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Abstract
Molybdenum diselenide (MoSe2) nanosheets are coated on the tapered region of microfiber (MF) to achieve active light control by light with order of mW. The MoSe2 nanosheets are illuminated by 405 nm and 980 nm lasers which change the conductivity of the MoSe2, thus the transmitted power of the guiding light (λ = 1550 nm) within the MF can be controlled. The transmitted optical power of the MF has a relative variation of ~2 dB (0.165 dB/mW) when the 405 nm light is illuminating on the MoSe2 nanosheets with a power ranging from 0 to 11.6 mW. The sensitivities of the 980 nm in-fiber and out-fiber experiments are 0.092 dB/mW and 0.851 dB/mW, respectively. The rise and fall times of the transient response are 0.4s and 0.6s, respectively. Therefore, the guiding light in our MF coated with MoSe2 can be effectively manipulated by the 405 and 980 nm light (order of mW). The MF coated with MoSe2 has potential applications in light sensing and all-optically controllable devices.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Layered molybdenum diselenide (MoSe2), two-dimensional (2D) transition metal dichalcogenides has attracted significant interest due to their unique electronic and optical properties [1–3]. The optoelectronic properties of the MoSe2 is layer-dependent, since a transition from an indirect to direct bandgap can be found when moving from bulk to monolayer forma at visible/near-infrared wavelengths [4]. The direct bandgap of monolayer MoSe2 offers advantages over graphene, a zero-gap material, for many optoelectronic applications [2, 5]. The MoSe2 exhibits saturable absorption when incident photon energy is larger than the bandgap, resulting from the free-carrier excitation from valence band to conduction band and hence Pauli-blocking [5]. Based on the saturable absorption, mode-locked and Q-switched lasers have been demonstrated over a broad wavelength range from the visible to the near infrared [5–8]. Recently, the spin-polarized electronic band structure is observed in doping MoSe2 [9]. The large values of the spin-splitting energies in the valence and conduction bands suggest that the MoSe2 can be relevant for spin- and valley- based applications [9–11]. It is demonstrated that the bandgap of the monolayer MoSe2 could be tuned by the uniaxial strain [12]. And the optoelectronic properties of the MoSe2 is controllable by the physical adsorption and chemical doping [5, 13]. Therefore, the MoSe2 can be used in tunable optical and electric devices [2, 14, 15].
Micro fiber (MF) is a kind of optical fiber with an extremely long distance in one dimension and well-defined microstructures in the other two dimensions [16]. Owing to its small diameter, optical MF can offer large fraction of evanescent and high-intensity surface field, enhancing the near-field interaction between the guided light and the surroundings [17]. Furthermore, other advantages including the easy preparation, low optical loss, tight optical confinement, and outstanding mechanical flexibility have made MF become a promise candidate for future optical and photonic devices [16, 17]. Many MF-based devices have been exploited such as Mach-Zehnder interferometers [18], optical ring resonators [17], compact filters [18], and all-optical modulators [19–22]. In the all-optical modulators, the MF is usually coated with 2D materials to achieve nonlinear respond [20]. In 2014, Li et al. have demonstrated an all-optical modulator based on graphene-clad microfiber, where a modulation depth of 38% and a response time of ∼2.2 ps have been achieved [21, 22]. In 2015, an all-optical fiber modulator based on a stereo graphene-MF structure has been demonstrated for two polarization states with a modulation depth being 7.5 dB and 2.5 dB, respectively [23]. It is shown lately that all-optical, all-fiber optical modulator with a Mach−Zehnder interferometer structure could significantly increase the modulation depth and overall transmittance [24]. Recently, other 2D materials, such as phosphorus [25] and MoS2 [26], have also been used for optical modulations.
Here, the active light control by light is investigated for a MF coated with MoSe2 nanosheets pumped by a 405 nm laser (out-fiber pumped) and a 980 nm (in-fiber and out-fiber pumped) laser. The light in the MF will interact with MoSe2 nanosheets through the evanescent wave, thus can be changed by pumped laser. The response sensitivities of MF coated with MoSe2 to the out-fiber pumped 405 nm laser is ~0.165 dB/mW. The sensitivities of the 980 nm in-fiber and out-fiber experiments are 0.092 dB/mW and 0.851 dB/mW, respectively. And the response times to the 980 nm pumped light are 0.4s and 0.6s in the rise and fall processes, respectively.
2. Fabrication
The concentration of MoSe2 alcohol suspension is 1mg/ml, and the average size is 100-1000 nm. Figure 1 shows the Raman spectrum of MoSe2 film coated on MF. The spectrum is excited by a 488 nm laser and measured with LabRAM HR Evolution (HORIBA JY, France) at room temperature. The out-of-plane A1g and in-plane E12g Raman active modes locate respectively at 240.7 and 287.9 cm−1, with a distance of 47.2 cm−1. The presence of the B2g mode at 352.6 cm−1 suggests that the MoSe2 film is in multilayer thickness [10].
The MF shown in Fig. 2(a) is manufactured from a telecom fiber by using “flame-brushing” technique. A standard single mode fiber (SMF, with a core diameter of 8 μm and a cladding diameter of 125 μm from Corning Inc.) is uncoated using a chemical solution of dichloromethane and acetone, and then is heated by a flame of millimetre dimensions and elongated at a drawing speed of 0.2 mm/s. As shown in Fig. 2(b), the MF with a diameter of ~12.5 μm in the uniform waist region is fabricated. Then the uniform waist region is immobilized onto a glass slide. In order to contain MoSe2 solution, a basin (15 mm × 5 mm × 1 mm) is constituted by using the UV adhesive (Loctite 352, Henkel Loctite Asia Pacific).
Fig. 2 (a) Schematic of the basin used in deposition of MoSe2 and configuration of a fixed MF on glass slide. (b) The enlarge view of MF.
The variation of the MF diameter is measured by an instrument (XS-01-05-001). As shown in Fig. 3, the length of MF is ~25 mm, and the waist region of the MF is estimated to be ~12.5 μm.
The MoSe2 alcohol suspension is dropped into the basin and evaporated for about 12 hours in ambient surrounding. Before the dropping, the MoSe2 suspension is treated by ultrasonication for 60 minutes to make the MoSe2 nanosheets evenly distributed. As the alcohol evaporates naturally, the MoSe2 nanosheets are self-assembled onto the waist region of the MF. During the self-assembly process of MoSe2, the optical transmitted power of the MF is monitored with a 1550nm distributed feedback (DFB) laser being the light source. As shown in Fig. 4, the transmitted power is about −7.4 dBm at the beginning. About 158 minutes later, the optical power is decreased abruptly to −38 dBm, which indicates that the MoSe2 solution becomes as MoSe2 film on the MF. The power is stable at −35 dBm after 333 minutes, which means the self-assembly process of MoSe2 is completed.
As shown in Fig. 5(a), the fabricated MF coated with MoSe2 is imaged by scanning electron microscopy (SEM). MoSe2 is distributed non-uniformly on the MF. The cross section of MF coated with MoSe2 and an enlarged view with higher magnification for the region marked by a white dotted line are given in Fig. 5(b), showing that the MoSe2 film are well coated on the MF. The thickness of the MoSe2 film is ~300 nm.
Fig. 5 (a) SEM image of the MF coated with MoSe2; (b) cross section SEM image of the MF coated with MoSe2 and enlarged view of the region marked by a dotted line.
3. Experimental results and discussions
To study the light-control-light properties of the MF coated with MoSe2, the visible light and near-infrared light are chosen to pump the MF coated with MoSe2 for comparison. A 405 nm semiconductor laser is chosen as the pumped source, since the MoSe2 is more absorptive at 405 nm than other wavelengths within visible spectrum range [5]. The 405 nm laser with order of mW emits spatial light and pumps the device in out-fiber configuration. For a comparison, 980 nm lasers are chosen among the near-infrared light to pump the MoSe2 coated MF in both in-fiber and out-fiber configurations, since it has a good modulating effect to the MoSe2 naosheets. The 980 nm laser is widely used in the generation of blue-green light sources and the pumping of the Er-doped fiber amplifiers/lasers [27].
In the 405 nm out-fiber pumped experiment, the light from a 1550 nm DFB laser is sent into the MF coated with MoSe2, and the transmitted power is measured by an optical power meter (6210 optical power meter, Accelink Technologies Co., Ltd). A 405 nm pump laser (LSR405NL, Lasever Inc.) is placed ~10 cm above the MF and focused by the cylindrical lens, as shown in Fig. 6. The experiments are performed with violet power ranging from 0 mW to 11.6 mW. The experimental humidity is fixed at 38% RH, while the temperature of MF is monitored by a thermocouple.
As shown in Fig. 7(a), the optical transmitted power of MF without MoSe2 vibrates only ~0.08 dB when the power of the violet laser (405 nm) ranges from 0 mW to 11.6 mW. There is no obvious relationship between the transmitted power of MF and the illuminate power of the violet laser. The illuminate of the violet laser has little influence on the optical transmitted power of the MF without MoSe2. However, for the MF coated with MoSe2, the transmitted power will change with illuminate power step by step. As shown in Fig. 7(b), the optical transmitted power has a relative variation of ~2 dB when the illuminating power of the violet laser increases from 0 mW to 11.6 mW. When the illuminate power decreases from 11.6 mW to 0 mW, a relative variation of the optical transmitted power is about ~1.8 dB.
Fig. 7 (a) Optical transmitted power change with different pump power of bare MF versus time. (b) Transmitted power of the MF coated with MoSe2 for different illuminated violet power.
During the laser pumping, the temperature of the MF with MoSe2 has been monitored by a thermocouple. The experimental results are shown in Fig. 8(a). When the illuminating power increases from 0 mW to 11.6 mW, the temperature of MF with MoSe2 increases from ~28.7 °C to ~35.2 °C accordingly. The MoSe2 coated MF is further measured by placing it in a humidity chamber (BPS-100CL, Shanghai Yiheng Instruments Co., Ltd). As shown in Fig. 8(b), when the temperature of the chamber is tuned from 28 °C to 36 °C in the absent of illumination, the vibration of the transmitted power is about 0.03 dB, which is negligible comparing with the power vibration by laser pumping. Therefore, our device is insensitive to the environment temperature.
Fig. 8 (a) The temperature of the MF with MoSe2 for different pump power. (b) Transmitted power of the MF with MoSe2 for different environment temperature.
To better show the relationship between the pump power of 405 nm laser and the transmitted power of the MF coated with MoSe2, the means values of the transmitted power for different steps of pump power are shown in Fig. 9. The response sensitivity of the transmitted power of the MF to pump light can be obtained by the slope of linear fitting curve. The sensitivities for the increasing and decreasing processes of the pump power is slightly different, which are ~0.165 dB/mW and ~0.158 dB/mW, respectively.
The difference of the sensitivity between the increasing and decreasing processes may be caused by the humidity of MoSe2 naosheets. As the experiment is performed in ambient condition, the humidity of the MoSe2 naosheets will be changed after the increasing process of the illumination, i.e., H2O molecules decrease. The H2O molecules act as electron acceptors, and tend to withdraw electrons from the MoSe2 surfaces [28], resulting in change of the electron concentration in the conduction band of MoSe2 [29]. Therefore, the different humidity will result in the difference of the sensitivity between the increasing and decreasing processes.
In the 980 nm in-fiber pumped experimental, the signal source and pump source from a 1550 nm DFB laser and 980 nm laser (Pump-LSB-980-500-SM, OPEAK) are multiplexed by a WDM (980 nm/1550 nm), and then sent into the MF. The transmitted power from MF is measured by an optical spectrum analyzer (OSA, Yokogawa-AQ6370D), as shown in Fig. 10. The experimental humidity is ~38%RH. The 980 nm laser power are sequentially changed from 0 mW, to 44.7 mW, 89.0 mW, 133.7 mW, 178.4 mW, 222.8 mW, 268.6 mW, and 314.2 mW, respectively.
The transmitted optical power of the MF without and with MoSe2 are shown respectively in Fig. 11(a) and 11(b) for the power of 980 nm laser changing from 0 mW to 314.2 mW. As shown in Fig. 9(a), the transmitted optical power of the MF without MoSe2 are almost unchanged. On the contrary, the transmitted power of the MF with MoSe2 undergoes a large variation (~30 dB) when the adjustable 980 nm laser changes from 0 mW to 314.2 mW. The transmitted optical power increases with the increasing 980 nm laser power.
Fig. 11 The optical transmitted power of the MF without (a) and with (b) MoSe2 for different 980 nm laser power.
We compare the light-control-light capabilities of MF with and without MoSe2. The dependences of the optical transmitted power on the pump powers are displayed in Fig. 12, where the experimental data have been linearly fitted. The response sensitivities of MF with and without MoSe2 are ~0.092 dB/mW and ~0.000026 dB/mW, respectively. Therefore, the response sensitivity has been improved 3538 times by the MoSe2.
Fig. 12 Relative optical transmitted power of the MF with and without MoSe2 for different 980nm laser power.
The 980 nm out-fiber pumped experiment is demonstrated by employing the experimental setup shown in Fig. 13 with the 980 nm laser (HW980ADX-34F, Shenzhen infrared laser technology Co., Ltd) as a pump. The experiment is performed with 980 nm power ranging from 0 mW to 17.7 mW, and the experimental humidity is fixed at 38% RH.
The optical transmitted power of the MF coated with MoSe2 has a relative variation of ~14.7 dB with respect to the increase of adjustable 980 nm laser from 0 mW to 17.7 mW as shown in Fig. 14(a). While the optical transmitted power has a relative variation of ~13.3 dB when the illuminating power decreases from 17.7 mW to 0 mW. The dependences of the optical transmitted powers to the 980 nm pump powers are displayed in Fig. 14(b). The sensitivities for the increasing and decreasing processes are ~0.851 dB/mW and ~0.767 dB/mW, respectively. The sensitivity (~0.851 dB/mW) in the 980 nm out-fiber pumped experiment is much higher than that (0.092 dB/mW) of the 980 nm in-fiber pumped experiment, indicating that the out-fiber pump may be more efficient in the light control than that the in-fiber pump. Moreover, the response sensitivity of the MoSe2 coated MF with the 980 nm pumped laser is about five times higher than that (~0.165 dB/mW) with the 405 nm pumped laser in the out-fiber pumped experiments. This illustrated that the 980 nm wavelength shows better modulating effect to the MoSe2 nanosheets than the 405 nm wavelength.
Fig. 14 (a) Transmitted power of the MF coated with MoSe2 for different illuminated out-fiber pumped 980 nm power. (b) Relative optical transmitted power of the MF coated with MoSe2 versus the 980 nm pump power.
To measure the transient response [30] of the MF coated with MoSe2, we use a chopper to carve the 980 nm light into pulses as shown in Fig. 15(a). The 980 nm light is in the ON (1.2 s) and OFF (1.2 s) state periodically with a power variation of ~15 mW. The modulated 1550 nm light is detected by a photo-detector (1811, New Focus), and the transformed electric signal is analyzed by an oscilloscope (DS1052E, Rigol). As shown in Fig. 15(b), the rise (fall) process corresponds to the presence (removal) of the pump light. The response time of the rise and fall process are 0.4 s and 0.6 s, respectively. In order to fulfill the critical needs in high-speed modulators and optical switches, the response of the MF coated with MoSe2 should be improved. Improving the production quality of MoSe2 to decrease impurities, optimizing the MoSe2 coating technology, and optimizing the diameter of the MF may improve the transient response.
Fig. 15 (a) Experimental measurement setup of the transient response to the 980 nm out-fiber pumped laser. (b) Transient response of the MF coated with MoSe2.
The active light control of MF coated with MoSe2 by the pumped lights can be explained as following: MoSe2 exhibits saturable absoption, since its band gap is smaller than the pumped photon energy in both cases of in-fiber and out-of-fiber pump [5]. The guiding light within MF will be absorbed by the MoSe2 nanosheets. The concentration of excited electrons-holes in MoSe2 increases with the pump power, resulting in a real part reduction of dynamic conductivity [31]. The reduced conductivity decreases the absorption of guiding light by the MoSe2 nanosheets [32], thus the transmitted power of MF increases. Therefore, the transmitted power of the MF coated with can be actively controlled MoSe2 by the pump lights.
4. Conclusions
The active light control by light has been demonstrated experimentally by fabricating a MF coated with MoSe2. Two different pumped ways, out-fiber pumped with a 405 nm laser and in-fiber pumped with a 980 nm laser, have been studied, which proves the flexibility of our device in light control. In the 405 nm out-fiber pumped experimental, the transmitted power of MF with MoSe2 has a relative variations of ~2 dB and ~1.8 dB in the increase and decrease processes of the pumped power, ranging from 0 mW to 11.6 mW. In the in-fiber pumped experiment, however, the relative power variation of the MF is up to ~30 dB, when the adjustable 980 nm laser changes from 0 to 314.2 mW. The sensitivities of the 980 nm in-fiber and out-fiber experiments are 0.092 dB/mW and 0.851 dB/mW, respectively. Therefore, the MF coated with MoSe2 is a promise candidate in controllable functionality devices.
Funding
National Natural Science Foundation of China (61505069; 61705086; 61475066; 61405075; 61675092), Science and technology projects of Guangdong Province (2014B010120002), Natural Science Foundation of Guangdong Province (2016A030310098, 2014A030313377, 2014A030310205, 2015A030313320), Planned Science & Technology Project of Guangzhou under Grant (201607010134, 201506010046, 201506010046, 201605030002, 20160404005).
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