Nonlinear optical and chemical effects in the irradiation of liquid benzene with femtosecond pulses Stanislav L. Kuzmin,* Michal J. Wesolowski, and Walter W. Duley Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West Waterloo, Ontario N2L 3G1, Canada *Corresponding author: [email protected] Received 20 September 2013; revised 14 October 2013; accepted 17 October 2013; posted 22 October 2013 (Doc. ID 198008); published 20 November 2013
We show that a spectral resonance between the π → π absorption band in liquid benzene and the third harmonic (TH) of a propagating 800 nm femtosecond laser beam causes large positive changes in the real refractive index at the TH wavelength. This produces an increase in the third-order optical susceptibility and leads to the enhancement of nonlinear optical effects including TH generation and self-focusing. Enhanced filamentation is observed in liquid benzene, but this effect is not seen in perdeuterated liquid benzene under similar irradiation conditions. Filamentation is associated with the decomposition of benzene molecules, plasma emission from the focal region, and the appearance of carbon nanoparticles. This indicates that a complex chemistry accompanies the onset of filamentation. Chemical products formed under these conditions have been characterized using combined gas chromatography mass spectroscopy techniques. We also find that the presence of a TH filament is indicated by the appearance of a photocurrent and increased electrical conductivity in the solution. This photocurrent is found to be 50–60 times smaller in C6 D6 where the π → π resonance with the TH is much weaker. The intensity dependence of this photocurrent confirms the role played by TH generation in the overall interaction. © 2013 Optical Society of America OCIS codes: (190.4710) Optical nonlinearities in organic materials; (260.3230) Ionization; (260.5130) Photochemistry; (260.5950) Self-focusing. http://dx.doi.org/10.1364/AO.52.008169
1. Introduction
The formation of filaments in the interaction of femtosecond (fs) laser pulses with homogeneous transparent media, such as gases and liquids, is a common phenomenon and well understood [1–6]. One of the specific features of this interaction is the absence of a second harmonic of the laser frequency due to the lack of second-order nonlinear optical interactions in centrosymmetric media [7]. Another characteristic feature is the generation and filamentation of the third harmonic (TH) component that couples with the filament produced at the fundamental frequency of the incident light [8]. It has been shown [1], that the intensity of the TH is proportional to the intensity of
1559-128X/13/338169-07$15.00/0 © 2013 Optical Society of America
the optical field at the fundamental frequency. Coupling between the initial and TH pulses is caused by cross-phase modulation of the refractive index, which imposes a constant phase difference inside the two-colored filament. Most of the previous studies on the defragmentation of benzene by high-intensity laser pulses considered only the irradiation of benzene in the gas phase [9–17] using techniques such as mass spectroscopy. A detailed theoretical analysis of the decomposition process has delineated possible reaction pathways for benzene fragmentation in vacuum [18]. There are fewer studies of the laser-induced fragmentation of benzene in the liquid state [19–23], despite the fact that the parameters describing laser irradiation in the gaseous and liquid phases will be dramatically different. Irradiation of gaseous molecules with highintensity fs laser pulses leads to fragmentation 20 November 2013 / Vol. 52, No. 33 / APPLIED OPTICS
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and the creation of positively charged ions. This situation will be different in the irradiation of molecular liquids with fs laser pulses, since the high-density conditions will result in the equilibration of the concentrations of positive and negative ions and the enhancement of ion–neutral and ion–ion recombination reactions. This is expected to facilitate the production and stabilization of a variety of chemical compounds characteristic of a “frozen-in” high temperature equilibrium. The synthesis of large complex molecules in liquids irradiated with high-energy fs pulses is also enhanced by the unique physical conditions that exist in this interaction. These conditions include the production of a high-pressure, high-temperature, ionized medium in and near the boundary of the filament, together with a shock wave that transfers energy into the surrounding medium. Turbulent motion in the ambient liquid introduces mechanical instabilities along the filament and results in mixing of the processed material with neutral molecules in the liquid. It has been shown [24] that 100 fs laser pulses can cause dramatic heating and plasma formation in a small focal region at incident energies as small as 100 nJ∕pulse. This leads to exposure of the surrounding region to plasma and the creation of a shock wave front that expands out of the focus. The pressure in the shock wave front can reach 1 TPa, which, combined with excitation temperatures ≥104 K, triggers molecular fragmentation initiating the synthesis of new products. There have been many studies of the conditions produced in the laser irradiation of benzene [9–23], but Nakamura et al. [25] were the first to show that prolonged irradiation of liquid benzene results in the generation of large quantities of carbon nanoparticles in solution. In that experiment, a Ti:sapphire laser with an optical amplifier emitted pulses with energy 5 mJ and pulse duration 100 fs. Later, the generation of carbon nanoparticles and the growth of the polyyne chains were reported using much lower laser intensities [26]. Nanoparticles appeared with the laser pulses’ energy nearly 100 times less than that observed by Nakamura. Although the generation of carbon nanoparticles was described in detail, the exact conditions required for their generation and the mechanism that governs their creation is yet to be fully understood. The current study was designed to clarify details of physics and physical–chemical dynamics of the process.
During irradiation, the photocurrent was measured using two plane parallel 0.2 mm × 4 mm × 40 mm stainless steel electrodes separated by 4 mm and centered on the laser focus. The voltage between the plates was kept at 650 V corresponding to an electric field of ∼160 V mm−1. The photocurrents were typically between 0.01 and 1 nA, with a decay time of 1–3 s following cessation of laser excitation. These fields were much smaller than those used in other studies [9–17,19–23] indicating that photoionization was efficient in the irradiation of benzene liquid with 30 fs laser pulses. This suggests that, while the initial ionization was likely due to the multiphoton ionization of benzene molecules, the resulting charges were gathered by other chemical species including carbon nanoparticles, and it was these particles that carried the current in the liquid. Soluble organic compounds formed as a result of irradiation have been studied using gas chromatography. Samples were collected after a 30 min irradiation of benzene by laser pulses at pulse energy of ∼1 mJ. To purify the sample, material extracted from the solution was centrifuged and passed through a ∼1 μm filter. Gas chromatography was combined with mass spectroscopy (GC/MS) in an Agilent 5975B inert Series GC/MS System. 3. Nonlinear Optical Processes During Enhanced Filamentation
Filamentation and other nonlinear optical phenomena in these experiments can be associated with the third-order susceptibility χ 3 [7]. Specific nonlinear third-order processes are described by different susceptibility coefficients. For example, self-focusing of the fundamental light wave and TH pulses are given by χ 3 ω ω − ω ω and χ 3 3ω 3ω − 3ω 3ω, respectively, while generation of the TH arises from the χ 3 ω ω ω 3ω term. Parametric amplification of the TH by the initial light wave is associated with χ 3 3ω ω − ω 3ω, and parametric amplification of the fundamental field by the TH term is due to χ 3 ω 3ω − 3ω ω. From nonlinear
2. Experiment
These irradiation experiments were carried out by focusing the output of a high-power Ti:sapphire laser (30 fs pulse duration, 1 kHz repetition rate) in liquid benzene. The pulse energy was varied from 0.1 to 2.2 mJ. At 2.2 mJ, this corresponded to a maximum intensity in the focal spot of ∼1022 W m−2. The emission spectrum of the laser output extended from 760 to 800 nm (FWHM) (Fig. 1). Liquid benzene (SigmaAldrich 99.8% purity) was contained in a 1 cm × 1 cm quartz cuvette. The focal length of the lens was 4 cm. 8170
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Fig. 1. Spectral components in the interaction.
optics [7], it is known that the susceptibility χ 3 ω ω − ω ω responsible for self-focusing of the initial light increases in the presence of the TH. This can be explained using a semiclassical model, as the expression for the third-order susceptibility at low light intensity with one resonance frequency can be written as [7] χ 3 ω1 ω2 ω3 ω4 K
4 Y
ω2 j1 res
1 ; (1) − ω2j iδωj
where K is a constant, ωres is the resonance frequency, and δ is the absorption coefficient. Thus, the third-order susceptibility is proportional to the product of four first-order susceptibilities calculated at different frequencies. When one ωj is close to the resonance frequency, ωres , then both the real and imaginary parts of the susceptibility have large values producing enhanced third-order nonlinearity together with high absorption at the resonance frequency. The absorption and linear refractive index are components of the same complex function, so they can be related via the Kramers–Kronig theorem. Changes in the real and imaginary components are given by [7] Δnω
ω πc
Z
Δβω0 0 dω ; 0 −∞ ω − ω ∞
(2)
where β is the absorption coefficient. It is apparent that changes in the real refractive index near the absorption line can be large. When the absorption becomes saturated, the effective absorption coefficient for high-intensity light is given by [7] β
β0 ; 1 I∕I s
(3)
where β0 is the absorption coefficient at low intensity, I is the intensity of the light, and I s is the saturation intensity. In this case, the medium becomes transparent at the light frequency. Saturation effects are especially strong when autofocusing occurs. Under these conditions, filamentation increases the intensity of the propagating beam enhancing the nonlinear refractive index, while the effective absorption becomes negligible. This condition exists in the present experiments for light at the TH frequency, resulting in a TH filament that is coupled to the filament produced by the propagation of light at 800 nm. Large changes in the refractive index at the TH frequency lead to an enhancement in the optical susceptibility at 800 nm increasing nonlinear effects at this wavelength. Due to the high absorption of the TH, a TH filament cannot exist when the 800 nm light intensity is relatively small. The TH filament then appears only when the intensity at 800 nm exceeds a threshold value
such that TH generation compensates for absorption at the TH frequency. When the 800 nm intensity approaches the saturation intensity, I s , then the absorption coefficient decreases and a TH filament can appear. Photochemical effects associated with the TH appear to initiate the generation of the chemical precursors involved in the growth of carbon nanoparticles in solution. As long-lived photocurrents are observed under these conditions, the presence of a TH filament can be inferred from the detection of carbon nanoparticles deposited on the electrodes immersed in the solution. The dependence of the photocurrent on laser power is shown in Fig. 2. Our measurements did not detect any photocurrent, ip , at laser focal intensities I < 2 × 1020 W m−2 corresponding to pulse energies 1014 W m−2 and the presence of a high-density ambient ensures that the overall concentration of positively charged products will be the same as that of product anions. Figure 5 shows GC/MS spectra of irradiated liquid C6 H6 in which ∼57 different organic compounds can be identified. A comparison of the mass spectrum of these compounds with the National Institute of Standards and Technology Mass Spectral Library [35] shows that most are common chemicals containing one, two, or three benzene rings terminated with or connected by chains. As expected, the largest molecular fraction arises from biphenyl, which is just two phenyl molecules joined by a single bond. This suggests that the phenyl radical C6 H5 may be a primary reaction product. In fact, the deuterated analog of biphenyl was also the most abundant product after irradiation of liquid C6 D6 , indicating that this structure is readily formed during irradiation. Table 1 lists the relative concentration of product molecules. It is apparent that a number of oxygenated and nitrogenated products are also present in addition to fully hydrogenated hydrocarbons. These appear as the irradiations are carried out in air. Small quantities of silicon and chlorine compounds occur as a result of chemical reactions at the walls of the cuvette.
Fig. 5. GC/MC chromatogram of molecular species generated in liquid C6H6 after fs irradiation. The chemical structures of some molecules are shown. 20 November 2013 / Vol. 52, No. 33 / APPLIED OPTICS
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Table 1. List of Synthesized Organic Molecules Identified with Probability >50% Arranged in Descending Order of Arbitrary Count Number
Nomenclature Biphenyl Benzene, (1,4–cyclohexadiene-1-yl) Tetradecanamide 5-phenylcyclohexane-1,3-dione 2-propenoic acid, pentadecyl ester 13-docosenamide Phenol p-Terphenyl 9-octadecanamide Benzaldehyde Phenylethyne Phthalic acid, 2-ethylhexyl tridecyl ester Silane [[4-[1,2-bis[(trimethylsilyl)oxy]ethyl]1,2-phenylene]bis(oxy)]bis trimethyl Styrene Naphthalene, 2-methyl Benzene, 1,4-diethynyl 2,5–cyclohexadiene-1-yl Benzene, 1-propynyl o-Terphenyl Acetophenone, 2-chloro Furan, 3-methyl p-Xylene 4-phenylbut-3-ene-1-yne Cinnamaldehyde 2,4,6-triazatricyclo[5,2,2,02,6]undec8-ene—3,5-dione, 4-methyl-1,7-diphenyl all-trans-1,6-diphenyl-1,3,5-hexatriene 1,4-ethenonaphtalene-1,4-dihydro 1,3,5,7-cyclooctatetraene Benzene, 1-ethenyl-3-methyl 1,2-benzenecarboxylic acid, butyl 2-ethylhexyl aster 1,1′-biphenyl,2-methyl Naphthalene
Quality Arbitrary Probability Counts 96 96 87 38 91 91 91 98 93 95 94 50 53
510 95 54 50 50 50 46 30 30 28 26 22 18
94 80 80 93 90 93 72 64 90 50 90 87
12 12 10 10 8 8 7 7 6 5 5 5
64 76 68 58 64
5 5 5 3 3
96 86
3 3
5. Conclusions
In this paper, we investigate the mechanism responsible for enhanced filamentation accompanying the propagation of fs laser pulses in liquid benzene and the effect this has on the generation of carbon nanoparticles. We show that enhanced filamentation is caused by a unique combination of optical conditions involving the generation and propagation of a TH component due to a narrow spectral resonance between the TH signal and the π → π absorption band in liquid C6 H6 . This resonance leads to a strong modulation of the refractive index at the wavelength of the TH component and the enhancement of nonlinear optical effects. The wavelength range corresponding to the increase in refractive index is nearly 3–5 times smaller than that of the absorption band and is located on the short wavelength edge of the π → π band in C6 H6 . Thus, a small shift in the wavelength of the π → π band as occurs in C6 D6 can detune this resonance, inhibiting filamentation. This appears to be the reason why enhanced filamentation does not occur in 8174
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the fs irradiation of C6 D6 and is consistent with the observation of a lower concentration of nanoparticles in this liquid. The intensity of the nonlinear optical self-focusing process facilitates generation of the TH component. We find that the photocurrent increases with the strength of the TH component as photochemical effects lead to the generation of a higher concentration of carbon nanoparticles in the liquid. The photocurrent is found to exhibit a threshold at an intensity of ∼2 × 1020 W m−2 corresponding to the saturation of absorption in the π → π band. Complex chemical reactions in the region of the laser focus and in the surrounding medium produce a variety of molecular species as well as carbon nanoparticles. Analysis of the composition of this material indicates that at least 57 distinct compounds are found after irradiation. This work was supported by the NSERC of Canada. References 1. W. Liu, S. Petit, A. Becker, N. Akozbek, C. M. Bowden, and S. L. Chin, “Intensity clamping of a femtosecond laser pulse in condensed matter,” Opt. Commun. 202, 189–197 (2002). 2. M. Centini, V. Roppo, E. Fazio, F. Pettazzi, C. Sibilia, J. W. Haus, J. V. Foreman, N. Akozbek, M. J. Bloemer, and M. Scalora, “Inhibition of linear absorption in opaque materials using phase-locked harmonic generation,” Phys. Rev. Lett. 101, 113905 (2008). 3. S. L. Chin, W. Liu, F. Theberge, Q. Luo, S. A. Hosseini, V. P. Kandidov, O. G. Kosareva, N. Aközbek, A. Becker, and H. Schroeder, Some Fundamental Concepts of Femtosecond Laser Filamentation (Springer, 2008). 4. A. Couairona and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441, 47–189 (2007). 5. C. D’Amico, B. Prade, M. Franco, and A. Mysyrowicz, “Femtosecond filament amplification in liquids,” Appl. Phys. B 85, 49–53 (2006). 6. H. Schroeder, S. A. Hosseini, Q. Luo, and S. L. Chin, “Selfsteepening is an abrupt process,” Opt. Commun. 266, 302–306 (2006). 7. R. W. Boyd, Nonlinear Optics (Academic, 2008). 8. N. Akozbek, A. Iwasaki, A. Becker, M. Scalora, S. L. Chin, and C. M. Bowden, “Third-harmonic generation and selfchanneling in air using high-power femtosecond laser pulses,” Phys. Rev. Lett. 89, 143901 (2002). 9. L. Zandee and R. B. Bernstein, “Resonance-enhanced multiphoton ionization and fragmentation of molecular beams: NO, I2, benzene and butadiene,” J. Chem. Phys. 71, 1359–1371 (1979). 10. A. Talebpour, A. D. Bandrauk, K. Vijayalakshmi, and S. L. Chin, “Dissociative ionization of benzene in intense ultra-fast laser pulses,” J. Phys. B 33, 4615–4626 (2000). 11. D. J. Smith, K. W. D. Ledingham, R. P. Singhal, H. S. Kilic, T. McCanny, A. J. Langley, P. F. Taday, and C. Kosmidis, “Timeof-flight mass spectrometry of aromatic molecules subjected to high intensity laser beams,” Rapid Commun. Mass Spectrom. 12, 813–820 (1998). 12. J. Momigny, C. Goffart, and L. D’or, “Photoionization studies by total ionization measurements. I. Benzene and its monohalogeno derivatives,” Int. J. Mass Spectrom. Ion Phys. 1, 53–68 (1968). 13. J. H. Miller, L. Andrews, P. A. Lund, and P. N. Schatz, “Argon matrix photolysis and photoionization studies of benzene. Absorption spectrum of benzene cation and benzene dimer cation,” J. Chem. Phys. 73, 4932–4939 (1980). 14. P. M. Johnson, “The multiphoton ionization spectrum of benzene,” J. Chem. Phys. 64, 4143–4148 (1976). 15. R. Itakura, J. Watanabe, A. Hishikawa, and K. Yamanouchi, “Ionization and fragmentation dynamics of benzene in intense
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25. T. Nakamura, Y. Mochidzuki, and S. Sato, Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies (Optical Society of America, 2007). 26. M. J. Wesolowski, S. Kuzmin, B. Moores, B. Wales, R. Karimi, A. A. Zaidi, Z. Leonenko, J. H. Sanderson, and W. W. Duley, “Polyyne synthesis and amorphous carbon nano-particle formation by femtosecond irradiation of benzene,” Carbon 49, 625–630 (2011). 27. T. V. K. Sarma and C. Santhamm, “The near ultraviolet absorption spectra of 2,3-, 2,4- and 2,5-dimethylbenzonitriles,” Spectrochim. Acta 43A, 929–937 (1987). 28. I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules (Academic, 1965). 29. T. Inagaki, “Absorption spectra of pure liquid benzene in the ultraviolet region,” J. Chem. Phys. 57, 2526–2530 (1972). 30. G. Herzberg, Molecular Spectra and Molecular Structure (Van Nostrand Reinhold, 1967), p. 178. 31. I. A. Shkrob and M. C. Sauer, “Electron localization in liquid acetonitrile,” J. Phys. Chem. A 106, 9120–9131 (2002). 32. V. S. Antonov and V. S. Letokhov, “Laser multiphoton and multistep photoionization of molecules and mass spectrometry,” Appl. Phys. 24, 89–106 (1981). 33. H. M. Boechat-Roberty, R. Neves, S. Pilling, A. F. Lago, and G. G. B. de Souza, “Dissociation of the benzene molecule by UV and soft x-rays in circumstellar environment,” Mon. Not. R. Astron. Soc. 394, 810–817 (2009). 34. W. Fub, W. E. Schmid, and S. A. Trushin, “Time-resolved dissociative intense-laser field ionization for probing dynamics: femtosecond photochemical ring opening of 1,3cyclohexadiene,” J. Chem. Phys. 112, 8347–8362 (2000). 35. Standard Reference Data, NIST: 100 Bureau Drive, Stop 2300 Gaithersburg, MD, 20899–2300.
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We show that a spectral resonance between the π → π absorption band in liquid benzene and the third harmonic (TH) of a propagating 800 nm femtosecond laser beam causes large positive changes in the real refractive index at the TH wavelength. This produces an increase in the third-order optical susceptibility and leads to the enhancement of nonlinear optical effects including TH generation and self-focusing. Enhanced filamentation is observed in liquid benzene, but this effect is not seen in perdeuterated liquid benzene under similar irradiation conditions. Filamentation is associated with the decomposition of benzene molecules, plasma emission from the focal region, and the appearance of carbon nanoparticles. This indicates that a complex chemistry accompanies the onset of filamentation. Chemical products formed under these conditions have been characterized using combined gas chromatography mass spectroscopy techniques. We also find that the presence of a TH filament is indicated by the appearance of a photocurrent and increased electrical conductivity in the solution. This photocurrent is found to be 50–60 times smaller in C6 D6 where the π → π resonance with the TH is much weaker. The intensity dependence of this photocurrent confirms the role played by TH generation in the overall interaction. © 2013 Optical Society of America OCIS codes: (190.4710) Optical nonlinearities in organic materials; (260.3230) Ionization; (260.5130) Photochemistry; (260.5950) Self-focusing. http://dx.doi.org/10.1364/AO.52.008169
1. Introduction
The formation of filaments in the interaction of femtosecond (fs) laser pulses with homogeneous transparent media, such as gases and liquids, is a common phenomenon and well understood [1–6]. One of the specific features of this interaction is the absence of a second harmonic of the laser frequency due to the lack of second-order nonlinear optical interactions in centrosymmetric media [7]. Another characteristic feature is the generation and filamentation of the third harmonic (TH) component that couples with the filament produced at the fundamental frequency of the incident light [8]. It has been shown [1], that the intensity of the TH is proportional to the intensity of
1559-128X/13/338169-07$15.00/0 © 2013 Optical Society of America
the optical field at the fundamental frequency. Coupling between the initial and TH pulses is caused by cross-phase modulation of the refractive index, which imposes a constant phase difference inside the two-colored filament. Most of the previous studies on the defragmentation of benzene by high-intensity laser pulses considered only the irradiation of benzene in the gas phase [9–17] using techniques such as mass spectroscopy. A detailed theoretical analysis of the decomposition process has delineated possible reaction pathways for benzene fragmentation in vacuum [18]. There are fewer studies of the laser-induced fragmentation of benzene in the liquid state [19–23], despite the fact that the parameters describing laser irradiation in the gaseous and liquid phases will be dramatically different. Irradiation of gaseous molecules with highintensity fs laser pulses leads to fragmentation 20 November 2013 / Vol. 52, No. 33 / APPLIED OPTICS
8169
and the creation of positively charged ions. This situation will be different in the irradiation of molecular liquids with fs laser pulses, since the high-density conditions will result in the equilibration of the concentrations of positive and negative ions and the enhancement of ion–neutral and ion–ion recombination reactions. This is expected to facilitate the production and stabilization of a variety of chemical compounds characteristic of a “frozen-in” high temperature equilibrium. The synthesis of large complex molecules in liquids irradiated with high-energy fs pulses is also enhanced by the unique physical conditions that exist in this interaction. These conditions include the production of a high-pressure, high-temperature, ionized medium in and near the boundary of the filament, together with a shock wave that transfers energy into the surrounding medium. Turbulent motion in the ambient liquid introduces mechanical instabilities along the filament and results in mixing of the processed material with neutral molecules in the liquid. It has been shown [24] that 100 fs laser pulses can cause dramatic heating and plasma formation in a small focal region at incident energies as small as 100 nJ∕pulse. This leads to exposure of the surrounding region to plasma and the creation of a shock wave front that expands out of the focus. The pressure in the shock wave front can reach 1 TPa, which, combined with excitation temperatures ≥104 K, triggers molecular fragmentation initiating the synthesis of new products. There have been many studies of the conditions produced in the laser irradiation of benzene [9–23], but Nakamura et al. [25] were the first to show that prolonged irradiation of liquid benzene results in the generation of large quantities of carbon nanoparticles in solution. In that experiment, a Ti:sapphire laser with an optical amplifier emitted pulses with energy 5 mJ and pulse duration 100 fs. Later, the generation of carbon nanoparticles and the growth of the polyyne chains were reported using much lower laser intensities [26]. Nanoparticles appeared with the laser pulses’ energy nearly 100 times less than that observed by Nakamura. Although the generation of carbon nanoparticles was described in detail, the exact conditions required for their generation and the mechanism that governs their creation is yet to be fully understood. The current study was designed to clarify details of physics and physical–chemical dynamics of the process.
During irradiation, the photocurrent was measured using two plane parallel 0.2 mm × 4 mm × 40 mm stainless steel electrodes separated by 4 mm and centered on the laser focus. The voltage between the plates was kept at 650 V corresponding to an electric field of ∼160 V mm−1. The photocurrents were typically between 0.01 and 1 nA, with a decay time of 1–3 s following cessation of laser excitation. These fields were much smaller than those used in other studies [9–17,19–23] indicating that photoionization was efficient in the irradiation of benzene liquid with 30 fs laser pulses. This suggests that, while the initial ionization was likely due to the multiphoton ionization of benzene molecules, the resulting charges were gathered by other chemical species including carbon nanoparticles, and it was these particles that carried the current in the liquid. Soluble organic compounds formed as a result of irradiation have been studied using gas chromatography. Samples were collected after a 30 min irradiation of benzene by laser pulses at pulse energy of ∼1 mJ. To purify the sample, material extracted from the solution was centrifuged and passed through a ∼1 μm filter. Gas chromatography was combined with mass spectroscopy (GC/MS) in an Agilent 5975B inert Series GC/MS System. 3. Nonlinear Optical Processes During Enhanced Filamentation
Filamentation and other nonlinear optical phenomena in these experiments can be associated with the third-order susceptibility χ 3 [7]. Specific nonlinear third-order processes are described by different susceptibility coefficients. For example, self-focusing of the fundamental light wave and TH pulses are given by χ 3 ω ω − ω ω and χ 3 3ω 3ω − 3ω 3ω, respectively, while generation of the TH arises from the χ 3 ω ω ω 3ω term. Parametric amplification of the TH by the initial light wave is associated with χ 3 3ω ω − ω 3ω, and parametric amplification of the fundamental field by the TH term is due to χ 3 ω 3ω − 3ω ω. From nonlinear
2. Experiment
These irradiation experiments were carried out by focusing the output of a high-power Ti:sapphire laser (30 fs pulse duration, 1 kHz repetition rate) in liquid benzene. The pulse energy was varied from 0.1 to 2.2 mJ. At 2.2 mJ, this corresponded to a maximum intensity in the focal spot of ∼1022 W m−2. The emission spectrum of the laser output extended from 760 to 800 nm (FWHM) (Fig. 1). Liquid benzene (SigmaAldrich 99.8% purity) was contained in a 1 cm × 1 cm quartz cuvette. The focal length of the lens was 4 cm. 8170
APPLIED OPTICS / Vol. 52, No. 33 / 20 November 2013
Fig. 1. Spectral components in the interaction.
optics [7], it is known that the susceptibility χ 3 ω ω − ω ω responsible for self-focusing of the initial light increases in the presence of the TH. This can be explained using a semiclassical model, as the expression for the third-order susceptibility at low light intensity with one resonance frequency can be written as [7] χ 3 ω1 ω2 ω3 ω4 K
4 Y
ω2 j1 res
1 ; (1) − ω2j iδωj
where K is a constant, ωres is the resonance frequency, and δ is the absorption coefficient. Thus, the third-order susceptibility is proportional to the product of four first-order susceptibilities calculated at different frequencies. When one ωj is close to the resonance frequency, ωres , then both the real and imaginary parts of the susceptibility have large values producing enhanced third-order nonlinearity together with high absorption at the resonance frequency. The absorption and linear refractive index are components of the same complex function, so they can be related via the Kramers–Kronig theorem. Changes in the real and imaginary components are given by [7] Δnω
ω πc
Z
Δβω0 0 dω ; 0 −∞ ω − ω ∞
(2)
where β is the absorption coefficient. It is apparent that changes in the real refractive index near the absorption line can be large. When the absorption becomes saturated, the effective absorption coefficient for high-intensity light is given by [7] β
β0 ; 1 I∕I s
(3)
where β0 is the absorption coefficient at low intensity, I is the intensity of the light, and I s is the saturation intensity. In this case, the medium becomes transparent at the light frequency. Saturation effects are especially strong when autofocusing occurs. Under these conditions, filamentation increases the intensity of the propagating beam enhancing the nonlinear refractive index, while the effective absorption becomes negligible. This condition exists in the present experiments for light at the TH frequency, resulting in a TH filament that is coupled to the filament produced by the propagation of light at 800 nm. Large changes in the refractive index at the TH frequency lead to an enhancement in the optical susceptibility at 800 nm increasing nonlinear effects at this wavelength. Due to the high absorption of the TH, a TH filament cannot exist when the 800 nm light intensity is relatively small. The TH filament then appears only when the intensity at 800 nm exceeds a threshold value
such that TH generation compensates for absorption at the TH frequency. When the 800 nm intensity approaches the saturation intensity, I s , then the absorption coefficient decreases and a TH filament can appear. Photochemical effects associated with the TH appear to initiate the generation of the chemical precursors involved in the growth of carbon nanoparticles in solution. As long-lived photocurrents are observed under these conditions, the presence of a TH filament can be inferred from the detection of carbon nanoparticles deposited on the electrodes immersed in the solution. The dependence of the photocurrent on laser power is shown in Fig. 2. Our measurements did not detect any photocurrent, ip , at laser focal intensities I < 2 × 1020 W m−2 corresponding to pulse energies 1014 W m−2 and the presence of a high-density ambient ensures that the overall concentration of positively charged products will be the same as that of product anions. Figure 5 shows GC/MS spectra of irradiated liquid C6 H6 in which ∼57 different organic compounds can be identified. A comparison of the mass spectrum of these compounds with the National Institute of Standards and Technology Mass Spectral Library [35] shows that most are common chemicals containing one, two, or three benzene rings terminated with or connected by chains. As expected, the largest molecular fraction arises from biphenyl, which is just two phenyl molecules joined by a single bond. This suggests that the phenyl radical C6 H5 may be a primary reaction product. In fact, the deuterated analog of biphenyl was also the most abundant product after irradiation of liquid C6 D6 , indicating that this structure is readily formed during irradiation. Table 1 lists the relative concentration of product molecules. It is apparent that a number of oxygenated and nitrogenated products are also present in addition to fully hydrogenated hydrocarbons. These appear as the irradiations are carried out in air. Small quantities of silicon and chlorine compounds occur as a result of chemical reactions at the walls of the cuvette.
Fig. 5. GC/MC chromatogram of molecular species generated in liquid C6H6 after fs irradiation. The chemical structures of some molecules are shown. 20 November 2013 / Vol. 52, No. 33 / APPLIED OPTICS
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Table 1. List of Synthesized Organic Molecules Identified with Probability >50% Arranged in Descending Order of Arbitrary Count Number
Nomenclature Biphenyl Benzene, (1,4–cyclohexadiene-1-yl) Tetradecanamide 5-phenylcyclohexane-1,3-dione 2-propenoic acid, pentadecyl ester 13-docosenamide Phenol p-Terphenyl 9-octadecanamide Benzaldehyde Phenylethyne Phthalic acid, 2-ethylhexyl tridecyl ester Silane [[4-[1,2-bis[(trimethylsilyl)oxy]ethyl]1,2-phenylene]bis(oxy)]bis trimethyl Styrene Naphthalene, 2-methyl Benzene, 1,4-diethynyl 2,5–cyclohexadiene-1-yl Benzene, 1-propynyl o-Terphenyl Acetophenone, 2-chloro Furan, 3-methyl p-Xylene 4-phenylbut-3-ene-1-yne Cinnamaldehyde 2,4,6-triazatricyclo[5,2,2,02,6]undec8-ene—3,5-dione, 4-methyl-1,7-diphenyl all-trans-1,6-diphenyl-1,3,5-hexatriene 1,4-ethenonaphtalene-1,4-dihydro 1,3,5,7-cyclooctatetraene Benzene, 1-ethenyl-3-methyl 1,2-benzenecarboxylic acid, butyl 2-ethylhexyl aster 1,1′-biphenyl,2-methyl Naphthalene
Quality Arbitrary Probability Counts 96 96 87 38 91 91 91 98 93 95 94 50 53
510 95 54 50 50 50 46 30 30 28 26 22 18
94 80 80 93 90 93 72 64 90 50 90 87
12 12 10 10 8 8 7 7 6 5 5 5
64 76 68 58 64
5 5 5 3 3
96 86
3 3
5. Conclusions
In this paper, we investigate the mechanism responsible for enhanced filamentation accompanying the propagation of fs laser pulses in liquid benzene and the effect this has on the generation of carbon nanoparticles. We show that enhanced filamentation is caused by a unique combination of optical conditions involving the generation and propagation of a TH component due to a narrow spectral resonance between the TH signal and the π → π absorption band in liquid C6 H6 . This resonance leads to a strong modulation of the refractive index at the wavelength of the TH component and the enhancement of nonlinear optical effects. The wavelength range corresponding to the increase in refractive index is nearly 3–5 times smaller than that of the absorption band and is located on the short wavelength edge of the π → π band in C6 H6 . Thus, a small shift in the wavelength of the π → π band as occurs in C6 D6 can detune this resonance, inhibiting filamentation. This appears to be the reason why enhanced filamentation does not occur in 8174
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the fs irradiation of C6 D6 and is consistent with the observation of a lower concentration of nanoparticles in this liquid. The intensity of the nonlinear optical self-focusing process facilitates generation of the TH component. We find that the photocurrent increases with the strength of the TH component as photochemical effects lead to the generation of a higher concentration of carbon nanoparticles in the liquid. The photocurrent is found to exhibit a threshold at an intensity of ∼2 × 1020 W m−2 corresponding to the saturation of absorption in the π → π band. Complex chemical reactions in the region of the laser focus and in the surrounding medium produce a variety of molecular species as well as carbon nanoparticles. Analysis of the composition of this material indicates that at least 57 distinct compounds are found after irradiation. This work was supported by the NSERC of Canada. References 1. W. Liu, S. Petit, A. Becker, N. Akozbek, C. M. Bowden, and S. L. Chin, “Intensity clamping of a femtosecond laser pulse in condensed matter,” Opt. Commun. 202, 189–197 (2002). 2. M. Centini, V. Roppo, E. Fazio, F. Pettazzi, C. Sibilia, J. W. Haus, J. V. Foreman, N. Akozbek, M. J. Bloemer, and M. Scalora, “Inhibition of linear absorption in opaque materials using phase-locked harmonic generation,” Phys. Rev. Lett. 101, 113905 (2008). 3. S. L. Chin, W. Liu, F. Theberge, Q. Luo, S. A. Hosseini, V. P. Kandidov, O. G. Kosareva, N. Aközbek, A. Becker, and H. Schroeder, Some Fundamental Concepts of Femtosecond Laser Filamentation (Springer, 2008). 4. A. Couairona and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441, 47–189 (2007). 5. C. D’Amico, B. Prade, M. Franco, and A. Mysyrowicz, “Femtosecond filament amplification in liquids,” Appl. Phys. B 85, 49–53 (2006). 6. H. Schroeder, S. A. Hosseini, Q. Luo, and S. L. Chin, “Selfsteepening is an abrupt process,” Opt. Commun. 266, 302–306 (2006). 7. R. W. Boyd, Nonlinear Optics (Academic, 2008). 8. N. Akozbek, A. Iwasaki, A. Becker, M. Scalora, S. L. Chin, and C. M. Bowden, “Third-harmonic generation and selfchanneling in air using high-power femtosecond laser pulses,” Phys. Rev. Lett. 89, 143901 (2002). 9. L. Zandee and R. B. Bernstein, “Resonance-enhanced multiphoton ionization and fragmentation of molecular beams: NO, I2, benzene and butadiene,” J. Chem. Phys. 71, 1359–1371 (1979). 10. A. Talebpour, A. D. Bandrauk, K. Vijayalakshmi, and S. L. Chin, “Dissociative ionization of benzene in intense ultra-fast laser pulses,” J. Phys. B 33, 4615–4626 (2000). 11. D. J. Smith, K. W. D. Ledingham, R. P. Singhal, H. S. Kilic, T. McCanny, A. J. Langley, P. F. Taday, and C. Kosmidis, “Timeof-flight mass spectrometry of aromatic molecules subjected to high intensity laser beams,” Rapid Commun. Mass Spectrom. 12, 813–820 (1998). 12. J. Momigny, C. Goffart, and L. D’or, “Photoionization studies by total ionization measurements. I. Benzene and its monohalogeno derivatives,” Int. J. Mass Spectrom. Ion Phys. 1, 53–68 (1968). 13. J. H. Miller, L. Andrews, P. A. Lund, and P. N. Schatz, “Argon matrix photolysis and photoionization studies of benzene. Absorption spectrum of benzene cation and benzene dimer cation,” J. Chem. Phys. 73, 4932–4939 (1980). 14. P. M. Johnson, “The multiphoton ionization spectrum of benzene,” J. Chem. Phys. 64, 4143–4148 (1976). 15. R. Itakura, J. Watanabe, A. Hishikawa, and K. Yamanouchi, “Ionization and fragmentation dynamics of benzene in intense
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