (MF). On the one hand, MF can be fabricated by tapering the standard communication fiber down to several micrometers, which enables a large portion of evanescent light leaking from the fiber core [11]. The strong evanescent light helps to boost light-matter interaction [12]. Compared with other fiber structures like side polished fiber (SPF) and microfiber resonators, MF shows its superiorities such as easy fabrication, long term stability and strong evanescent field [13]. What is more, the geometric configurations of MF could be easily customized and optimized with the help of numerical simulations [14]. However, the properties such as refractive index of silica (SiO2) based fibers cannot be greatly tuned by light due to its intrinsic material limitation. Therefore, in most cases, bare SiO2 based MF structures are not suitable for developing high performance all-optical active components such as highly sensitive fiber-optic devices and sensors.
On the other hand, the emerging of different types of 2D nanosheets with atomic thin layers exhibit excellent electronic, optical, physical and chemical properties [15–22]. Their layer-dependent structures and variable band gap energy make them promising candidates in the applications of novel optoelectronic and photonic devices [23–25]. Most recently, 2D materials have demonstrated a lot of novel applications. For example, 2D TMD nanosheets could be utilized in the biomedical applications like tumor imaging and therapy due to its high specific surface area and photo/thermo conversion efficiency [26]. 2D materials can also be integrated into van der Waals heterostructures for building transistors [27] and photodetectors [28]. In addition, new class of 2D materials such as 2D metal carbides and nitrides (MXenes) are reported in the applications of energy storage [29]. What is more, some applications of 2D materials in nonlinear optics are reported. For example, few-layer black phosphorus [30] and graphene [31] are employed as saturable absorbers in ultrafast lasers.
Here we propose a device combining 2D materials whose properties can be tuned by light with MF in order to build an all optical tunable component. As the choice of material, a representative of semiconducting transition metal dichalcogenides (TMD)-tungsten disulfide (WS2) is exploited here. It has excellent properties such as tunable band gap with variable thickness, high carrier mobility, high nonlinear susceptibility and broadband light absorption [32–36]. WS2 shows direct band gap in the visible to near-infrared range, thus the all-light-control operations of WS2-based devices can be efficiently achieved in the same wavelength range [37]. What is more, the coating of WS2 onto MF is cost-effective. The zero or narrow band gap of other 2D materials like graphene and black phosphorus greatly limits their applications. In addition, the stability problem of black phosphorus makes it hard to meet the demanding requirements of long term stability [38]. As a new class of 2D materials, 2D metal carbides and nitrides (MXenes) come with many outstanding merits such as good conductivity and tunable band gap. However, the fabrication of MXenes and the combination of MXenes with MF require complicated and costly fabrication processes [39]. In particular, WS2 is an ideal material for investigating the nonlinear optical effects-based photonic components [40]. For example, by integrating WS2 nanosheets into an erbium-doped fiber, a WS2-based mode-locked fiber laser with pulse duration of 1.32 ps is demonstrated [41]. Generation of extraordinary second harmonic of monolayer WS2 is also investigated in the last few years where an unusual large second order nonlinear susceptibility of 4.5 nm/V is reported [42].
By coating WS2 nanosheets onto MF, a WS2 coated MF structure with broadband (spanning the wavelength range of ~100 nm) all optical tunable functionality is achieved and presented here. The interaction between the evanescent light of MF and WS2 nanosheets enable the transmitted light amplitude tuning via both external and internal pump light excitations. The highest tunability is up to ~0.3744 dB/mW via external 405 nm pump light excitation. Experimental light amplitude tuning by using external pump light excitations, internal pump light excitations and their response time measurements are carried out and will be presented in the following sections.
2. Device fabrication
Figure 1 is the schematic illustration of WS2 coated MF structure. The output intensity of the MF can be tuned by using different pump lasers. Several steps are required for the fabrication of this structure. In this section, the device fabrication steps which include MF fabrication and the deposition of WS2 nanosheets onto the MF will be presented.
Before coating WS2 nanosheets onto the MF, characterizations of WS2 nanosheets such as Raman and UV-VIS spectra are performed for the commercially available WS2 nanosheets dispersions (MK NANO). The nanosheets are few layers (varying from one to ten layers) with a concentration of about 1 mg/ml and they are fabricated by lithium ion intercalation method. The averaged lateral size of such WS2 nanosheets is about 50~200 nm.
Figure 2(a) shows the Raman absorption spectrum of WS2 nanosheets. It is obtained under a 514.5 nm laser (Ar+ Laser) illumination and the spectrum is recorded at room temperature by LabRAM HR Evolution (HORIBA JY, France). Two prominent peaks one at 356 cm−1 and the other at 420 cm−1 can be seen from Fig. 2(a). It corresponds to the in-plane E12g and out-of-plane A1g modes respectively [43]. The amplitude comparison and the peak position of these two modes here indicate that the WS2 nanosheets are multilayers [44]. The absorption spectrum of WS2 shown in Fig. 2(b) is measured by an UV-VIS spectrophotometer (UV-2600, SHIMADZU). Overall, the WS2 nanosheets have absorption properties over a broad wavelength range. Some typical characteristic absorption peaks of WS2 which result from the direct exciton transitions can be observed from Fig. 2(b). The absorption is strong at short wavelength from 200 to 500 nm and it is relatively weak at the wavelength from 600 to 800 nm. The absorption spectrum of WS2 indicates that the all-optical-control operation of WS2-based devices can be efficiently operated in broadband wavelengths.
The horizontal cross section of a MF is schematically shown in Fig. 3(a) which consists of two tapering sections and one flat part referred to as the waist region. Experimentally, the MF is fabricated by “flame-brushing” technique [12]. It begins with a single mode fiber (SMF-28e from Corning Inc.) with a cladding diameter of 125 μm. The thin waist and the tapered shape are realized by heating the fiber with a flame and it is stretched by two electrical motors. The taper shape can be adjusted by controlling the drawing speed of the electrical motors. One microscopic diameter measurement (captured by Zeiss Axio Scope A1 microscope) of the fabricated MF around the tapered region is shown in Fig. 3(b). It shows that the MF has a diameter (d) of ~9.7 μm around the waist region. Figure 3(c) shows a microscopic view of the MF around the waist region. From Fig. 3(b) and Fig. 3(c), we can see that the length of tapered part of MF is ~30 mm, and the diameter of the waist region in the MF is ~9.7 μm. In order to find a suitable waist diameter for hosting the WS2 nanosheets, other two MFs with different waist diameters are also fabricated. One has a smaller waist diameter of ~6.4 μm, while the other has a larger waist diameter of ~15.7 μm.
Fig. 3 (a) Schematical drawing of horizontal cross section of a MF. (b) The diameter of a MF along the X propagation direction measured by microscopes. (c) An enlarged view of a bare MF measured by microscopes.
Before depositing the WS2 nanosheets onto these MFs, a basin as schematically shown in Fig. 4(a) is constructed in order to fix the MF on a glass slide. The basin is fabricated by a UV curing adhesive (Loctite 352, Henkel Loctite Asia Pacific) and it is cured by a UV light illumination (365 nm, USHIO SP7-250DB) for ~10 minutes. The construction of this UV basin not only helps to maintain the stability and mechanical strength of the structure, but it also facilitates the WS2 nanosheets coating onto the MF. Then, a 40 minutes ultrasonic treatment is employed to the WS2 dispersions in order to obtain quasi-evenly distributed nanosheets. Immediately after the ultrasonic process, a pipette is employed to transfer about 0.2 ml of the prepared WS2 solutions into the basin. The MF with WS2 solutions on the basin is then set aside for about ten hours for the solvent evaporation. During this process, the WS2 is gradually self-assembled onto the MF. Meanwhile, a 1550 nm distributed feedback (DFB) laser is connected to the input facet of the structure. The output power is collected by an optical power meter during the WS2 nanosheets coating process and the result for the three MFs is shown in Fig. 4(b).
Fig. 4 (a) A schematical drawing of a basin and a fixed MF on a glass slide. (b) The variation of transmitted optical power of the three MFs with different waist during the deposition of WS2.
From Fig. 4(b) one can observe that the smaller the MF waist diameter is, the larger the transmitted power drop is for the WS2 nanosheets coated MF. The decreased transmitted optical power is due to the interaction between the evanescent light outside the MF and the deposited WS2 nanosheets. For a MF with a waist diameter of 9.7 μm, a power drop of 37 dB is observed [red curve with circle in Fig. 4(b)]. Whereas, for the MFs with diameter of 6.4 μm and 15.7 μm, the power drop are about 55 dB [black curve with square in Fig. 4(b)] and 5.8 dB [blue curve with triangle in Fig. 4(b)] respectively. A larger transmitted power drop indicates that the interaction between MF and WS2 nanosheets is stronger. However, too strong interaction might lead to little transmitted power can be detected at the output. Therefore, by making a compromise between strong interaction and appropriate amount of light at the output, WS2 coated MF where the MF diameter of 9.7 μm is chosen for the later light amplitude tuning experiments via pump light excitations.
Scanning electron microscopy (SEM) imaging is performed for the chosen WS2 nanosheets coated MF structure and the results are shown in Fig. 5. Figure 5(a) shows the WS2 nanosheets precipitate around the MF with a diameter of 9.7 μm which is consistent with that measured by optical microscope. The inset in Fig. 5(a) shows an enlarged image of the WS2 nanosheets on the MF from which the closely self-packed WS2 nanosheets can be seen. Figure 5(b) is a cross section view of the WS2 coated MF structure and the inset shows the thickness of deposited WS2 nanosheets which is about 135 nm.
Fig. 5 (a) SEM images of the MF coated with WS2 and an enlarged view of the region marked by a dotted rectangle. (b) SEM images of the cross section view of MF coated with WS2 and an enlarged view of the region marked by a dotted rectangle.
The mechanism of the light amplitude tuning in WS2 coated MF is obtained thanks to the evanescent light that leaks out from the MF and the redistribution of the evanescent light around the interface between the coated material and MF. This is substantiated by the simulated mode field distributions of the MF and the WS2 coated MF as shown in Fig. 6. The structure is approximated as two-dimensional cross section which is schematically shown in Fig. 6(a) and the simulation is performed by finite element method (FEM) in COMSOL. The calculation window is considered as a rectangle with a size of 20 μm x 20 μm. A 4.85 μm radius of MF and a WS2 cladding layer thickness of 135 nm are considered. The meshing size is 30 nm and the simulations are performed at a fixed wavelength of 1550 nm. The MF core refractive index is taken as 1.45 while the WS2 cladding refractive index is taken as 3.14 + 0.55i [45,46] where the imaginary part accounts for the absorption. Figure 6(b) and Fig. 6(c) are the electric field distributions of the optical mode in the bare MF and WS2 coated MF respectively. The solid white circles in Fig. 6(b) and Fig. 6(c) denote the outer surface of MF. By integrating the electric field distribution along the whole cross section of Fig. 6(b) and Fig. 6(c), the output electric energy decreases from 5.8779 x 108 W/m2 to 4.4655 x 107 W/m2. The field cross section along the fiber core center along the X-direction [white dashed lines shown in Fig. 6(b) and Fig. 6(c)] for the bare MF and the WS2 coated MF are shown in Fig. 6(d). The blue curve in Fig. 6(d) corresponding to field profile of bare MF shows that part of guiding energy leaks outside the MF as evanescent wave thus facilitating light-matter interaction. The red curve in Fig. 6(d) corresponding to field profile of WS2 coated MF clearly shows the redistribution of the electric field around the outer interface of MF and the WS2 nanosheets. There is a discontinuity of the electric field along the interfaces between MF and the WS2 nanosheets. In the following section, different pump light sources are employed to tune the refractive index of WS2 nanosheets, therefore changing the electric field distribution at the outer interfaces of the structure. Consequently, the light amplitude of WS2 coated MF structure can be tuned.
Fig. 6 (a) Theoretical model of the WS2 coated MF and the considered MF core radius is 4.85 μm and the WS2 thickness is 135 nm. (b) Mode field distribution of the bare MF at 1550 nm where the white solid circle represent the border of the MF. (c) Mode field distribution of WS2 coated MF at 1550 nm where the solid circle represents the interface between border of the MF and the WS2 nanosheets. (d) Normalized field profile [Normalize to the maximum field intensity of Fig. 5(b) and Fig. 5(c)] and it is plotted along the dashed white line shown in Fig. 5(b) and Fig. 5(c).
3. Experimental details and results
In this section, the transmitted light amplitude tuning of the WS2 coated MF structure under external and internal pump light excitations will be presented. The experimental setup for the external pump light excitation is shown in Fig. 7 where the sample (either the WS2 coated MF or the bare MF) is fixed at the basin. Both ends of the MF are spliced to optical connectors facilitating light incident and light output collection. Light from TLS (ANDO-AQ4321D) is connected to one connectors of the MF while the other end of the MF is connected to an OSA (YOKOGAWA-AQ6317C). Two semiconductor lasers, one at 405 nm and the other at 660 nm will be employed as external pump light sources. In order to control the size of the beam on the WS2 coated MF structure, the external 405/660 nm pump light is placed vertically above the sample at a distance of ~10 cm and it is focused by a cylindrical lens.
Fig. 7 A schematical drawing of the experimental setup for 405/660 nm external pump light amplitude tuning on the MF with WS2.
The external pump light experiment is firstly performed on the bare MF and the results are shown in Fig. 8. Figure 8(a) shows the transmitted light amplitude of the bare MF at the wavelength ranging from 1520 nm to 1620 nm. Curves with different colors correspond to measurements under 405 nm violet light illumination at the power of 0, 4.9, 7.6, 9.9 and 12.5 mW respectively. Likewise, the transmission spectra with 660 nm pump light excitation of bare MF under the pump light power of 0, 6.7, 10.9, 15.5 and 20.3 mW are shown in Fig. 8(b). The maximum variations of the transmitted power for the 405 nm and 660 nm pump light are only 0.03 and 0.04 dB respectively. The above results reveal that the bare MF without active material can hardly enable light amplitude tuning functionalities.
Fig. 8 Variations of the transmitted power in the bare MF under (a) 405 nm and (b) 660 nm external pump light excitations with different powers.
By employing the same experimental setup and the same pump light power variation, experiment of light amplitude tuning for the WS2 coated MF is performed. The results of the transmission spectra under 405 nm violet pump light and 660 nm red pump light are shown in Fig. 9(a) and Fig. 9(b) respectively. In Fig. 9(a), the power variation can reach up to 4.68 dB under only 12.5 mW violet pump light excitation which is achieved at the wavelength of 1570 nm. For the 660 nm pump light excitation in Fig. 9(b), the maximum power variation is about 3.24 dB under a 20.3 mW of red pump light excitation which is obtained at the wavelength of 1620 nm. The average variations of transmitted power are 3.99 dB and 2.71 dB for the case of 405 nm and 660 nm pump light excitations respectively. In order to investigate how sensitive does the transmitted power variation versus pump light powers, the transmitted light under different pump light power at five different wavelengths (1520, 1545, 1570, 1595 and 1620 nm) are chosen for analysis. These data are analyzed by linear fitting and the results are shown in Fig. 10. Figure 10(a) corresponds to the power variation rate of violet pump powers analyzed at different wavelengths where the largest is ~0.3744 dB/mW and its correlation coefficient is 99.04%. Figure 10(b) corresponds to the power variation rate under red pump light excitation analyzed at different wavelengths where the largest one is ~0.1596 dB/mW and its correlation coefficient is 99.68%.
Fig. 9 Variations of the transmitted power in the WS2 coated MF under (a) 405 nm and (b) 660 nm external pump light excitations with different powers.
Fig. 10 Relative variations of power in WS2 coated MF with different (a) 405 nm and (b) 660 nm laser powers.
The above external pump light experiments show the feasibilities of tuning the output light amplitude in the WS2 coated MF structure. Encouraging by these results, another experiment of internally introducing the pump light sources is also performed. The experimental setup is shown in Fig. 11. The tunable probe light and the pump light sources (either 980 nm or 1458 nm pump lasers) are coupled via a wavelength division multiplexer (WDM). The multiplexed signal then passes through the WS2 coated MF and the output spectra are recorded by an OSA.
Fig. 11 A schematical drawing of 980/1458 nm internal pump light amplitude tuning experimental setup for the WS2 coated MF.
Experiments are performed with two different internal pump light sources. One is 980 nm laser where the excitation power varies at 0, 50, 95.6, 143.4 and 184.1 mW. The other is 1458 nm laser where the excitation power varies at 0, 10.1, 18.4, 25.7 and 34.3 mW. Experimental results for the bare MF under internal pump light power excitations are similar with that of external pump light excitations where only negligible transmitted power variations can be found. This is reasonable since silica based fiber do not favor light amplitude tuning under pump light excitations.
Contrary to the bare MF, the light amplitude tuning functionalities can be achieved with the internal pump light excitations for the WS2 coated MF as can be seen by the experimental results shown in Fig. 12. The transmitted power of the structure increases as the increasing pump light illumination power. Figure 12(a) corresponds to the transmission spectra of the WS2 coated MF under 980 nm internal pump light excitation at the same power variation as that for the bare MF case. An increase of up to ~8.95 dB in the transmitted power is obtained under 184.1 mW of 980 nm pump light excitation. Figure 12(b) corresponds to the transmission spectra of the WS2 coated MF under 1458 nm internal pump light excitation where its power varies as the same for the 980 nm pump light excitation case. The increase of transmitted power is up to ~1.52 dB under 34.3 mW of 1458 nm pump light excitation.
Fig. 12 Variations of the transmitted power in WS2 coated MF with different pump light powers at (a) 980 nm and (b) 1458 nm.
In order to investigate the transmitted power variation rate under internal pump light excitation at different wavelengths, a linear fit of the power variation versus different pump light powers at five different wavelengths (1520, 1545, 1570, 1595 and 1620 nm) are performed. The results are shown in Fig. 13. Figure 13(a) corresponds to the linear fitting results under 980 nm pump light excitation where a maximum power variation rate of ~0.0486 dB/mW is obtained at the wavelength of 1520 nm, and the average power variation rate along the whole investigated wavelength range is ~0.0423 dB/mW. Figure 13(b) corresponds to the linear fitting results under 1458 nm pump light excitation where the maximum power variation rate of ~0.0443 dB/mW is obtained at 1520 nm. The average power variation rate along the whole investigated wavelength range is ~0.0405 dB/mW.
Fig. 13 Relative variations of power in WS2 coated MF with different pump light powers at (a) 980 nm and (b) 1458 nm.
4. Discussions and response time measurement
In retrospect, the above experiments demonstrate that the light amplitude tuning functionalities are achieved under different pump light sources by ways of either internal or external pump light excitations. Quantitatively, the power variation rate in the WS2 coated MF has greatly improved compared with the bare MF. In addition, the tuning wavelength range presents a broadband characteristic which covers around 100 nm. The power variation rate is related to the ways of pump light excitation which is larger in the case of external pump light excitations.
In order to further characterize the structure, an experiment measuring the response time is performed where the experimental setup is shown in Fig. 14. A signal generator (AFG3102, Tektronix) is employed for the on and off state control of pump light signal with a period (T) of 0.1s. A 1550 nm DFB laser is connected to the input facet of the sample. The output light from the sample passes through a photo detector (Model 1811, New Focus) and finally it is collected by an oscilloscope. A 405 nm laser is employed as the external pump light source and its power varies at 28, 42 and 61 mW. A 660 nm laser is also employed as the pump source and its power changes from 97, 122 and 157 mW.
Fig. 14 Experimental measurement setup of the optical response to the different 405/660 nm external pump light.
The response time measurements are repeated for over hundreds of periods for different violet/red pump powers. They all show good repeatability. One typical response depicted in Fig. 15(a) and Fig. 15(b) show the averaged rise time is ~20.5 ms, and the averaged fall time is ~19.6 ms. Upon analyzing, we can see that the response time has little dependence on the pump light power. But with different light illuminations, the fabricated device shows a slightly different response speed where under the 405 nm power excitation is faster than 660 nm power excitation. This is because the response time measurement has dependencies on the pump light wavelengths or the methods of controlling ON-OFF state of the pump light which is consistent with what is reported in [47]. The response time of the above measurements has already taken into account the response time of the system including photo detector and oscilloscope. Therefore, it is a lower limit for the WS2 coated MF structure.
Fig. 15 (a) Response time of the MF coated with WS2 under different 405 nm laser illuminations. (b) Response time of the MF coated with WS2 under different 660 nm laser illuminations.
The dominant factor that accounts for the obtained experimental phenomena might be results from the thermo-optic effect considering the obtained moderate rise/fall response time: In the bare MF case, pump light illumination cannot redistribute the evanescent light around the outer surface of the MF due to the negligible thermal conductivity of silica (1.44 W m−1 K−1) [48]. Therefore, it cannot tune the output light power amplitude. In the case of WS2 coated MF, pump light illumination is absorbed by the WS2 nanosheets which results in heating the WS2 coated MF structure [49]. The rise of temperature in the structure results in a decrease of the real part refractive index (nr) of the WS2 nanosheets [50]. Consequently, the effective index (neff) of the guided mode in the WS2 coated MF structure is affected [51]. The rise of temperature in the structure results in a decrease of the real part refractive index of the WS2 nanosheets and the variation of the effective index will then lead to an increase of the output power which is substantiated by the following simulations. By using the FEM in COMSOL and employing the same theoretical model displayed in Fig. 6(a), simulations of the variation of output power versus the variation of the WS2 refractive index are performed. The pump light illuminations on the WS2 coated MF lead to an increase of temperature which results in a decrease of the real part of the WS2 refractive index. As shown in Fig. 16(a), when the real part of the WS2 refractive index nr decrease from 3.14 down to 2.54, the guided mode effective index of WS2 coated MF neff increases from 1.4441 to 1.4455. The electric field distributions corresponding to two different mode effective indexes (one for neff = 1.4455 and the other is for neff = 1.4441) are shown in Fig. 16(b) and Fig. 16(c). Qualitatively, the field distribution also has some variations for the two cases. By integrating the electric field distributions along the whole cross section of the two cases, we obtain mode field energy of 4.4655 x 107 W/m2 for neff = 1.4455 [Corresponding to Fig. 16(b)] and 7.626 x 107 W/m2 for neff = 1.4441 [Corresponding to Fig. 16(c)]. Since the detected output power is proportional to the electric field integration, a higher value indicates a larger detected power by increasing pump light illumination which is consistent with the experimental results in Fig. 9 and Fig. 12.
Fig. 16 (a) The real part of the mode effective index of the WS2 coated MF waveguide varies with the real part of the refractive index of WS2. (b) The electric field distribution of WS2 coated MF with neff = 1.4455. (c) The electric field distribution of WS2 coated MF with neff = 1.4441.
As to the different output power variation rates for different pump light wavelengths in the external pump light excitations, this might be due to the absorption strength is larger for the 405 nm than that in 660 nm therefore a higher power tunability. Regarding to the relatively low power tunability in the internal pump light excitation case than that of external pump light excitation, the coupling loss in the internal pump light excitations might account for the lower tuning efficiency. Other minor factors such as photons-generated excitons concentration variation might also play a role in the light amplitude tuning functionality which will results in a much faster device response time than that of thermo-optic effect if it is dominant working principles [52,53].
In the literature, other kinds of materials such as TiO2 [54], MoSe2 [55], graphene and its derivatives like reduced graphene oxide (rGO) [56–58] are also combined with MF for the demonstrations of all-optical tunable functionalities. Table 1 shows the performance comparison of different types of light amplitude tuning structures. In terms of the power variation rate, the all-optical light amplitude tuning structure demonstrated here yields the highest power variation rate of ~0.3744 dB/mW shown in bold fonts in Table 1.
Table 1. Comparison of different all-optical light amplitude tuning structures
In terms of the response time, the demonstrated structure here yields a moderate one of ~20.5 ms, we believe that the speed of the fabricated structure is limited by factors such as inhomogeneous WS2 deposition and unoptimised MF interaction length and MF diameter etc. Potential ways of improving response speed might achieve via employing monolayer nanosheets coating (such as a bi-layer graphene coated MF structure demonstrated in [57] where a response time of 1x10−6 s is achieved), optimizing the film quality of WS2 and the geometric configurations of MF etc. Therefore, the response time of WS2 coated MF structure could be decreased when employing nanofabrication methods, optimizing the geometric configurations of MF and improving the quality of WS2 nanosheets.
5. Conclusion
In conclusion, a WS2 coated MF is demonstrated here for all optical light amplitude tuning via both external and internal pump light excitations. A power variation rate of ~0.3744 dB/mW is achieved and the fabricated device can be efficiently tuned in a broad-spectrum range of ~100 nm from 1520 nm to 1620 nm. The high tuning efficiency and broadband characterizations are promising for future all optical active device applications. However, the moderate response time of ~20.5/19.6 ms which we believed is limited to the fabrication method employed here should be improved for the real applications. Our future research will focus on employing monolayer film deposition and optimizing the MF diameter in order to improve its response time.
Funding
National Natural Science Foundation of China (61505069, 61475066, 61675092, 61705087, 61705089, 61775084); National Major Project of China (J-GFZX0205010501.12, GFZX0205010501.24-J); Guangdong Special Support Program (2016TQ03X962); Natural Science Foundation of Guangdong Province (2015A030306046, 2016A030310098, 2016A030311019); Science and Technology Project of Guangzhou (201605030002, 201607010134, 201704030105).
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