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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 6 — Jun. 1, 2012
  • pp: 825–838
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Saturable absorption of multi-walled carbon nanotubes/hybrid-glass composites

Mariana Pokrass, Zeev Burshtein, Raz Gvishi, and Menachem Nathan  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 6, pp. 825-838 (2012)
http://dx.doi.org/10.1364/OME.2.000825


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Abstract

The saturated optical absorption of multi-walled carbon nanotube (MWNT)-doped hybrid-glass composites was studied at 1064 nm with nanosecond long pulses using the Z-scan technique. Increased transmission was demonstrated for high intensity pulses. The results were modeled for the limiting cases of a slow or a fast saturable absorber, both referring to a three-level system. The ground-state absorption cross section was estimated as 2.3x10−18 cm2, in good agreement with that of a single carbon atom. The excited state absorption cross section for carbon nanotubes (CNTs) was estimated for the first time as 6.9x10−19 cm3. The absorber's density of 2.8x1018 cm−3 is smaller by approximately a factor of 5 compared to the nominal density of carbon atoms incorporated into the glass. This could be a result of the broad band-like states formation in the MWNTs. The above figures were obtained by assuming that the excited state decay time was longer than the pulse duration of ~1 ns.

© 2012 OSA

1. Introduction

Carbon nanotubes (CNTs) have been widely explored for their nonlinear optical properties. In the visible, specifically at 532 and 800 nm, enhanced absorption and/or scattering were observed at high light intensities, in liquid suspensions [1

1. S. M. O’Flaherty, S. V. Hold, M. E. Brennan, M. Cadek, A. Drury, J. N. Coleman, and W. J. Blau, “Nonlinear optical response of multiwalled carbon-nanotube dispersions,” J. Opt. Soc. Am. B 20(1), 49–58 (2003). [CrossRef]

8

8. J. Wang and W. J. Blau, “Solvent effect on optical limiting properties of single-walled carbon nanotube dispersions,” J. Phys. Chem. C 112(7), 2298–2303 (2008). [CrossRef]

] as well as in solids [5

5. Z. Hongbing, C. Wenzhe, W. Minquan, Zhengchan, and Z. Chunlin, “Optical limiting effects of multi-walled carbon nanotubes suspension and silica xerogel composite,” Chem. Phys. Lett. 382(3-4), 313–317 (2003). [CrossRef]

,6

6. C. Zheng, W. Chen, X. Ye, S. Cai, X. Xiao, and M. Wang, “Preparation and optical limiting properties of carbon nanotubes coated with Au nanoparticle composites embedded in silica gel-glass,” Mater. Lett. 65(2), 150–152 (2011). [CrossRef]

,9

9. H. Xu, Y. Pan, Y. Zhu, H. Kou, and J. Guo, “Transparent multi-walled carbon nanotube-silica composite prepared by hot-pressed sintering and its nonlinear optical properties,” J. Alloy. Comp. 481(1-2), L4–L7 (2009). [CrossRef]

11

11. W. Feng, W. Yi, H. Wu, M. Ozaki, and K. Yoshino, “Enhancement of third-order optical nonlinearities by conjugated polymer-bonded carbon nanotubes,” J. Appl. Phys. 98(3), 034301 (2005). [CrossRef]

]. These effects were termed “optical limiting” by virtually all workers. In the near infrared (NIR), mainly saturable absorption was observed [12

12. H. I. Elim, W. Ji, G. H. Ma, K. Y. Lim, C. H. Sow, and C. H. A. Huan, “Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films,” Appl. Phys. Lett. 85(10), 1799–1801 (2004). [CrossRef]

22

22. Y. W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett. 92(2), 021115 (2008). [CrossRef]

], although several studies showed the occurrence of optical limiting at 1064 nm [2

2. L. Vivien, E. Anglaret, D. Riehl, F. Hache, F. Bacou, M. Andrieux, F. Lafonta, C. Journet, C. Goze, M. Brunet, and P. Bernier, “Optical limiting properties of singlewall carbon nanotubes,” Opt. Commun. 174(1-4), 271–275 (2000). [CrossRef]

5

5. Z. Hongbing, C. Wenzhe, W. Minquan, Zhengchan, and Z. Chunlin, “Optical limiting effects of multi-walled carbon nanotubes suspension and silica xerogel composite,” Chem. Phys. Lett. 382(3-4), 313–317 (2003). [CrossRef]

]. Saturable absorption was studied using single-walled carbon nanotubes (SWNTs), which exhibit discrete band transitions [19

19. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Ultrafast fiber pulsed lasers incorporating carbon nanotubes,” IEEE J. Sel. Top. Quantum Electron. 10(1), 137–146 (2004). [CrossRef]

,23

23. J. A. Fagan, J. R. Simpson, B. J. Bauer, S. H. De Paoli Lacerda, M. L. Becker, J. Chun, K. B. Migler, A. R. Hight Walker, and E. K. Hobbie, “Length-dependent optical effects in single-wall carbon nanotubes,” J. Am. Chem. Soc. 129(34), 10607–10612 (2007). [CrossRef] [PubMed]

25

25. D. J. Styers-Barnett, S. P. Ellison, C. Park, K. E. Wise, and J. M. Papanikolas, “Ultrafast dynamics of single-walled carbon nanotubes dispersed in polymer films,” J. Phys. Chem. A 109(2), 289–292 (2005). [CrossRef] [PubMed]

] and a controllable band-gap that depends on the tubes diameter [24

24. H. Kataura, Y. Kumazawa, Y. Maniwa, I. Umezu, S. Suzuki, Y. Ohtsuka, and Y. Achiba, “Optical properties of single-wall carbon nanotubes,” Synth. Met. 103(1-3), 2555–2558 (1999). [CrossRef]

]. Applications of the SWNTs for laser mode-locking [15

15. D. V. Khudyakov, A. S. Lobach, and V. A. Nadtochenko, “Nonlinear optical absorption of single-wall carbon nanotubes in carboxymethylcellulose thin polymerfilm and its application as a saturable absorber for mode-locking in pulsed Nd:glass laser,” Appl. Opt. 48(8), 1624–1627 (2009). [CrossRef] [PubMed]

17

17. H. R. Chen, Y. G. Wang, C. Y. Tsai, K. H. Lin, T. Y. Chang, J. Tang, and W. F. Hsieh, “High-power, passively mode-locked Nd:GdVO4 laser using single-walled carbon nanotubes as saturable absorber,” Opt. Lett. 36(7), 1284–1286 (2011). [CrossRef] [PubMed]

,19

19. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Ultrafast fiber pulsed lasers incorporating carbon nanotubes,” IEEE J. Sel. Top. Quantum Electron. 10(1), 137–146 (2004). [CrossRef]

21

21. Y. Sakakibara, A. G. Rozhin, H. Kataura, Y. Achiba, and M. Tokumoto, “Carbon nanotube-poly(vinylalcohol) nanocomposite film devices: applications for femtosecond fiber laser mode lockers and optical amplifier noise suppressors,” Jpn. J. Appl. Phys. 44(4A), 1621–1625 (2005). [CrossRef]

] noise suppression [18

18. S. Set, H. Yaguchi, M. Jablonski, Y. Tanaka, Y. Sakakibara, A. Rozhin, and M. Tohmata, “A noise suppressing saturable absorber at 1550 nm based on carbon nsnotube technology,” in Optical Fiber Communications Conference (2003), pp. 723–725.

,21

21. Y. Sakakibara, A. G. Rozhin, H. Kataura, Y. Achiba, and M. Tokumoto, “Carbon nanotube-poly(vinylalcohol) nanocomposite film devices: applications for femtosecond fiber laser mode lockers and optical amplifier noise suppressors,” Jpn. J. Appl. Phys. 44(4A), 1621–1625 (2005). [CrossRef]

] and pulse shaping [22

22. Y. W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett. 92(2), 021115 (2008). [CrossRef]

] were demonstrated utilizing their fast saturable absorption property.

Any CNT composite production requires dispersion of the CNT species within the hosting matrix. It has been established, that dispersion of multi-walled carbon nanotubes (MWNTs) is achieved more easily and effectively than SWNTs [26

26. M. Sennett, E. Welsh, J. B. Wright, W. Z. Li, J. G. Wen, and Z. F. Ren, “Dispersion and alignment of carbon nanotubes in polycarbonate,” Appl. Phys., A Mater. Sci. Process. 76(1), 111–113 (2003). [CrossRef]

28

28. S. Razdan, “Novel polyelectrolyte complex based carbon nanotube composite architectures,” PhD thesis (Rensselaer Polytechnic Institute, Troy, New-York, 2008).

]. Thus MWNTs are advantageous in terms of composite production ease, and not less importantly, in terms of price. However, the saturable absorption of MWNTs was studied only briefly [12

12. H. I. Elim, W. Ji, G. H. Ma, K. Y. Lim, C. H. Sow, and C. H. A. Huan, “Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films,” Appl. Phys. Lett. 85(10), 1799–1801 (2004). [CrossRef]

]. In this work we present a study of saturable absorption of MWNT/hybrid organic-inorganic sol-gel glass composites prepared by a Fast-Sol-Gel (FSG) method [29

29. R. Gvishi, “Fast sol–gel technology: from fabrication to applications,” J. Sol-Gel Sci. Technol. 50(2), 241–253 (2009). [CrossRef]

]. Analysis of the nonlinear properties was performed by studying two limiting cases: the slow saturable absorber, analyzed by the modified Frantz-Nodvik equation, and the fast saturable absorber, analyzed by the steady-state solution of Hercher's rate equations [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

]. The models allow estimation of ground-, and excited-state absorption cross sections, σgs and σes respectively, the absorber's effective density N, and the excited state decay time τ.

The matrix in which the MWNTs were incorporated is a silica-based optically transparent material prepared by the FSG route. The process facilitates and shortens the production of organic-inorganic hybrid glassy materials. These hybrid glasses exhibit tunable properties that vary between those of a silicon rubber and those of a silica glass [29

29. R. Gvishi, “Fast sol–gel technology: from fabrication to applications,” J. Sol-Gel Sci. Technol. 50(2), 241–253 (2009). [CrossRef]

]. A variety of optical properties of the glasses, such as refractive index, thermo-optic coefficient, infrared absorption and humidity sensitivity, have been published [29

29. R. Gvishi, “Fast sol–gel technology: from fabrication to applications,” J. Sol-Gel Sci. Technol. 50(2), 241–253 (2009). [CrossRef]

,31

31. M. Pokrass, Z. Burshtein, and R. Gvishi, “Thermo-optic coefficient in some hybrid organic/inorganic fast sol–gel glasses,” Opt. Mater. 32(9), 975–981 (2010). [CrossRef]

,32

32. M. Pokrass, I. Gouzman, G. Bar, and R. Gvishi, “Infrared and X-ray photoelectron spectroscopy studies of hybrid organic/inorganic fast sol–gel glasses,” Opt. Mater. 34(2), 341–346 (2011). [CrossRef]

]. The process is based on an accelerated sol–gel reaction of a combination of organically modified silicon alkoxides (ORMOSILs) with traditional alkoxides at elevated temperature and pressure. The composites were prepared by solution mixing [33

33. P. J. Harris, ed., Carbon Nanotube Science (Cambridge University Press, New York, 2009).

], which is a common fabrication technique of CNT/polymer composites [34

34. P.-C. Ma, N. A. Siddiqui, G. Marom, and J.-K. Kim, “Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review,” Composites, Part A 41(10), 1345–1367 (2010). [CrossRef]

].

The composite MWNT/hybrid glasses were studied for their nonlinear optical properties at 1064 nm, using the Z-scan technique [35

35. M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]

]. In this method, a sample in the form of a thin slab is moved along the converging axis (Z-direction) of a Gaussian beam emanating from a pulsed laser. This results in variation of the laser beam power density impinging the sample, reaching a maximum at the beam waist position. Analysis of the transmitted beam intensity as a function of the sample position Z reveals the nonlinear interactions between the light and the sample material.

In the past, analysis of saturable absorption in CNTs was performed using an empirical formula [36

36. R. W. Boyd, ed., Nonlinear Optics (Academic, San-Diego, CA, 2003).

38

38. O. Svelto, ed., Principles of Lasers (Springer, New York, 1998).

]
α(I)=α01+(I/Is)+αns,
(1)
where I is the input optical pulse intensity, α(I)is the intensity-dependent absorption coefficient, α0 and αns are the limits of the linear saturable-, and non-saturable absorption coefficients, respectively, and Is is the saturation intensity (the intensity necessary to reduce the saturable absorption coefficient to a half). In our work, we present a refined analysis of the saturable absorption process based on solutions of the basic rate equations that describe the energy state population and the photon density kinetics in saturable absorbers [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

]. These solutions consider two limiting cases: a slow saturable absorber yielding a modified Frantz-Nodvik equation, and a fast saturable absorber yielding a modified Hercher equation. A more detailed description of the models and the resulting solutions is presented in section 2.4.1.

2. Experimental

2.1. Materials

The precursors used for the Fast-Sol-Gel reaction were tetramethylorthosilicate (TMOS), methyltrimethoxysilane (MTMS), and 0.1 N HCl as the catalyst. The precursors (98% purity) and HCl (99% purity) were purchased from Sigma–Aldrich. Ethanol absolute (dehaydrated, 99.9% purity) and tetrahydrofuran (THF) high performance liquid chromatography (HPLC)-grade were purchased from BioLab Ltd. MWNTs of 10-30 nm diameter, 1-10 μm long were purchased from Nanostructured and Amorphous Materials Inc. at 90% purity. All chemicals were used as purchased.

2.2. Sample preparation

A solution mixing approach was adopted [39

39. P. C. Ma, N. A. Siddiqui, G. Marom, and J. K. Kim, “Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review,” Composites, Part A 41(10), 1345–1367 (2010). [CrossRef]

] to achieve a homogeneous dispersion of the MWNTs in the composite. The MWNTs were dispersed in ethanol at a concentration of 1 mg/ml using a KUDOS model SK2210HP ultrasonic bath (frequency 53 kHz) for 1 hour. Correspondingly, a hybrid organic-inorganic glass, comprising 19.3 wt.% organic content, was prepared according to the process described in [29

29. R. Gvishi, “Fast sol–gel technology: from fabrication to applications,” J. Sol-Gel Sci. Technol. 50(2), 241–253 (2009). [CrossRef]

,32

32. M. Pokrass, I. Gouzman, G. Bar, and R. Gvishi, “Infrared and X-ray photoelectron spectroscopy studies of hybrid organic/inorganic fast sol–gel glasses,” Opt. Mater. 34(2), 341–346 (2011). [CrossRef]

]. The resulting viscous gel was diluted with THF at a 1:1 weight ratio. When kept refrigerated below ~10°C, the diluted gel remained stable for several months. The composite gel was prepared by mixing 2 ml of MWNT/ethanol suspension with 20 g of a diluted gel at ambient temperature until evaporation of the solvents. The viscous composite gel was then poured into polystyrene cups and covered with perforated lids. The samples were cured for 48 hrs at ambient temperature, followed by an additional 48 hrs at 65°C. The resultant MWNTs concentration was 0.02 wt.%. Prior to optical characterization, the samples were polished to form thin slabs with thicknesses ranging between 1.0 and 1.6 mm.

2.3. Characterization

Environmental scanning electron microscopy (E-SEM) was used to study the morphology of different stages of the composite material formation. The E-SEM was equipped with Quanta 200 field-emission gun. For inspection of the MWNTs powder, several grains were deposited on a double-sided carbon tape; for inspection of MWNT/ethanol suspension, a drop was spread on a microscope slide and dried; inspection of the composite hybrid glass was performed by examining its cross section. All inspections were performed under vacuum (10−2 - 10−5 mbar). Optical transmission spectra were measured using a Jasco model V-570 spectrophotometer in the 400–2500 nm range with 2 nm resolution.

2.4. Nonlinear transmission

The samples were irradiated with 1 ns-long pulses at 1064 nm, ~10 kHz repetition rate, from a diode-pumped, passively Q-switched fiber-pumped microchip laser PULSELAS-P-1064-300-FC-CS model, manufactured by ALPHALAS GmbH. The average power output was approximately 200 mW. The power was measured with an OPHIR Laserstar model power meter. A reference beam for monitoring the pulse intensity was obtained using a 3.5% reflecting quartz beam-splitter. The sample was moved along the Z-axis using a Newport 426/431 series linear stage of 10 µm resolution. The beam waist radius was w0=13.5μm. The input and transmitted signals were measured with THORLABS PDA55 model Si detectors, with their output displayed on a TEXTRONIX TDS360 model digital real-time oscilloscope. To avoid overheating of the irradiated sample spot, the incident beam was blocked between successive transmission measurements. A schematic layout of the optical set-up is provided in Fig. 1
Fig. 1 Schematic description of the Z-scan measurement system.
.

2.4.1. Analysis of the nonlinear transmission

Analysis of the Fresnel-reflection-corrected light transmission was performed according to expressions developed for slow and fast saturable absorbers [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

]. These expressions relate to a three-level optical system as shown in Fig. 2
Fig. 2 Schematic diagram of a three-level system used for modeling of saturable absorption. Optical transitions are marked with solid arrows; non-radiative intra-band transitions are marked with a wiggled one.
.

Species at the S(0) ground state band are excited by the incident photons to the S(1) first excited state band. The probability for this process is given by the ground-state absorption cross section σgs. Electrons at the first excited band level can non-radiatively decay to the S(1) lowest state level at a very fast rate. Subsequent photons may then induce further excitation to the S(2) higher lying band levels, with a cross section σes. The effective spontaneous decay time τ of S(1) may be assumed to be either very long or very short compared to the light pulse duration. Increased light intensity brings about a reduction in the ground-state species concentration. If σes<σgs, the pulsed light transmission increases. In practical saturable absorbers, the transmission never reaches 100%, which is a result of the photon absorption by species in their S(1) excited state.

2.4.1.1. Slow saturable absorber – the modified Frantz-Nodvik Solution

The term “slow absorber” is used for the case where the light pulse duration is very short compared to the excited state decay time τ. An analytical solution for the transmission T of a real saturable absorber is given in a closed form [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

] as
T(L)=T0+TFN(L)T01T0(TmaxT0);T0=eNσgsL;Tmax=eNσesL,
(2)
where L is the sample thickness, T0 is the transmission in the low pulse intensity limit, Tmax is the maximal transmission achieved at very high pulse intensities, N is the absorbers density and TFN is the transmission of an “ideal” saturable absorber (σes0)
TFN=1σgsE(0)ln{1+T0[eσgsE(0)1]},
(3)
whereE(0) is the input beam fluence in units of photons per unit area. This expression is known as the Frantz-Nodvik equation [40

40. L. M. Frantz and J. S. Nodvik, “Theory of pulse propagation in a laser amplifier,” J. Appl. Phys. 34(8), 2346–2349 (1963). [CrossRef]

]. In the case of a transversely Gaussian beam distribution, the half peak beam fluence should appear in Eq. (3) instead of E(0) [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

].

2.4.1.2. Fast saturable absorber – the modified Hercher Solution

This case occurs when the relaxation of excited atoms is very fast, namely the excited-state decay time τ is short compared to the pulse duration. The analytical solution for the photon flux density (photons per unit time per unit area) is given in a closed form [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

] as
I(0)=S[1(T0/T)1/D](T0/T)1/DT;S=1τσgs;D=Δσσes,
(4)
whereI(0) is the input beam intensity in units of photons per unit area per unit time, and σσgsσes. This expression is known as Hercher’s equation [41

41. M. Hercher, “An analysis of saturable absorbers,” Appl. Opt. 6(5), 947–954 (1967). [CrossRef] [PubMed]

]. In the case of a transversely Gaussian beam distribution, the half peak beam intensity should appear in Eq. (4) instead of I(0) [30

30. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kikta, “Excited-state absorption studies of Cr4+ ions in several Garnet host crystals,” IEEE J. Quantum Electron. 34(2), 292–299 (1998). [CrossRef]

]. Although Eq. (4) constitutes a closed equation for T as a function of I(0), this expression should be very useful for curve fitting of measured data, as one may readily swap between the I(0) and Taxes.

3. Results and discussion

3.1. Morphology

MWNTs were imaged by E-SEM over several steps of the composite glass processing. Figure 3(a)
Fig. 3 E-SEM images of (a) a MWNTs powder grain, as purchased; (b) MWNT/ethanol suspension after drying; (c) a MWNT/hybrid-glass composite surface
shows a grain of the MWNTs powder along with two other particles. The grain appears as a very dense particle of an approximately 25 µm diameter, composed of tightly agglomerated nanotubes. The image is a typical micrograph of the as-purchased MWNTs powder. As such, the MWNTs cannot be directly used for the composite glass preparation. During the first processing step, the MWNTs are dispersed in ethanol with the aid of ultrasonication. During ultrasonication, vibrational energy is delivered to the system, inducing disentangling of the agglomerates. Dispersed nanotubes of the MWNT/ethanol suspension are shown in Fig. 3(b). The agglomeration of the nanotubes appears to have been reduced dramatically. The nanotubes appear quite isolated, with lengths ranging between approximately 1 and 6 µm. The large distribution of lengths implies that the nanotubes were also fractioned by the ultrasonication [42

42. M. Moniruzzaman and K. I. Winey, “Polymer nanocomposites containing carbon nanotubes,” Macromolecules 39(16), 5194–5205 (2006). [CrossRef]

,43

43. K. L. Lu, R. M. Lago, Y. K. Chen, M. L. H. Green, P. J. F. Harris, and S. C. Tsang, “Mechanical damage of carbon nanotubes by ultrasound,” Carbon 34(6), 814–816 (1996). [CrossRef]

]. A cross-section of a MWNT/hybrid-glass is shown in Fig. 3(c). The smooth background demonstrates the uniform and nonporous glassy matrix surface, with some nanotube edges sticking out.

In spite of the said dispersion process, occasionally, some inhomogeneous distribution of the MWNTs in the glasses was obtained. It manifested itself in the appearance of several “dark dots”, each 10-50 μm in diameter, across the uniform slab. Throughout our optical transmission experiments we verified that only optically uniform regions were used.

3.2. Vis-NIR spectroscopy

The effective absorption spectrum of the MWNTs’ suspension in ethanol is shown in Fig. 4
Fig. 4 Vis-NIR absorption spectrum of a 0.3 mg/ml MWNTs suspension in ethanol.
. The Nd:YAG 1064 nm laser wavelength is indicated on the abscissa for reference in our following experiments. The spectrum is entirely structureless, exhibiting a decrease in transmission with shortening wavelength. Light scattering is most likely the major factor causing the transmission reduction with wavelength shortening [24

24. H. Kataura, Y. Kumazawa, Y. Maniwa, I. Umezu, S. Suzuki, Y. Ohtsuka, and Y. Achiba, “Optical properties of single-wall carbon nanotubes,” Synth. Met. 103(1-3), 2555–2558 (1999). [CrossRef]

], thus bringing about the “effective” absorption coefficient to grow in that direction. The effective absorption spectrum for 0.3 mg/ml MWNT concentration in the 400-1500 nm range follows an empirical relation
α(λ)[cm1]=21.082.6ln(λ[cm1]),
(5)
where α is the effective adsorption coefficient and λ is the wavelength. The lack of any pronounced structure is characteristic of MWNTs [2

2. L. Vivien, E. Anglaret, D. Riehl, F. Hache, F. Bacou, M. Andrieux, F. Lafonta, C. Journet, C. Goze, M. Brunet, and P. Bernier, “Optical limiting properties of singlewall carbon nanotubes,” Opt. Commun. 174(1-4), 271–275 (2000). [CrossRef]

] as well as of long SWNTs; in the latter, pronounced absorption peaks in the vicinity of 1000 nm were observed for only very short (<1 µm) tubes [23

23. J. A. Fagan, J. R. Simpson, B. J. Bauer, S. H. De Paoli Lacerda, M. L. Becker, J. Chun, K. B. Migler, A. R. Hight Walker, and E. K. Hobbie, “Length-dependent optical effects in single-wall carbon nanotubes,” J. Am. Chem. Soc. 129(34), 10607–10612 (2007). [CrossRef] [PubMed]

]. The absorption of the composite glass compared to a pure hybrid glass is shown in Fig. 5
Fig. 5 Vis-NIR absorption spectrum MWNT/hybrid-glass containing 0.02 wt.% MWNT, compared to a pure hybrid glass. Both spectra corrected for Fresnel reflections.
. As in Fig. 4, the Nd:YAG 1064 nm laser wavelength is indicated on the abscissa for reference in our following experiments. The composite glass exhibits the characteristic absorption bands of silica and its organic residues [32

32. M. Pokrass, I. Gouzman, G. Bar, and R. Gvishi, “Infrared and X-ray photoelectron spectroscopy studies of hybrid organic/inorganic fast sol–gel glasses,” Opt. Mater. 34(2), 341–346 (2011). [CrossRef]

], superimposed on those of the MWNT background. The MWNT background spectrum exhibits a general agreement with that of the MWNT/ethanol suspension provided in Fig. 4, namely the absorption decreases with λ. However, the exact wavelength dependence is weaker than in Fig. 4; i.e. it does not follow the empiric relation of Eq. (5). We have not studied this behavioral difference further. We assume that light scattering at 1064 nm is negligible thus the absorption coefficient of the MWNTs/hybrid-glass is α=3.7cm1at 1064 nm for the particular MWNTs’ concentration in the measured sample.

3.3. Nonlinear transmission

The results of Fig. 6 were modeled by the limiting cases of a slow or a fast saturable absorber expression, and are shown in Figs. 7
Fig. 7 Optical transmission of MWNT/hybrid-glass containing 0.02 wt.% MWNT as function of the peak beam fluence (same results as Fig. 6). Fit curve is by the modified Frantz-Nodvik model (Eqs. (2)-(3)) for a slow saturable absorber (SA).
and 8
Fig. 8 Transmission of MWNT/hybrid-glass containing 0.02 wt.% MWNT as function of peak beam power (same results as Fig. 6). Fit curve by Eq. (4) for a fast saturable absorber (SA).
respectively. The figures show the measured data of the composite hybrid glass transmission as function of the peak pulse fluence and the peak pulse intensity, and their fittings for each model. As the peak pulse fluence/intensity increases, the transmission exhibits an increase which is initially linear, but which then saturates. At the turn to saturation, most of the absorbing species in the glass have been excited to the S(1) state level by the first arriving photons. Later photons impinging the absorbing species exhibit a higher probability for traversing the sample. The fact that the saturated value never reaches 100% indicates that excited-state absorption by an S(1)S(2) transition sets in as a result of the enhanced population in the S(1) band (Fig. 2).

In Fig. 7 we show the same experimental data of Fig. 6 and the calculated fit according to the slow saturable absorber model (Eqs. (2) and (3)), where the optical transmission is plotted vs. the peak beam fluence. Justification for attempting this model is based on excited state lifetimes of ~3-10 ns estimated by Miyauchi et al. [44

44. Y. Miyauchi, H. Hirori, K. Matsuda, and Y. Kanemitsu, “Radiative lifetimes and coherence lengths of one-dimensional excitons in single-walled carbon nanotubes,” Phys. Rev. B 80(8), 081410 (2009). [CrossRef]

], which are in fact quite longer than our pulse duration of only 1 ns. Moreover, Park et al. [45

45. J. Park, P. Deria, and M. J. Therien, “Dynamics and transient absorption spectral signatures of the single-wall carbon nanotube electronically excited triplet state,” J. Am. Chem. Soc. 133(43), 17156–17159 (2011). [CrossRef] [PubMed]

] have recently found that the triplet excited-state lifetime is even orders of magnitude longer, ~15 µs. As a word of caution, however, we should add that both latter values were estimated for SWNTs and not for MWNTs.

The physical parameters used for the fit curve in Fig. 7 (see inset in the figure frame) exhibit reasonable agreement with those obtained by several other researchers, at least in the order of magnitude. Particularly, the estimated ground state absorption cross section is σgs=2.3×1018cm2, in good agreement with that of a single carbon atom, σgs=1×1018cm2, measured at λ=800nm [46

46. A. Hagen, M. Steiner, M. B. Raschke, C. Lienau, T. Hertel, H. Qian, A. J. Meixner, and A. Hartschuh, “Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes,” Phys. Rev. Lett. 95(19), 197401 (2005). [CrossRef] [PubMed]

]. However, our value is still an order of magnitude smaller than the value σgs=1×1017cm2 measured at λ=567nm [47

47. S. Berciaud, L. Cognet, and B. Lounis, “Luminescence decay and the absorption cross section of individual single-walled carbon nanotubes,” Phys. Rev. Lett. 101(7), 077402 (2008). [CrossRef] [PubMed]

]. Both latter values were reported per single carbon atom in SWNTs. In MWNTs, Ni and Bandaru [48

48. C. Ni and P. R. Bandaru, “Enhanced optical absorption cross-section characteristics of multi-wall carbon nanotubes,” Carbon 47(12), 2898–2903 (2009). [CrossRef]

] measured σgs1×1017cm2 per carbon atom at 473 and 633 nm, which is approximately a factor of 5 larger than their measured value per single atom in SWNTs. Thus the σgs value we have estimated by the slow saturable absorber model is within the range of previously reported values per carbon atom in CNTs. The excitation wavelength and the embedding matrix [49

49. G. K. Lim, Z. L. Chen, J. Clark, R. G. S. Goh, W. H. Ng, H. W. Tan, R. H. Friend, P. K. H. Ho, and L. L. Chua, “Giant broadband nonlinear optical absorption response in dispersed graphene single sheets,” Nat. Photonics 5(9), 554–560 (2011). [CrossRef]

] may play an important role in determining CNTs’ absorption cross sections.

For the excited state absorption cross section we have further obtained (within the slow absorber model) the value σes=6.9×1019cm2. To the best of our knowledge, this is the first time an estimation of σes is reported for CNTs. With respect to other carbon-nanotube species, the singlet excited state absorption cross section of fullerenes at 532-540 nm was reported to be in the range of 818×1018cm2 [50

50. Z.-B. Liu, J.-G. Tian, W.-P. Zang, W.-Y. Zhou, C.-P. Zhang, J.-Y. Zheng, Y.-C. Zhou, and H. Xu, “Large optical nonlinearities of new organophosphorus fullerene derivatives,” Appl. Opt. 42(35), 7072–7076 (2003). [CrossRef] [PubMed]

], which is larger than their ground state absorption cross section. Thus their samples exhibit a reverse saturable absorption phenomenon.

Another issue to consider is the fact that the estimated absorbers density N=2.8×1018cm3 is about a factor of 5 smaller than the nominal density of carbon atoms incorporated into the glass, N=1.3×1019cm3. This discrepancy may perhaps be rationalized by assuming that band formation in the MWNTs causes a reduction in the effective density of states.

It should be noted, that fitting attempts of our data to a dielectric function of a constant, obviously negative Im(χ(3)), always failed. This strongly supports our modeling approach of excited state absorption. Still, for the benefit of researchers who are used to assessment of nonlinear optical properties through the Im(χ(3)) parameter, we should indicate that its order of magnitude is around |10−10| esu, similar to values obtained by others [15

15. D. V. Khudyakov, A. S. Lobach, and V. A. Nadtochenko, “Nonlinear optical absorption of single-wall carbon nanotubes in carboxymethylcellulose thin polymerfilm and its application as a saturable absorber for mode-locking in pulsed Nd:glass laser,” Appl. Opt. 48(8), 1624–1627 (2009). [CrossRef] [PubMed]

].

With respect to technological applications, it should be mentioned that saturable absorption allows applications for Q-switching, mode-locking, pulse-shaping and noise suppressing in fast pulsing lasers [13

13. C.-C. Liu, Y.-G. Wang, J. Liu, L.-H. Zheng, L.-B. Su, and J. Xu, “Ultrafast laser performance of Yb3+: Sc2SiO5 crystal with a single-walled carbon nanotube absorber,” Opt. Commun. 285(6), 1352–1355 (2012). [CrossRef]

,15

15. D. V. Khudyakov, A. S. Lobach, and V. A. Nadtochenko, “Nonlinear optical absorption of single-wall carbon nanotubes in carboxymethylcellulose thin polymerfilm and its application as a saturable absorber for mode-locking in pulsed Nd:glass laser,” Appl. Opt. 48(8), 1624–1627 (2009). [CrossRef] [PubMed]

22

22. Y. W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett. 92(2), 021115 (2008). [CrossRef]

].

4. Conclusions

Novel MWNT/hybrid-organic-inorganic glass composites containing 0.02 wt.% MWNTs were prepared by solutions mixing in a fast-sol-gel route. The composites had an approximately 50% visible light transmission along with a strong “bleaching” effect when irradiated with high intensity, 1 ns-long pulses at 1064 nm. This nonlinear phenomenon is attributed to saturated absorption of the MWNTs, and is analyzed using both slow and fast saturable absorber models of a three-level energy scheme. Both models yield very good fits with reasonable physical parameters. Quite arbitrarily, we choose the slow absorber model as representative. The ground state absorption cross section was estimated as σgs=2.3×1018cm2, which is in good agreement with the known value per single carbon atom. The cross section for excited state absorption was estimated as σes=6.9×1019cm2, and is reported for the first time for CNTs. The saturable absorption phenomenon allows possible applications of the MWNT/hybrid-glass composites as promising candidates for applications in fast laser passive mode lockers and limiters. Obviously, direct measurement of the excited-state decay time τ for every studied sample is of critical importance, both for obtaining accurate estimates of the absorption cross-sections, and for properly assessing the material suitability in any technological application.

References and links

1.

S. M. O’Flaherty, S. V. Hold, M. E. Brennan, M. Cadek, A. Drury, J. N. Coleman, and W. J. Blau, “Nonlinear optical response of multiwalled carbon-nanotube dispersions,” J. Opt. Soc. Am. B 20(1), 49–58 (2003). [CrossRef]

2.

L. Vivien, E. Anglaret, D. Riehl, F. Hache, F. Bacou, M. Andrieux, F. Lafonta, C. Journet, C. Goze, M. Brunet, and P. Bernier, “Optical limiting properties of singlewall carbon nanotubes,” Opt. Commun. 174(1-4), 271–275 (2000). [CrossRef]

3.

L. Vivien, E. Anglaret, D. Riehl, F. Bacou, C. Journet, C. Goze, M. Andrieux, M. Brunet, F. Lafonta, P. Bernier, and F. Hache, “Single-wall carbon nanotubes for optical limiting,” Chem. Phys. Lett. 307(5-6), 317–319 (1999). [CrossRef]

4.

C. Zheng, M. Feng, and H. Zhan, “The synthesis of carbon nanotube based composites with conducting, semiconducting, and insulating coatings and their optical limiting properties,” Carbon 48(13), 3750–3759 (2010). [CrossRef]

5.

Z. Hongbing, C. Wenzhe, W. Minquan, Zhengchan, and Z. Chunlin, “Optical limiting effects of multi-walled carbon nanotubes suspension and silica xerogel composite,” Chem. Phys. Lett. 382(3-4), 313–317 (2003). [CrossRef]

6.

C. Zheng, W. Chen, X. Ye, S. Cai, X. Xiao, and M. Wang, “Preparation and optical limiting properties of carbon nanotubes coated with Au nanoparticle composites embedded in silica gel-glass,” Mater. Lett. 65(2), 150–152 (2011). [CrossRef]

7.

N. Kamaraju, S. Kumar, A. K. Sood, S. Guha, S. Krishnamurthy, and C. N. R. Rao, “Large nonlinear absorption and refraction coefficients of carbon nanotubes estimated from femtosecond z-scan measurements,” Appl. Phys. Lett. 91(25), 251103 (2007). [CrossRef]

8.

J. Wang and W. J. Blau, “Solvent effect on optical limiting properties of single-walled carbon nanotube dispersions,” J. Phys. Chem. C 112(7), 2298–2303 (2008). [CrossRef]

9.

H. Xu, Y. Pan, Y. Zhu, H. Kou, and J. Guo, “Transparent multi-walled carbon nanotube-silica composite prepared by hot-pressed sintering and its nonlinear optical properties,” J. Alloy. Comp. 481(1-2), L4–L7 (2009). [CrossRef]

10.

L. Zhang, J. Thomas, S. D. Allen, and Y.-X. Wang, “Optical power limiting and nonlinear absorption effects of polymer functionalized carbon nanotube thin films,” Opt. Eng. 49(6), 063801 (2010). [CrossRef]

11.

W. Feng, W. Yi, H. Wu, M. Ozaki, and K. Yoshino, “Enhancement of third-order optical nonlinearities by conjugated polymer-bonded carbon nanotubes,” J. Appl. Phys. 98(3), 034301 (2005). [CrossRef]

12.

H. I. Elim, W. Ji, G. H. Ma, K. Y. Lim, C. H. Sow, and C. H. A. Huan, “Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films,” Appl. Phys. Lett. 85(10), 1799–1801 (2004). [CrossRef]

13.

C.-C. Liu, Y.-G. Wang, J. Liu, L.-H. Zheng, L.-B. Su, and J. Xu, “Ultrafast laser performance of Yb3+: Sc2SiO5 crystal with a single-walled carbon nanotube absorber,” Opt. Commun. 285(6), 1352–1355 (2012). [CrossRef]

14.

A. G. Rozhin, Y. Sakakibara, H. Kataura, S. Matsuzaki, K. Ishida, Y. Achiba, and M. Tokumoto, “Anisotropic saturable absorption of single-wall carbon nanotubes aligned in polyvinyl alcohol,” Chem. Phys. Lett. 405(4-6), 288–293 (2005). [CrossRef]

15.

D. V. Khudyakov, A. S. Lobach, and V. A. Nadtochenko, “Nonlinear optical absorption of single-wall carbon nanotubes in carboxymethylcellulose thin polymerfilm and its application as a saturable absorber for mode-locking in pulsed Nd:glass laser,” Appl. Opt. 48(8), 1624–1627 (2009). [CrossRef] [PubMed]

16.

G. Qin, T. Suzuki, and Y. Ohishi, “Widely tunable passively mode-locked fiber laser with carbon nanotube films,” Opt. Rev. 17(3), 97–99 (2010). [CrossRef]

17.

H. R. Chen, Y. G. Wang, C. Y. Tsai, K. H. Lin, T. Y. Chang, J. Tang, and W. F. Hsieh, “High-power, passively mode-locked Nd:GdVO4 laser using single-walled carbon nanotubes as saturable absorber,” Opt. Lett. 36(7), 1284–1286 (2011). [CrossRef] [PubMed]

18.

S. Set, H. Yaguchi, M. Jablonski, Y. Tanaka, Y. Sakakibara, A. Rozhin, and M. Tohmata, “A noise suppressing saturable absorber at 1550 nm based on carbon nsnotube technology,” in Optical Fiber Communications Conference (2003), pp. 723–725.

19.

S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Ultrafast fiber pulsed lasers incorporating carbon nanotubes,” IEEE J. Sel. Top. Quantum Electron. 10(1), 137–146 (2004). [CrossRef]

20.

S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, M. Jablonski, and S. Y. Set, “Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers,” Opt. Lett. 29(14), 1581–1583 (2004). [CrossRef] [PubMed]

21.

Y. Sakakibara, A. G. Rozhin, H. Kataura, Y. Achiba, and M. Tokumoto, “Carbon nanotube-poly(vinylalcohol) nanocomposite film devices: applications for femtosecond fiber laser mode lockers and optical amplifier noise suppressors,” Jpn. J. Appl. Phys. 44(4A), 1621–1625 (2005). [CrossRef]

22.

Y. W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett. 92(2), 021115 (2008). [CrossRef]

23.

J. A. Fagan, J. R. Simpson, B. J. Bauer, S. H. De Paoli Lacerda, M. L. Becker, J. Chun, K. B. Migler, A. R. Hight Walker, and E. K. Hobbie, “Length-dependent optical effects in single-wall carbon nanotubes,” J. Am. Chem. Soc. 129(34), 10607–10612 (2007). [CrossRef] [PubMed]

24.

H. Kataura, Y. Kumazawa, Y. Maniwa, I. Umezu, S. Suzuki, Y. Ohtsuka, and Y. Achiba, “Optical properties of single-wall carbon nanotubes,” Synth. Met. 103(1-3), 2555–2558 (1999). [CrossRef]

25.

D. J. Styers-Barnett, S. P. Ellison, C. Park, K. E. Wise, and J. M. Papanikolas, “Ultrafast dynamics of single-walled carbon nanotubes dispersed in polymer films,” J. Phys. Chem. A 109(2), 289–292 (2005). [CrossRef] [PubMed]

26.

M. Sennett, E. Welsh, J. B. Wright, W. Z. Li, J. G. Wen, and Z. F. Ren, “Dispersion and alignment of carbon nanotubes in polycarbonate,” Appl. Phys., A Mater. Sci. Process. 76(1), 111–113 (2003). [CrossRef]

27.

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28.

S. Razdan, “Novel polyelectrolyte complex based carbon nanotube composite architectures,” PhD thesis (Rensselaer Polytechnic Institute, Troy, New-York, 2008).

29.

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30.

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31.

M. Pokrass, Z. Burshtein, and R. Gvishi, “Thermo-optic coefficient in some hybrid organic/inorganic fast sol–gel glasses,” Opt. Mater. 32(9), 975–981 (2010). [CrossRef]

32.

M. Pokrass, I. Gouzman, G. Bar, and R. Gvishi, “Infrared and X-ray photoelectron spectroscopy studies of hybrid organic/inorganic fast sol–gel glasses,” Opt. Mater. 34(2), 341–346 (2011). [CrossRef]

33.

P. J. Harris, ed., Carbon Nanotube Science (Cambridge University Press, New York, 2009).

34.

P.-C. Ma, N. A. Siddiqui, G. Marom, and J.-K. Kim, “Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review,” Composites, Part A 41(10), 1345–1367 (2010). [CrossRef]

35.

M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]

36.

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37.

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38.

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39.

P. C. Ma, N. A. Siddiqui, G. Marom, and J. K. Kim, “Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review,” Composites, Part A 41(10), 1345–1367 (2010). [CrossRef]

40.

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41.

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42.

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43.

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44.

Y. Miyauchi, H. Hirori, K. Matsuda, and Y. Kanemitsu, “Radiative lifetimes and coherence lengths of one-dimensional excitons in single-walled carbon nanotubes,” Phys. Rev. B 80(8), 081410 (2009). [CrossRef]

45.

J. Park, P. Deria, and M. J. Therien, “Dynamics and transient absorption spectral signatures of the single-wall carbon nanotube electronically excited triplet state,” J. Am. Chem. Soc. 133(43), 17156–17159 (2011). [CrossRef] [PubMed]

46.

A. Hagen, M. Steiner, M. B. Raschke, C. Lienau, T. Hertel, H. Qian, A. J. Meixner, and A. Hartschuh, “Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes,” Phys. Rev. Lett. 95(19), 197401 (2005). [CrossRef] [PubMed]

47.

S. Berciaud, L. Cognet, and B. Lounis, “Luminescence decay and the absorption cross section of individual single-walled carbon nanotubes,” Phys. Rev. Lett. 101(7), 077402 (2008). [CrossRef] [PubMed]

48.

C. Ni and P. R. Bandaru, “Enhanced optical absorption cross-section characteristics of multi-wall carbon nanotubes,” Carbon 47(12), 2898–2903 (2009). [CrossRef]

49.

G. K. Lim, Z. L. Chen, J. Clark, R. G. S. Goh, W. H. Ng, H. W. Tan, R. H. Friend, P. K. H. Ho, and L. L. Chua, “Giant broadband nonlinear optical absorption response in dispersed graphene single sheets,” Nat. Photonics 5(9), 554–560 (2011). [CrossRef]

50.

Z.-B. Liu, J.-G. Tian, W.-P. Zang, W.-Y. Zhou, C.-P. Zhang, J.-Y. Zheng, Y.-C. Zhou, and H. Xu, “Large optical nonlinearities of new organophosphorus fullerene derivatives,” Appl. Opt. 42(35), 7072–7076 (2003). [CrossRef] [PubMed]

51.

L. Huang, H. N. Pedrosa, and T. D. Krauss, “Ultrafast ground-state recovery of single-walled carbon nanotubes,” Phys. Rev. Lett. 93(1), 017403 (2004). [CrossRef]

52.

Y.-C. Chen, N. R. Raravikar, L. S. Schadler, P. M. Ajayan, Y.-P. Zhao, T.-M. Lu, G.-C. Wang, and X.-C. Zhang, “Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm,” Appl. Phys. Lett. 81(6), 975–977 (2002). [CrossRef]

OCIS Codes
(160.2750) Materials : Glass and other amorphous materials
(160.4330) Materials : Nonlinear optical materials
(160.6060) Materials : Solgel

ToC Category:
Nonlinear Optical Materials

History
Original Manuscript: March 14, 2012
Revised Manuscript: April 22, 2012
Manuscript Accepted: May 5, 2012
Published: May 22, 2012

Virtual Issues
Nanocarbon for Photonics and Optoelectronics (2012) Optical Materials Express

Citation
Mariana Pokrass, Zeev Burshtein, Raz Gvishi, and Menachem Nathan, "Saturable absorption of multi-walled carbon nanotubes/hybrid-glass composites," Opt. Mater. Express 2, 825-838 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-6-825


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References

  1. S. M. O’Flaherty, S. V. Hold, M. E. Brennan, M. Cadek, A. Drury, J. N. Coleman, and W. J. Blau, “Nonlinear optical response of multiwalled carbon-nanotube dispersions,” J. Opt. Soc. Am. B20(1), 49–58 (2003). [CrossRef]
  2. L. Vivien, E. Anglaret, D. Riehl, F. Hache, F. Bacou, M. Andrieux, F. Lafonta, C. Journet, C. Goze, M. Brunet, and P. Bernier, “Optical limiting properties of singlewall carbon nanotubes,” Opt. Commun.174(1-4), 271–275 (2000). [CrossRef]
  3. L. Vivien, E. Anglaret, D. Riehl, F. Bacou, C. Journet, C. Goze, M. Andrieux, M. Brunet, F. Lafonta, P. Bernier, and F. Hache, “Single-wall carbon nanotubes for optical limiting,” Chem. Phys. Lett.307(5-6), 317–319 (1999). [CrossRef]
  4. C. Zheng, M. Feng, and H. Zhan, “The synthesis of carbon nanotube based composites with conducting, semiconducting, and insulating coatings and their optical limiting properties,” Carbon48(13), 3750–3759 (2010). [CrossRef]
  5. Z. Hongbing, C. Wenzhe, W. Minquan, Zhengchan, and Z. Chunlin, “Optical limiting effects of multi-walled carbon nanotubes suspension and silica xerogel composite,” Chem. Phys. Lett.382(3-4), 313–317 (2003). [CrossRef]
  6. C. Zheng, W. Chen, X. Ye, S. Cai, X. Xiao, and M. Wang, “Preparation and optical limiting properties of carbon nanotubes coated with Au nanoparticle composites embedded in silica gel-glass,” Mater. Lett.65(2), 150–152 (2011). [CrossRef]
  7. N. Kamaraju, S. Kumar, A. K. Sood, S. Guha, S. Krishnamurthy, and C. N. R. Rao, “Large nonlinear absorption and refraction coefficients of carbon nanotubes estimated from femtosecond z-scan measurements,” Appl. Phys. Lett.91(25), 251103 (2007). [CrossRef]
  8. J. Wang and W. J. Blau, “Solvent effect on optical limiting properties of single-walled carbon nanotube dispersions,” J. Phys. Chem. C112(7), 2298–2303 (2008). [CrossRef]
  9. H. Xu, Y. Pan, Y. Zhu, H. Kou, and J. Guo, “Transparent multi-walled carbon nanotube-silica composite prepared by hot-pressed sintering and its nonlinear optical properties,” J. Alloy. Comp.481(1-2), L4–L7 (2009). [CrossRef]
  10. L. Zhang, J. Thomas, S. D. Allen, and Y.-X. Wang, “Optical power limiting and nonlinear absorption effects of polymer functionalized carbon nanotube thin films,” Opt. Eng.49(6), 063801 (2010). [CrossRef]
  11. W. Feng, W. Yi, H. Wu, M. Ozaki, and K. Yoshino, “Enhancement of third-order optical nonlinearities by conjugated polymer-bonded carbon nanotubes,” J. Appl. Phys.98(3), 034301 (2005). [CrossRef]
  12. H. I. Elim, W. Ji, G. H. Ma, K. Y. Lim, C. H. Sow, and C. H. A. Huan, “Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films,” Appl. Phys. Lett.85(10), 1799–1801 (2004). [CrossRef]
  13. C.-C. Liu, Y.-G. Wang, J. Liu, L.-H. Zheng, L.-B. Su, and J. Xu, “Ultrafast laser performance of Yb3+: Sc2SiO5 crystal with a single-walled carbon nanotube absorber,” Opt. Commun.285(6), 1352–1355 (2012). [CrossRef]
  14. A. G. Rozhin, Y. Sakakibara, H. Kataura, S. Matsuzaki, K. Ishida, Y. Achiba, and M. Tokumoto, “Anisotropic saturable absorption of single-wall carbon nanotubes aligned in polyvinyl alcohol,” Chem. Phys. Lett.405(4-6), 288–293 (2005). [CrossRef]
  15. D. V. Khudyakov, A. S. Lobach, and V. A. Nadtochenko, “Nonlinear optical absorption of single-wall carbon nanotubes in carboxymethylcellulose thin polymerfilm and its application as a saturable absorber for mode-locking in pulsed Nd:glass laser,” Appl. Opt.48(8), 1624–1627 (2009). [CrossRef] [PubMed]
  16. G. Qin, T. Suzuki, and Y. Ohishi, “Widely tunable passively mode-locked fiber laser with carbon nanotube films,” Opt. Rev.17(3), 97–99 (2010). [CrossRef]
  17. H. R. Chen, Y. G. Wang, C. Y. Tsai, K. H. Lin, T. Y. Chang, J. Tang, and W. F. Hsieh, “High-power, passively mode-locked Nd:GdVO4 laser using single-walled carbon nanotubes as saturable absorber,” Opt. Lett.36(7), 1284–1286 (2011). [CrossRef] [PubMed]
  18. S. Set, H. Yaguchi, M. Jablonski, Y. Tanaka, Y. Sakakibara, A. Rozhin, and M. Tohmata, “A noise suppressing saturable absorber at 1550 nm based on carbon nsnotube technology,” in Optical Fiber Communications Conference (2003), pp. 723–725.
  19. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Ultrafast fiber pulsed lasers incorporating carbon nanotubes,” IEEE J. Sel. Top. Quantum Electron.10(1), 137–146 (2004). [CrossRef]
  20. S. Yamashita, Y. Inoue, S. Maruyama, Y. Murakami, H. Yaguchi, M. Jablonski, and S. Y. Set, “Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers,” Opt. Lett.29(14), 1581–1583 (2004). [CrossRef] [PubMed]
  21. Y. Sakakibara, A. G. Rozhin, H. Kataura, Y. Achiba, and M. Tokumoto, “Carbon nanotube-poly(vinylalcohol) nanocomposite film devices: applications for femtosecond fiber laser mode lockers and optical amplifier noise suppressors,” Jpn. J. Appl. Phys.44(4A), 1621–1625 (2005). [CrossRef]
  22. Y. W. Song, S. Yamashita, and S. Maruyama, “Single-walled carbon nanotubes for high-energy optical pulse formation,” Appl. Phys. Lett.92(2), 021115 (2008). [CrossRef]
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