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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 10 — May. 9, 2011
  • pp: 9950–9955

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Large energy laser pulses with high repetition rate by graphene Q-switched solid-state laser

Xian-lei Li, Jin-long Xu, Yong-zhong Wu, Jing-liang He, and Xiao-peng Hao  »View Author Affiliations


Optics Express, Vol. 19, Issue 10, pp. 9950-9955 (2011)
http://dx.doi.org/10.1364/OE.19.009950


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Abstract

We demonstrated that the graphene could be used as an effective saturable absorber for Q-switched solid-state lasers. A graphene saturable absorber mirror was fabricated with large and high-quality graphene sheets deprived from the liquid phase exfoliation. Using this mirror, 105-ns pulses and 2.3-W average output power are obtained from a passively Q-switched Nd:GdVO4 laser. The maximum pulse energy is 3.2 μJ. The slope efficiency is as high as 37% approximating to 40% of the continue-wave laser, indicating a low intrinsic loss of the graphene.

© 2011 OSA

1. Introduction

Q-switching, also known as giant pulse formation, allows the production of light pulses with extremely high peak power, much higher than would be generated by the same laser if it were operating in a continuous wave mode. This technique finds its industry and science applications requiring high pulse energy, such as medicine, geochemistry and material processing. Previously, the passively Q-switched lasers with semiconductor saturable absorber mirrors (SESAMs) as Q-switching elements were actively reported [1

B. Braun, F. X. Kärtner, U. Keller, J. P. Meyn, and G. Huber, “Passively Q-switched 180-ps Nd:LaSc(3)(BO(3))(4) microchip laser,” Opt. Lett. 21(6), 405–407 (1996). [CrossRef] [PubMed]

–4

G. J. Spühler, S. Reffert, M. Haiml, M. Moser, and U. Keller, “Output-coupling semiconductor saturable absorber mirror,” Appl. Phys. Lett. 78(18), 2733 (2001). [CrossRef]

]. However, these SESAMs require complex fabrication and packaging which limit their widespread use [5

U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]

]. So it is crucial to search new saturable absorber materials with low cost, broad absorption band, and low intrinsic loss.

Recent progress reveals that graphene can be used as a modulation element in pulsed laser. Graphene enjoys clear advantages over conventional semiconductor saturable absorbers in ultrafast photonics such as the ultrafast carrier dynamics [6

P. A. George, J. Strait, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Ultrafast optical-pump terahertz-probe spectroscopy of the carrier relaxation and recombination dynamics in epitaxial graphene,” Nano Lett. 8(12), 4248–4251 (2008). [CrossRef]

,7

J. M. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92(4), 042116 (2008). [CrossRef]

], large optical absorption, and modulation depth [8

Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

,9

R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008). [CrossRef] [PubMed]

]. The modulation depth is as higher as 66.5% for three layers graphene sheets, and almost linearly drops with the increase of the layers [8

Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

]. The large modulation depth is favorable for short pulses [10

J. J. Zayhowski and P. L. Kelley, “Optimization of Q-switched lasers,” IEEE J. Quantum Electron. 27(9), 2220–2225 (1991). [CrossRef]

]. And the controllable modulation depth allows one to adjust the pulse duration. Previous work has proved that graphene is an excellent saturable absorber in mode-locked fiber lasers and solid-state lasers [8

Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

,11

Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]

–15

W. D. Tan, C. Y. Su, R. J. Knize, G. Q. Xie, L. J. Li, and D. Y. Tang, “Mode locking of ceramic Nd:yttrium aluminum garnet with graphene as a saturable absorber,” Appl. Phys. Lett. 96(3), 031106 (2010). [CrossRef]

]. Very recently, graphene Q-switching has also been reported. Yu et al obtained 159.2-nJ single pulse energy and 161-ns pulse duration from a Nd:YAG laser Q-switched by graphene grown on silicon carbide [16

H. H. Yu, X. F. Chen, H. J. Zhang, X. G. Xu, X. B. Hu, Z. P. Wang, J. Y. Wang, S. D. Zhuang, and M. H. Jiang, “Large energy pulse generation modulated by graphene epitaxially grown on silicon carbide,” ACS Nano 4(12), 7582–7586 (2010). [CrossRef] [PubMed]

]. Popa et al. demonstrated the performance of the graphene Q-switched fiber laser with a single pulse energy of 40 nJ at 1.5 µm [17

D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, and A. C. Ferrari, “Graphene Q-switched, tunable fiber laser,” Appl. Phys. Lett. 98(7), 073106 (2011). [CrossRef]

]. Here, we report on the application of a graphene-based saturable absorber mirror (SAM) in diode-pumped passively Q-switched Nd:GdVO4 laser. 3.2-μJ pulse energy and 105-ns pulse duration are obtained with a stable Q-switching operation.

2. Preparation and characterization of graphene

In order to obtain graphene sheets with size of tens of microns, we pretreated worm-like exfoliated graphite (WEG) with oxidant before exfoliating. Exfoliated graphite was pre-oxidized in a mixture of concentrated sulphuric acid, potassium peroxodisulfate, phosphorus oxide (P2O5) at 90 °C under stirring. On the completion of 4 hours, the mixture was poured to a large beaker containing excessive de-ionized water, followed by filtration and washing until the pH of the filtrate was close to neutral. The as-obtained graphite was dried at 80 °C for 24 hours. The dried graphite was ultrasonicated in 1-methyl-2-pyrrolidinone (NMP) in a sealed glass vial for 2 hours. The resulting dispersion was left for 3 days to precipitate out any insoluble particles. The supernatant solution was collected for characterization. The scanning electron microscope (SEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the product. Graphene sheets with lateral size over 20 µm can be clearly seen in Fig. 1(a) and 1(b). The selected area electron diffraction (SEAD) pattern in Fig. 1(c), shows the typical six-fold symmetry expected for graphite/graphene. The intensity of the pattern also suggests the area is a monolayer graphene due to the fact that the intensity ratio of I{1100}/I{2110} > 1 is a unique feature for monolayer graphene [18

J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, D. Obergfell, S. Roth, C. Girit, and A. Zettl, “On the roughness of single-and bi-layer graphene membranes,” Solid State Commun. 143(1–2), 101–109 (2007). [CrossRef]

]. The edge-on image of graphene in Fig. 1(d) indicates an intergraphene spacing of 0.34 nm.

Fig. 1 (a) SEM images of graphene sheets. (b) HRTEM images of graphene sheets. (c) SEAD pattern shows the six-fold rotational symmetry (d) HRTEM image of graphene edge where fringes are observed and interlaminar spacing is 0.34-nm.

3. Results and discussion

The graphene sheets were directly spin-coated onto a plane BK7 glass reflector coated with SiO2/TiO2 dielectrical layers, which had a reflectivity of ~95% with a broad band as in Fig. 2(a) . The transmission of the graphene SAM is measured at different locations. The curves of maximum and minimum values are given in Fig. 2(a), respectively. The transmission of the graphene SAM can be described as
T= To (1−α)n
where To, α, and n are the initial transmission of the substrate, the absorption of the monolayer graphene, and the number of the coated graphene layers, respectively. The measured transmission is between ~95.2% and 96.1% at 1063 nm. Thus it can be concluded that the layers of the coated graphene range from 2 to 10.

Fig. 2 (a) Transmissivity spectra of the BK7 substrate and graphene SAM. (b) Experimental setup of the Q-switched laser. (c) Average output power versus incident pump power for continuous-wave and Q-switching (Q-S) operation. (d) Pulse width and repetition rate versus incident pump power for Q-switching operation.

The schematic arrangement of the Q-switched laser is shown in Fig. 2(b). A 17-mm-long two-mirror resonator was used to evaluate the performance of the graphene SAM. The gain medium was a 3 × 3 × 5 mm3 a-cut Nd:GdVO4 with the Nd3+ doping level of 0.5 at.%. To remove the stored heat, we wrapped the crystal with indium foil and mounted it in a copper heat sink with the temperature kept at 21°C by water cooling. The crystal was end-pumped by a fiber-coupled laser diode array emitting at 808 nm with 400 μm in diameter and 0.22 in numerical aperture. The input coupler was a concave mirror with a curvature radius of 200 mm. It was antireflection coated at 808 nm and high-reflection coated at 1063 nm.

Initially, we investigated the performance of the continuous-wave (CW) Nd:GdVO4 laser with a BK7 reflector (the same as the substrate of the graphene SAM) as output coupler. The laser operation was realized at the threshold pump power of 0.18 W. The output power is plotted in Fig. 2(c) as a function of the incident pump power (P in). 2.5-W output power was obtained under the incident pump power of 6.5 W, resulting in an optical-to-optical efficiency of 38% and a slope efficiency of 40%. No self Q-switching was observed during the experiment. The laser emission centered at 1063 nm with a full width at half maximum (FWHM) of ~0.8 nm. These results revealed the good laser properties of our Nd:GdVO4.

When the graphene SAM was substituted for the BK7 reflector as shown in Fig. 2(b), the pulsed laser oscillation was achieved as soon as the incident pump power exceeded the threshold of 0.22 W. The relationship between the average output power and incident pump power is plotted in Fig. 2(c). It can be seen the average output power increases linearly with the incident pump power. No pump saturation was observed even if the incident pump power increased to 6.5 W. Under this incident pump power, an average output power of 2.3 W was obtained, slightly lower than that under continuous-wave condition by a factor of 8%. The corresponding optical-to-optical and slope efficiencies were 35% and 37%, respectively. Such a good performance means that the intrinsic loss of the graphene is at a very low level. The pulse width (τ) and repetition rate (f) depending on the incident pump power were recorded by a digital oscilloscope and presented in Fig. 2(d). The figure shows a rapid drop from 1435 ns to a minimum data of 105 ns in pulse width with the increase of the pump power from threshold to 6.5 W, while an increase in repetition rate from 305 to 704 kHz was observed. The high repetition rate may be due to the ultrafast relaxation time of graphene (0.4~1.7 ps [7

J. M. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92(4), 042116 (2008). [CrossRef]

]) and the relatively large stimulated emission cross section of Nd:GdVO4. [19

T. Jensen, V. G. Ostroumov, J. P. Meyn, G. Huber, A. I. Zagumennyi, and I. A. Shcherbakov, “Spectroscopic characterization and laser performance of diode-laser-pumped Nd:GdVO4 ,” Appl. Phys. B 58(5), 373–379 (1994). [CrossRef]

]. According to the average output power and pulse repetition rate, the maximum single pulse energy of 3.2 μJ was realized under the incident pump power of 5.3 W. However, it should be pointed out the pulse width and repetition rate in Fig. 2(d) under the incident pump power below 2.9 W are the approximate average value, because in this pump region the Q-switched operation was far from stable (the pulse trains under the pump power of 0.9 W is presented in Fig. 3(a) as an example). This is reasonable, considering that the graphene could not be fully saturated under low intracavity power. The fluctuation of the measurements was within ~20% of the average value. The Q-switching operation turned to a stable regime under an incident pump power level higher than 2.9 W (such as Fig. 3(b) recorded at the pump power of 3.2 W), corresponding to an intracavity intensity of ~0.926 MWcm−2 on the graphene sheets, which was close to the saturation intensity of 0.87 MWcm−2 reported in Ref. [8

Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

,12

H. Zhang, Q. L. Bao, D. Y. Tang, L. M. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett. 95(14), 141103 (2009). [CrossRef]

]. The temporal pulse trains and single pulse profile with repetition rate of 704 kHz and pulse duration of 105 ns were obtained under the output power of 2.3 W, as depicted in Fig. 3(c) and Fig. 3(d). The beam quality was found to near the diffraction limit through the experiment. With a commercial beam quality analyzer, the radial and tangential M2 were measured to be 1.16 and 1.18 under the maximum output power of 2.3 W. The emission wavelength of the Q-switched laser still centered at 1063 nm, but the FHWM was 1.0 nm which was a little broader than 0.8 nm of previous continuous-wave laser. This can be attributed to two reasons. One is the spontaneous transition of the large accumulated inversion population to the lower sub levels of excited level. When graphene is saturated, the transition from the lower sub levels to the ground level would emit photons at long wavelength. The other is the extremely large normal dispersion of graphene [8

Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

].

Fig. 3 Q-switched pulse train under the incident pump power of 0.9 W (a), under the incident pump power of 3.2 W (b), and under the incident pump power of 6.5 W (c). (d) 105-ns Q-switched pulse profile under the incident pump power of 6.5 W.

For Q-switched lasing with a graphene SAM, the modulation depth related to the number of the graphene layers plays an important role in the pulse duration. A high modulation depth can shorten the pulse duration. In addition, low output transmittivity is usually beneficial to the energy storage and low laser threshold. But a high output transmittivity is favorable for high power laser from the viewpoint of reducing the intracavity fluence to avoid optical damage and resist multiple pulses. Thus future design of graphene SAM for the generation of high energy Q-switched pulses should be focused on the optimization of the layer number of graphene and the SAM transmittivity.

4. Conclusion

In this article, the efficient performance of the graphene SAM on the Q-switched solid-state lasers has been demonstrated. 2.3 W of average output power and 3.2 μJ of pulse energy are obtained. Our results show that graphene can be applied to generate high-energy stable pulses at a repetition rate in the tens to hundreds kHz range.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 11074148, 50823009, and 51021062) and National Basic Research Program of China (2009CB930503), the Key Project of Chinese Ministry of Education (No. 109096).

References and links

1.

B. Braun, F. X. Kärtner, U. Keller, J. P. Meyn, and G. Huber, “Passively Q-switched 180-ps Nd:LaSc(3)(BO(3))(4) microchip laser,” Opt. Lett. 21(6), 405–407 (1996). [CrossRef] [PubMed]

2.

B. Braun, F. X. Kärtner, G. Zhang, M. Moser, and U. Keller, “56-ps passively Q-switched diode-pumped microchip laser,” Opt. Lett. 22(6), 381–383 (1997). [CrossRef] [PubMed]

3.

R. Fluck, R. Häring, R. Paschotta, E. Gini, H. Melchior, and U. Keller, “Eyesafe pulsed microchip laser using semiconductor saturable absorber mirrors,” Appl. Phys. Lett. 72(25), 3273 (1998). [CrossRef]

4.

G. J. Spühler, S. Reffert, M. Haiml, M. Moser, and U. Keller, “Output-coupling semiconductor saturable absorber mirror,” Appl. Phys. Lett. 78(18), 2733 (2001). [CrossRef]

5.

U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]

6.

P. A. George, J. Strait, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Ultrafast optical-pump terahertz-probe spectroscopy of the carrier relaxation and recombination dynamics in epitaxial graphene,” Nano Lett. 8(12), 4248–4251 (2008). [CrossRef]

7.

J. M. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92(4), 042116 (2008). [CrossRef]

8.

Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]

9.

R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008). [CrossRef] [PubMed]

10.

J. J. Zayhowski and P. L. Kelley, “Optimization of Q-switched lasers,” IEEE J. Quantum Electron. 27(9), 2220–2225 (1991). [CrossRef]

11.

Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]

12.

H. Zhang, Q. L. Bao, D. Y. Tang, L. M. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett. 95(14), 141103 (2009). [CrossRef]

13.

Y. W. Song, S. Y. Jang, W. S. Han, and M. K. Bae, “Graphene mode-lockers for fiber lasers functioned with evanescent field interacton,” Appl. Phys. Lett. 96(5), 051122 (2010). [CrossRef]

14.

L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, Q. L. Bao, and K. P. Loh, “Dissipative soliton operation of an ytterbium-doped fiber laser mode locked with atomic multilayer graphene,” Opt. Lett. 35(21), 3622–3624 (2010). [CrossRef] [PubMed]

15.

W. D. Tan, C. Y. Su, R. J. Knize, G. Q. Xie, L. J. Li, and D. Y. Tang, “Mode locking of ceramic Nd:yttrium aluminum garnet with graphene as a saturable absorber,” Appl. Phys. Lett. 96(3), 031106 (2010). [CrossRef]

16.

H. H. Yu, X. F. Chen, H. J. Zhang, X. G. Xu, X. B. Hu, Z. P. Wang, J. Y. Wang, S. D. Zhuang, and M. H. Jiang, “Large energy pulse generation modulated by graphene epitaxially grown on silicon carbide,” ACS Nano 4(12), 7582–7586 (2010). [CrossRef] [PubMed]

17.

D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, and A. C. Ferrari, “Graphene Q-switched, tunable fiber laser,” Appl. Phys. Lett. 98(7), 073106 (2011). [CrossRef]

18.

J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, D. Obergfell, S. Roth, C. Girit, and A. Zettl, “On the roughness of single-and bi-layer graphene membranes,” Solid State Commun. 143(1–2), 101–109 (2007). [CrossRef]

19.

T. Jensen, V. G. Ostroumov, J. P. Meyn, G. Huber, A. I. Zagumennyi, and I. A. Shcherbakov, “Spectroscopic characterization and laser performance of diode-laser-pumped Nd:GdVO4 ,” Appl. Phys. B 58(5), 373–379 (1994). [CrossRef]

OCIS Codes
(140.3540) Lasers and laser optics : Lasers, Q-switched
(140.3580) Lasers and laser optics : Lasers, solid-state
(160.4330) Materials : Nonlinear optical materials
(230.4170) Optical devices : Multilayers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 28, 2011
Revised Manuscript: April 29, 2011
Manuscript Accepted: May 4, 2011
Published: May 5, 2011

Citation
Xian-lei Li, Jin-long Xu, Yong-zhong Wu, Jing-liang He, and Xiao-peng Hao, "Large energy laser pulses with high repetition rate by graphene Q-switched solid-state laser," Opt. Express 19, 9950-9955 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-9950


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References

  1. B. Braun, F. X. Kärtner, U. Keller, J. P. Meyn, and G. Huber, “Passively Q-switched 180-ps Nd:LaSc(3)(BO(3))(4) microchip laser,” Opt. Lett. 21(6), 405–407 (1996). [CrossRef] [PubMed]
  2. B. Braun, F. X. Kärtner, G. Zhang, M. Moser, and U. Keller, “56-ps passively Q-switched diode-pumped microchip laser,” Opt. Lett. 22(6), 381–383 (1997). [CrossRef] [PubMed]
  3. R. Fluck, R. Häring, R. Paschotta, E. Gini, H. Melchior, and U. Keller, “Eyesafe pulsed microchip laser using semiconductor saturable absorber mirrors,” Appl. Phys. Lett. 72(25), 3273 (1998). [CrossRef]
  4. G. J. Spühler, S. Reffert, M. Haiml, M. Moser, and U. Keller, “Output-coupling semiconductor saturable absorber mirror,” Appl. Phys. Lett. 78(18), 2733 (2001). [CrossRef]
  5. U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]
  6. P. A. George, J. Strait, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Ultrafast optical-pump terahertz-probe spectroscopy of the carrier relaxation and recombination dynamics in epitaxial graphene,” Nano Lett. 8(12), 4248–4251 (2008). [CrossRef]
  7. J. M. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. G. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92(4), 042116 (2008). [CrossRef]
  8. Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
  9. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008). [CrossRef] [PubMed]
  10. J. J. Zayhowski and P. L. Kelley, “Optimization of Q-switched lasers,” IEEE J. Quantum Electron. 27(9), 2220–2225 (1991). [CrossRef]
  11. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, and A. C. Ferrari, “Graphene mode-locked ultrafast laser,” ACS Nano 4(2), 803–810 (2010). [CrossRef] [PubMed]
  12. H. Zhang, Q. L. Bao, D. Y. Tang, L. M. Zhao, and K. P. Loh, “Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker,” Appl. Phys. Lett. 95(14), 141103 (2009). [CrossRef]
  13. Y. W. Song, S. Y. Jang, W. S. Han, and M. K. Bae, “Graphene mode-lockers for fiber lasers functioned with evanescent field interacton,” Appl. Phys. Lett. 96(5), 051122 (2010). [CrossRef]
  14. L. M. Zhao, D. Y. Tang, H. Zhang, X. Wu, Q. L. Bao, and K. P. Loh, “Dissipative soliton operation of an ytterbium-doped fiber laser mode locked with atomic multilayer graphene,” Opt. Lett. 35(21), 3622–3624 (2010). [CrossRef] [PubMed]
  15. W. D. Tan, C. Y. Su, R. J. Knize, G. Q. Xie, L. J. Li, and D. Y. Tang, “Mode locking of ceramic Nd:yttrium aluminum garnet with graphene as a saturable absorber,” Appl. Phys. Lett. 96(3), 031106 (2010). [CrossRef]
  16. H. H. Yu, X. F. Chen, H. J. Zhang, X. G. Xu, X. B. Hu, Z. P. Wang, J. Y. Wang, S. D. Zhuang, and M. H. Jiang, “Large energy pulse generation modulated by graphene epitaxially grown on silicon carbide,” ACS Nano 4(12), 7582–7586 (2010). [CrossRef] [PubMed]
  17. D. Popa, Z. Sun, T. Hasan, F. Torrisi, F. Wang, and A. C. Ferrari, “Graphene Q-switched, tunable fiber laser,” Appl. Phys. Lett. 98(7), 073106 (2011). [CrossRef]
  18. J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, D. Obergfell, S. Roth, C. Girit, and A. Zettl, “On the roughness of single-and bi-layer graphene membranes,” Solid State Commun. 143(1–2), 101–109 (2007). [CrossRef]
  19. T. Jensen, V. G. Ostroumov, J. P. Meyn, G. Huber, A. I. Zagumennyi, and I. A. Shcherbakov, “Spectroscopic characterization and laser performance of diode-laser-pumped Nd:GdVO4,” Appl. Phys. B 58(5), 373–379 (1994). [CrossRef]

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