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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 23 — Nov. 9, 2009
  • pp: 21060–21065
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Elongation of femtosecond filament by molecular alignment in air

Hua Cai, Jian Wu, Hao Li, Xueshi Bai, and Heping Zeng  »View Author Affiliations


Optics Express, Vol. 17, Issue 23, pp. 21060-21065 (2009)
http://dx.doi.org/10.1364/OE.17.021060


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Abstract

We study the influence of the molecular alignment on the plasma channel length of femtosecond filament at 800 nm in air. The filament length is observed to be nearly doubled when the high-energy femtosecond laser pulse is properly tuned to match the perpendicular revivals of the molecular alignment, which is different from the low-energy femtosecond laser pulse where the filament is promoted for parallel molecular alignment revivals. These are understood to be the loosened or tightened focusing condition of the propagation of the femtosecond laser pulse by the cross-(de)focusing effect from the prealigned molecules.

© 2009 OSA

1. Introduction

The self-guided propagation of infrared femtosecond laser pulses in transparent optical nonlinear media has sparked many interests and important applications [1

1. A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 ( 2007). [CrossRef]

7

7. X. Xie, J. Dai, and X. C. Zhang, “Coherent control of THz wave generation in ambient air,” Phys. Rev. Lett. 96(7), 075005 ( 2006). [CrossRef] [PubMed]

] such as atmospheric remote sensing [4

4. J. Yu, D. Mondelain, G. Ange, R. Volk, S. Niedermeier, J.-P. Wolf, J. Kasparian, and R. Sauerbrey, “Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air,” Opt. Lett. 26(8), 533–535 ( 2001). [CrossRef] [PubMed]

,5

5. J. Kasparian, M. Rodriguez, G. Méjean, J. Yu, E. Salmon, H. Wille, R. Bourayou, S. Frey, Y.-B. André, A. Mysyrowicz, R. Sauerbrey, J.-P. Wolf, and L. Wöste, “White-light filaments for atmospheric analysis,” Science 301(5629), 61–64 ( 2003). [CrossRef] [PubMed]

], trigger and guide electric discharges [6

6. M. Rodriguez, R. Sauerbrey, H. Wille, L. Wöste, T. Fujii, Y.-B. André, A. Mysyrowicz, L. Klingbeil, K. Rethmeier, W. Kalkner, J. Kasparian, E. Salmon, J. Yu, and J.-P. Wolf, “Triggering and guiding megavolt discharges by use of laser-induced ionized filaments,” Opt. Lett. 27(9), 772–774 ( 2002). [CrossRef] [PubMed]

], and remote terahertz source [7

7. X. Xie, J. Dai, and X. C. Zhang, “Coherent control of THz wave generation in ambient air,” Phys. Rev. Lett. 96(7), 075005 ( 2006). [CrossRef] [PubMed]

]. For most applications, a long-distance filament is desired, and it was shown that, for constant input pulse energy, the length of the filament could be longest for an optimum pulse duration [8

8. A. Couairon, “Filamentation length of powerful laser pulses,” Appl. Phys. B 76(7), 789–792 ( 2003). [CrossRef]

], which was also shown to be sensitive to the temporal chirp [9

9. I. S. Golubtsov, V. P. Kandidov, and O. G. Kosareva, “Initial phase modulation of a high-power femtosecond laser pulse as a tool for controlling its filamentation and generation of a supercontinuum in air,” Quantum Electron. 33(6), 525–530 ( 2003). [CrossRef]

] of femtosecond laser pulses. For molecular gas, we have recently numerically demonstrated [10

10. J. Wu, H. Cai, H. Zeng, and A. Couairon, “Femtosecond filamentation and pulse compression in the wake of molecular alignment,” Opt. Lett. 33(22), 2593–2595 ( 2008). [CrossRef] [PubMed]

] that the length of the filament could be significantly increased by properly tuning the femtosecond laser pulse to match the molecular alignment revivals after an impulsive excitation of an ultrashort pump pulse. The controllable orientation of molecules with respect to the field polarization [11

11. H. Stapelfeldt and T. Seideman, “Colloquium: aligning molecules with strong laser pulses,” Rev. Mod. Phys. 75(2), 543–557 ( 2003). [CrossRef]

] provides us an additional degree of freedom to control the filament propagation and interaction, such as filament formation [12

12. F. Calegari, C. Vozzi, S. Gasilov, E. Benedetti, G. Sansone, M. Nisoli, S. De Silvestri, and S. Stagira, “Rotational Raman effects in the wake of optical filamentation,” Phys. Rev. Lett. 100(12), 123006 ( 2008). [CrossRef] [PubMed]

,13

13. S. Varma, Y.-H. Chen, and H. M. Milchberg, “Trapping and destruction of long-range high-intensity optical filaments by molecular quantum wakes in air,” Phys. Rev. Lett. 101(20), 205001 ( 2008). [CrossRef] [PubMed]

], filament induced supercontinuum generation [14

14. J. Wu, H. Cai, Y. Peng, and H. Zeng, “Controllable supercontinuum generation by the quantum wake of molecular alignment,” Phys. Rev. A 79(4), 041404 ( 2009). [CrossRef]

,15

15. H. Cai, J. Wu, Y. Peng, and H. Zeng, “Comparison study of supercontinuum generation by molecular alignment of N2 and O2.,” Opt. Express 17(7), 5822–5828 ( 2009). [PubMed]

], couplings of crossly overlapped filaments [16

16. A. C. Bernstein, M. McCormick, G. M. Dyer, J. C. Sanders, and T. Ditmire, “Two-beam coupling between filament-forming beams in air,” Phys. Rev. Lett. 102(12), 123902 ( 2009). [CrossRef] [PubMed]

], and kicking of intense filaments [17

17. J. Wu, Y. Tong, X. Yang, H. Cai, P. Lu, H. Pan, and H. Zeng, “Interaction of two parallel femtosecond filaments at different wavelengths in air,” Opt. Lett. (in press.

].

In this paper, we experimentally demonstrate that the length of the filament channel could be significantly increased by propagating the energetic femtosecond laser pulse in prealigned diatomic molecules in air. When the high-energy femtosecond pulse was properly tuned to match the perpendicular revivals of the molecular alignment, the filament length was nearly doubled with respect to the randomly orientated molecules. The basic underlying physics is that the cross-(de)focusing effect of the prealigned molecules modulated the focusing condition and hence the filament dynamics.

2. Experiment setup

Our experiments, as schematically shown in Fig. 1
Fig. 1 (a) Schematic illustration of the experimental setup, 1: lens (f = 5 cm, D = 5 cm), 2: dielectric mirror, 3: UG11, 4: slit. The calculated molecular alignment metric <<cos 2 θ >> versus pump-probe delay in (b) N2, (c) O2, and (d) air.
, were carried out with an amplified Ti:Sapphire laser system (1 kHz, 800 nm, 35 fs). The original beam diameter of the pump and probe pulses were 12 mm (1/e2), and only the p-polarized pump beam was down-collimated with a minification of 2:1 to achieve a larger interaction volume, which were recombined collinearly with the s-polarized probe pulse by a thin film polarizer (TFP). The time delay between the pump and probe pulses was tuned by using a motorizing translation in the probe arm. The final pulse-energies after the TFP for the pump and probe pulses were measured to be 1.43 and 2.26 mJ, which were focused with two separated lenses of f = 1 m and f = 2 m, respectively. These two lenses were positioned properly with respect to the TFP so that the pump filament was located at the onset of the probe one to control the onset of the probe filament through the molecular alignment. By using a half wave plate just before the TFP in the pump arm, the pulse energy of the pump could be adjusted from 0.1 to 1.43 mJ. We determined the filament channel length by measuring the transverse fluorescence emission perpendicular to the filament propagation. Our detection setup comprised of a fused-silica lens, a slit, and a photomultiplier tube (PMT). In order to eliminate the influence of the input pulse and supercontinuum emission, a dielectric mirror (HR 800 nm @ 0°) and a bandpass filter (UG11, transmission 200-400 nm) were placed just in front of the PMT. The whole detection setup was installed on a breadboard mounted on a motorizing translation stage, which could be moved along the filament propagation direction. The chopper frequency signal was used to provide the reference input for the lock-in amplifier, and only the output signal from the PMT at the chopping frequency was processed by the lock-in amplifier. Hence, by setting a chopper in the pump or probe arm, only the fluorescence emission from the pump or probe pulse was measured, respectively.

3. Results and discussions

Figure 2
Fig. 2 The measured filament channels of the probe pulse when it is turned to various molecular alignment revivals as labeled in Fig. 1. The filament channels of the pump (navy curve) and probe (blue curve) pulses when they propagate alone are also shown.
shows the measured filament channels of the probe pulse when it was tuned to experience various molecular alignment revivals. The zero position of the propagation axis in the figure was defined to be at the distance of 80 cm after the TFP and the termination of the filament [3

3. Y. Chen, F. Théberge, O. Kosareva, N. Panov, V. P. Kandidov, and S. L. Chin, “Evolution and termination of a femtosecond laser filament in air,” Opt. Lett. 32(24), 3477–3479 ( 2007). [CrossRef] [PubMed]

] was defined to be the position with fluorescence intensity smaller than 2.9% of the plateau region in this paper. The filament channels for the pump and probe pulses when they propagated alone in the air are also shown in Fig. 2.

We calculated the corresponding molecular alignment revival <<cos 2 θ >> in N2, O2 and air by considering the coherent superposition of the molecular rotational wave packets excited by the pump pulse as described in Ref [18

18. J. Wu, H. Cai, A. Couairon, and H. Zeng, “Few-cycle shock X-wave generation by filamentation in prealigned molecules,” Phys. Rev. A 80(1), 013828 ( 2009). [CrossRef]

], which were respectively shown in Figs. 1(b), 1(c) and 1(d), and θ is the angle between the molecular axis and the probe polarization. The revival signals <<cos 2 θ >> greater and smaller than 1/3 account for, respectively, the transient molecular orientations parallel and perpendicular to the field polarization of the probe pulse, which equals to 1/3 for randomly orientated molecules.

The cross-(de)focusing effect from the molecular alignment could be adjusted by changing the intensity of the pump pulse, which could be used to continuously control the probe filament. As shown in Fig. 3
Fig. 3 The measured probe filament channels with different pump intensities in air for molecular alignment revival at delay-B. The dashed and solid curves stand for the cases when the pump pulse is turned off and turned on, respectively.
, for the perpendicular alignment revival at delay-B, the length of the probe filament channel decreased gradually as the decreasing pump energy from 1.4 to 1.1 mJ. According to the decreased cross-defocusing effect from the perpendicularly orientated molecules, the onset positions of the probe filament moved backward as the pump intensity decreased. No noticeable elongation of the probe filament was observed when the pump energy was lower than 1.0 mJ for the negligible influence of the molecular alignment.

In the above discussion, the observed filament elongation was a combined contribution of molecular N2 and O2 in air, especially at delay-B where the longest filament channel was observed. In order to individually show the molecular alignment effect of N2 and O2 on the filament length control, we then tuned the probe pulse to the half revivals of molecular N2 and O2 in air around 4.2 and 5.8 ps, respectively. The measured probe filaments were shown in Fig. 4
Fig. 4 The measured probe filament channel when it is tuned to match the individual revivals of molecular (a) N2 and (b) O2 in air, and random orientation (labeled as r). The energies of the pump and probe pulses were 1.43 and 2.26 mJ, respectively
. Similar to the above observations around 8.0~9.0 ps, the movement of the onset positions of the probe filament due to the cross-(de)focusing effect from the plasma and molecular alignment revivals were also observed. As compared to molecular O2, the change of the onset position of the probe filament was more significant around the molecular alignment of N2. This was consistent with the fact that much more N2 (~80%) are contained than O2 (~20%) in air, which therefore influenced the probe filament more drastically. Correspondingly, the probe filament elongation was more significant around the perpendicular alignment revival of N2 at delay-D than that of O2 at delay-F, which both were shorter than that at delay-B (as shown in Fig. 2) where the molecular alignment revival of N2 and O2 cooperated together to dominate the probe filament.

Figure 5
Fig. 5 The measured fluorescence intensity variations of the probe filament in air at the propagation distances of (a, b) 20, and (c, d) 50 cm as a function of the time delay.
shows the measured fluorescence intensity variations of the probe filament at different propagation distances as a function of the pump-probe time delay. As shown in Figs. 5(a) and 5(b) around the onset positions of the probe filament at a propagation distance of 20 cm, the fluorescence intensity decreased at delays D and F, and increased at delays E and G, which were in accordance with the forward and backward movements of the onset positions of the probe filament (Fig. 4). As shown in Figs. 5(c) and 5(d) at a propagation distance of 50 cm, the fluorescence intensity of the probe filament channel was increased only at delays D and F, indicating that the probe filament channels were significantly elongated only at these time delays with perpendicular orientated N2/O2 molecules.

4. Conclusion

Acknowledgments

This work was funded in part by National Natural Science Fund (10990101 & 10525416 & 10804032), National Key Project for Basic Research (2006CB806005), Projects from Shanghai Science and Technology Commission (08ZR1407100 & 09QA1402000), and Shanghai Educational Development Foundation (2008CG29).

References and links

1.

A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 ( 2007). [CrossRef]

2.

C. Marceau, Y. Chen, F. Théberge, M. Châteauneuf, J. Dubois, and S. L. Chin, “Ultrafast birefringence induced by a femtosecond laser filament in gases,” Opt. Lett. 34(9), 1417–1419 ( 2009). [CrossRef] [PubMed]

3.

Y. Chen, F. Théberge, O. Kosareva, N. Panov, V. P. Kandidov, and S. L. Chin, “Evolution and termination of a femtosecond laser filament in air,” Opt. Lett. 32(24), 3477–3479 ( 2007). [CrossRef] [PubMed]

4.

J. Yu, D. Mondelain, G. Ange, R. Volk, S. Niedermeier, J.-P. Wolf, J. Kasparian, and R. Sauerbrey, “Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air,” Opt. Lett. 26(8), 533–535 ( 2001). [CrossRef] [PubMed]

5.

J. Kasparian, M. Rodriguez, G. Méjean, J. Yu, E. Salmon, H. Wille, R. Bourayou, S. Frey, Y.-B. André, A. Mysyrowicz, R. Sauerbrey, J.-P. Wolf, and L. Wöste, “White-light filaments for atmospheric analysis,” Science 301(5629), 61–64 ( 2003). [CrossRef] [PubMed]

6.

M. Rodriguez, R. Sauerbrey, H. Wille, L. Wöste, T. Fujii, Y.-B. André, A. Mysyrowicz, L. Klingbeil, K. Rethmeier, W. Kalkner, J. Kasparian, E. Salmon, J. Yu, and J.-P. Wolf, “Triggering and guiding megavolt discharges by use of laser-induced ionized filaments,” Opt. Lett. 27(9), 772–774 ( 2002). [CrossRef] [PubMed]

7.

X. Xie, J. Dai, and X. C. Zhang, “Coherent control of THz wave generation in ambient air,” Phys. Rev. Lett. 96(7), 075005 ( 2006). [CrossRef] [PubMed]

8.

A. Couairon, “Filamentation length of powerful laser pulses,” Appl. Phys. B 76(7), 789–792 ( 2003). [CrossRef]

9.

I. S. Golubtsov, V. P. Kandidov, and O. G. Kosareva, “Initial phase modulation of a high-power femtosecond laser pulse as a tool for controlling its filamentation and generation of a supercontinuum in air,” Quantum Electron. 33(6), 525–530 ( 2003). [CrossRef]

10.

J. Wu, H. Cai, H. Zeng, and A. Couairon, “Femtosecond filamentation and pulse compression in the wake of molecular alignment,” Opt. Lett. 33(22), 2593–2595 ( 2008). [CrossRef] [PubMed]

11.

H. Stapelfeldt and T. Seideman, “Colloquium: aligning molecules with strong laser pulses,” Rev. Mod. Phys. 75(2), 543–557 ( 2003). [CrossRef]

12.

F. Calegari, C. Vozzi, S. Gasilov, E. Benedetti, G. Sansone, M. Nisoli, S. De Silvestri, and S. Stagira, “Rotational Raman effects in the wake of optical filamentation,” Phys. Rev. Lett. 100(12), 123006 ( 2008). [CrossRef] [PubMed]

13.

S. Varma, Y.-H. Chen, and H. M. Milchberg, “Trapping and destruction of long-range high-intensity optical filaments by molecular quantum wakes in air,” Phys. Rev. Lett. 101(20), 205001 ( 2008). [CrossRef] [PubMed]

14.

J. Wu, H. Cai, Y. Peng, and H. Zeng, “Controllable supercontinuum generation by the quantum wake of molecular alignment,” Phys. Rev. A 79(4), 041404 ( 2009). [CrossRef]

15.

H. Cai, J. Wu, Y. Peng, and H. Zeng, “Comparison study of supercontinuum generation by molecular alignment of N2 and O2.,” Opt. Express 17(7), 5822–5828 ( 2009). [PubMed]

16.

A. C. Bernstein, M. McCormick, G. M. Dyer, J. C. Sanders, and T. Ditmire, “Two-beam coupling between filament-forming beams in air,” Phys. Rev. Lett. 102(12), 123902 ( 2009). [CrossRef] [PubMed]

17.

J. Wu, Y. Tong, X. Yang, H. Cai, P. Lu, H. Pan, and H. Zeng, “Interaction of two parallel femtosecond filaments at different wavelengths in air,” Opt. Lett. (in press.

18.

J. Wu, H. Cai, A. Couairon, and H. Zeng, “Few-cycle shock X-wave generation by filamentation in prealigned molecules,” Phys. Rev. A 80(1), 013828 ( 2009). [CrossRef]

19.

J. B. Ashcom, R. R. Gattass, C. B. Schaffer, and E. Mazur, “Numerical aperture dependence of damage and supercontinuum generation from femtosecond laser pulses in bulk fused silica,” J. Opt. Soc. Am. B 23(11), 2317–2322 ( 2006). [CrossRef]

OCIS Codes
(260.5950) Physical optics : Self-focusing
(320.7110) Ultrafast optics : Ultrafast nonlinear optics
(020.2649) Atomic and molecular physics : Strong field laser physics

ToC Category:
Ultrafast Optics

History
Original Manuscript: September 21, 2009
Revised Manuscript: October 10, 2009
Manuscript Accepted: October 31, 2009
Published: November 4, 2009

Citation
Hua Cai, Jian Wu, Hao Li, Xueshi Bai, and Heping Zeng, "Elongation of femtosecond filament by molecular alignment in air," Opt. Express 17, 21060-21065 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-23-21060


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References

  1. A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 (2007). [CrossRef]
  2. C. Marceau, Y. Chen, F. Théberge, M. Châteauneuf, J. Dubois, and S. L. Chin, “Ultrafast birefringence induced by a femtosecond laser filament in gases,” Opt. Lett. 34(9), 1417–1419 (2009). [CrossRef] [PubMed]
  3. Y. Chen, F. Théberge, O. Kosareva, N. Panov, V. P. Kandidov, and S. L. Chin, “Evolution and termination of a femtosecond laser filament in air,” Opt. Lett. 32(24), 3477–3479 (2007). [CrossRef] [PubMed]
  4. J. Yu, D. Mondelain, G. Ange, R. Volk, S. Niedermeier, J.-P. Wolf, J. Kasparian, and R. Sauerbrey, “Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air,” Opt. Lett. 26(8), 533–535 (2001). [CrossRef] [PubMed]
  5. J. Kasparian, M. Rodriguez, G. Méjean, J. Yu, E. Salmon, H. Wille, R. Bourayou, S. Frey, Y.-B. André, A. Mysyrowicz, R. Sauerbrey, J.-P. Wolf, and L. Wöste, “White-light filaments for atmospheric analysis,” Science 301(5629), 61–64 (2003). [CrossRef] [PubMed]
  6. M. Rodriguez, R. Sauerbrey, H. Wille, L. Wöste, T. Fujii, Y.-B. André, A. Mysyrowicz, L. Klingbeil, K. Rethmeier, W. Kalkner, J. Kasparian, E. Salmon, J. Yu, and J.-P. Wolf, “Triggering and guiding megavolt discharges by use of laser-induced ionized filaments,” Opt. Lett. 27(9), 772–774 (2002). [CrossRef] [PubMed]
  7. X. Xie, J. Dai, and X. C. Zhang, “Coherent control of THz wave generation in ambient air,” Phys. Rev. Lett. 96(7), 075005 (2006). [CrossRef] [PubMed]
  8. A. Couairon, “Filamentation length of powerful laser pulses,” Appl. Phys. B 76(7), 789–792 (2003). [CrossRef]
  9. I. S. Golubtsov, V. P. Kandidov, and O. G. Kosareva, “Initial phase modulation of a high-power femtosecond laser pulse as a tool for controlling its filamentation and generation of a supercontinuum in air,” Quantum Electron. 33(6), 525–530 (2003). [CrossRef]
  10. J. Wu, H. Cai, H. Zeng, and A. Couairon, “Femtosecond filamentation and pulse compression in the wake of molecular alignment,” Opt. Lett. 33(22), 2593–2595 (2008). [CrossRef] [PubMed]
  11. H. Stapelfeldt and T. Seideman, “Colloquium: aligning molecules with strong laser pulses,” Rev. Mod. Phys. 75(2), 543–557 (2003). [CrossRef]
  12. F. Calegari, C. Vozzi, S. Gasilov, E. Benedetti, G. Sansone, M. Nisoli, S. De Silvestri, and S. Stagira, “Rotational Raman effects in the wake of optical filamentation,” Phys. Rev. Lett. 100(12), 123006 (2008). [CrossRef] [PubMed]
  13. S. Varma, Y.-H. Chen, and H. M. Milchberg, “Trapping and destruction of long-range high-intensity optical filaments by molecular quantum wakes in air,” Phys. Rev. Lett. 101(20), 205001 (2008). [CrossRef] [PubMed]
  14. J. Wu, H. Cai, Y. Peng, and H. Zeng, “Controllable supercontinuum generation by the quantum wake of molecular alignment,” Phys. Rev. A 79(4), 041404 (2009). [CrossRef]
  15. H. Cai, J. Wu, Y. Peng, and H. Zeng, “Comparison study of supercontinuum generation by molecular alignment of N2 and O2.,” Opt. Express 17(7), 5822–5828 (2009). [PubMed]
  16. A. C. Bernstein, M. McCormick, G. M. Dyer, J. C. Sanders, and T. Ditmire, “Two-beam coupling between filament-forming beams in air,” Phys. Rev. Lett. 102(12), 123902 (2009). [CrossRef] [PubMed]
  17. J. Wu, Y. Tong, X. Yang, H. Cai, P. Lu, H. Pan, and H. Zeng, “Interaction of two parallel femtosecond filaments at different wavelengths in air,” Opt. Lett. (in press.
  18. J. Wu, H. Cai, A. Couairon, and H. Zeng, “Few-cycle shock X-wave generation by filamentation in prealigned molecules,” Phys. Rev. A 80(1), 013828 (2009). [CrossRef]
  19. J. B. Ashcom, R. R. Gattass, C. B. Schaffer, and E. Mazur, “Numerical aperture dependence of damage and supercontinuum generation from femtosecond laser pulses in bulk fused silica,” J. Opt. Soc. Am. B 23(11), 2317–2322 (2006). [CrossRef]

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