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Influence of dopants on supercontinuum generation during the femtosecond laser filamentation in water
Accepted Manuscript Research paper Influence of dopants on supercontinuum generation during the femtosecond laser filamentation in water He Li, Zhe Shi, Xiaowei Wang, Laizhi Sui, Suyu Li, Mingxing Jin PII: DOI: Reference:
S0009-2614(17)30467-0 http://dx.doi.org/10.1016/j.cplett.2017.05.029 CPLETT 34818
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
14 April 2017 10 May 2017
Please cite this article as: H. Li, Z. Shi, X. Wang, L. Sui, S. Li, M. Jin, Influence of dopants on supercontinuum generation during the femtosecond laser filamentation in water, Chemical Physics Letters (2017), doi: http:// dx.doi.org/10.1016/j.cplett.2017.05.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of dopants on supercontinuum generation during the femtosecond laser filamentation in water He Lia,b, Zhe Shia,b, Xiaowei Wanga,b, Laizhi Suia,b, Suyu Lia,b,* Mingxing Jina,b* a
Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012,
China b
Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy
(Jilin University), Changchun 130012, China c
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
and Engineering, Jilin University, Changchun 130012, China Our experiments show that lactose as one of organic substance and nitric acid as one of inorganic substance added into distilled water can influence the supercontinuum generation in aqueous solution irradiated by the ultrashort femtosecond laser pulses. It is found that once the dopants are added into the water, the supercontinuum generation is suppressed to different extent, and the supercontinuum suppression is enhanced by increasing the concentrations of lactose solution and nitric acid solution. Through the analysis, we find that the capture of electrons by the solvent, and spectral absorption or scattering by the solution may also result in the supercontinuum suppression. These studies will be helpful to understanding of the supercontinuum generation during femtosecond filamentation in liquid samples. Keywords: asymmetric broadening, supercontinuum suppression, absorption spectra 1. Introduction Laser filamentation is well known to be a dynamically balanced process between the self-focusing resulted from Kerr effect and plasma defocusing resulted from the multiphoton/tunning ionization [1,2]. Many nonlinear effects, such as fluorescence emission [3,4], terahertz radiation [5,6] and supercontinuum generation [7,8] etc. are involved in the process of filamentation. Among these nonlinear effects, the supercontinuum generation attracts a great deal of interest due to its potential and *
Corresponding authors at: Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012,
China. E-mail addresses: sylee@ jlu.edu.cn (S. Li), [email protected] (M. Jin).
promising applications in remote sensing [9], biomedical imaging [10], cavity ring-down spectroscopy [11], generation of few-cycle femtosecond pulses [12], and so on. To the best of our knowledge, a series of experimental and theoretical studies of the generation of white light during the filamentation of a high-power femtosecond laser pulse in water have been carried out [13-15]. Since Jimbo et al. measured the supercontinuum generation in water by the addition of small quantities of metal ions like Zn 2+ and K + [16], the effect of dopants on supercontinuum generation has been
a hot research area. In recently years, Santhosh et al. have researched the supercontinuum broadening in water doped with small quantities of proteins [17,18]. It is demonstrated that the dopants weaken the plasma effect by scavenging electrons which is a major cause for supercontinuum suppression. Meanwhile, there are other factors that may result in supercontinuum suppression. For example, it is known that there is an obvious light absorption by water molecules when light beam passes through water [19]. The dopants added to water may lead to the absorption or scattering of light. Under this circumstance, supercontinuum is suppressed, however, which is less studied. In this letter, we experimentally study the supercontinuum generated from the interaction between femtosecond laser pulses and lactose solution and nitric acid solution with different concentrations, and discuss the influence of these two kinds of dopants on supercontinuum generation. Based on this study, we attempt to discuss the mechanism for the supercontinuum suppression in aqueous solution. 2. Experiment setup
H 800nm@1kHz
Laser
M
G
I
L Sample cell
Spectrometer Avantes
Fig. 1. Schematic of experimental setup to measure the supercontinuum generated during the femtosecond filamentation in different liquid samples. H: half-wave plate; G: Glan prism; L: focusing lens; I: integrating sphere.
The experimental setup is shown in Figure 1. A Ti: sapphire laser system is used to generate laser pulses at central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1 kHz. The combination of a half-wave plate and a Glan laser polarizer attenuates laser energy to the desired value. The lens L with focal length of 400 mm is used to focus the laser beam into a quartz sample cell (50 mm×10 mm×40 mm) filled with liquid samples (e.g. distilled water, lactose solution and nitric acid solution, etc.). To avoid any supercontinuum contribution from the way of sample cell, the laser beam is focused close to the center of the sample cell. The generated supercontinuum is collected by integrating sphere and then guided to the Avantes spectrometer (AvaSpec-1650F-USB2) through an optical fiber. 3. Result and Discussion 3.1 The supercontinuum suppression in distilled water It
is
known that
the
critical power
for
self-focusing
in water
is
Pcr = l 2 2p n0 n2 = 4.4 MW , where n0 and n2 denote the linear and nonlinear refractive indices of medium, respectively. As the nonlinear refractive index in water is larger than that in air, supercontinuum can be generated when the laser pulse with lower incident power injects into water. A typical white light spectrum obtained with triply distilled water is shown in Ref. [18]. As is known, the spectral broadening is essentially symmetric around 800 nm, which is mainly due to self-phase modulation (SPM) that arises from the Kerr nonlinearity effect. In addition, asymmetry generates towards the blue side of the spectrum. This component arises due to processes such as space-time focusing, self-steepening, and plasma formation that arises from free electrons are generated by multiphoton ionization [17,20]. In Ref. [21], asymmetric broadening by considering separately the contributions to the refractive index from the Kerr nonlinearity ∆nk and the plasma component ∆np has been modeled. Considering plane waves propagating in the nonlinear medium, the frequency deviation at a certain propagation distance z can be written as [22]:
dw (t ) = -kz
¶(Dnk + Dn p ) ¶t
(1)
where ∆nk denotes the index from the Kerr nonlinearity, ∆np denotes the index from the plasma, z is along the propagation axis, k is the wave number, and t denotes time. In practice, Eq. (1) can be rewritten in the following form:
dw (t ) = -a
¶I (t ) + bI K (t ) ¶t
(2)
where a = 4p n2 kz / c , b = 2p ne2 b ( K ) N0 kz / (mew02 ) are time-dependent and b ( K ) is the K-photon absorption coefficient of water. n2 is the nonlinear refractive index, N0 is the number of initially present neutral particles in the focal volume, and me is electron mass. In Ref. 21, it was pointed out that the symmetrical spectral broadening can be observed at low laser energies (0.4–1.8 µJ) and the asymmetric broadening at higher laser energies (2–200 µJ). The symmetrical spectral broadening can be attributed to the frequency shift due to the nonlinear refractive index n2 of the material [i.e., the first term in Eq. (2)] which is resulted from the interaction of the laser with the bound electrons: If n2 is positive, lower frequency components (i.e., red-shift Dw- ) are generated in the leading edge and higher frequency components (i.e., blue-shift Dw+ ) are generated in the trailing edge of the pulse. The reason of the asymmetrical spectral broadening is that through the interaction of the laser with (quasi-) free electrons [namely the second term in Eq. (2)] generated via multiphoton excitation causes the spectral broadening, it results in the blue-shift of spectrum only. In our work, the energy of incident laser pulse is relatively high (higher than 2 μJ), and only the asymmetrical spectral broadening can be observed, which will be discussed in the following part.
G
G
WU_ WU] G
u¡G
XUW
WU[ WUY WUW [WW [\W \WW \\W ]WW ]\W ^WW ^\W
~GOP Fig. 2. Typical supercontinuum (from 400 nm to 770 nm) generated when the triply-distilled water is irradiated by the femtosecond laser pulse. The incident pulse energy is 110 μJ.
Fig. 2 shows the typical supercontinuum (from 400 nm to 770 nm) generated when the triply-distilled water is irradiated by the femtosecond laser pulse. The relatively sharp and obvious dip that can be observed is due to the absorption on the higher frequency side. It is known that the water can emit by monochromatic light of frequency ν0 with an intense continuum. H2O molecules within the laser focal volume are stimulated so that they emit at frequency ν 0 and, at the same time, absorb the emission at frequencies corresponding to (ν 0+νm) and (ν0-νm), where hn m is the energy separation between different quantum states in H2O. The position of the dip is irrespective of conditions like levels of dopants introduced into the water sample, or variations in laser intensity and focusing conditions that have been represented in Ref. [18]. This phenomenon may be mainly caused by inverse Raman effect [23]. 3.2 The supercontinuum suppression in lactose solution In our experiments, 0.04 g lactose is dissolved in 10 mL distilled water as the original solution, then we dilute the solution with distill water according to the proportion of 100, 200 and 400 times. Fig. 3(a) presents the supercontinuum produced in water doped with different quantities of lactose when the incident pulse energy is 10.7 μJ. The solutions with four kinds of concentrations act as the samples to study the variation of the properties of white light spectra. It can be clearly seen in Fig. 3(a) that the blue-side components of the white light spectra are suppressed with addition
of lactose, and the degree of suppression is mainly related to concentrations of lactose solution: the higher the concentrations of lactose solution, the more the supercontinuum is suppressed. As is known, the Kerr self-focusing increases the local intensity of laser pulse which makes it intense enough to ionize the medium forming large amounts of plasmas, and thus generating the supercontinuum. Kolesnik et al. [24] have shown that in water, linear chromatic dispersion, multiphoton-induced (MPI) and plasma defocusing play a major role in suppression of supercontinuum. In our experiment, the suppression occurs only on the blue side of the spectra, which indicates that the process of MPI free-electron generation is affected by addition of lactose in water. The density of electrons may be decreased with the increase of lactose quantity in water, which leads to the weaker plasma effect. Thus, the blue-sided components of the supercontinuum are suppressed. That is to say, lactose plays the electron scavenging action in water. The electron scavenging mechanism contains most likely two kinds of process: one is the simple electron attachment, giving rise to anion formation, and the other one is the dissociative attachment process wherein a low-energy electron is temporarily attached to the lactose via resonance. The resulting temporary negative ion rapidly dissociates into fragments, one of which is an anion [18]. The lactose may participate in either or both processes after being added into distilled water, which gives rise to the supercontinuum suppression. The suppression is enhanced along with increasing concentrations of lactose solution in both cases. To demonstrate that the electron scavenging effect ‘captures’ the solvated electrons thereby removing the source of excess electrons and suppressing the supercontinuum, we study the generation of supercontinuum in water doped with acetone, as shown in Fig. 3(b). It was reported that acetone plays the role of electron scavenging in capturing the excess electrons which are provided by the water inonization [25,26]. From Fig. 3(b) we can see that the supercontinuum is suppressed after acetone being added into distilled water, indicating that plasma effects become weaker due to the fact that efficiency of the electron scavenging action increases with the increase of lactose quantity in water.
Normalized intensity
1.0 (a)
1.0 (b)
0.8
0.8
0.6
0.6
0.4
0.4
0.2 0.0 400
500
600
1:1 1:100 1:200 1:400
0.2
700
0.0 400
water acetone
500
600
700
Wavelength (nm) Fig. 3. (a) Supercontinuum produced in water doped with different quantities of lactose when the incident pulse energy is 10.7 μJ. (b) Supercontinuum produced in distilled water and in water doped with acetone. The doping level is 1 % and the incident pulse energy is 110 μJ.
It is also important to note that the effect of lactose doping levels on supercontinuum suppression is not exclusively dependent on electron scavenging. As is known, when light beam passes through water, some of the light may be absorbed by water molecules. In this experiment, the generated supercontinuum may be absorbed by lactose solution. Hence, we also conduct a series of careful measurements to study the absorption spectrum of lactose. The variation of absorption spectrum of lactose solution with different concentrations is shown in Fig. 4. In this part, standard light source irradiated on the solution is used to study the variation of absorption spectrum of water doped with different quantity of lactose. It can be seen that the enhancement of absorption spectrum is along with the increase of the quantity of lactose, which suggests that lactose may play a role in light absorption, and the denser the concentration of solution, the stronger of the absorption is. Meanwhile, there may also exist scattering in the absorption process which affects the variation of spectrum. In a word, another cause for the supercontinuum suppression in Fig. 3(a) is the absorption or scattering of light by lactose in the water.
G
WUY`
WUY^ WUY] G
hGOvkP
WUY_
WUY\ GXaX GXaXWW GXaYWW GXa[WW
WUY[ WUYZ [WW
[\W
\WW
\\W
]WW
]\W
^WW
^\W
~GOP Fig. 4. Absorption spectrum of lactose solution with different concentrations. The black, red, blue and magenta curves correspond to the proportion 1:1, 1:100, 1:200 and 1:400.
3.3 The supercontinuum suppression in nitric acid solution Here, we study the supercontinuum generation in inorganic solution. The nitric acid solution with concentration of 65% is used as the original solution, and it is diluted by distilled water according to the proportion of 9, 19, 39 and 99 times. Fig. 5 shows the supercontinuum generated by the nitric acid solution with different concentrations irradiated by femtosecond laser pulses. It can be seen from the figure that the supercontinuum shows the similar variation tendency to that shown in Fig. 3(a), which is suppressed more intensely with increasing concentrations of the nitric acid solution. In the following part, we will discuss the cause of supercontinuum suppression in nitric acid solution. The reaction in nitric acid solution can be described by the equation [27,28]: N O2 O H
O2H
N3 O
H O 2 H
(3)
It is known that the formation of the anion (bitrate radical, NO3- ) should capture electrons, thus decreasing the qualities of electrons. From this reaction, the larger the quantity of NO3- , the fewer the electrons are, therefore, the density of electrons generated by ionization of water decreases with increasing concentration of nitric acid solution. Consequently, one cause for the supercontinuum suppression in nitric acid solution shown in Fig. 5 is the capture of electron by nitric acid.
Normalized intensity
1.0
1:9 1:19 1:40 1:99
0.8 0.6 0.4 0.2 0.0 400
450
500
550
600
650
700
750
Wavelength (nm) Fig. 5. Supercontinuum produced in water doped with different quantities of nitric acid when the incident pulse energy is 10.7 μJ. The solid black, red, blue and magenta curves correspond to the proportion 1:9, 1:19, 1:39 and 1:99.
On the other hand, we measure the absorption spectra of nitric acid with different concentrations, as shown in Fig. 6. The absorption spectra are obtained by irradiating the nitric acid solution with standard light source. It can be seen from the figure that the variation tendency of absorption spectrum of nitric acid solution is similar to that of lactose solution. The absorption of light is enhanced along with the increase of the concentrations of solution. The result demonstrates that the generated supercontinuum may be absorbed by nitric acid solution: the denser the concentration of solution, the stronger of the absorption is. Therefore, we can say that another cause for supercontinuum suppression in Fig. 5 is the absorption of light by nitric acid solution. Certainly, the scattering of light by nitric acid solution may also cause the supercontinuum suppression. The factors studied in our experiments are only part of causes for the suppression of the asymmetric supercontinuum. There also may be other factors that cause spectral suppression, such as refractive index of a liquid and laser parameters, which are important factors that determine the characteristics of supercontinuum. Ref. [29] has clearly identified the refractive index changes caused by NaCI solution with different concentrations. Therefore, adding lactose and nitric acid into water may cause the change of refractive index, which may be one of the causes for supercontinuum
suppression. In our future work, we will further quantitatively study how each factor affects the supercontinuum, and which plays the predominant role. G
WUXX
WUW` WUW_ WUW^ WUW]
G
hGOvkP
WUXW
GXa` GXaX` GXa[W GXa``
WUW\ [WW [\W \WW \\W ]WW ]\W ^WW ^\W
~GOP Fig. 6. Absorption spectrum of nitric acid. The solid black, red, blue and magenta curves correspond to the proportion 1:9, 1:19, 1:39 and 1:99.
4. Conclusion In summary, the supercontinuum generation during the propagation high-intensity, ultrashort-duration laser pulses in distilled water, lactose solution and nitric acid solution is investigated. Supercontinuum suppression can be observed during the femtosecond fliamentation in lactose solution and nitric acid solution, and the increase of lactose and nitric acid quantity enhances the supercontinuum suppression. We find that two factors may result in the suppression of supercontinuum: one is the capture of electrons by the solvent (either organic or inorganic), and the other one is spectral absorption or scattering by the solution. In addition, the refractive index may be changed by adding lactose and nitric acid that affects the suppression. These studies will be helpful to understanding of the supercontinuum suppression during femtosecond filamentation in these solutions. In this letter, we only qualitatively study the absorption of different concentrations of solutions. In future work, we attempt to quantitatively study the causes for the variation of white light spectra. Acknowledgments This work was supported by the Research Fund for the National Basic Research Program of China (973 Program, Grant No. 2013CB922200), the National Natural Science Foundation of China (Grant Nos. 11474129, 11504129, 11674124 and
11674128) and 2017M610190).
the
China
Postdoctoral
Science
Foundation
(Grant
No.
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[17] C. Santhosh, A.K. Dharmadhikari, K. Alti, J.A. Dharmadhikari, D. Mathur, J. Biomed. Opt. 12 (2007) 020510. [18] C. Santhosh, A.K. Dharmadhikari, J.A. Dharmadhikari, K. Alti, D. Mathur, Appl. Phys. B: Lasers Opt. 99 (2010) 427– 432. [19] P.C. Teh, Y.H. Ho, S.C. Lee, P.K. Lo, K.C. Lai, K.H. Yeap, Asia Communications and Photonics Conference. Optical Society of America, 2015. [20] L. Bergé, S. Skupin, R. Nuter, J. Kasparian, J.P. Wolf, Rep. Prog. Phys. 70 (2007) 1633-1713. [21] V.P. Kandidov, O.G. Kosareva, I.S. Golubtsov, W. Liu, A. Becker, N. Akozbek, C.M. Bowden, S.L. Chin, Appl. Phys. B: Lasers Opt. 77 (2003) 149–165. [22] C. Rulliere, Femtosecond Laser Pulses (Springer, Berlin 1998) p. 47. [23] W. J. Jones, B.P. Stoicheff, Phys. Rev. Lett. 13 (1964) 657. [24] M. Kolesik, G. Katona, J.V. Moloney, E.M. Wright, Phys. Rev. Lett. 91 (2003) 043905. [25] J.T. Allan, G. Scholes, Nature 187 (1960) 218–220. [26] R. Y. N. Gengler, D. S. Badali, D. Zhang, K. Dimos, K. Spyrou, D. Gournis, R. J. D. Miller, Nat. Commun. 4 (2013) 2560. [27] M. Leuchs, G. Zundel, Can. J. Chem. 58 (1980) 311-322. [28] M. Leuchs, G. Zundel, J. Phys. Chem. C 82 (1978) 1632-1635. [29] J. Räty, K.E. Peiponen, Talanta 137 (2015) 143–147.
Highlights 1. The addition of lactose and nitric acid in distilled water causes the supercontinuum suppression. 2. The supercontinuum suppression is enhanced by increasing the concentrations of solution. 3. The capture of electrons by the solvent, and spectral absorption or scattering by the solution may be the cause for supercontinuum suppression.
S0009-2614(17)30467-0 http://dx.doi.org/10.1016/j.cplett.2017.05.029 CPLETT 34818
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
14 April 2017 10 May 2017
Please cite this article as: H. Li, Z. Shi, X. Wang, L. Sui, S. Li, M. Jin, Influence of dopants on supercontinuum generation during the femtosecond laser filamentation in water, Chemical Physics Letters (2017), doi: http:// dx.doi.org/10.1016/j.cplett.2017.05.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of dopants on supercontinuum generation during the femtosecond laser filamentation in water He Lia,b, Zhe Shia,b, Xiaowei Wanga,b, Laizhi Suia,b, Suyu Lia,b,* Mingxing Jina,b* a
Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012,
China b
Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy
(Jilin University), Changchun 130012, China c
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
and Engineering, Jilin University, Changchun 130012, China Our experiments show that lactose as one of organic substance and nitric acid as one of inorganic substance added into distilled water can influence the supercontinuum generation in aqueous solution irradiated by the ultrashort femtosecond laser pulses. It is found that once the dopants are added into the water, the supercontinuum generation is suppressed to different extent, and the supercontinuum suppression is enhanced by increasing the concentrations of lactose solution and nitric acid solution. Through the analysis, we find that the capture of electrons by the solvent, and spectral absorption or scattering by the solution may also result in the supercontinuum suppression. These studies will be helpful to understanding of the supercontinuum generation during femtosecond filamentation in liquid samples. Keywords: asymmetric broadening, supercontinuum suppression, absorption spectra 1. Introduction Laser filamentation is well known to be a dynamically balanced process between the self-focusing resulted from Kerr effect and plasma defocusing resulted from the multiphoton/tunning ionization [1,2]. Many nonlinear effects, such as fluorescence emission [3,4], terahertz radiation [5,6] and supercontinuum generation [7,8] etc. are involved in the process of filamentation. Among these nonlinear effects, the supercontinuum generation attracts a great deal of interest due to its potential and *
Corresponding authors at: Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012,
China. E-mail addresses: sylee@ jlu.edu.cn (S. Li), [email protected] (M. Jin).
promising applications in remote sensing [9], biomedical imaging [10], cavity ring-down spectroscopy [11], generation of few-cycle femtosecond pulses [12], and so on. To the best of our knowledge, a series of experimental and theoretical studies of the generation of white light during the filamentation of a high-power femtosecond laser pulse in water have been carried out [13-15]. Since Jimbo et al. measured the supercontinuum generation in water by the addition of small quantities of metal ions like Zn 2+ and K + [16], the effect of dopants on supercontinuum generation has been
a hot research area. In recently years, Santhosh et al. have researched the supercontinuum broadening in water doped with small quantities of proteins [17,18]. It is demonstrated that the dopants weaken the plasma effect by scavenging electrons which is a major cause for supercontinuum suppression. Meanwhile, there are other factors that may result in supercontinuum suppression. For example, it is known that there is an obvious light absorption by water molecules when light beam passes through water [19]. The dopants added to water may lead to the absorption or scattering of light. Under this circumstance, supercontinuum is suppressed, however, which is less studied. In this letter, we experimentally study the supercontinuum generated from the interaction between femtosecond laser pulses and lactose solution and nitric acid solution with different concentrations, and discuss the influence of these two kinds of dopants on supercontinuum generation. Based on this study, we attempt to discuss the mechanism for the supercontinuum suppression in aqueous solution. 2. Experiment setup
H 800nm@1kHz
Laser
M
G
I
L Sample cell
Spectrometer Avantes
Fig. 1. Schematic of experimental setup to measure the supercontinuum generated during the femtosecond filamentation in different liquid samples. H: half-wave plate; G: Glan prism; L: focusing lens; I: integrating sphere.
The experimental setup is shown in Figure 1. A Ti: sapphire laser system is used to generate laser pulses at central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1 kHz. The combination of a half-wave plate and a Glan laser polarizer attenuates laser energy to the desired value. The lens L with focal length of 400 mm is used to focus the laser beam into a quartz sample cell (50 mm×10 mm×40 mm) filled with liquid samples (e.g. distilled water, lactose solution and nitric acid solution, etc.). To avoid any supercontinuum contribution from the way of sample cell, the laser beam is focused close to the center of the sample cell. The generated supercontinuum is collected by integrating sphere and then guided to the Avantes spectrometer (AvaSpec-1650F-USB2) through an optical fiber. 3. Result and Discussion 3.1 The supercontinuum suppression in distilled water It
is
known that
the
critical power
for
self-focusing
in water
is
Pcr = l 2 2p n0 n2 = 4.4 MW , where n0 and n2 denote the linear and nonlinear refractive indices of medium, respectively. As the nonlinear refractive index in water is larger than that in air, supercontinuum can be generated when the laser pulse with lower incident power injects into water. A typical white light spectrum obtained with triply distilled water is shown in Ref. [18]. As is known, the spectral broadening is essentially symmetric around 800 nm, which is mainly due to self-phase modulation (SPM) that arises from the Kerr nonlinearity effect. In addition, asymmetry generates towards the blue side of the spectrum. This component arises due to processes such as space-time focusing, self-steepening, and plasma formation that arises from free electrons are generated by multiphoton ionization [17,20]. In Ref. [21], asymmetric broadening by considering separately the contributions to the refractive index from the Kerr nonlinearity ∆nk and the plasma component ∆np has been modeled. Considering plane waves propagating in the nonlinear medium, the frequency deviation at a certain propagation distance z can be written as [22]:
dw (t ) = -kz
¶(Dnk + Dn p ) ¶t
(1)
where ∆nk denotes the index from the Kerr nonlinearity, ∆np denotes the index from the plasma, z is along the propagation axis, k is the wave number, and t denotes time. In practice, Eq. (1) can be rewritten in the following form:
dw (t ) = -a
¶I (t ) + bI K (t ) ¶t
(2)
where a = 4p n2 kz / c , b = 2p ne2 b ( K ) N0 kz / (mew02 ) are time-dependent and b ( K ) is the K-photon absorption coefficient of water. n2 is the nonlinear refractive index, N0 is the number of initially present neutral particles in the focal volume, and me is electron mass. In Ref. 21, it was pointed out that the symmetrical spectral broadening can be observed at low laser energies (0.4–1.8 µJ) and the asymmetric broadening at higher laser energies (2–200 µJ). The symmetrical spectral broadening can be attributed to the frequency shift due to the nonlinear refractive index n2 of the material [i.e., the first term in Eq. (2)] which is resulted from the interaction of the laser with the bound electrons: If n2 is positive, lower frequency components (i.e., red-shift Dw- ) are generated in the leading edge and higher frequency components (i.e., blue-shift Dw+ ) are generated in the trailing edge of the pulse. The reason of the asymmetrical spectral broadening is that through the interaction of the laser with (quasi-) free electrons [namely the second term in Eq. (2)] generated via multiphoton excitation causes the spectral broadening, it results in the blue-shift of spectrum only. In our work, the energy of incident laser pulse is relatively high (higher than 2 μJ), and only the asymmetrical spectral broadening can be observed, which will be discussed in the following part.
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Fig. 2 shows the typical supercontinuum (from 400 nm to 770 nm) generated when the triply-distilled water is irradiated by the femtosecond laser pulse. The relatively sharp and obvious dip that can be observed is due to the absorption on the higher frequency side. It is known that the water can emit by monochromatic light of frequency ν0 with an intense continuum. H2O molecules within the laser focal volume are stimulated so that they emit at frequency ν 0 and, at the same time, absorb the emission at frequencies corresponding to (ν 0+νm) and (ν0-νm), where hn m is the energy separation between different quantum states in H2O. The position of the dip is irrespective of conditions like levels of dopants introduced into the water sample, or variations in laser intensity and focusing conditions that have been represented in Ref. [18]. This phenomenon may be mainly caused by inverse Raman effect [23]. 3.2 The supercontinuum suppression in lactose solution In our experiments, 0.04 g lactose is dissolved in 10 mL distilled water as the original solution, then we dilute the solution with distill water according to the proportion of 100, 200 and 400 times. Fig. 3(a) presents the supercontinuum produced in water doped with different quantities of lactose when the incident pulse energy is 10.7 μJ. The solutions with four kinds of concentrations act as the samples to study the variation of the properties of white light spectra. It can be clearly seen in Fig. 3(a) that the blue-side components of the white light spectra are suppressed with addition
of lactose, and the degree of suppression is mainly related to concentrations of lactose solution: the higher the concentrations of lactose solution, the more the supercontinuum is suppressed. As is known, the Kerr self-focusing increases the local intensity of laser pulse which makes it intense enough to ionize the medium forming large amounts of plasmas, and thus generating the supercontinuum. Kolesnik et al. [24] have shown that in water, linear chromatic dispersion, multiphoton-induced (MPI) and plasma defocusing play a major role in suppression of supercontinuum. In our experiment, the suppression occurs only on the blue side of the spectra, which indicates that the process of MPI free-electron generation is affected by addition of lactose in water. The density of electrons may be decreased with the increase of lactose quantity in water, which leads to the weaker plasma effect. Thus, the blue-sided components of the supercontinuum are suppressed. That is to say, lactose plays the electron scavenging action in water. The electron scavenging mechanism contains most likely two kinds of process: one is the simple electron attachment, giving rise to anion formation, and the other one is the dissociative attachment process wherein a low-energy electron is temporarily attached to the lactose via resonance. The resulting temporary negative ion rapidly dissociates into fragments, one of which is an anion [18]. The lactose may participate in either or both processes after being added into distilled water, which gives rise to the supercontinuum suppression. The suppression is enhanced along with increasing concentrations of lactose solution in both cases. To demonstrate that the electron scavenging effect ‘captures’ the solvated electrons thereby removing the source of excess electrons and suppressing the supercontinuum, we study the generation of supercontinuum in water doped with acetone, as shown in Fig. 3(b). It was reported that acetone plays the role of electron scavenging in capturing the excess electrons which are provided by the water inonization [25,26]. From Fig. 3(b) we can see that the supercontinuum is suppressed after acetone being added into distilled water, indicating that plasma effects become weaker due to the fact that efficiency of the electron scavenging action increases with the increase of lactose quantity in water.
Normalized intensity
1.0 (a)
1.0 (b)
0.8
0.8
0.6
0.6
0.4
0.4
0.2 0.0 400
500
600
1:1 1:100 1:200 1:400
0.2
700
0.0 400
water acetone
500
600
700
Wavelength (nm) Fig. 3. (a) Supercontinuum produced in water doped with different quantities of lactose when the incident pulse energy is 10.7 μJ. (b) Supercontinuum produced in distilled water and in water doped with acetone. The doping level is 1 % and the incident pulse energy is 110 μJ.
It is also important to note that the effect of lactose doping levels on supercontinuum suppression is not exclusively dependent on electron scavenging. As is known, when light beam passes through water, some of the light may be absorbed by water molecules. In this experiment, the generated supercontinuum may be absorbed by lactose solution. Hence, we also conduct a series of careful measurements to study the absorption spectrum of lactose. The variation of absorption spectrum of lactose solution with different concentrations is shown in Fig. 4. In this part, standard light source irradiated on the solution is used to study the variation of absorption spectrum of water doped with different quantity of lactose. It can be seen that the enhancement of absorption spectrum is along with the increase of the quantity of lactose, which suggests that lactose may play a role in light absorption, and the denser the concentration of solution, the stronger of the absorption is. Meanwhile, there may also exist scattering in the absorption process which affects the variation of spectrum. In a word, another cause for the supercontinuum suppression in Fig. 3(a) is the absorption or scattering of light by lactose in the water.
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WUY[ WUYZ [WW
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3.3 The supercontinuum suppression in nitric acid solution Here, we study the supercontinuum generation in inorganic solution. The nitric acid solution with concentration of 65% is used as the original solution, and it is diluted by distilled water according to the proportion of 9, 19, 39 and 99 times. Fig. 5 shows the supercontinuum generated by the nitric acid solution with different concentrations irradiated by femtosecond laser pulses. It can be seen from the figure that the supercontinuum shows the similar variation tendency to that shown in Fig. 3(a), which is suppressed more intensely with increasing concentrations of the nitric acid solution. In the following part, we will discuss the cause of supercontinuum suppression in nitric acid solution. The reaction in nitric acid solution can be described by the equation [27,28]: N O2 O H
O2H
N3 O
H O 2 H
(3)
It is known that the formation of the anion (bitrate radical, NO3- ) should capture electrons, thus decreasing the qualities of electrons. From this reaction, the larger the quantity of NO3- , the fewer the electrons are, therefore, the density of electrons generated by ionization of water decreases with increasing concentration of nitric acid solution. Consequently, one cause for the supercontinuum suppression in nitric acid solution shown in Fig. 5 is the capture of electron by nitric acid.
Normalized intensity
1.0
1:9 1:19 1:40 1:99
0.8 0.6 0.4 0.2 0.0 400
450
500
550
600
650
700
750
Wavelength (nm) Fig. 5. Supercontinuum produced in water doped with different quantities of nitric acid when the incident pulse energy is 10.7 μJ. The solid black, red, blue and magenta curves correspond to the proportion 1:9, 1:19, 1:39 and 1:99.
On the other hand, we measure the absorption spectra of nitric acid with different concentrations, as shown in Fig. 6. The absorption spectra are obtained by irradiating the nitric acid solution with standard light source. It can be seen from the figure that the variation tendency of absorption spectrum of nitric acid solution is similar to that of lactose solution. The absorption of light is enhanced along with the increase of the concentrations of solution. The result demonstrates that the generated supercontinuum may be absorbed by nitric acid solution: the denser the concentration of solution, the stronger of the absorption is. Therefore, we can say that another cause for supercontinuum suppression in Fig. 5 is the absorption of light by nitric acid solution. Certainly, the scattering of light by nitric acid solution may also cause the supercontinuum suppression. The factors studied in our experiments are only part of causes for the suppression of the asymmetric supercontinuum. There also may be other factors that cause spectral suppression, such as refractive index of a liquid and laser parameters, which are important factors that determine the characteristics of supercontinuum. Ref. [29] has clearly identified the refractive index changes caused by NaCI solution with different concentrations. Therefore, adding lactose and nitric acid into water may cause the change of refractive index, which may be one of the causes for supercontinuum
suppression. In our future work, we will further quantitatively study how each factor affects the supercontinuum, and which plays the predominant role. G
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~GOP Fig. 6. Absorption spectrum of nitric acid. The solid black, red, blue and magenta curves correspond to the proportion 1:9, 1:19, 1:39 and 1:99.
4. Conclusion In summary, the supercontinuum generation during the propagation high-intensity, ultrashort-duration laser pulses in distilled water, lactose solution and nitric acid solution is investigated. Supercontinuum suppression can be observed during the femtosecond fliamentation in lactose solution and nitric acid solution, and the increase of lactose and nitric acid quantity enhances the supercontinuum suppression. We find that two factors may result in the suppression of supercontinuum: one is the capture of electrons by the solvent (either organic or inorganic), and the other one is spectral absorption or scattering by the solution. In addition, the refractive index may be changed by adding lactose and nitric acid that affects the suppression. These studies will be helpful to understanding of the supercontinuum suppression during femtosecond filamentation in these solutions. In this letter, we only qualitatively study the absorption of different concentrations of solutions. In future work, we attempt to quantitatively study the causes for the variation of white light spectra. Acknowledgments This work was supported by the Research Fund for the National Basic Research Program of China (973 Program, Grant No. 2013CB922200), the National Natural Science Foundation of China (Grant Nos. 11474129, 11504129, 11674124 and
11674128) and 2017M610190).
the
China
Postdoctoral
Science
Foundation
(Grant
No.
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Highlights 1. The addition of lactose and nitric acid in distilled water causes the supercontinuum suppression. 2. The supercontinuum suppression is enhanced by increasing the concentrations of solution. 3. The capture of electrons by the solvent, and spectral absorption or scattering by the solution may be the cause for supercontinuum suppression.