- Email: [email protected]
Q-switched Erbium-doped fiber laser using MoSe2 as saturable absorber
Optics & Laser Technology 79 (2016) 20–23
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Q-switched Erbium-doped fiber laser using MoSe2 as saturable absorber H. Ahmad a,n, M. Suthaskumar a, Z.C. Tiu a, A. Zarei a, S.W. Harun b a b
Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 23 October 2015 Accepted 5 November 2015 Available online 21 November 2015
A Q-switched Erbium-doped fiber laser by using MoSe2 thin film as saturable absorber is experimentally demonstrated. The bulk MoSe2 is processed into few layer MoSe2 based on liquid phase exfoliation technique, and further fabricated into thin film by using polyvinyl alcohol polymer. Q-switching operation is obtained from pump power range of 22.4–102.0 mW. The pulse repetition rate shows an increasing trend from 16.9 kHz to 32.8 kHz, whereas the pulse width exhibits a decreasing trend from 59.1 μs to 30.4 μs. The highest pulse energy of 57.9 nJ is obtained at pump power of 102.0 mW. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Q-switched Ultrafast laser
1. Introduction Two-dimensional (2D) materials exhibit great nonlinear optical (NLO) responses that have attracted intense interest in photonic field. Graphene, which is the first 2D nano-material to be discovered, has shown impressive NLO [1] and widely used as a saturable absorber (SA) to generate pulsed laser [2–7]. The success of graphene has broadened the study to different types of 2D materials. In the post graphene era, transition metal dichalcogenides (TMDs) materials have shown great potential as next generation 2D materials [8,9]. In general, TMDs exhibit formula of MX2, where M refers to transition metals (e.g. Tungsten (W) and Molybdenum (Mo)) while X refers to chalcogen atoms (e.g. Sulfur (S) and Selenium (Se)). TMDs attract considerable attention as future optical materials because they exhibit layer-dependent optical properties [10]. For instance, TMDs can transform from indirect bandgap to direct bandgap at near-infrared wavelengths when changing from bulk TMDs to monolayer TMDs. Moreover, TMDs also exhibit other important optical properties such as high nonlinearity, great photoluminescence, ultrafast carrier dynamics and strong optical absorption [11,12]. Sulfide-based TMDs and selenide-based TMDs possess similar characteristics. The main advantage of selenide-based TMDs over sulfide-based TMDs is that selenide-based TMDs have heavier chalcogenide atoms that lead to reduced bandgap energies [13]. To date, most of the pulsed laser generations with TMD materials as n
Corresponding author. E-mail address: [email protected] (H. Ahmad).
http://dx.doi.org/10.1016/j.optlastec.2015.11.007 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
saturable absorber are sulfide-based whereas the exploration of selenide-based TMDs as saturable absorber is yet to be fully explored. In this work, we have experimentally investigated one of the selenide-based TMDs, MoSe2 for short-pulse generation. The fabrication from bulk MoSe2 to few layer MoSe2–polyvinyl alcohol (PVA) thin film is reported. Furthermore, the MoSe2–PVA composite film is used as saturable absorber to generate Q-switching operation in Erbium-doped fiber laser.
2. MoSe2 thin film fabrication The few layer MoSe2 used in this experiment were prepared with liquid phase exfoliation (LPE) method [14]. In brief, the N-methyl-2-Pyrrolidine (NMP) solvent for the exfoliation of TMDs is mixed with bulk powder with an initial concentration of 5 mg/ ml. The solution is treated with high power ultrasonicator for 8 h. The suspension is centrifuged at 3000 rpm for 60 min and the top 2/3 supernatant solution is pipetted out for further characterization. The obtained supernatant is diluted to 10 vol% and the linear absorption characterization is carried out using SPECORD 210-Plus UV–vis Spectrophotometer. As seen from Fig. 1, the two observed peaks at 697 nm and 800 nm match perfectly with the previously reported values [15]. These two bands correspond to the interband excitonic transitions at the K point which indicates a pristine 2H poly type. Next, the crystalline nature of both the bulk and exfoliated MoSe2 were characterized with X-Ray Diffraction (XRD) analysis using Bruker D8 Advanced equipment at an excitation wavelength of 1.5406 Å. As seen from Fig. 2, all the labeled peaks of the bulk MoSe2 are indexed to rhombohedral MoSe2
H. Ahmad et al. / Optics & Laser Technology 79 (2016) 20–23
21
Fig. 1. The linear optical absorption spectrum measured by UV–vis Spectrophotometer after diluting the sample to 10 vol%.
Fig. 3. The Raman spectroscopy characterization of bulk and few-layer MoSe2.
Fig. 2. The XRD pattern of the bulk and few layer MoSe2.
(JCPDS no: 06-0097). The few-layer MoSe2 showed a high [0 0 2] orientation and disappearance of some characteristic peaks, which attests that the bulk MoS2 had been successfully exfoliated to fewlayer MoSe2. The few-layer solution were then drop casted onto silica wafers to conduct the Raman spectroscopy using Renishaw inVia confocal Raman Microscope at an excitation wavelength of 488 nm and 3.5 mW power. As depicted in Fig. 3, the out of plane vibration (Ag1 ) for bulk MoSe2 is centered at 240 cm 1, whereas the few layer MoSe2 is centered at 235 cm 1. This shows a peak shift for fewlayer MoSe2 as compared to the bulk and further confirms the exfoliation of few layers. Next, the few layer MoSe2 solution were processed to become thin film. The few layer MoSe2 solution were placed in a bath sonicator for 10 min. Then, 15 ml of the few layer MoSe2 solution were added with 200 mg of polyvinyl alcohol (PVA) dissolved in 15 ml of deionised (DI) water (concentration of 10 mg/ml). The 30 ml solution mixture was stirred using magnetic stirrer and heated continuously at a fixed temperature of 80 °C till the solution were reduced to approximately 10 ml. This process takes approximately 6 h to finish. This was followed by drying the
Fig. 4. Configuration of the proposed EDFL.
remaining solution on a glass substrate in an oven at 80 °C for another four hours to obtain the MoSe2 thin film.
3. Experimental arrangement The experimental set-up of the proposed EDFL is illustrated in Fig. 4. The ring resonator consists of a 3 m long Erbium-doped fiber (EDF) as the gain medium, a wavelength division multiplexer (WDM), an isolator, 95:5 output coupler (OC1) and 50:50 output coupler (OC2). The EDF used has a doping concentration of 2000 ppm and GVD parameter of about 21.64 (ps/nm)/km. This fiber was pumped by a 974 nm laser diode via the WDM. Other fibers in the cavity is a standard SMF (18 (ps/nm)/km), which constituted the rest of the ring. Unidirectional operation of the ring was achieved with the use of an isolator. The output of the laser is collected from the cavity via a 95:5 coupler and retains 95% of the light in the ring cavity to oscillate. The optical spectrum
22
H. Ahmad et al. / Optics & Laser Technology 79 (2016) 20–23
analyser (OSA) with a spectral resolution of 0.02 nm is used to analyze the spectrum of the proposed EDFL whereas the oscilloscope (OSC) is employed in conjunction with a 1.2 GHz bandwidth photodetector to capture the output pulse train of the Q-switched emission.
4. Results and discussion The oscillator started the Q-switching operation after reaching the threshold pump power of 22.4 mW. Operation in this regime occurred with pump power range up to 102.0 mW. Stable selfstarting Q-switched pulse trains against pump power were observed as shown in Fig. 5. As the pump power further increases beyond 102.0 mW, the pulse train becomes unstable and disappears. As shown in Fig. 5, the time interval between pulse reduces while the pulse amplitude increases as the pump power increases from 22.4 to 102.0 mW. Fig. 6 shows the optical spectrum of Q-switching operation at pump power of 102.0 mW. The operating wavelength centered at 1562.3 nm throughout the Q-switching operation. Besides, the optical spectrum becomes slightly broader in Q-switching operation as compared to CW operation. Fig. 7 shows the repetition rate and pulse width against pump power. The dependence of the pulse repetition rate can be seen to increase almost linearly against the pump power, whereas the pulse width decreases also almost linearly against the pump power. By launching higher pump power into the cavity, it fastens the gain population excitation process to achieve the saturation state. Therefore, higher pulse repetition rate with narrower pulse width is achieved. The trend of pulse repetition rate and pulse width agrees well with the passive Q-switching theory. The pulse repetition rate of the Q-switched EDFL can be widely tuned from 16.9 kHz to 32.8 kHz by varying the pump power from 22.4 mW to 102.0 mW. Besides, the pulse width reduces from 59.1 μs to 30.4 μs as the pump power increases in the range of Q-switching operation. Thus, the highest repetition rate of 32.8 kHz and shortest pulse width of 30.4 μs is obtained at pump power of 102.0 mW. It is observed that the pulse train becomes unstable and disappears as the pump power is increased above 102.0 mW which may be caused by the limitation of MoSe2 SA recovery time. On the other hand, the average output power is measured to calculate the corresponding single-pulse energy. Fig. 8 shows the relationship of average output power and pulse energy of the
Fig. 6. Optical spectrum of the proposed Q-switched EDFL when the pump is fixed at 102.0 mW.
Fig. 7. Pulse repetition rate and pulse width of the proposed Q-switched EDFL against the pump power.
Fig. 8. Output power and pulse energy of the proposed Q-switched EDFL against the pump power.
Fig. 5. Q-switched pulse evolution of the proposed Q-switched EDFL against pump power.
Q-switched EDFL against pump power. As shown in the figure, average output power increases almost linearly from 0.28 mW to 1.9 W as the pump power increases from 22.4 mW to 102.0 mW. From the gradient of output power against pump power, the Q-switched EDFL efficiency is obtained to be around 5%. Furthermore, the pulse energy exhibited increasing trend from 16.6 nJ to 57.9 nJ in the range of Q-switching operation. The pulse energy started to reach saturation state at pump power of 88.6 mW as shown in Fig. 8. This indicated that the film MoSe2 reached the damage threshold of Q-switching operation. When the pump power higher than 102.0 mW, the film MoSe2 not able to fully recover. Therefore, the film MoSe2 not able to perform the saturable absorption and become transparent in the cavity.
H. Ahmad et al. / Optics & Laser Technology 79 (2016) 20–23
23
5. Conclusion
References
Q-switched EDFL by using MoSe2 thin film as saturable absorber is experimentally demonstrated. The bulk MoSe2 is processed to be few layer MoSe2 based on LPE technique, and further fabricated into thin film by using PVA polymer. Q-switching operation is obtained from pump power range of 22.4–102.0 mW. The pulse repetition rate shows an increasing trend from 16.9 kHz to 32.8 kHz, whereas the pulse width exhibits a decreasing trend from 59.1 μs to 30.4 μs. The highest pulse energy of 57.9 nJ is obtained at pump power of 102.0 mW.
[1] Euan Hendry, J. Peter, J. Moger, A.K. Savechenko, S.A. Mikhailov, Phys. Rev. Lett. 11 (9) (2010) 097401. [2] Z.C. Tiu, F. Ahmad, S.J. Tan, H. Ahmad, S.W. Harun, Indian J. Phys. 88 (7) (2014) 727. [3] Z.P. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F.Q. Wang, F. Bonaccorso, D. M. Basko, A.C. Ferrari, ACS Nano 4 (2) (2010) 803. [4] Q.L. Bao, H. Zhang, Y. Wang, Z.H. Ni, Y.L. Yan, Z.X. Shen, K.P. Loh, D.Y. Tang, Adv. Funct. Mater. 19 (19) (2009) 3077. [5] J.L. Xu, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Opt. Lett. 36 (10) (2011) 1948. [6] I.H. Baek, H.W. Lee, S. Bae, B.H. Hong, Y.H. Ahn, D. Yeom, F. Rotermund, Appl. Phys. Express 5 (3) (2012) 032701. [7] Z.C. Tiu, F. Ahmad, S.J. Tan, H. Ahmad, S.W. Harun, Optik 126 (17) (2015) 1495. [8] Manish Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, Nat. Chem. 5 (4) (2013) 263. [9] Q.H. Wang, K.K. Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Nat. Nanotechnol. 7 (11) (2012) 699. [10] S.X. Wang, H.H. Yu, H.J. Zhang, A.Z. Wang, W.,M.W. Zhao, Y.X. Chen, L.M. Mei, J. Y. Wang, Adv. Mater. 26 (21) (2014) 3538. [11] J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (73) (1969) 193. [12] X. Huang, Z.Y. Zeng, H. Zhang, Chem. Soc. Rev. 42 (5) (2013) 1934. [13] K.G. Zhou, H.L. Zhang, Small 11 (6) (2015) 694. [14] Coleman, et al., Science 6017 (2011) 568. [15] R.I. Woodward, R.C.T. Howe, T.H. Runcorn, G. Hu, F. Torrisi, E.J.R. Kelleher, T. Hassan, arXiv preprint arXiv 1503 2015 08003.
Acknownledgement We would like to acknowledge the generous funding from University of Malaya through the Grant RU007/2015 and LRGS (2015)/NGOD/UM/KPT.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Q-switched Erbium-doped fiber laser using MoSe2 as saturable absorber H. Ahmad a,n, M. Suthaskumar a, Z.C. Tiu a, A. Zarei a, S.W. Harun b a b
Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 22 September 2015 Received in revised form 23 October 2015 Accepted 5 November 2015 Available online 21 November 2015
A Q-switched Erbium-doped fiber laser by using MoSe2 thin film as saturable absorber is experimentally demonstrated. The bulk MoSe2 is processed into few layer MoSe2 based on liquid phase exfoliation technique, and further fabricated into thin film by using polyvinyl alcohol polymer. Q-switching operation is obtained from pump power range of 22.4–102.0 mW. The pulse repetition rate shows an increasing trend from 16.9 kHz to 32.8 kHz, whereas the pulse width exhibits a decreasing trend from 59.1 μs to 30.4 μs. The highest pulse energy of 57.9 nJ is obtained at pump power of 102.0 mW. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Q-switched Ultrafast laser
1. Introduction Two-dimensional (2D) materials exhibit great nonlinear optical (NLO) responses that have attracted intense interest in photonic field. Graphene, which is the first 2D nano-material to be discovered, has shown impressive NLO [1] and widely used as a saturable absorber (SA) to generate pulsed laser [2–7]. The success of graphene has broadened the study to different types of 2D materials. In the post graphene era, transition metal dichalcogenides (TMDs) materials have shown great potential as next generation 2D materials [8,9]. In general, TMDs exhibit formula of MX2, where M refers to transition metals (e.g. Tungsten (W) and Molybdenum (Mo)) while X refers to chalcogen atoms (e.g. Sulfur (S) and Selenium (Se)). TMDs attract considerable attention as future optical materials because they exhibit layer-dependent optical properties [10]. For instance, TMDs can transform from indirect bandgap to direct bandgap at near-infrared wavelengths when changing from bulk TMDs to monolayer TMDs. Moreover, TMDs also exhibit other important optical properties such as high nonlinearity, great photoluminescence, ultrafast carrier dynamics and strong optical absorption [11,12]. Sulfide-based TMDs and selenide-based TMDs possess similar characteristics. The main advantage of selenide-based TMDs over sulfide-based TMDs is that selenide-based TMDs have heavier chalcogenide atoms that lead to reduced bandgap energies [13]. To date, most of the pulsed laser generations with TMD materials as n
Corresponding author. E-mail address: [email protected] (H. Ahmad).
http://dx.doi.org/10.1016/j.optlastec.2015.11.007 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
saturable absorber are sulfide-based whereas the exploration of selenide-based TMDs as saturable absorber is yet to be fully explored. In this work, we have experimentally investigated one of the selenide-based TMDs, MoSe2 for short-pulse generation. The fabrication from bulk MoSe2 to few layer MoSe2–polyvinyl alcohol (PVA) thin film is reported. Furthermore, the MoSe2–PVA composite film is used as saturable absorber to generate Q-switching operation in Erbium-doped fiber laser.
2. MoSe2 thin film fabrication The few layer MoSe2 used in this experiment were prepared with liquid phase exfoliation (LPE) method [14]. In brief, the N-methyl-2-Pyrrolidine (NMP) solvent for the exfoliation of TMDs is mixed with bulk powder with an initial concentration of 5 mg/ ml. The solution is treated with high power ultrasonicator for 8 h. The suspension is centrifuged at 3000 rpm for 60 min and the top 2/3 supernatant solution is pipetted out for further characterization. The obtained supernatant is diluted to 10 vol% and the linear absorption characterization is carried out using SPECORD 210-Plus UV–vis Spectrophotometer. As seen from Fig. 1, the two observed peaks at 697 nm and 800 nm match perfectly with the previously reported values [15]. These two bands correspond to the interband excitonic transitions at the K point which indicates a pristine 2H poly type. Next, the crystalline nature of both the bulk and exfoliated MoSe2 were characterized with X-Ray Diffraction (XRD) analysis using Bruker D8 Advanced equipment at an excitation wavelength of 1.5406 Å. As seen from Fig. 2, all the labeled peaks of the bulk MoSe2 are indexed to rhombohedral MoSe2
H. Ahmad et al. / Optics & Laser Technology 79 (2016) 20–23
21
Fig. 1. The linear optical absorption spectrum measured by UV–vis Spectrophotometer after diluting the sample to 10 vol%.
Fig. 3. The Raman spectroscopy characterization of bulk and few-layer MoSe2.
Fig. 2. The XRD pattern of the bulk and few layer MoSe2.
(JCPDS no: 06-0097). The few-layer MoSe2 showed a high [0 0 2] orientation and disappearance of some characteristic peaks, which attests that the bulk MoS2 had been successfully exfoliated to fewlayer MoSe2. The few-layer solution were then drop casted onto silica wafers to conduct the Raman spectroscopy using Renishaw inVia confocal Raman Microscope at an excitation wavelength of 488 nm and 3.5 mW power. As depicted in Fig. 3, the out of plane vibration (Ag1 ) for bulk MoSe2 is centered at 240 cm 1, whereas the few layer MoSe2 is centered at 235 cm 1. This shows a peak shift for fewlayer MoSe2 as compared to the bulk and further confirms the exfoliation of few layers. Next, the few layer MoSe2 solution were processed to become thin film. The few layer MoSe2 solution were placed in a bath sonicator for 10 min. Then, 15 ml of the few layer MoSe2 solution were added with 200 mg of polyvinyl alcohol (PVA) dissolved in 15 ml of deionised (DI) water (concentration of 10 mg/ml). The 30 ml solution mixture was stirred using magnetic stirrer and heated continuously at a fixed temperature of 80 °C till the solution were reduced to approximately 10 ml. This process takes approximately 6 h to finish. This was followed by drying the
Fig. 4. Configuration of the proposed EDFL.
remaining solution on a glass substrate in an oven at 80 °C for another four hours to obtain the MoSe2 thin film.
3. Experimental arrangement The experimental set-up of the proposed EDFL is illustrated in Fig. 4. The ring resonator consists of a 3 m long Erbium-doped fiber (EDF) as the gain medium, a wavelength division multiplexer (WDM), an isolator, 95:5 output coupler (OC1) and 50:50 output coupler (OC2). The EDF used has a doping concentration of 2000 ppm and GVD parameter of about 21.64 (ps/nm)/km. This fiber was pumped by a 974 nm laser diode via the WDM. Other fibers in the cavity is a standard SMF (18 (ps/nm)/km), which constituted the rest of the ring. Unidirectional operation of the ring was achieved with the use of an isolator. The output of the laser is collected from the cavity via a 95:5 coupler and retains 95% of the light in the ring cavity to oscillate. The optical spectrum
22
H. Ahmad et al. / Optics & Laser Technology 79 (2016) 20–23
analyser (OSA) with a spectral resolution of 0.02 nm is used to analyze the spectrum of the proposed EDFL whereas the oscilloscope (OSC) is employed in conjunction with a 1.2 GHz bandwidth photodetector to capture the output pulse train of the Q-switched emission.
4. Results and discussion The oscillator started the Q-switching operation after reaching the threshold pump power of 22.4 mW. Operation in this regime occurred with pump power range up to 102.0 mW. Stable selfstarting Q-switched pulse trains against pump power were observed as shown in Fig. 5. As the pump power further increases beyond 102.0 mW, the pulse train becomes unstable and disappears. As shown in Fig. 5, the time interval between pulse reduces while the pulse amplitude increases as the pump power increases from 22.4 to 102.0 mW. Fig. 6 shows the optical spectrum of Q-switching operation at pump power of 102.0 mW. The operating wavelength centered at 1562.3 nm throughout the Q-switching operation. Besides, the optical spectrum becomes slightly broader in Q-switching operation as compared to CW operation. Fig. 7 shows the repetition rate and pulse width against pump power. The dependence of the pulse repetition rate can be seen to increase almost linearly against the pump power, whereas the pulse width decreases also almost linearly against the pump power. By launching higher pump power into the cavity, it fastens the gain population excitation process to achieve the saturation state. Therefore, higher pulse repetition rate with narrower pulse width is achieved. The trend of pulse repetition rate and pulse width agrees well with the passive Q-switching theory. The pulse repetition rate of the Q-switched EDFL can be widely tuned from 16.9 kHz to 32.8 kHz by varying the pump power from 22.4 mW to 102.0 mW. Besides, the pulse width reduces from 59.1 μs to 30.4 μs as the pump power increases in the range of Q-switching operation. Thus, the highest repetition rate of 32.8 kHz and shortest pulse width of 30.4 μs is obtained at pump power of 102.0 mW. It is observed that the pulse train becomes unstable and disappears as the pump power is increased above 102.0 mW which may be caused by the limitation of MoSe2 SA recovery time. On the other hand, the average output power is measured to calculate the corresponding single-pulse energy. Fig. 8 shows the relationship of average output power and pulse energy of the
Fig. 6. Optical spectrum of the proposed Q-switched EDFL when the pump is fixed at 102.0 mW.
Fig. 7. Pulse repetition rate and pulse width of the proposed Q-switched EDFL against the pump power.
Fig. 8. Output power and pulse energy of the proposed Q-switched EDFL against the pump power.
Fig. 5. Q-switched pulse evolution of the proposed Q-switched EDFL against pump power.
Q-switched EDFL against pump power. As shown in the figure, average output power increases almost linearly from 0.28 mW to 1.9 W as the pump power increases from 22.4 mW to 102.0 mW. From the gradient of output power against pump power, the Q-switched EDFL efficiency is obtained to be around 5%. Furthermore, the pulse energy exhibited increasing trend from 16.6 nJ to 57.9 nJ in the range of Q-switching operation. The pulse energy started to reach saturation state at pump power of 88.6 mW as shown in Fig. 8. This indicated that the film MoSe2 reached the damage threshold of Q-switching operation. When the pump power higher than 102.0 mW, the film MoSe2 not able to fully recover. Therefore, the film MoSe2 not able to perform the saturable absorption and become transparent in the cavity.
H. Ahmad et al. / Optics & Laser Technology 79 (2016) 20–23
23
5. Conclusion
References
Q-switched EDFL by using MoSe2 thin film as saturable absorber is experimentally demonstrated. The bulk MoSe2 is processed to be few layer MoSe2 based on LPE technique, and further fabricated into thin film by using PVA polymer. Q-switching operation is obtained from pump power range of 22.4–102.0 mW. The pulse repetition rate shows an increasing trend from 16.9 kHz to 32.8 kHz, whereas the pulse width exhibits a decreasing trend from 59.1 μs to 30.4 μs. The highest pulse energy of 57.9 nJ is obtained at pump power of 102.0 mW.
[1] Euan Hendry, J. Peter, J. Moger, A.K. Savechenko, S.A. Mikhailov, Phys. Rev. Lett. 11 (9) (2010) 097401. [2] Z.C. Tiu, F. Ahmad, S.J. Tan, H. Ahmad, S.W. Harun, Indian J. Phys. 88 (7) (2014) 727. [3] Z.P. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F.Q. Wang, F. Bonaccorso, D. M. Basko, A.C. Ferrari, ACS Nano 4 (2) (2010) 803. [4] Q.L. Bao, H. Zhang, Y. Wang, Z.H. Ni, Y.L. Yan, Z.X. Shen, K.P. Loh, D.Y. Tang, Adv. Funct. Mater. 19 (19) (2009) 3077. [5] J.L. Xu, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Opt. Lett. 36 (10) (2011) 1948. [6] I.H. Baek, H.W. Lee, S. Bae, B.H. Hong, Y.H. Ahn, D. Yeom, F. Rotermund, Appl. Phys. Express 5 (3) (2012) 032701. [7] Z.C. Tiu, F. Ahmad, S.J. Tan, H. Ahmad, S.W. Harun, Optik 126 (17) (2015) 1495. [8] Manish Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, Nat. Chem. 5 (4) (2013) 263. [9] Q.H. Wang, K.K. Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Nat. Nanotechnol. 7 (11) (2012) 699. [10] S.X. Wang, H.H. Yu, H.J. Zhang, A.Z. Wang, W.,M.W. Zhao, Y.X. Chen, L.M. Mei, J. Y. Wang, Adv. Mater. 26 (21) (2014) 3538. [11] J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (73) (1969) 193. [12] X. Huang, Z.Y. Zeng, H. Zhang, Chem. Soc. Rev. 42 (5) (2013) 1934. [13] K.G. Zhou, H.L. Zhang, Small 11 (6) (2015) 694. [14] Coleman, et al., Science 6017 (2011) 568. [15] R.I. Woodward, R.C.T. Howe, T.H. Runcorn, G. Hu, F. Torrisi, E.J.R. Kelleher, T. Hassan, arXiv preprint arXiv 1503 2015 08003.
Acknownledgement We would like to acknowledge the generous funding from University of Malaya through the Grant RU007/2015 and LRGS (2015)/NGOD/UM/KPT.