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Vibration-selective coherent anti-Stokes Raman scattering with linearly chirped white-light pulses
Chemical Physics Letters 485 (2010) 45–48
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Vibration-selective coherent anti-Stokes Raman scattering with linearly chirped white-light pulses Tatsuya Kasajima, Keiichi Yokoyama *, Leo Matsuoka, Atsushi Yokoyama Quantum Beam Science Directorate, Kansai Photon Science Institute, Japan Atomic Energy Agency, 8-1-7 Umemidai, Kizugawa, Kyoto 619-0215, Japan
a r t i c l e
i n f o
Article history: Received 9 October 2009 In final form 9 December 2009 Available online 16 December 2009
a b s t r a c t A practical extension of selective excitation using broadband laser is reported. A specific molecular vibration is excited by stimulated Raman scattering induced by a pair of linearly chirped white-light pulses (650–900 nm). The white-light pulse is generated by filamentation produced in a focused Ti:sapphire laser beam (30 fs, 1.8 mJ/pulse). The excited amplitude is probed by coherent anti-Stokes Raman scattering using the third pulse with a narrow bandwidth (769.9 ± 1.5 nm). As a demonstration, the N2 and O2 molecules are respectively excited at different time intervals of the pulse pair without changing the wavelength region of the light source. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Coherent anti-Stokes Raman scattering (CARS) taking advantage of a broadband character of the femtosecond laser pulse is being developed to facilitate the spectroscopic measurement and to study the laser manipulation, or the quantum control, of molecules [1–12]. For the spectroscopic measurement of molecules, multiplex CARS has been developed [3–5]. In the multiplex CARS, a broadband Stokes pulse simultaneously excites various kinds of molecular vibrations. Namely, broader Stokes pulses excite more vibrations at once. In principle, the Stokes pulse with the spectral coverage of >4161 cm1, which is the fundamental vibration frequency of the hydrogen molecule [13], covers all of the fundamental frequencies of Raman active molecular vibrations. The broadest Stokes pulse reported so far covers >3500 cm1 [5]. For the quantum control of molecules, vibration-selective broadband CARS has been demonstrated by Hellerer et al. [6], Pestov et al. [7], and Langbein et al. [8]. They introduced a scheme to excite a specific mode in molecular vibrations with a spectral resolution beyond the bandwidth of the light source (Fig. 1a). To select vibrations, they tuned the time interval of a pair of linearly chirped broadband pulses, instead of the frequency difference between narrowband pump and Stokes pulses. In this Letter, we refer to such CARS technique as linearly-chirped-pump and -Stokes CARS (LCPS-CARS). The same Raman excitation method as LCPSCARS has also been used by other groups [14,15]. In the present study, we exploit a variant of LCPS-CARS using a white-light pulse as the identical source of pump and Stokes pulses. The broadband spectrum of the white-light pulse makes * Corresponding author. Fax: +81 774 71 3338. E-mail address: [email protected] (K. Yokoyama). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.12.032
the Raman excitation applicable to a wide range of vibrational frequencies as well as the broadband Stokes pulse used in multiplex CARS. LCPS-CARS with white-light pulses is not only a scheme of selective excitation applicable to various kinds of molecules without changing the light source, but also an elementary scheme to help the development of the controlling technique for complex molecular processes by shaping broadband pulses. In addition, the pump and Stokes pulses made of an identical pulse are preferable to those prepared in different ways, because the chirp rates for both pulses must be the same in LCPS-CARS, although the higherorder chirps still exist even by this method and it can degrade the selectivity. White-light pulses are frequently generated by focusing femtosecond laser pulses (driver pulses) into condensed- or gas-phase media to produce a channel of plasma, called filamentation [16]. The spectral amplitude and phase of such white-light pulses are strongly affected by the pulse parameter of the driver pulse, the medium, and the focusing condition. In the present experiment, we chose filamentation in gas-phase media without waveguides, because less quadratic chirp and larger pulse energy can be expected than the other method. Nevertheless, it is not so clear whether such white-light pulses are usable for LCPS-CARS. In particular, the spectral amplitude of the white-light pulse usually shows a complex structure probably due to the complexity of the filamentation process. In this Letter, we also pay attention to the influence of the complex structure on LCPS-CARS. 2. Experimental methods The experimental setup is shown in Fig. 2. Femtosecond pulses from a commercial Ti:sapphire laser oscillator (Femtolasers, Femtosource Scientific Pro) were amplified by a home-built
46
T. Kasajima et al. / Chemical Physics Letters 485 (2010) 45–48
Fig. 1. Schemes of LCPS-CARS. The difference between instantaneous angular frequencies of pump and Stokes pulses, x12 , is constant when both pulses are linearly chirped with the same chirp rate. x12 can be controlled by the delay time between the pump and Stokes pulses, s12 . Consequently, a specific molecular vibration in resonance with x12 can be selectively excited by tuning s12 . (a) The LCPS-CARS scheme with pump and Stokes pulses prepared in different ways to cover different spectral regions. (b) The LCPS-CARS scheme with a white-light pulse as the identical source of pump and Stokes pulses, which requires no additional wavelength conversion, can be applied widely, and guarantees the same chirp rate.
regenerative amplifier system, which provided output pulses (driver pulses) with the center wavelength of 780 nm, the pulse duration around 30 fs, and the repetition rate of 1 kHz. The driver pulse of 1.8 mJ was focused into 1-atm gaseous argon filled in a glass tube (1 inch diameter and 2.6 m long) to generate a white-light pulse with its spectral coverage of >4300 cm1. A planoconvex lens with the focal length of 500 mm and a convex mirror with the focal length of 300 mm were used for focusing the driver pulses. The distance between the lens and mirror was 260 mm, resulting in the effective focal length of 1.2 m from the mirror. The spectra of the driver pulse and the white-light pulse were measured with a fiber spectrometer (Ocean Optics, USB4000). The white-light pulse was divided into three pulses with two different kinds of beamsplitters, giving the pump, Stokes, and probe pulses, respectively. The white-light pulse for the probe pulse was spectrally filtered by a bandpass filter (the center wavelength of 769.9 nm and the FWHM of 1.5 nm) to obtain CARS signals in the frequency domain. The spectra of the driver pulse, the white-light pulse, and the probe pulse are presented in Fig. 3. Two optical delay stages were placed in the pump and Stokes beam lines to control the optical path lengths of them. The three pulses were delivered to a
Fig. 2. Experimental setup for the present LCPS-CARS. L is a planoconvex lens. CVMs are convex mirrors. CCMs are concave mirrors. BSs are beamsplitters. F is a bandpass filter.
folded BOXCARS arrangement in which the distance between the pump and probe beam lines was 30 mm, and were focused into the sample point with a spherical concave mirror with the focal length of 250 mm. The diameter of the three pulses was 10 mm just before the spherical concave mirror. The BOXCARS arrangement is usable even with the extremely wide spectral coverage of >4300 cm1 because the phase-matching condition in the gas
Fig. 3. (a) A typical spectrum of the driver laser pulse. (b) Spectra of a white-light pulse (dotted line) and a probe pulse (solid line). The white-light spectrum has complex structure with many peaks. The ranges of wavelength acting as the pump (solid lines) and Stokes (dotted lines) lights are shown in the bottom, indicating that the pump light is weak.
T. Kasajima et al. / Chemical Physics Letters 485 (2010) 45–48
Fig. 4. (a) Observed pump-Stokes SFG autocorrelation function and a fitting curve. The solid line shows the autocorrelation function obtained by the SFG autocorrelation measurement between pump and Stokes pulses, and its FWHM is 190 fs. The dotted line presents the calculated autocorrelation function of the white-light pulse estimated from the observed spectrum (Fig. 3b) with an assumption of the linear chirp. The chirp rate, 3:01 103 fs2 (11.1 cm1/fs), was obtained from the best fit. (b) Observed Stokes-probe and pump–probe SFG cross-correlation functions. FWHM of them is 290 fs. The probe delay time of s23 ¼ 500 fs is sufficient to get rid of the nonresonant background because no SFG cross-correlation signals are measured around the delay time.
phase is not so restrictive as that in the condensed phase. Although, we tried a spherical lens instead of the spherical concave mirror, we were not able to obtain any CARS signals due to the chromatic aberration of the lens. Alternatively, if we have a pulse shaper, a single-beam arrangement [11,12] also can be used owing to the less-restrictive phase-matching condition in the gas phase, which will make the beam geometry simpler. Sum-frequency-generation (SFG) cross-correlations of the pump-Stokes, pump–probe, and Stokes-probe pulses were measured by placing a BBO crystal at the sample point. The results are shown in Fig. 4. The delay times of the pump and probe pulses relative to the Stokes pulse were denoted as s12 and s23 , respectively. The positions of the delay stages for s12 ¼ s23 ¼ 0 were determined so as to maximize respective SFG signals. The pulse energies at the sample point were 440 lJ/pulse for the pump pulse, 300 lJ/pulse for the Stokes pulse, and <1 lJ/pulse for the probe pulse, respectively. The target systems were chosen to be the N2 and O2 molecules in the air, so that no gas cell was used. The CARS spectra were measured with the fiber spectrometer. We did not explicitly apply chirp on the whitelight pulse. The pump and Stokes pulses were chirped by the group velocity dispersions of the glass tube window, the beamsplitters, and the air in the optical paths.
47
Fig. 5. The s12 dependence of CARS signals. The N2 and O2 CARS signals were respectively observed in the delay range of 240 fs < s12 < 210 fs and at s12 ¼ 140 fs and 130 fs. The result indicates that we can select x12 between 2330 cm1 or 1555 cm1 by tuning s12 .
The CARS spectra were measured at s23 ¼ 500 fs to minimize the nonresonant background produced mostly around s12 ¼ s23 ¼ 0. This method using the delayed narrowband probe pulse has been reported by Kano and Hamaguchi [4], Prince et al. [9], and Pestov et al. [10]. We call it a hybrid CARS after Pestov et al. [10]. In the present experiment, we confirmed that the hybrid CARS is effective to identify the CARS signals, owing to both the improvement of the spectral resolution and the suppression of the nonresonant background. 3. Results and discussion Narrow peaks at two different wavelengths were observed in the CARS spectra at different ranges of s12 , 240 fs < s12 < 210 fs and at s12 ¼ 140 fs and 130 fs (Fig. 5). These peaks are assigned to the N2 and O2 CARS signals, respectively, by their Raman shifts (mN0 2 ¼ 2330 cm1 and mO0 2 ¼ 1555 cm1). It is obvious that the selective excitation is successfully performed by the control of s12 . Here, the minus sign of s12 means that the pump pulse is followed by the Stokes pulse. In the delay range of 200 fs < s12 < 160 fs, we were not able to identify any signals due to the large nonresonant background. The background was surely suppressed to some extent by the hybrid CARS technique mentioned above. However, the remaining
48
T. Kasajima et al. / Chemical Physics Letters 485 (2010) 45–48
nonresonant background completely disturbed the detection of the CARS signals in the range of 200 fs < s12 < 160 fs, while slightly raised the baselines at s12 ¼ 130 fs and 120 fs. This irregular rise of the nonresonant background probably comes from the poor flatness or complex structure in the spectrum of the white-light pulse (Fig. 3b). The spectral component necessary for the pump light (650–700 nm) is very weak compared to the central component of the spectrum (740–830 nm) which is responsible for most of the nonresonant background. In consequence, it is suggested that the spectral flatness of the white-light pulse would be important to obtain the CARS spectra continuously. We verify that the vibration-selective excitation (Fig. 5) would have been achieved by the same mechanism as LCPS-CARS (Fig. 1b). The s12 difference between x12 ¼ 2330 cm1 and 1555 cm1, Ds12 , is calculated from the estimated chirp rate of the white-light pulse and is compared to the observed Ds12 which was determined to be 80 fs from Fig. 5. The estimation is carried out using the observed white-light spectrum IðxÞ (Fig. 3b) and the observed pump-Stokes SFG autocorrelation function (Fig. 4a) as follows. In the frequency domain, the electric field of the linearly chirped white-light pulse can be written by
pffiffiffiffiffiffiffiffiffiffi 1 EðxÞ ¼ A IðxÞ exp i /2 ðx x0 Þ2 2
ð1Þ
where A is the proportional constant, /2 is the inverse of the linearchirp rate, and x0 is the carrier angular frequency (2:44 1015 s1). The electric field in the time domain EðtÞ is obtained by the inverse Fourier transform of EðxÞ. For estimation of /2 , A and /2 are adjusted in the least-square fit of the autocorrelation function
IAC ðs12 Þ ¼
Z
1
jEðtÞj2 jEðt þ s12 Þj2 dt
ð2Þ
1
to the observed SFG cross-correlation function. As a result, the chirp rate 1=/2 has been determined to be 3:01 103 fs2 (11.1 cm1/fs), which gives the best fit function shown in Fig. 4a. From this chirp rate, Ds12 is calculated as
Ds12 ¼
mN0 2 mO0 2 11:1
70 fs;
ð3Þ
which agrees well with the experimental value of 80 fs. It is concluded that the observed vibration-selective excitation was achieved by the LCPS-CARS mechanism. The pump-Stokes excitation bandwidth is estimated to be 460 cm1 as the Fourier transform limit of the temporal envelope of the pump and stokes pulses. Together with the current chirp rate of 11.1 cm1/fs, the CARS signal is expected to be observed over the
range of Ds12 = 41 fs, which agrees with the experimental results in Fig. 5. In the present demonstration, we made no attempt to get high selectivity to distinguish closer vibrations. As the next step, selectivity should be improved. The precise pulse shaping to get precise linearity and equality of chirp rate should be important for the higher selectivity. Also, a large chirp rate of the pump and Stokes pulses must be important to achieve the high selectivity, because it leads to the effective reduction of their instantaneous bandwidths. 4. Summary We have demonstrated the LCPS-CARS using white-light pulses generated by filamentation as a simple and universal technique for vibration-selective Raman excitation, which should be applicable to all of the Raman active molecular vibrations. However, the complex structure seen in the spectrum of the white-light pulse is found to cause irregular nonresonant background, which may disturb the detection of the CARS signals. To manage this problem, it is important to make the white-light spectrum as flat as possible. Acknowledgements This work was supported by Grants-in-Aid for Scientific Research (B) (No. 20360423) from the Japan Society for the Promotion of Science (JSPS). References [1] M. Dantus, V.V. Lozovoy, Chem. Rev. 104 (2004) 1813. [2] P. Nuernberger, G. Vogt, T. Brixner, G. Gerber, Phys. Chem. Chem. Phys. 9 (2007) 2470. [3] T.W. Kee, H. Zhao, M.T. Cicerone, Opt. Express 14 (2006) 3631. [4] H. Kano, H. Hamaguchi, J. Raman Spectrosc. 37 (2006) 411. [5] H. Kano, H. Hamaguchi, Anal. Chem. 79 (2007) 8967. [6] T. Hellerer, A.M.K. Enejder, A. Zumbusch, Appl. Phys. Lett. 85 (2004) 25. [7] D. Pestov, X. Wang, R.K. Murawski, G.O. Ariunbold, V.A. Sautenkov, A.V. Sokolov, J. Opt. Soc. B 25 (2008) 768. [8] W. Langbein, I. Rocha-Mendoza, P. Borri, Appl. Phys. Lett. 95 (2009) 081109. [9] B.D. Prince, A. Chakraborty, B.M. Prince, H.U. Staufferb, J. Chem. Phys. 125 (2006) 044502. [10] D. Pestov et al., Science 316 (2007) 265. [11] H. Li, D.A. Harris, B. Xu, P.J. Wrzesinski, V.V. Lozovoy, M. Dantus, Opt. Express 16 (2008) 5499. [12] S. Roy, P. Wrzesinski, D. Pestov, T. Gunaratne, M. Dantus, J.R. Gord, Appl. Phys. Lett. 95 (2009) 074102. [13] B.P. Stoicheff, Can. J. Phys. 35 (1957) 730. [14] E. Gershgoren, R.A. Bartels, J.T. Fourkas, R. Tobey, M.M. Murnane, H.C. Kapteyn, Opt. Lett. 28 (2003) 361. [15] M. Zhi, A.V. Sokolov, New J. Phys. 10 (2008) 025032. [16] J. Kasparian, J.-P. Wolf, Opt. Express 16 (2008) 466.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Vibration-selective coherent anti-Stokes Raman scattering with linearly chirped white-light pulses Tatsuya Kasajima, Keiichi Yokoyama *, Leo Matsuoka, Atsushi Yokoyama Quantum Beam Science Directorate, Kansai Photon Science Institute, Japan Atomic Energy Agency, 8-1-7 Umemidai, Kizugawa, Kyoto 619-0215, Japan
a r t i c l e
i n f o
Article history: Received 9 October 2009 In final form 9 December 2009 Available online 16 December 2009
a b s t r a c t A practical extension of selective excitation using broadband laser is reported. A specific molecular vibration is excited by stimulated Raman scattering induced by a pair of linearly chirped white-light pulses (650–900 nm). The white-light pulse is generated by filamentation produced in a focused Ti:sapphire laser beam (30 fs, 1.8 mJ/pulse). The excited amplitude is probed by coherent anti-Stokes Raman scattering using the third pulse with a narrow bandwidth (769.9 ± 1.5 nm). As a demonstration, the N2 and O2 molecules are respectively excited at different time intervals of the pulse pair without changing the wavelength region of the light source. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Coherent anti-Stokes Raman scattering (CARS) taking advantage of a broadband character of the femtosecond laser pulse is being developed to facilitate the spectroscopic measurement and to study the laser manipulation, or the quantum control, of molecules [1–12]. For the spectroscopic measurement of molecules, multiplex CARS has been developed [3–5]. In the multiplex CARS, a broadband Stokes pulse simultaneously excites various kinds of molecular vibrations. Namely, broader Stokes pulses excite more vibrations at once. In principle, the Stokes pulse with the spectral coverage of >4161 cm1, which is the fundamental vibration frequency of the hydrogen molecule [13], covers all of the fundamental frequencies of Raman active molecular vibrations. The broadest Stokes pulse reported so far covers >3500 cm1 [5]. For the quantum control of molecules, vibration-selective broadband CARS has been demonstrated by Hellerer et al. [6], Pestov et al. [7], and Langbein et al. [8]. They introduced a scheme to excite a specific mode in molecular vibrations with a spectral resolution beyond the bandwidth of the light source (Fig. 1a). To select vibrations, they tuned the time interval of a pair of linearly chirped broadband pulses, instead of the frequency difference between narrowband pump and Stokes pulses. In this Letter, we refer to such CARS technique as linearly-chirped-pump and -Stokes CARS (LCPS-CARS). The same Raman excitation method as LCPSCARS has also been used by other groups [14,15]. In the present study, we exploit a variant of LCPS-CARS using a white-light pulse as the identical source of pump and Stokes pulses. The broadband spectrum of the white-light pulse makes * Corresponding author. Fax: +81 774 71 3338. E-mail address: [email protected] (K. Yokoyama). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.12.032
the Raman excitation applicable to a wide range of vibrational frequencies as well as the broadband Stokes pulse used in multiplex CARS. LCPS-CARS with white-light pulses is not only a scheme of selective excitation applicable to various kinds of molecules without changing the light source, but also an elementary scheme to help the development of the controlling technique for complex molecular processes by shaping broadband pulses. In addition, the pump and Stokes pulses made of an identical pulse are preferable to those prepared in different ways, because the chirp rates for both pulses must be the same in LCPS-CARS, although the higherorder chirps still exist even by this method and it can degrade the selectivity. White-light pulses are frequently generated by focusing femtosecond laser pulses (driver pulses) into condensed- or gas-phase media to produce a channel of plasma, called filamentation [16]. The spectral amplitude and phase of such white-light pulses are strongly affected by the pulse parameter of the driver pulse, the medium, and the focusing condition. In the present experiment, we chose filamentation in gas-phase media without waveguides, because less quadratic chirp and larger pulse energy can be expected than the other method. Nevertheless, it is not so clear whether such white-light pulses are usable for LCPS-CARS. In particular, the spectral amplitude of the white-light pulse usually shows a complex structure probably due to the complexity of the filamentation process. In this Letter, we also pay attention to the influence of the complex structure on LCPS-CARS. 2. Experimental methods The experimental setup is shown in Fig. 2. Femtosecond pulses from a commercial Ti:sapphire laser oscillator (Femtolasers, Femtosource Scientific Pro) were amplified by a home-built
46
T. Kasajima et al. / Chemical Physics Letters 485 (2010) 45–48
Fig. 1. Schemes of LCPS-CARS. The difference between instantaneous angular frequencies of pump and Stokes pulses, x12 , is constant when both pulses are linearly chirped with the same chirp rate. x12 can be controlled by the delay time between the pump and Stokes pulses, s12 . Consequently, a specific molecular vibration in resonance with x12 can be selectively excited by tuning s12 . (a) The LCPS-CARS scheme with pump and Stokes pulses prepared in different ways to cover different spectral regions. (b) The LCPS-CARS scheme with a white-light pulse as the identical source of pump and Stokes pulses, which requires no additional wavelength conversion, can be applied widely, and guarantees the same chirp rate.
regenerative amplifier system, which provided output pulses (driver pulses) with the center wavelength of 780 nm, the pulse duration around 30 fs, and the repetition rate of 1 kHz. The driver pulse of 1.8 mJ was focused into 1-atm gaseous argon filled in a glass tube (1 inch diameter and 2.6 m long) to generate a white-light pulse with its spectral coverage of >4300 cm1. A planoconvex lens with the focal length of 500 mm and a convex mirror with the focal length of 300 mm were used for focusing the driver pulses. The distance between the lens and mirror was 260 mm, resulting in the effective focal length of 1.2 m from the mirror. The spectra of the driver pulse and the white-light pulse were measured with a fiber spectrometer (Ocean Optics, USB4000). The white-light pulse was divided into three pulses with two different kinds of beamsplitters, giving the pump, Stokes, and probe pulses, respectively. The white-light pulse for the probe pulse was spectrally filtered by a bandpass filter (the center wavelength of 769.9 nm and the FWHM of 1.5 nm) to obtain CARS signals in the frequency domain. The spectra of the driver pulse, the white-light pulse, and the probe pulse are presented in Fig. 3. Two optical delay stages were placed in the pump and Stokes beam lines to control the optical path lengths of them. The three pulses were delivered to a
Fig. 2. Experimental setup for the present LCPS-CARS. L is a planoconvex lens. CVMs are convex mirrors. CCMs are concave mirrors. BSs are beamsplitters. F is a bandpass filter.
folded BOXCARS arrangement in which the distance between the pump and probe beam lines was 30 mm, and were focused into the sample point with a spherical concave mirror with the focal length of 250 mm. The diameter of the three pulses was 10 mm just before the spherical concave mirror. The BOXCARS arrangement is usable even with the extremely wide spectral coverage of >4300 cm1 because the phase-matching condition in the gas
Fig. 3. (a) A typical spectrum of the driver laser pulse. (b) Spectra of a white-light pulse (dotted line) and a probe pulse (solid line). The white-light spectrum has complex structure with many peaks. The ranges of wavelength acting as the pump (solid lines) and Stokes (dotted lines) lights are shown in the bottom, indicating that the pump light is weak.
T. Kasajima et al. / Chemical Physics Letters 485 (2010) 45–48
Fig. 4. (a) Observed pump-Stokes SFG autocorrelation function and a fitting curve. The solid line shows the autocorrelation function obtained by the SFG autocorrelation measurement between pump and Stokes pulses, and its FWHM is 190 fs. The dotted line presents the calculated autocorrelation function of the white-light pulse estimated from the observed spectrum (Fig. 3b) with an assumption of the linear chirp. The chirp rate, 3:01 103 fs2 (11.1 cm1/fs), was obtained from the best fit. (b) Observed Stokes-probe and pump–probe SFG cross-correlation functions. FWHM of them is 290 fs. The probe delay time of s23 ¼ 500 fs is sufficient to get rid of the nonresonant background because no SFG cross-correlation signals are measured around the delay time.
phase is not so restrictive as that in the condensed phase. Although, we tried a spherical lens instead of the spherical concave mirror, we were not able to obtain any CARS signals due to the chromatic aberration of the lens. Alternatively, if we have a pulse shaper, a single-beam arrangement [11,12] also can be used owing to the less-restrictive phase-matching condition in the gas phase, which will make the beam geometry simpler. Sum-frequency-generation (SFG) cross-correlations of the pump-Stokes, pump–probe, and Stokes-probe pulses were measured by placing a BBO crystal at the sample point. The results are shown in Fig. 4. The delay times of the pump and probe pulses relative to the Stokes pulse were denoted as s12 and s23 , respectively. The positions of the delay stages for s12 ¼ s23 ¼ 0 were determined so as to maximize respective SFG signals. The pulse energies at the sample point were 440 lJ/pulse for the pump pulse, 300 lJ/pulse for the Stokes pulse, and <1 lJ/pulse for the probe pulse, respectively. The target systems were chosen to be the N2 and O2 molecules in the air, so that no gas cell was used. The CARS spectra were measured with the fiber spectrometer. We did not explicitly apply chirp on the whitelight pulse. The pump and Stokes pulses were chirped by the group velocity dispersions of the glass tube window, the beamsplitters, and the air in the optical paths.
47
Fig. 5. The s12 dependence of CARS signals. The N2 and O2 CARS signals were respectively observed in the delay range of 240 fs < s12 < 210 fs and at s12 ¼ 140 fs and 130 fs. The result indicates that we can select x12 between 2330 cm1 or 1555 cm1 by tuning s12 .
The CARS spectra were measured at s23 ¼ 500 fs to minimize the nonresonant background produced mostly around s12 ¼ s23 ¼ 0. This method using the delayed narrowband probe pulse has been reported by Kano and Hamaguchi [4], Prince et al. [9], and Pestov et al. [10]. We call it a hybrid CARS after Pestov et al. [10]. In the present experiment, we confirmed that the hybrid CARS is effective to identify the CARS signals, owing to both the improvement of the spectral resolution and the suppression of the nonresonant background. 3. Results and discussion Narrow peaks at two different wavelengths were observed in the CARS spectra at different ranges of s12 , 240 fs < s12 < 210 fs and at s12 ¼ 140 fs and 130 fs (Fig. 5). These peaks are assigned to the N2 and O2 CARS signals, respectively, by their Raman shifts (mN0 2 ¼ 2330 cm1 and mO0 2 ¼ 1555 cm1). It is obvious that the selective excitation is successfully performed by the control of s12 . Here, the minus sign of s12 means that the pump pulse is followed by the Stokes pulse. In the delay range of 200 fs < s12 < 160 fs, we were not able to identify any signals due to the large nonresonant background. The background was surely suppressed to some extent by the hybrid CARS technique mentioned above. However, the remaining
48
T. Kasajima et al. / Chemical Physics Letters 485 (2010) 45–48
nonresonant background completely disturbed the detection of the CARS signals in the range of 200 fs < s12 < 160 fs, while slightly raised the baselines at s12 ¼ 130 fs and 120 fs. This irregular rise of the nonresonant background probably comes from the poor flatness or complex structure in the spectrum of the white-light pulse (Fig. 3b). The spectral component necessary for the pump light (650–700 nm) is very weak compared to the central component of the spectrum (740–830 nm) which is responsible for most of the nonresonant background. In consequence, it is suggested that the spectral flatness of the white-light pulse would be important to obtain the CARS spectra continuously. We verify that the vibration-selective excitation (Fig. 5) would have been achieved by the same mechanism as LCPS-CARS (Fig. 1b). The s12 difference between x12 ¼ 2330 cm1 and 1555 cm1, Ds12 , is calculated from the estimated chirp rate of the white-light pulse and is compared to the observed Ds12 which was determined to be 80 fs from Fig. 5. The estimation is carried out using the observed white-light spectrum IðxÞ (Fig. 3b) and the observed pump-Stokes SFG autocorrelation function (Fig. 4a) as follows. In the frequency domain, the electric field of the linearly chirped white-light pulse can be written by
pffiffiffiffiffiffiffiffiffiffi 1 EðxÞ ¼ A IðxÞ exp i /2 ðx x0 Þ2 2
ð1Þ
where A is the proportional constant, /2 is the inverse of the linearchirp rate, and x0 is the carrier angular frequency (2:44 1015 s1). The electric field in the time domain EðtÞ is obtained by the inverse Fourier transform of EðxÞ. For estimation of /2 , A and /2 are adjusted in the least-square fit of the autocorrelation function
IAC ðs12 Þ ¼
Z
1
jEðtÞj2 jEðt þ s12 Þj2 dt
ð2Þ
1
to the observed SFG cross-correlation function. As a result, the chirp rate 1=/2 has been determined to be 3:01 103 fs2 (11.1 cm1/fs), which gives the best fit function shown in Fig. 4a. From this chirp rate, Ds12 is calculated as
Ds12 ¼
mN0 2 mO0 2 11:1
70 fs;
ð3Þ
which agrees well with the experimental value of 80 fs. It is concluded that the observed vibration-selective excitation was achieved by the LCPS-CARS mechanism. The pump-Stokes excitation bandwidth is estimated to be 460 cm1 as the Fourier transform limit of the temporal envelope of the pump and stokes pulses. Together with the current chirp rate of 11.1 cm1/fs, the CARS signal is expected to be observed over the
range of Ds12 = 41 fs, which agrees with the experimental results in Fig. 5. In the present demonstration, we made no attempt to get high selectivity to distinguish closer vibrations. As the next step, selectivity should be improved. The precise pulse shaping to get precise linearity and equality of chirp rate should be important for the higher selectivity. Also, a large chirp rate of the pump and Stokes pulses must be important to achieve the high selectivity, because it leads to the effective reduction of their instantaneous bandwidths. 4. Summary We have demonstrated the LCPS-CARS using white-light pulses generated by filamentation as a simple and universal technique for vibration-selective Raman excitation, which should be applicable to all of the Raman active molecular vibrations. However, the complex structure seen in the spectrum of the white-light pulse is found to cause irregular nonresonant background, which may disturb the detection of the CARS signals. To manage this problem, it is important to make the white-light spectrum as flat as possible. Acknowledgements This work was supported by Grants-in-Aid for Scientific Research (B) (No. 20360423) from the Japan Society for the Promotion of Science (JSPS). References [1] M. Dantus, V.V. Lozovoy, Chem. Rev. 104 (2004) 1813. [2] P. Nuernberger, G. Vogt, T. Brixner, G. Gerber, Phys. Chem. Chem. Phys. 9 (2007) 2470. [3] T.W. Kee, H. Zhao, M.T. Cicerone, Opt. Express 14 (2006) 3631. [4] H. Kano, H. Hamaguchi, J. Raman Spectrosc. 37 (2006) 411. [5] H. Kano, H. Hamaguchi, Anal. Chem. 79 (2007) 8967. [6] T. Hellerer, A.M.K. Enejder, A. Zumbusch, Appl. Phys. Lett. 85 (2004) 25. [7] D. Pestov, X. Wang, R.K. Murawski, G.O. Ariunbold, V.A. Sautenkov, A.V. Sokolov, J. Opt. Soc. B 25 (2008) 768. [8] W. Langbein, I. Rocha-Mendoza, P. Borri, Appl. Phys. Lett. 95 (2009) 081109. [9] B.D. Prince, A. Chakraborty, B.M. Prince, H.U. Staufferb, J. Chem. Phys. 125 (2006) 044502. [10] D. Pestov et al., Science 316 (2007) 265. [11] H. Li, D.A. Harris, B. Xu, P.J. Wrzesinski, V.V. Lozovoy, M. Dantus, Opt. Express 16 (2008) 5499. [12] S. Roy, P. Wrzesinski, D. Pestov, T. Gunaratne, M. Dantus, J.R. Gord, Appl. Phys. Lett. 95 (2009) 074102. [13] B.P. Stoicheff, Can. J. Phys. 35 (1957) 730. [14] E. Gershgoren, R.A. Bartels, J.T. Fourkas, R. Tobey, M.M. Murnane, H.C. Kapteyn, Opt. Lett. 28 (2003) 361. [15] M. Zhi, A.V. Sokolov, New J. Phys. 10 (2008) 025032. [16] J. Kasparian, J.-P. Wolf, Opt. Express 16 (2008) 466.