Carbon nanotubes for optical limiting

Carbon 40 (2002) 1789–1797

Carbon nanotubes for optical limiting L. Vivien a , *, P. Lanc¸on a , D. Riehl a , F. Hache b , E. Anglaret c a DGA /DCE /CTA /Laser, Optique et Thermooptique, Arcueil, France Ecole Polytechnique, Laboratoire d’ Optique Quantique, Palaiseau, France c ´ , Montpellier, France Universite´ de Montpellier II, Groupe de Dynamique des Phases, Condensees b

Abstract This paper reviews the optical limiting properties of carbon nanotubes. The nonlinear optical properties of nanotubes were investigated in water and in chloroform suspensions. Nonlinear transmittance measurements were reported for various pulse durations and wavelengths and show that carbon nanotubes are good candidates for effective optical limiting over broad temporal and laser energy ranges. Z-Scans and pump-probe time-resolved experiments were achieved to identify the origin of optical limiting in nanotubes. The main phenomenon is a strong nonlinear scattering, originating from solvent vapour bubble growth and sublimation of nanotubes at high fluences. Heat transfer from particles to solvent is particularly effective as compared to carbon black suspensions because of the large surface area of the carbon nanotubes.  2002 Published by Elsevier Science Ltd. Keywords: A. Carbon nanotubes; B. Optical properties

1. Introduction The proliferation of lasers and systems employing lasers is associated to potential adverse effects from these bright, coherent light sources. Nowadays, laser sources are widely used in many applications, not only in the laboratory, but also in many areas of industry and medicine as well as for military applications and they constitute a potential hazard for eyes and other optical sensors (CCD, thermal camera, . . . ). This includes the possibility of damage from pulsed lasers, as well as temporary blinding by continuouswave lasers. With nearly every wavelength being emitted by these sources, there is a need to develop optical limiters and tunable filters which can suppress undesired radiation. Optical limiters are effective devices to decrease transmittance at high intensity or fluence. They have been studied extensively for applications in optical pulse shaping and smoothing and pulse compression [1,2]. However, the greatest interest in optical limiters in recent years has been for optical sensors protection [3–9]. In an ideal optical limiter, the linear transmittance at low input fluence must be reasonable (at least of 70%) and the output energy must *Corresponding author. Institut d’Electronique Fondamentale, CNRS / UMR 8622, Universite´ Paris Sud, Bat. 220 / P, 119, 91405 Orsay, France. Tel.: 133-1-6915-4070; fax: 133-1-6915-4000. E-mail address: [email protected] (L. Vivien).

remain at high fluences below the damage threshold of sensors or eyes. The optical damage threshold must be as high as possible and the activating threshold as low as possible. Ideal nonlinear optical limiters must protect optical sensors or eyes against laser pulses of any wavelength (from visible to infrared radiation) and pulse duration (from a few picosecond to continuous waves). To realize this, all existing nonlinear materials require a tightly-focused beam to initiate the effect. A limiter must therefore be incorporated into an adapted optical system. Several nonlinear effects lead to optical limiting: nonlinear absorption (reverse saturable absorption [10], multiphotonics absorption [11]), nonlinear refraction (electronics [12] or thermal effects [13]) and nonlinear scattering (solvent bubble formation and / or particle sublimation [14–16] or mismatched indices [17]). Since their discovery by Iijma in 1991 [18], carbon nanotubes have been the object of extensive physical studies concerning essentially their fascinating electronic and mechanical properties [19]. Their optical limiting properties have been evidenced and investigated recently [20–31] and they were shown to be good candidates for optical limiting applications. The aim of this article is to review the nonlinear optical properties and optical limiting behaviour of carbon nanotube suspensions. Sun et al. [20] and Vivien et al. [22] reported the first studies of multiwall carbon nanotube (MWNT) and single-wall carbon

0008-6223 / 02 / $ – see front matter  2002 Published by Elsevier Science Ltd. PII: S0008-6223( 02 )00046-5

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nanotube (SWNT) suspensions, respectively, and compared their optical limiting performances with those of carbon black particles suspensions (CBS) and fullerenes C 60 . Nanotubes were shown to exhibit nonlinear behaviours very different to C 60 (reverse saturable absorption), but very similar to those of CBS, i.e. nonlinear scattering due to bubble growth and plasma formation. Mishra et al. carried out nonlinear transmittance, Z-scan and nonlinear scattering measurements on single-wall carbon nanotubes in various solvents (ethylene glycol, water and ethanol) [23]. They discussed extensively the mechanisms responsible of optical limiting behavior and especially the nonlinear refraction due to thermal effect. Recently, Riggs et al. reported nonlinear transmittance results for MWNT and SWNT in suspensions (using surfactants) and in solutions (functionnalized or polymer-bound nanotubes) in water and chloroform. The solutions were found to exhibit much weaker optical limiting responses as compared to the suspensions, possibly because of different nonlinear mechanisms when working with single tubes in solutions as compared to bundled tubes in suspensions [30]. Hereafter, we review the results of our own investigations and we especially focus on the physical origin of optical limiting. The materials are presented in Section 2. Section 3 reports Z-scan experiments which were achieved to identify the nonlinear mechanisms responsible for optical limiting in nanotubes. In Section 4, we present time-resolved pump-probe experiments for two pump wavelengths (532 and 1064 nm) and discuss the origin of nonlinear scattering in suspensions of carbon nanotubes. The study of the light emission observed during pumpprobe experiments is presented in Section 5. Lastly, we present nonlinear transmittance measurements and compare the optical limiting performances of nanotubes for various solvents, laser wavelengths and pulse durations in Section 6.

2. Materials SWNTs were synthesized by the electric arc discharge technique [32]. The electrodes were a mixture of catalysts (4.2 and 1 at.% of Ni and Y, respectively) to prepare SWNT. The SWNTs were purified in a three-step procedure, i.e. acid treatment at about 100 8C, tangential filtration and annealing around 1600 8C in inert atmosphere [33]. After purification, the amount of nanotubes in the

Fig. 1. Linear transmittance of single wall carbon nanotube suspended in water–surfactant (dashed line) and in chloroform (solid line) from 400 to 1200 nm.

samples is about 90% for SWNTs. SWNTs are selfassembled into crystalline nanobundles of a few tens of tubes [32]. Their mean diameter is of about 1.4 nm with a dispersion of 60.2 nm [32,34] and their length is in the micronic range. For optical measurements, SWNT were suspended in water in presence of a few percentage of surfactant (Triton X100) or in chloroform without surfactant. We used 5-mm cells for Z-scan experiments and 2-mm cells for the other experiments. The thermodynamic properties of the solvents are summarized in Table 1. Carbon nanotube suspensions are stable over several weeks and are colorless as compared for example with fullerenes C 60 and C 70 . Fig. 1 displays typical linear transmittance of the suspensions between 400 and 1200 nm. A broadband high-transmission range is observed from visible to near infrared (400–1100 nm). Before each experiment, the linear transmittance was adjusted at about 70%. The two absorption bands at 950 and 1200 nm in water correspond to solvent absorption. In chloroform suspensions, the absorption peaks are also from the solvent.

3. Z-Scan experiments The Z-scan technique is a sensitive and simple characterisation method of the nonlinear optical properties of materials (nonlinear absorption, refraction or scattering) [31]. It is based on self-focusing or self-defocusing of an

Table 1 Thermodynamic parameters of the two solvents used in the experiments. r is density, Cp calorific capacity, k heating conductivity, T boil boiling temperature, DHv ap vaporization energy, s surface tension, m viscosity and Pv vapour pressure at 293 K Solvents

r (kg m 23 )

Cp (J cm 23 K 21 )

k (W m 21 K)

T boil (K)

DHv ap (kJ g 21 )

s (N m 21 )

m (Pa s)

Pv (293 K) (Pa)

Water Chloroform

997 1480

4.18 0.95

0.608 0.119

373.2 334.3

2.425 0.262

73?10 23 27?10 23

10 23 10 24

3.2 ? 10 3 2.7 ? 10 4

L. Vivien et al. / Carbon 40 (2002) 1789 – 1797

optical beam by a sample. The sample is moved along the propagation direction (Z axis) of a focused beam, and the variation of the far-field intensity allows to identify the nonlinear optical mechanism(s). The Z-scan experiments were achieved with a Nd:YAG laser at 532 nm and 1064 nm with a f / 30 focusing geometry [22,24]. An aperture was placed in front of the output detector to separate the nonlinear effects. With a small aperture, three mechanisms can be possibly identified: nonlinear absorption, nonlinear scattering and nonlinear refraction. With an open aperture, one usually only observes a dip in the transmission curve, which can be the signature of either nonlinear absorption or nonlinear scattering. The experimental results for SWNT suspended in water are displayed in Fig. 2 for various incident energies. When the aperture is open (Fig. 2a and b), one measures a strong and symmetric loss of transmittance around Z 5 0. Scattering can be observed at naked eyes and nonlinear scattering is therefore mainly responsible for this feature, as confirmed by further experiments described in the next sections. On the basis of this experiment, nonlinear absorption cannot be ruled out but no evidence of absorption could be made in other experiments. When the aperture is closed (Fig. 2c and d),

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one observes a peak–valley signal which is the signature of nonlinear refraction with a negative nonlinear index, typical of thermal effects [22,24]. At larger energies, the strong scattering contribution dominates the Z-scan profile.

4. Pump-probe time-resolved experiments The pump-probe set-up is sketched in Fig. 3 [26–28]. An horizontally polarized pump beam is emitted by a Q-switched injected Nd:YAG laser delivering Gaussian pulses of 5 ns FWHM at 1064 and 532 nm in single shot mode. The input energy is recorded with a pyroelectric detector and the temporal profile is measured using a fast photocathode (rise time: 270 ps). The pump beam, passing through a polarizing cube to preserve horizontal polarization, is focused into the sample using a 200-mm focal length achromatic doublet ( f/ 40 focusing geometry). The beam waist measured with a CCD camera is w 0 545 mm at 1064 nm and w 0 515 mm at 532 nm. The samples are probed at 632.8 nm by a continuous He–Ne laser beam, vertically polarized in order to allow separation from the pump beam. This probe beam is expanded by an afocal

Fig. 2. Open aperture Z-scan experiments at 532 m (a) and 1064 nm (c) for water suspensions. Closed aperture Z-scan experiments at 532 m (b) and 1064 nm (d) for water suspensions (see Ref. [24]).

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Fig. 3. Pump-probe set-up. Pc, polariser cube; PT, fast photocathode; PM, photomultiplier; Pv and Ph, vertical and horizontal polarisation, respectively; f1 , 200 mm focal length achromatic doublet; S, sample. (A) aperture, F633 nm : interferential filters, F1064 nm : infrared filter (2000  IEEE).

telescope to obtain a beam diameter of 5 mm and directed into the optical path of the pump beam using a polarizing cube. The probe beam is focused on the samples down to a radius of 25 mm (below 15 mm) at 1064 nm (532 nm). A fast photomultiplier tube (PMT) equipped with an interferential filter centered at 632.8 nm and connected to a digital scope is used to analyze the probe transmission through SWNT samples. Time resolved measurements of light emission from the samples are simultaneously performed using a similar photomultiplier tube (Hamamatsu model H5783-01, rise time 650 ps) over its whole sensitivity range (400–800 nm), after filtering the pump scattered signal by two KG3 filters (infrared filters). Timeresolved emission measurements are reported in the next section. The spectral profile of the emission was not investigated in the present work. Fig. 4 presents the probe transmittance at a nanosecond timescale for the two solvents, the two wavelengths and different input pump fluences. At low input fluence (between 40 and 100 mJ / cm 2 for water suspensions and between a few and 100 mJ / cm 2 for chloroform suspensions at 1064 nm), we observe a probe perturbation which occurs a few nanoseconds after the pump pulse. The decrease of the probe transmittance occurs too late to induce any optical limiting property for 5-ns width pulses (see Fig. 8b). Such a behavior is not compatible with a nonlinear absorption effect (multiphoton absorption or reverse saturable absorption), but is rather characteristic of vapor bubble growth due to heat transfer from the carbon nanotubes to the surrounding liquid by thermal conduction. Scattering of the probe on these bubbles explains the drop

in the probe transmittance. At 532 nm, the probe perturbation occurs at a few mJ / cm 2 for both solvents. Because of pump parasits (central sensitivity of our photomultiplicator tube), we did not determine precisely the threshold of the solvent bubble growth. At the optical limiting threshold, the perturbation begins at the top of the incident pump pulse and develops much faster, the minimum probe transmittance being reached within a few nanoseconds. This behaviour suggests another nonlinear mechanism, like a sublimation of the nanotubes or microplasma formation, as also suggested by the observation of a short white light flash in the focal zone of the samples. At larger incident fluences, the probe perturbation arises even earlier, i.e. several nanoseconds before the top of the incident pulse. After a few microseconds, the probe transmittance exhibits more spectacular features, especially at high incident pump fluences (Fig. 5). One observes strong oscillations of the signal, with increasing periods for increasing fluences. These fluctuations are characteristic of the formation and collapsing of cavitation bubbles which scatter the probe beam, each local maximum of the signal corresponding to a minimum bubble radius. The cavitation phenomenon is due to the coalescence of laser-initiated microbubbles, leading to macroscopic bubbles. It can be pictured as a spring oscillating around its position of equilibrium. Such an oscillating signal can arise either from a single bubble or from a cumulative effect (several bubbles oscillating in phase). Initially the internal pressure of the bubble exceeds strongly the hydrostatic equilibrium pressure leading to fast expansion, followed by collapsing when the gas pressure falls below the equilibrium pressure. Then several

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Fig. 4. Comparison of probe transmittance for nanosecond timescale. SWNT suspended: in (a) water and in (b) chloroform at 1064 nm, (c) in water and (d) in chloroform at 532 nm. The pump pulse intensity is in solid line.

Fig. 5. Comparison of probe transmittance for microsecond timescale. SWNT suspended: in (a) water and (b) chloroform at 1064 nm, (c) in water and (d) in chloroform at 532 nm. The pump pulse intensity is in solid line.

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rebounds occur, the decrease of the period indicating a decrease of the modulation amplitude of the bubble radius after successive rebounds due to solvent viscosity and surface tension effects. The hydrostatic equilibrium is finally reached after a couple of oscillations (Fig. 5, low fluences). The probe transmittance recovers its initial value when the bubbles escape from the scattering volume and this may occur on a longer time scale, up to a few milliseconds depending on the incident pump fluence. The cavitation threshold for single-wall carbon nanotubes in water at 1064 nm is very low, about 7 J / cm 2 . As a comparison, carbon black suspensions do not produce cavitation bubbles in water, even at 50 J / cm 2 . That difference can be related to the larger specific surface of nanotubes bundles as compared to quasispherical carbon black particles, allowing a more efficient heat transfer to the solvent and subsequent evaporation, and possibly to a larger absorption of SWNT at 1064 nm. At 532 nm the cavitation threshold is around 200 mJ / cm 2 . In chloroform suspensions at 1064 and at 532 nm, the cavitation threshold is 150 and 45 mJ / cm 2 solvent bubble growth is facilitated in chloroform and occurs more rapidly. Chloroform suspensions are certainly good candidates for optical limiting even for longer pulse durations.

5. Emission study Together with the pump-probe experiments, we coupled emission studies at 1064 nm. The aim of this experiment was to determine the origin of the short white light emission observed in the focal zone of the samples even at naked eyes. We performed a study of that emission via time resolved measurements, using a fast photomultiplier tube. The PMT, whose spectral sensitivity ranges from 400 to 800 nm and whose time response is about 500 ps, collected the light emitted in the direction perpendicular to that of the 1064 nm pump beam [26]. The emission temporal profiles were recorded with pump fluences ranging from 0.06 J / cm 2 (much below the limiting threshold) to about 20 J / cm 2 , with both water [26] and chloroform SWNT suspensions. For both, the dynamical behaviour is nearly the same, with typical durations of about 15 ns. However, the fluence dependence of the emission intensity versus pump fluence exhibits striking features, as shown on Fig. 6. Below the optical limiting threshold, the peak emission increases very fast with pump fluence. At larger fluences the increase is slowed down and the crossover between these two regimes is observed to be rather rough. The pump fluence at the crossover corresponds exactly to

Fig. 6. Maximum of the emission intensity as a function of input fluence for water (filled circles) and chloroform suspensions (open circles) at 1064 nm and 5 ns pulse width (see Ref. [28]).

L. Vivien et al. / Carbon 40 (2002) 1789 – 1797 Table 2 Comparison of optical limiting (O.L.) properties for water and chloroform, for 1000–1064 nm (from Fig. 7) and 532 nm (from Fig. 8) and for various pulse durations 532 nm

1000–1064 nm

3 ns

5 ns

5 ns

80 ns

O.L. threshold (mJ / cm 2 )

Water Chloroform

335 135

150 40

350 150

200 13

T at 4 J / cm 2 (%)

Water Chloroform

40 30

24 13

58 42

20 13

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times of the nonlinear scattering centers. In Fig. 7, we compare the nonlinear transmittance results for measurements with 5-ns pulses at 1064 nm and 80-ns pulses at 1000 nm for both solvents. With short pulses, only the sublimation effect contributes to optical limiting, above a threshold of about 150 mJ / cm 2 . Pump-probe experiments showed that bubble formation occurs from about 10 mJ / cm 2 , but only a few tens of nanoseconds after the pulse, thus it was not efficient in optical limiting for the 5-ns pulse width. By contrast, for 80-ns pulses, solvent bubble growth is expected to contribute efficiently to optical limiting at low fluences. In Fig. 5, one observes that limitation occurs at much lower fluences. In addition, a discontinuity is measured around 150 mJ / cm 2 for chloroform suspensions and around 350 mJ / cm 2 for water

the optical limiting threshold, for both water and chloroform suspensions (see results in Table 2, for Nd:Yag at 1064 nm pump wavelength). We assign this crossover to a phase transition of the nanotubes, i.e. a sublimation which explosively generates hot carbon vapour cavities. Below limiting threshold, one observes blackbody emission which indicates a strong heating of SWNT. At larger fluences, the excess of absorbed energy is consumed principally by the change of state which leads to a relative saturation of the temperature and thus of the emission signal. The short lifetime of the emission seems to rule out microplasmas formation, even if this effect probably occurs anyhow at larger incident fluences, as suggested by the very strong and fluctuating emission signal measured above about 10 J / cm 2 . We also emphasize that the peak emission intensity at the limiting threshold is the same for both solvents, confirming that the emission is only due to SWNTs. In contrast, the input fluence at the crossover is smaller for chloroform. This relates to its thermodynamical properties and especially its low heating conductivity which allows a more effective heating of the nanotubes as compared to water suspensions and therefore leads to sublimation at lower fluences. From the pump-probe and emission studies, the origin of optical limiting for SWNTs suspensions was determined. Two different mechanisms are effective: bubble formation due to solvent evaporation and sublimation of carbon nanotubes.

6. Nonlinear transmittance experiments The experimental set-up for nonlinear transmittance measurements consists in a f/ 30 focusing geometry, using a Q-switched Nd:YAG laser generating 5-ns pulses at 1064 nm or 532 nm and a titanium:sapphire laser generating 80-ns pulses at 1000 nm. In the sections above, we identified the thermal effects responsible of optical limiting. These thermal effects are expected to be very sensitive to pulse duration with respect to the characteristic growth

Fig. 7. Nonlinear transmittance for 80 ns (1000 nm, open circles) and 5 ns (1064 nm, filled circles) pulses for SWNT suspended in water (a) and in chloroform (b).

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suspensions which corresponds exactly to the sublimation threshold. This separates two regimes characterized by two different fluence-dependence of the transmittance. We associate these two regimes to the two different mechanisms discussed above: vapor bubble growth, which is effective from 15 to 150 mJ / cm 2 and from 150 to 350 mJ / cm 2 in chloroform and in water suspensions, respectively. Sublimation of carbon nanotubes occurs at 150 and at 350 mJ / cm 2 in chloroform and in water suspensions, respectively. In Fig. 7b, one observes a third regime in water suspensions above 10 J / cm 2 which we assign to a microplasma formation as proposed in Section 5, but this mechanism have not been definitely demonstrated yet. The longer the pulse duration, the better the efficiency of optical limiting. Finally, we also compare optical limiting for 3- and 5-ns pulses at 532 nm (Fig. 8). Optical limiting is observed to be less effective for 3-ns pulses. This confirms that no

significant nonlinear absorption occurs in carbon nanotubes suspensions by contrast with fullerene C 60 . The 3-ns pulse corresponds to the limit of optical limiting efficiency for SWNT suspensions which was recently measured in picosecond pump-probe experiments [35]. The whole optical limiting results are summarized in Table 2 (optical limiting threshold and induced optical density at 4 J / cm 2 ). This emphasizes the general better optical limiting performances for SWNT suspended in chloroform as compared to water and for longer pulse durations which favor thermal effects.

7. Conclusion The performances and physical origins of optical limiting in carbon nanotubes were investigated in various nonlinear optical limiting experiments. As evidenced in Z-scan experiments, nonlinear scattering is the dominant phenomenon but nonlinear refraction due to thermal effects occurs as well. As far as scattering is concerned, pumpprobe experiments allowed to identify two mechanisms: solvent microbubble growth and sublimation of carbon nanotubes. The former is due to heat transfer from nanotubes to surrounding liquid leading to solvent bubble growth and the latter corresponds to a phase transition (from solid to gas) of nanotubes generating explosive growth of hot carbon vapour cavities. As expected for such thermal effects, optical limiting is more effective for longer pulse durations. This is confirmed in nonlinear transmittance experiments. The thermodynamical properties of the solvent are also relevant parameters. Chloroform was found to be more effective than water for huge limiting effects. In the future, further optimizations of the optical limiting performances of nanotubes may be achieved by the cumulation of different nonlinear effects, for example by functionalizing carbon nanotubes with reverse saturable absorbents.

Acknowledgements This work is a part of the Ph.D. thesis of L.V. supported ´´ ´ ´ by the DGA (Delegation Generale pour l’Armement, Defense Ministry of France). The authors greatly appreciate the high-quality samples from S. Tahir, C. Goze, C. Journet and P. Bernier. We thank M. Andrieux and F. Lafonta for their help and fruitful discussion. E.A. acknowledges the DGA for financial support.

References Fig. 8. Nonlinear transmittance for 3 ns (a) and 5 ns (b) pulses at 532 nm for SWNT suspended in (a) water (open circles) and in (b) chloroform (filled circles).

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