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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 5 — Mar. 1, 2010
  • pp: 4371–4379
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Optimized continuum from a photonic crystal fiber for broadband time-resolved coherent anti-Stokes Raman scattering

Young Jong Lee, Sapun H. Parekh, Yeon Ho Kim, and Marcus T. Cicerone  »View Author Affiliations


Optics Express, Vol. 18, Issue 5, pp. 4371-4379 (2010)
http://dx.doi.org/10.1364/OE.18.004371


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Abstract

We demonstrate an optimization of continuum generation in a commercially available photonic crystal fiber and show that this continuum can be used to simultaneously measure vibrational dephasing times over an unprecedented frequency range of Raman modes. The dephasing time measurement is based on 2-pulse 3-color coherent anti-Stokes Raman scattering (CARS), and requires a continuum pulse that is coherent over a broad spectral bandwidth. We demonstrate that a continuum with the required characteristics can be generated from a photonic crystal fiber by appropriately conditioning the chirp of the excitation pulse and controlling its pulse energy. We are able to simultaneously measure vibrational dephasing times of multiple Raman modes (covering 500 cm−1 to 3100 cm−1) of acetonitrile and benzonitrile using the optimized continuum with broadband time-resolved CARS.

© 2010 OSA

1. Introduction

2. Experimental setup

3. Results and discussion

We can also use input pulse power to optimize the continuum. Increasing input pulse power increases the bandwidth of the continuum until, at an input power of 350 mW, the long wavelength end reaches 1200 nm. Beyond this “saturation” input power, additional power only leads to more complex structure in the time domain [Figs. 3(c) and 3(d)] and results in less efficient CARS generation. Theoretical and numerical studies have shown that the majority of spectral broadening occurs within the initial stages of propagation. The length of the initial stage depends on various parameters of laser and fiber, including laser power. For a fixed fiber length, higher peak power will cause the initial broadening to occur earlier, imparting greater fiber-induced dispersion to the output continuum, and eventually lead to soliton splitting [21

21. J. M. Dudley and S. Coen, “Numerical simulations and coherence properties of supercontinuum generation in photonic crystal and tapered optical fibers,” IEEE J. Sel. Top. Quantum Electron. 8(3), 651–659 (2002). [CrossRef]

]. This scenario is consistent with the data of Fig. 2 as well as Fig. 3. At a higher power [Figs. 3(c) and 3(d)] the average GDD of the continuum is −200 fs2, while that by a lower power [Fig. 3(a)] is + 500 fs2. From the average dispersion of the fiber (~70 ps/(km·nm) [20]) in the continuum wavelength range, we can equate this difference in chirp of output continuum to a difference in the propagation within in the fiber of ~2 cm; at higher power, the pulse seems to be generated ~2 cm earlier than at low power. Thus, by tuning both chirp and power of the input pulse, we have some control over the continuum chirp, and we can pre-compensate for the dispersion of an objective lens, yielding a mean continuum GDD of near-zero at the sample position. The GDD of a continuum output can be varied to a value as large as 4500 fs2, depending on where the continuum broadening occurs in the 12 cm fiber while the GDD of typical high numerical aperture objective lenses are 2000 fs2 ~6500 fs2 [22

22. J. B. Guild, C. Xu, and W. W. Webb, “Measurement of group delay dispersion of high numerical aperture objective lenses using two-photon excited fluorescence,” Appl. Opt. 36(1), 397–401 (1997). [CrossRef] [PubMed]

]. This fortuitous GDD cancellation removes necessity of additional pulse compression or pulse shaping for the continuum pulse, allowing maximum power delivery for the continuum pulse into the sample position even though the higher order dispersion is still not compensated.

Based on needs for generation of 3-color CARS, an appropriate amount of power and GDD of the excitation pulse should be chosen for both a reasonable Raman shift range and signal intensity. For the remainder of this study, an input pulse power of 350 mW and the GDD of −19000 fs2 are used to generate the continuum pulse for broad and strong 3-color CARS generation. The input pulse polarization is matched to the polarization axis of the fiber, which provides the highest throughput and polarization extinction of the output continuum, which will maximize the efficiency of CARS signal generation. The output power of the continuum is measured as 40 mW after the 850 nm longpass filter and the power is reduced with a neutral density filter to an appropriate level at the sample position. It should be noted that one may achieve a continuum with similar bandwidth, but possibly a greater degree of coherence using different length or different dispersion of fiber.

Figure 4
Fig. 4 Time-resolved CARS spectroscopy of neat (a) acetonitrile and (b) benzonitrile. CARS spectra at Δt = 0 and 1 ps are shown for (c) acetonitrile and (d) benzonitrile. Peak intensities at time delay, Δt = 1 ps, are plotted as a function of narrowband pulse power at the sample position for (e) acetonitrile and (f) benzonitrile. Log-log plots of the peak intensities at Δt = 1 ps are fitted to log(I) = slope*log(P) + constant, where I is the intensity, P is the average power of the narrowband pulse. The fitted slopes are all close to unity, confirming that the signal at Δt = 1 ps is generated by the 3-color CARS mechanism. The average power of the continuum pulse was constant at 14 mW, and the average power of the narrowband pulse was 1 mW at the sample position for (a)-(d). For each time scan, the time-independent baseline was measured at Δt = −2 ps and subtracted from the total CARS spectrum data. The baseline was (10 to 20) % of the peak signal. The exposure time was 600 ms.
shows time resolved CARS results from neat acetonitrile and benzonitrile using the optimized continuum pulse in the 3-color arrangement. The time resolved data show clear differences in the measured CARS spectra of Δt = 0 ps and 1 ps. At Δt = 0 ps, NRB of both 2- and 3-color CARS interferes with resonant signal and produces dispersive line shapes overlaid on a broad baseline. At Δt = 1 ps, the NRB contribution diminishes and only 3-color resonant CARS signal remains. The log-log plots of the peak height at Δt = 1 ps in Figs. 4(e) and 4(f) show a linear dependence on the narrowband pulse power, which confirms that the time-delayed spectra are generated by the 3-color CARS mechanism. The frequency range shown in the time-resolved spectra is noteworthy - in previously reported NRB-free CARS spectra using similar time-resolved techniques, Raman peaks are barely analyzable at frequencies greater than 2300 cm−1 [5

5. Y. J. Lee and M. T. Cicerone, “Vibrational dephasing time imaging by time-resolved broadband coherent anti-Stokes Raman scattering microscopy,” Appl. Phys. Lett. 92(4), 041108 (2008). [CrossRef]

]. This higher-frequency region includes CH stretch resonances, whose dephasing time will be impacted by biologically important phenomena, such as membrane phase changes. The optimized time-resolved broadband CARS technique presented here provides access to vibrational dephasing times of multiple Raman modes from 500 cm−1 to 3100 cm−1 in a single time delay scan without any additional laser tuning. This broad frequency range is not available in the conventional non-collinear coherent time-resolved techniques in a bulk sample due to the strict phase matching condition. Figure 5
Fig. 5 Dephasing time measurements of various Raman modes of neat (a-c) acetonitrile and (d-f) benzonitrile. The dotted lines indicate time profiles of the nonresonant background signal from a glass coverslip at the same Raman shift in each figure. The data of time delay (Δt) later than 1 ps are fitted to a single exponential function, I(t) = exp(−2Δt/T 2), where T 2 is the vibrational dephasing time. The average powers of the continuum and narrowband pulses were 14 mW and 1 mW, respectively, at the sample position. The exposure time was 600 ms.
shows time evolution of low, medium, and high frequency Raman modes, extracted from the data shown in Figs. 4(a) and 4(b). The bandwidth of the narrowband pulse can be easily adjusted to give the desired temporal and spectral resolution considering the dephasing time of a mode and its frequency proximity to adjacent Raman modes. For Figs. 4 and 5, the narrowband pulse is shaped to a Gaussian function with the full-width-half-maximum (FWHM) of 50 cm−1, which corresponds to 300 fs in the time domain. For the instrumental response functions (IRF), the time profiles are measured from a glass cover slip (a nonresonant medium) under the same measurement condition as the sample liquids, and the IRF FWHM ranges between 500 fs and 550 fs (the dotted lines in Fig. 5). Data only after Δt > 1 ps are used for analysis to avoid the interference from NRB contribution when the pulses overlap. The time profiles are fitted to single exponential functions, It) = exp(−2Δt/T2), where T2 is the vibrational dephasing time [7

7. A. Morresi, L. Mariani, M. R. Distefano, and M. G. Giorgini, “Vibrational-Relaxation Processes in Isotropic Molecular Liquids - A Critical Comparison,” J. Raman Spectrosc. 26(3), 179–216 (1995). [CrossRef]

].

It should be noted that the time profile measured by this 2-pulse time-resolved CARS technique contains not only the vibrational dephasing process but other dynamic processes, including vibrational depopulation and rotational motion [7

7. A. Morresi, L. Mariani, M. R. Distefano, and M. G. Giorgini, “Vibrational-Relaxation Processes in Isotropic Molecular Liquids - A Critical Comparison,” J. Raman Spectrosc. 26(3), 179–216 (1995). [CrossRef]

]. The vibrational depopulation time, T1, can cause the measured dephasing time, T2, to be shorter than the pure dephasing time, T2*, as 1/T2 = 1/(2T1) + 1/T2*. For most liquids and solutions at room temperature, T1 is considerably longer than T2 and the perturbation of T2 by T1 is negligible [6

6. A. Laubereau and W. Kaiser, “Vibrational Dynamics of Liquids and Solids Investigated by Picosecond Light-Pulses,” Rev. Mod. Phys. 50(3), 607–665 (1978). [CrossRef]

,29

29. H. Hamaguchi and T. L. Gustafson, “Ultrafast Time-Resolved Spontaneous and Coherent Raman-Spectroscopy - the Structure and Dynamics of Photogenerated Transient Species,” Annu. Rev. Phys. Chem. 45(1), 593–622 (1994). [CrossRef]

]. For example, the T1 values of liquid acetonitrile are 45 ps, 80 ps, and 5 ps for the CC stretch, CN stretch, and CH stretch modes, respectively [30

30. J. C. Deäk, L. K. Iwaki, and D. D. Dlott, “Vibrational energy redistribution in polyatomic liquids: Ultrafast IR-Raman spectroscopy of acetonitrile,” J. Phys. Chem. A 102(42), 8193–8201 (1998). [CrossRef]

]. Similarly, rotational motion of molecules can affect the time profile of CARS signals when the anisotropy tensor element (or the depolarization ratio) of a Raman mode is large and the time scale of rotation is comparable to T2. However, in many practical cases, the values of anisotropy tensor elements are small and the rotation times and T2 are sufficiently dissimilar that contribution of molecular rotation may be negligible [6

6. A. Laubereau and W. Kaiser, “Vibrational Dynamics of Liquids and Solids Investigated by Picosecond Light-Pulses,” Rev. Mod. Phys. 50(3), 607–665 (1978). [CrossRef]

]. For example, the depolarization ratio of the CN stretch modes are 0.04 and 0.045 for acetonitrile and benzonitrile, respectively [31

31. D. Bhattacharjee, A. G. Purkayastha, T. N. Misra, and S. K. Nandy, “Raman Spectral Study of Vibrational Relaxation of the CN Stretching Band of Acetonitrile and Benzonitrile,” Journal of Raman Spectroscopy 27(6), 457–461 (1996). [CrossRef]

], and the reorientational relaxation time is 5.58 ps for the CN mode of neat benzonitrile [28

28. H. Okamoto, R. Inaba, K. Yoshihara, and M. Tasumi, “Femtosecond Time-Resolved Polarized Coherent Anti-Stokes Raman Studies on Reorientational Relaxation in Benzonitrile,” Chem. Phys. Lett. 202(1-2), 161–166 (1993). [CrossRef]

]. To minimize the rotational motion contribution experimentally, the magic angle polarization configuration can be applied [32

32. B. Dick, “Response Function-Theory of Time-Resolved CARS and CSRS of Rotating Molecules in Liquids Under General Polarization Conditions,” Chem. Phys. 113(1), 131–147 (1987). [CrossRef]

] by using a set of accurately adjusted waveplates, but complete suppression of the contribution may still be difficult to achieve due to polarization mixing associated with tight focusing at the sample position.

4. Conclusion

References and links

1.

T. C. Bakker Schut, R. Wolthuis, P. J. Caspers, and G. J. Puppels, “Real-time tissue characterization on the basis of in vivo Raman spectra,” J. Raman Spectrosc. 33(7), 580–585 (2002). [CrossRef]

2.

T. W. Kee and M. T. Cicerone, “Simple approach to one-laser, broadband coherent anti-Stokes Raman scattering microscopy,” Opt. Lett. 29(23), 2701–2703 (2004). [CrossRef] [PubMed]

3.

H. Kano and H. Hamaguchi, “Femtosecond coherent anti-Stokes Raman scattering spectroscopy using supercontinuum generated from a photonic crystal fiber,” Appl. Phys. Lett. 85(19), 4298–4300 (2004). [CrossRef]

4.

Y. J. Lee, Y. Liu, and M. T. Cicerone, “Characterization of three-color CARS in a two-pulse broadband CARS spectrum,” Opt. Lett. 32(22), 3370–3372 (2007). [CrossRef] [PubMed]

5.

Y. J. Lee and M. T. Cicerone, “Vibrational dephasing time imaging by time-resolved broadband coherent anti-Stokes Raman scattering microscopy,” Appl. Phys. Lett. 92(4), 041108 (2008). [CrossRef]

6.

A. Laubereau and W. Kaiser, “Vibrational Dynamics of Liquids and Solids Investigated by Picosecond Light-Pulses,” Rev. Mod. Phys. 50(3), 607–665 (1978). [CrossRef]

7.

A. Morresi, L. Mariani, M. R. Distefano, and M. G. Giorgini, “Vibrational-Relaxation Processes in Isotropic Molecular Liquids - A Critical Comparison,” J. Raman Spectrosc. 26(3), 179–216 (1995). [CrossRef]

8.

H. Kano and H. Hamaguchi, “Dispersion-compensated supercontinuum generation for ultrabroadband multiplex coherent anti-Stokes Raman scattering spectroscopy,” J. Raman Spectrosc. 37(1-3), 411–415 (2006). [CrossRef]

9.

D. Pestov, R. K. Murawski, G. O. Ariunbold, X. Wang, M. C. Zhi, A. V. Sokolov, V. A. Sautenkov, Y. V. Rostovtsev, A. Dogariu, Y. Huang, and M. O. Scully, “Optimizing the laser-pulse configuration for coherent Raman spectroscopy,” Science 316(5822), 265–268 (2007). [CrossRef] [PubMed]

10.

B. von Vacano and M. Motzkus, “Time-resolving molecular vibration for microanalytics: single laser beam nonlinear Raman spectroscopy in simulation and experiment,” Phys. Chem. Chem. Phys. 10(5), 681–691 (2008). [CrossRef] [PubMed]

11.

A. Volkmer, L. D. Book, and X. S. Xie, “Time-resolved coherent anti-Stokes Raman scattering microscopy: Imaging based on Raman free induction decay,” Appl. Phys. Lett. 80(9), 1505–1507 (2002). [CrossRef]

12.

J. P. Ogilvie, E. Beaurepaire, A. Alexandrou, and M. Joffre, “Fourier-transform coherent anti-Stokes Raman scattering microscopy,” Opt. Lett. 31(4), 480–482 (2006). [CrossRef] [PubMed]

13.

K. B. Shi, P. Li, and Z. W. Liu, “Broadband coherent anti-Stokes Raman scattering spectroscopy in supercontinuum optical trap,” Appl. Phys. Lett. 90(14), 141116 (2007). [CrossRef]

14.

J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]

15.

B. Schenkel, R. Paschotta, and U. Keller, “Pulse compression with supercontinuum generation in microstructure fibers,” J. Opt. Soc. Am. B 22(3), 687–693 (2005). [CrossRef]

16.

M. A. Foster, A. L. Gaeta, Q. Cao, and R. Trebino, “Soliton-effect compression of supercontinuum to few-cycle durations in photonic nanowires,” Opt. Express 13(18), 6848–6855 (2005). [CrossRef] [PubMed]

17.

N. Nishizawa and J. Takayanagi, “Octave spanning high-quality supercontinuum generation in all-fiber system,” J. Opt. Soc. Am. B 24(8), 1786–1792 (2007). [CrossRef]

18.

L. B. Fu, B. K. Thomas, and L. Dong, “Efficient supercontinuum generations in silica suspended core fibers,” Opt. Express 16(24), 19629–19642 (2008). [CrossRef] [PubMed]

19.

Certain equipment is identified in this Letter to specify adequately the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply that the equipment is necessarily the best available for this purpose.

20.

http://nktphotonics.com/files/files/datasheet_fw-800.pdf

21.

J. M. Dudley and S. Coen, “Numerical simulations and coherence properties of supercontinuum generation in photonic crystal and tapered optical fibers,” IEEE J. Sel. Top. Quantum Electron. 8(3), 651–659 (2002). [CrossRef]

22.

J. B. Guild, C. Xu, and W. W. Webb, “Measurement of group delay dispersion of high numerical aperture objective lenses using two-photon excited fluorescence,” Appl. Opt. 36(1), 397–401 (1997). [CrossRef] [PubMed]

23.

H. W. Hubble, T. S. Lai, and M. A. Berg, “Raman free-induction-decay measurements in low viscosity and supercooled toluene: Vibrational dephasing by shear fluctuations,” J. Chem. Phys. 114(8), 3662–3673 (2001). [CrossRef]

24.

R. Inaba, H. Okamoto, K. Yoshihara, and M. Tasumi, “Observation of the dephasing of the C.tplbond.N stretching vibration in liquid nitriles by femtosecond time-resolved coherent anti-Stokes Raman scattering,” J. Phys. Chem. 96(21), 8385–8390 (1992). [CrossRef]

25.

D. Vanden Bout, L. Muller, and M. Berg, “Ultrafast Raman echoes in liquid acetonitrile,” Phys. Rev. Lett. 67(26), 3700–3703 (1991). [CrossRef] [PubMed]

26.

M. Fickenscher and A. Laubereau, “High-Precision Femtosecond CARS of Simple Liquids,” J. Raman Spectrosc. 21(12), 857–861 (1990). [CrossRef]

27.

R. Inaba, K. Tominaga, M. Tasumi, K. A. Nelson, and K. Yoshihara, “Observation of Homogeneous Vibrational Dephasing in Benzonitrile by Ultrafast Raman Echoes,” Chem. Phys. Lett. 211(2-3), 183–188 (1993). [CrossRef]

28.

H. Okamoto, R. Inaba, K. Yoshihara, and M. Tasumi, “Femtosecond Time-Resolved Polarized Coherent Anti-Stokes Raman Studies on Reorientational Relaxation in Benzonitrile,” Chem. Phys. Lett. 202(1-2), 161–166 (1993). [CrossRef]

29.

H. Hamaguchi and T. L. Gustafson, “Ultrafast Time-Resolved Spontaneous and Coherent Raman-Spectroscopy - the Structure and Dynamics of Photogenerated Transient Species,” Annu. Rev. Phys. Chem. 45(1), 593–622 (1994). [CrossRef]

30.

J. C. Deäk, L. K. Iwaki, and D. D. Dlott, “Vibrational energy redistribution in polyatomic liquids: Ultrafast IR-Raman spectroscopy of acetonitrile,” J. Phys. Chem. A 102(42), 8193–8201 (1998). [CrossRef]

31.

D. Bhattacharjee, A. G. Purkayastha, T. N. Misra, and S. K. Nandy, “Raman Spectral Study of Vibrational Relaxation of the CN Stretching Band of Acetonitrile and Benzonitrile,” Journal of Raman Spectroscopy 27(6), 457–461 (1996). [CrossRef]

32.

B. Dick, “Response Function-Theory of Time-Resolved CARS and CSRS of Rotating Molecules in Liquids Under General Polarization Conditions,” Chem. Phys. 113(1), 131–147 (1987). [CrossRef]

OCIS Codes
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(300.6230) Spectroscopy : Spectroscopy, coherent anti-Stokes Raman scattering
(300.6500) Spectroscopy : Spectroscopy, time-resolved

ToC Category:
Nonlinear Optics

History
Original Manuscript: January 4, 2010
Revised Manuscript: January 28, 2010
Manuscript Accepted: January 28, 2010
Published: February 17, 2010

Virtual Issues
Vol. 5, Iss. 6 Virtual Journal for Biomedical Optics

Citation
Young Jong Lee, Sapun H. Parekh, Yeon Ho Kim, and Marcus T. Cicerone, "Optimized continuum from a photonic crystal fiber for broadband time-resolved coherent anti-Stokes Raman scattering," Opt. Express 18, 4371-4379 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-5-4371


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References

  1. T. C. Bakker Schut, R. Wolthuis, P. J. Caspers, and G. J. Puppels, “Real-time tissue characterization on the basis of in vivo Raman spectra,” J. Raman Spectrosc. 33(7), 580–585 (2002). [CrossRef]
  2. T. W. Kee and M. T. Cicerone, “Simple approach to one-laser, broadband coherent anti-Stokes Raman scattering microscopy,” Opt. Lett. 29(23), 2701–2703 (2004). [CrossRef] [PubMed]
  3. H. Kano and H. Hamaguchi, “Femtosecond coherent anti-Stokes Raman scattering spectroscopy using supercontinuum generated from a photonic crystal fiber,” Appl. Phys. Lett. 85(19), 4298–4300 (2004). [CrossRef]
  4. Y. J. Lee, Y. Liu, and M. T. Cicerone, “Characterization of three-color CARS in a two-pulse broadband CARS spectrum,” Opt. Lett. 32(22), 3370–3372 (2007). [CrossRef] [PubMed]
  5. Y. J. Lee and M. T. Cicerone, “Vibrational dephasing time imaging by time-resolved broadband coherent anti-Stokes Raman scattering microscopy,” Appl. Phys. Lett. 92(4), 041108 (2008). [CrossRef]
  6. A. Laubereau and W. Kaiser, “Vibrational Dynamics of Liquids and Solids Investigated by Picosecond Light-Pulses,” Rev. Mod. Phys. 50(3), 607–665 (1978). [CrossRef]
  7. A. Morresi, L. Mariani, M. R. Distefano, and M. G. Giorgini, “Vibrational-Relaxation Processes in Isotropic Molecular Liquids - A Critical Comparison,” J. Raman Spectrosc. 26(3), 179–216 (1995). [CrossRef]
  8. H. Kano and H. Hamaguchi, “Dispersion-compensated supercontinuum generation for ultrabroadband multiplex coherent anti-Stokes Raman scattering spectroscopy,” J. Raman Spectrosc. 37(1-3), 411–415 (2006). [CrossRef]
  9. D. Pestov, R. K. Murawski, G. O. Ariunbold, X. Wang, M. C. Zhi, A. V. Sokolov, V. A. Sautenkov, Y. V. Rostovtsev, A. Dogariu, Y. Huang, and M. O. Scully, “Optimizing the laser-pulse configuration for coherent Raman spectroscopy,” Science 316(5822), 265–268 (2007). [CrossRef] [PubMed]
  10. B. von Vacano and M. Motzkus, “Time-resolving molecular vibration for microanalytics: single laser beam nonlinear Raman spectroscopy in simulation and experiment,” Phys. Chem. Chem. Phys. 10(5), 681–691 (2008). [CrossRef] [PubMed]
  11. A. Volkmer, L. D. Book, and X. S. Xie, “Time-resolved coherent anti-Stokes Raman scattering microscopy: Imaging based on Raman free induction decay,” Appl. Phys. Lett. 80(9), 1505–1507 (2002). [CrossRef]
  12. J. P. Ogilvie, E. Beaurepaire, A. Alexandrou, and M. Joffre, “Fourier-transform coherent anti-Stokes Raman scattering microscopy,” Opt. Lett. 31(4), 480–482 (2006). [CrossRef] [PubMed]
  13. K. B. Shi, P. Li, and Z. W. Liu, “Broadband coherent anti-Stokes Raman scattering spectroscopy in supercontinuum optical trap,” Appl. Phys. Lett. 90(14), 141116 (2007). [CrossRef]
  14. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]
  15. B. Schenkel, R. Paschotta, and U. Keller, “Pulse compression with supercontinuum generation in microstructure fibers,” J. Opt. Soc. Am. B 22(3), 687–693 (2005). [CrossRef]
  16. M. A. Foster, A. L. Gaeta, Q. Cao, and R. Trebino, “Soliton-effect compression of supercontinuum to few-cycle durations in photonic nanowires,” Opt. Express 13(18), 6848–6855 (2005). [CrossRef] [PubMed]
  17. N. Nishizawa and J. Takayanagi, “Octave spanning high-quality supercontinuum generation in all-fiber system,” J. Opt. Soc. Am. B 24(8), 1786–1792 (2007). [CrossRef]
  18. L. B. Fu, B. K. Thomas, and L. Dong, “Efficient supercontinuum generations in silica suspended core fibers,” Opt. Express 16(24), 19629–19642 (2008). [CrossRef] [PubMed]
  19. Certain equipment is identified in this Letter to specify adequately the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply that the equipment is necessarily the best available for this purpose.
  20. http://nktphotonics.com/files/files/datasheet_fw-800.pdf
  21. J. M. Dudley and S. Coen, “Numerical simulations and coherence properties of supercontinuum generation in photonic crystal and tapered optical fibers,” IEEE J. Sel. Top. Quantum Electron. 8(3), 651–659 (2002). [CrossRef]
  22. J. B. Guild, C. Xu, and W. W. Webb, “Measurement of group delay dispersion of high numerical aperture objective lenses using two-photon excited fluorescence,” Appl. Opt. 36(1), 397–401 (1997). [CrossRef] [PubMed]
  23. H. W. Hubble, T. S. Lai, and M. A. Berg, “Raman free-induction-decay measurements in low viscosity and supercooled toluene: Vibrational dephasing by shear fluctuations,” J. Chem. Phys. 114(8), 3662–3673 (2001). [CrossRef]
  24. R. Inaba, H. Okamoto, K. Yoshihara, and M. Tasumi, “Observation of the dephasing of the C.tplbond.N stretching vibration in liquid nitriles by femtosecond time-resolved coherent anti-Stokes Raman scattering,” J. Phys. Chem. 96(21), 8385–8390 (1992). [CrossRef]
  25. D. Vanden Bout, L. Muller, and M. Berg, “Ultrafast Raman echoes in liquid acetonitrile,” Phys. Rev. Lett. 67(26), 3700–3703 (1991). [CrossRef] [PubMed]
  26. M. Fickenscher and A. Laubereau, “High-Precision Femtosecond CARS of Simple Liquids,” J. Raman Spectrosc. 21(12), 857–861 (1990). [CrossRef]
  27. R. Inaba, K. Tominaga, M. Tasumi, K. A. Nelson, and K. Yoshihara, “Observation of Homogeneous Vibrational Dephasing in Benzonitrile by Ultrafast Raman Echoes,” Chem. Phys. Lett. 211(2-3), 183–188 (1993). [CrossRef]
  28. H. Okamoto, R. Inaba, K. Yoshihara, and M. Tasumi, “Femtosecond Time-Resolved Polarized Coherent Anti-Stokes Raman Studies on Reorientational Relaxation in Benzonitrile,” Chem. Phys. Lett. 202(1-2), 161–166 (1993). [CrossRef]
  29. H. Hamaguchi and T. L. Gustafson, “Ultrafast Time-Resolved Spontaneous and Coherent Raman-Spectroscopy - the Structure and Dynamics of Photogenerated Transient Species,” Annu. Rev. Phys. Chem. 45(1), 593–622 (1994). [CrossRef]
  30. J. C. Deäk, L. K. Iwaki, and D. D. Dlott, “Vibrational energy redistribution in polyatomic liquids: Ultrafast IR-Raman spectroscopy of acetonitrile,” J. Phys. Chem. A 102(42), 8193–8201 (1998). [CrossRef]
  31. D. Bhattacharjee, A. G. Purkayastha, T. N. Misra, and S. K. Nandy, “Raman Spectral Study of Vibrational Relaxation of the CN Stretching Band of Acetonitrile and Benzonitrile,” Journal of Raman Spectroscopy 27(6), 457–461 (1996). [CrossRef]
  32. B. Dick, “Response Function-Theory of Time-Resolved CARS and CSRS of Rotating Molecules in Liquids Under General Polarization Conditions,” Chem. Phys. 113(1), 131–147 (1987). [CrossRef]

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