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Abstract
All-optical light-control-light functionality is realized in a layered tungsten disulfide (WS2) nanosheet coated microfiber knot resonator (MKR) structure. Mainly due to the photon generated excitons induced refractive index variation in WS2 nanosheets, a large variation in the transmitted power (∆T) can be observed under external violet/red laser excitation. The ∆T variation rates can reach up to ~0.4 dB/mW under violet pump light excitation whereas the state of the art light-control-light structures usually has a variation rate of less than 0.25 dB/mW. In terms of the response time, the averaged rise/fall time is ~0.12/0.1 s. The demonstrated structure has the advantages of easy fabrication, low cost and high sensitivity, therefore, it might be a promising candidate for building future all-fiber-optics based functional devices and all-optical circuitry.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fibers have long been the backbone of modern communication system. One way of extending the functionality of fibers is to thin down its core size to form microfibers (MF). These MFs and MF based structures help to enhance the light-matter interaction via a large fraction of evanescent light outside the fiber core [1]. Nevertheless SiO2 based bare MF structures, due to the intrinsic material limitation, in most cases cannot meet the demand for high performance applications such as tunable light-control-light ability [2,3] and highly sensitive sensors etc [4]. On the other hand, a large panel of two dimensional (2D) layered nanosheets characterizing by their layer dependent band structures and suitable band gap energy, are promising candidates for building photonic components with different functionalities such as ultra-fast nonlinear photonics [5,6], quantum emitters [7] and ultra-short pulse generations [8,9], etc.
One particularly interesting applications is the all-optical control of light functionality which can be realized by simply coating 2D layered nanosheets onto the MF region [10]. These kind of combinational all-optical tunable structures have been demonstrated in a bi-layer graphene covered MF structure yielding a tuning efficiency of 0.007 dB/mW [11]. An improved configuration of the stucture is demonstrated in a mono-layer graphene coated MF structure where a tuning efficiency of 0.2 dB/mW is achieved [10]. Other kinds of materials such as nematic liquid crystals [12], MoSe2 [13] and WS2 [14] are also employed to combine with MF based structures for the demonstration of all-optical tunable functionalities. However, the demonstrated light-control-light structure typically has low sensitivity not exceeding 0.25 dB/mW and varying response time with the lowest up to several seconds of rise/fall time [12].
This paper aims at building a structure with light-control-light ability of high sensitivity and relatively moderate response time. To achieve this, a microfiber knot resonator (MKR) is chosen. It has excellent resonance properties such as high Q, high extinction ratio (ER) and comb shape resonance spectra which are exploited here for the enhancement of light-control-light functionality with higher sensitivity [15]. In terms of the coated material, a layered 2D transition metal dichalcogenide tungsten disulfide (WS2) is a promising material to develop functional optoelectronic devices because it has high electron mobility, high on/off ratio and tunable refractive index by light absorption in the visible regime [16,17].
By coating few layered WS2 nanosheets onto MKR, an all-optical control of light functionality is realized and presented here. Thanks to the photo-induced refractive index change of WS2 [3], the transmitted power of the MKR with WS2 can be quasi-linearly tuned by the external violet/red pump light power. A high sensitivity corresponding to the ∆T variation rate under violet pump power excitation of 0.4 dB/mW is achieved. In terms of the response time, the MKR with WS2 structure yields a rise time of around 0.12 s. The device fabrication, characterization and experiments of all-optical control of light functionality will be presented in the following sections.
2. Device fabrication
In this section the fabrication of the MKR structure, the characterization of the WS2 nanosheets and the deposition of the WS2 nanosheets onto the MKR will be presented. In order to make a compromise between a large fraction of the evanescent light and the optical transmitted power, the diameter of the MF is chosen between 5 ~10 µm. An appropriate core size will make sure that some portion of evanescent light leaks outside the MF for light-matter interaction. On the other hand its core size should not be too small in order to guarantee that there is still light guides to the output facet of MKR for later transmitted power tuning experiments. The MF (which is utilized to form the MKR) is fabricated by manually stretching a SMF-28 (Corning) by heat-flame taper-drawing method [1]. A microscopic image of the fabricated MF is shown in the inset of Fig. 1(a) where the MF diameter is about 8.1 µm. Afterwards, the MF is assembled into an intertwisted MKR structure with the aid of translation stages and microscopes. The fabricated MKR is then packed onto a MgF2 crystal substrate with a high degree of cleanliness.
Fig. 1 (a) Microscopic image of the MKR with a loop diameter of about 968.2 µm and the inset is a microscopic image of MF where the diameter is about 8.1 µm in the waist region. (b) Absorption spectrum of WS2 nanosheets. The inset is the Raman spectrum of WS2 nanosheets.
A microscopic image of the MKR with a red light excitation is shown in Fig. 1(a) which depicts the MKR structure with a loop diameter ~968.2 µm. We can see from these images that the MF has a low surface roughness and the MKR is assembled with good quality. The optical transmission, the experiments of transmitted power tuning with respect to different pump light power are performed for the SiO2 based bare MKR structure (i. e. without the WS2 nanosheets) and the results will be presented in the next section.
Before coating the WS2 nanosheets onto the MKR, characterization is performed on the commercially available WS2 dispersion [18]. The WS2 dispersion is fabricated by lithium ion intercalation exfoliation method with a concentration of 1mg/ml. The WS2 nanosheets have averaged lateral sizes of about 0.05 ~1 µm, while the thickness varies from one till ten layers. Raman and UV-Vis absorption spectrum are performed for the characterization of WS2 nanosheets and the results are shown in the inset of Fig. 1(b) and Fig. 1(b). The Raman spectrum of WS2 nanosheets is shown in the inset of Fig. 1(b) which is measured under 514.5 nm laser excitation. Due to the weak van der Waals interlayer interactions that affects the optical phonon modes of intralayer bonding and lattice vibrations, two major peaks which is typical to WS2 nanosheets in the inset of Fig. 1(b) can be found. One located at 355.8 cm−1 is associated to an in-plane optical mode which originates from the longitudinal acoustic mode at the symmetric point M of the Brillouin zone. The other is the A1g mode which is associated to the out of plane vibrations of the sulfur atoms. The position of these peaks also suggest that the WS2 nanosheets employed here are multi-layers [16]. The UV-Vis absorption spectrum of the WS2 nanosheets is shown in Fig. 1(b) from which we can see that it has strong absorption at the wavelength range between 200 nm and 700 nm. A local absorption peak locates at around 624 nm which is believed originating from the d-d type transitions at the center of the Brillouin zone. The absorption property is relatively strong at the wavelength of 405 nm (violet light) and 660 nm (red light) which will then be utilized as the pump light lasers for tuning the transmitted power of the MKR with coated WS2 [19].
After characterizing the WS2 dispersion, sonication process is performed at room temperature for about 30 minutes in order to avoid agglomeration effect of the nanosheets. Immediately after the ultrasonic process is finished, a pipette is employed to transfer the dispersion to the MF regions of the MKR. The structure of the MKR with WS2 is built after the solvent is evaporated and it reaches a stable state. Figure 2 shows SEM images of a small part of the MKR circumference from which we can see that the WS2 nanosheets are successfully coated onto the MKR structure. From Fig. 2(a) we can see that the width of the MF is around 8.1 µm which is consistent with the one measured by the optical microscope. The inset in Fig. 2(a) is a high magnification view of the coated WS2 nanosheets. It shows that the WS2 nanosheets have a good coverage onto the MF region of the MKR. Figure 2(b) shows a cross sections view of the MKR with WS2 structure from which we can deduce that the WS2 coating is about 300 ~400 nm in thickness. Optical characterization and external violet/red pump light for transmitted light power tuning in both structures of the MKR with and without WS2 will be presented in the following section.
Fig. 2 SEM images of a small part in the MKR coated with WS2 nanosheets (a) which shows the MF of about 8.145 µm in diameter and the inset shows an enlarged view of the coated material, (b) which shows a cross-section view of the MF with WS2 nanosheets.
3. Experimental details, results and discussion
In this section, optical characterization where the transmitted power is tuned by external violet/red pump light power for the structures of MKR with and without WS2 will be presented. The experimental set up for these characterization is shown in Fig. 3. The sample is fixed inside a basin (made of UV adhesive) onto a MgF2 substrate. Light from TLS (ANDO-AQ4321D, wavelengths ranging from 1520 nm to 1620 nm) is connected to one end of the MKR structure while the other end of the MKR is collected by an OSA (YOKOGAWA-AQ6317C). The 405 nm violet /660 nm red laser diode is placed vertically above the sample (either MKR with or without WS2) at a distance of about 10 cm and it is focused by a cylindrical lens.
The experiments of transmitted power tuning is firstly performed on the MKR without WS2 structure (in terms of chronological order, this is performed before the WS2 coating process onto the the MKR structure). The output spectra under violet pump power varying at 0, 5.08, 13.0, 18.75 and 23.6 mW are recorded and the results at around 1565.7 nm (corresponds to the λres of the maximum ER of ~15.27 dB and the largest Q of ~97,856 within the considered wavelength range) is shown in Fig. 4(a). From Fig. 4(a) we can see that under different violet pump power excitation there is a transmitted power variation ∆T of less than 0.5 dB while the λres position hardly changes. By changing the the pump light source to a 660 nm laser light in Fig. 3, experiments of transmitted power tuning are then performed with the red pump light power varying at 0, 10.8, 29.2, 100 and 125 mW for the MKR without WS2 structure. The results are shown in Fig. 4(b) with the wavelength ranging from 1565 nm to 1568 nm. Similar to the violet pump light excitation, both the transmitted power and the λres have little or no variation under different pump power in Fig. 4(b). The maximum ∆T under red light power of 125 mW is less than 0.6 dB within the considered wavelength range while the λres hardly changes. These results indicate that a MKR made of only silica based MF cannot enable transmitted power tuning which is consistent with what has been reported in [4,20].
Fig. 4 (a) Transmission of the MKR structure (black curve) where the largest obtained ER is ~15.2 dB at the resonance wavelength around 1565.7 nm. The red, blue, magenta and red wine coloured curves correspond to the transmission with external violet pump power of 5.08, 13.0, 18.75 and 23.6 mW. The inset is an enlarged view around the resonance dip. (b) Transmission of the MKR without WS2 structure at different external red pump light power excitation. The black, red, blue, magenta and red wine coloured curves correspond to the transmission with external violet pump power of 0, 10.8, 29.2, 100 and 125 mW.
By employing the same experimental setup in Fig. 3, measurements are then performed on the MKR with WS2 structure. The output spectra under the same violet pump light power variation range for the MKR with WS2 structure at around 1577.4 nm (corresponds to the λres of the maximum ER of about 23 dB) is shown in Fig. 5(a). Contrary to small or no variation of ∆T and λres position in MKR without WS2, from Fig. 5(a) we can see that with the increase of external vioet pump light power, the transmitted power becomes larger (i.e. the ER becomes smaller) and there is a red shift of the λres position. Quantitatively, the resonance Qfactor decreases following the increase of the pump light power as shown in the inset of Fig. 5(a). The Q factor decreases from 87,635 (with pump light off) down to 47,801 under 23.6 mW violet pump light excitation. If we employ an amplitude investigation at a fixed wavelength λfixed of 1577.44 nm, the largest ∆T is about 10 dB corresponding to 23.6 mW violet pump power which is shown in Fig. 5(a). Whereas under 23.6 mW violet pump power, there is about 0.02 nm λres shift.
Fig. 5 (a) Transmission of the MKR with WS2 structure at different external violet pump light power excitation. The black, red, blue, magenta and red wine coloured curves correspond to the transmission with external violet pump power of 0, 5.08, 13.0, 18.75 and 23.6 mW. The inset shows the Q factor varies with respect to different violet pump light power. (b) Transmission of the MKR with WS2 structure at different external red pump light power excitation. The black, red, blue, magenta and red wine coloured curves correspond to the transmission with external red pump power of 0, 10.8, 29.2, 100 and 125 mW. The inset shows the Q factor varies with respect to different red pump light power.
Replacing the violet pump light by the red light laser, experiments are performed on the MKR with WS2 structure where the excitation power varies at the range of 0, 10.8, 29.2, 100 and 125 mW. Figure 5(b) corresponds to the measured output spectrum around the wavelength of 1577.4 nm for different red light power. The trends of ∆T and λres shift versus the increase of red light powers is the same with the case of violet pump light excitation. The resonance Q factor, similar to the violet pump light excitation case, also decreases as the increase of the pump light power as shown in the inset of Fig. 5(b). The Q factor decreases down to 47,802 under 125 mW red pump light excitation. The largest ∆T is about 17.1 dB under 125 mW red light excitation which is shown in Fig. 5(b). Whereas under the same red pump light power excitation, there is about 0.03 nm of the λres shift.
Variations in both the transmitted power and the λres position can be found in the above experiments of violet/red pump light excitation in the MKR with WS2 structure. The physical mechanism favoring these variations might probably be explained as follows: The strong absorption property of WS2 at 405 nm violet/ 660 nm red light will lead to the excitation of electron-hole pairs in WS2. These photon generated carriers will then affect the real and imaginary parts of the conductivity in WS2. And the change of conductivity will affect the refractive index of material [21]. The transmitted power is mainly related to the real part of the conductivity through the change of resonance condition. The pump light photon induced variation in carriers distribution will lead to an increase in the real part of the WS2 conductivity, therefore the light absorption i.e. the loss factor on the MKR will increase. As a consequence, the resonance condition of the MKR structure changes and it shows as a decrease in the resonance ER. On the other hand, a red shift in the λres under violet/red pump light excitation indicates that the mode effective index becomes larger which is related to the imaginary part of the conductivity in WS2. The red shift of the λres might suggest that with the increase of the pump light power, the imaginary part of the WS2 conductivity gets larger. Since at the 405 nm regime, the WS2 nanosheets present a stronger absorption than that at 660 nm, it therefore leads to a larger variation in the WS2 conductivity. Consequently, the ∆T variation and λres shift is larger at 405 nm pump light excitation [3,17,19,22].
Correspondingly, the linear fit of ∆T versus different violet/red light excitation power is shown in Fig. 6(a). The ∆T variation rate for the violet pump light excitation is shown as red curve in Fig. 6(a) and it yields a sensitivity of 0.4 dB/mW. The blue curve in Fig. 6(a) corresponds to the ∆T variation rate for the red pump light excitation and it yields a variation rate of 0.14 dB/mW.
Fig. 6 (a) Linear fit of ∆T versus different pump light powers. The red curve corresponds to the violet pump light excitation while the blue curve corresponds to the red pump light excitation. The inset shows the resonance curve normalized by the ER for the MKR with WS2 structure (black curve) and the simulated resonance curve by coupled mode theory (dashed red curve). (b) Experimental setup for the response time measurement.
Numerical analysis according to the coupled mode theory is performed and the result is shown in the inset of Fig. 6(a). The output intensity of the MKR can be described by [23]
where γ is the coupling loss at the twisted knot of the MKR; β = k0Re(neff) is the mode propagation constant, k0 and Re(neff) are the wave number in vacuum and real part of the effective index respectively. κr is the coupling coefficient which can be evaluated from F = πκr1/2/(1 - κr). F is the finesse of the experimental measured resonance. The fitting result according to Eq. (1) is shown as dashed red curve in the inset of Fig. 6(a). In order to have almost the same signal intensity between the simulated and the measured results, the signal obtained in measurements are divided by the largest ER. The fitting result are close to that of the measurement. The fitting result indicates that the coupling loss of γ = 0.45 and Re(neff) = 1.5. The fitting protocol by Eq. (1) indicates that the resonance wavelength shift is mainly due to the variation of Re(neff) while the resonance ER variation is dominantly determined by κr. The dip of the fitting result is close to that of the measurement. These is consistent with the physical mechanism explained in the previous paragraph for the obtained experimental results in Fig. 5.
In order to further characterize the structure of MKR with WS2, an experiment aiming at measuring its response time is performed where the experimental set up is shown in Fig. 6(b). A chopper is employed for the on and off state control of the pump light signal. A TLS is connected to the input facet of the sample. The transmitted light from the sample then passes through a photo detector and at the end it is collected by an oscilloscope.
In order to investigate whether the response time is related to the pump light wavelength or pump light power, experiments of varying the pump light wavelength (at 405 nm or 660 nm laser) and varying pump light powers are carried out. The probe light is chosen at 1522 nm and the chopper is controlled by a T = 50 ms periodic square wave. The response time measurement processes are repeated for over several tens of periods for different pump light power. They all show high reproducibility. The response obtained from the oscilloscope is shown in Fig. 7. Figure 7(a) corresponds to measurements at 405 nm violet pumping light with the power of 25.3, 33.9 and 45.8 mW. Whereas Fig. 7(b) corresponds to measurements at 660 nm red pumping light with the power of 139.7, 149.5 and 155 mW. From Fig. 7, we can see that the response time has little dependence on the pump light wavelength or pump light power. Upon analyzing, the averaged rise time of the sample is ~0.12 s, and the averaged fall time is ~0.1 s. Possible methods of improving this moderated obtained response time might be developing a more precise and homogeneous nanosheets coating, optimizing the size of the MKR, optimizing the operating wavelength etc.
Fig. 7 (a) Response time under the violet light power of 25.3, 33.9 and 45.8 mW excitation. (b) Response time under the red light power of 139.7, 149.5 and 155 mW excitation.
In retrospect, we have demonstrated the realization of the light-control-light capability in a MKR with WS2 structure. Under 125 mW red pump light excitation, the transmitted power variation ∆T is less than 0.6 dB in the MKR without WS2 structure, whereas in the MKR with
WS2 structure a ∆T yields up to 17.1 dB. Consequently, the ∆T variation yields at least 28 fold enhancement. Table. 1 shows the performance comparison of different types of light-control-light structures. In terms of the ∆T variation rate, the demonstrated structure as shown in bold characters in table. 1 yields the highest sensitivity of 0.4 dB/mW with respect to others. Even though the response time of our device is a moderate one, it still has large room for improvement if further optimization such as the region of the nanosheets coating, the diameters of the MF and the MKR etc are performed.
Table 1. Comparison of different all-optical control of light devices
4. Conclusion
To conclude, an all-optical control of light functionality has been demonstrated in the structure of MKR with WS2 nanosheets. Due to the variations in both the real and imaginary part of WS2 conductivity, the tuning of both the transmittance and the λres under different violet/red pump light powers can be achieved. A high ∆T variation rate under violet pump light of 0.4 dB/mW is obtained. In terms of the response time, the structure has a moderate averaged rise/fall time ~0.12/0.1 s. Future further improvement such as developing more precise control over the fabrication of MKR structure and a more controllable process of the WS2 nanosheets deposition might lead to the performance improvement of the structure.
Funding
National Natural Science Foundation of China (61505069, 61705089, 61775084, 61705087, 61475066, 61475066, 61675092), Guangdong Special Support Program (2016TQ03X962), Guangdong Natural Science Funds for Distinguish Young Scholar (2015A030306046), Science Foundation of Guangdong Province (2016A030310098, 2016A030311019), Science & Technology Project of Guangzhou (201607010134, 201704030105, 201605030002, 201604040005) and Rail Transit Healthy Operation Cooperative Innovation Center of Zhuhai (55560307).
Acknowledgments
We thank Huazhuo Dong and Dongquan Li for their help in drawing the experimental setup and Zhongmin Wang for his helpful discussion on the experimental details.
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