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					               UNIVERSITY OF MARYLAND AT COLLEGE PARK




    Trapping and destruction of long range high
  intensity optical/plasma filaments by molecular
                   quantum wakes

                 S. Varma, Y.-H. Chen, and H. M. Milchberg
               Institute for Research in Electronics and Applied Physics
                      Dept. of Electrical and Computer Engineering
                                     Dept. of Physics




HEDLP - 2008                                        Support: DoE, NSF, JHU-APL
      Some applications of filaments

• directed energy

• triggering and guiding of lightening

• remote detection: LIDAR, LIBS

• directed, remote THz generation
                Introduction to Filamentation
•   High power, femtosecond laser beams propagating through air form
    extremely long filaments due to nonlinear self-focusing ((3)) dynamically
    balanced by ionization and defocusing.



                                                                 0

                                                neff = n0 + ngas + nplasma




                   Pcr ~ 2/8n0n2
         What does a filament look like?




                                                        5 mm


0.8Pcr      1.3Pcr      1.8Pcr     2.3Pcr      2.8Pcr
                                               3.5 mJ



           Filament images at increasing power
          (Pcr occurs at 1.25 mJ for a 130fs pulse)
      “prompt” and “delayed” optical
       response of air constituents

Prompt electronic response                      Delayed inertial response




                         Laser polarization
            + + +                                           + + +
       +            +                                   +           +
                                                                        + + +
                                                                +               +
        -           -                                   -           -
            - - -                                           - - -
                                                                    -           -
                                                                        - - -

 Atoms: 1% argon                              Molecules: 78% nitrogen, 21% oxygen
            Laser field alignment of linear gas molecules


Classical picture
                                 induced         molecular axis
                                 dipole
                   E     p      moment    p//                       -laser field applies a net
                                                                      torque to the molecule
                                                                     -molecular axis aligns along
                                                                      the E field
                                                                     -delayed response (ps)
                                                                      due to inertia
           intense laser field
            (~1013 W/cm2)
     random                                                           time-dependent
    orientation           E “some” alignment                       refractive index shift
                                                                     2 N                   1
                                                         n(t )             cos 2  t  
                                                                      n0                    3

                                                                               degree of alignment

                                                     < >t : time-dependent ensemble average
  cos 2   1/ 3                   cos 2   1/ 3                 n0=n(random orientation)
    Field alignment and “revivals” of rotational wavepacket



Quantum description
   of rigid rotor         j, m exp(i j t )          eigenstate

                                               even

              where    j  E j /  2πcBj ( j  1) (j: ≥0 integer)
                      B  h(8 2 cI ) 1 (“rotational constant”)
                      I : moment of inertia

    Rotational wavepacket
                            j ,m a j ,m j, m exp(i j t )

                      An intense fs laser pulse “locks” the
                      relative phases of the rotational states in
                      the wavepacket
      Quantum revival of rotational response

The time-delayed nonlinear response is composed of many
   quantized rotational excitations which coherently beat.




t=0                                                  t = Tbeat
           We can expect the index of refraction
           to be maximally disturbed at each beat.
             Single-shot Supercontinuum Spectral Interferometry
            (SSSI) – Imagine a streak camera with 10fs resolution!

                                           A pump pulse generates transient
                                           refractive index n (r, t)
                                     x

   Pump pulse                                    Imaging lens




   Probe Ref.                                    z     Probe    Ref.    CCD
                                                                                Imaging
                                  medium
                                                                              spectrometer
                          y

Probe and Ref.
• Temporally stretched (chirp) for long
temporal field of view (~ 2 ps).
                                              Extract probe (x, t) to obtain n(x, t).
• ~100 nm bandwidth supercontinuum
gives ~10 fs resolution.
        Experimental setup and sample interferogram




0 ps               Sample interferogram                         ~ 2 ps
 N2O gas




                                                       250 mm
652nm                                                           723nm

  Chen, Varma, York and Milchberg, Opt. Express 15, 11341 (2007)
Rotational wavepacket of D2 and H2 molecules



                                         P=7.8 atm
                                         I=4.4x1013 W/cm2
                                         room temperature
        Rotational quantum “wakes” in air

                                             TN2 , ¾TO2




                                              Vg pump


          vg pump

SSSI measurement showing alignment and anti-alignment “wake”
       traveling at the group velocity of the pump pulse.
Pump-probe filament experiment


                               2m
                           filament

          f/300 focusing               Object
                                       plane




                                       Polarizing
                                      beamsplitter



                                                     CCD
Filaments are trapped/enhanced or destroyed
          TN2 , ¾TO2


                       A
                                     B
5 mm




                C                D


       8.08.0              8.4
                           8.4           8.8 8.8(ps)
                                                 (ps)
              Trapped filaments are ENHANCED

      White light generation, filament length and spectral
                   broadening are enhanced.

Aligning filament (left) and probing        Both beams collinear, probe
    filament (right), misaligned        filament coincident with alignment
                                             wake of N2 and O2 in air




                                       CCD camera saturation
                    Conclusions

• SSSI enables us to probe refractive index transients with
  ~10fs resolution over 2ps in a single shot, allowing us to
  observe room-temperature molecular alignment.
• A high intensity laser filament propagating in the
  quantum wake of molecular alignment can be
  controllably and stably trapped and enhanced, or
  destroyed.
• Applications: directed energy, remote sensing, etc...
                             Pump power
Response near t=0
                           scan (probe=3.4Pcr)
          A




                                                  Increasing aligning pulse energy
  laser
                                        0.68Pcr


                                        1.12Pcr


                                        1.72Pcr



          A                             2.20Pcr



                                        2.60Pcr


                                        3.72Pcr
                    (ps)
                                        (ps)
                  Spectral broadening
The spatio-temporally varying refractive index of the wake of
molecular alignment causes predictable spectral modulation and
broadening of the probe filament.
           Filament spectrum v. delay


             B                                  Alignment v. delay
    A                         D
                    C                   E

                                                         C
                                                  A           E

                                                       B     D
              Molecular rotational wavepacket revivals
    mode-locking analogy: coherent sum of longitudinal modes

                                                                   typ. spectrum

                                                                                   modes




             pulse width ≈ (round trip time) / (# of modes)


                                               T/2                      T=8.2ps
Example:                            T/4                   3T/4
   N2


                            nitrogen

                                                     ps
                          peak width ≈T / jmax(jmax+1) ~ 40 fs for N2
              1D spatially resolved temporal evolution of O2 alignment

                                                          • pump peak intensity:
         0T                0.25T              0.5T           2.7x1013 W/cm2
                                                                • 5.1 atm O2 at
                                                              room temperature
                                                                • T=11.6 ps
   x
 (mm)
                 (fs)


                   0.75T             1T               1.25T




x (mm)




                                                                  (ps)
             Introduction to Filamentation
• High power, femtosecond laser beams that propagate through air
  form extremely long filaments due to nonlinear self-focusing ((3))
  dynamically balanced by ionization and defocusing.




• Filaments can propagate through air up to 100s of meters, and are
  useful for remote excitation, ionization and sensing.
           Rotational wavepacket of H2 molecules at room temperature

Experiment:


                                      Fourier
                                    transform                           BH 2=61.8 cm1
                                                                        T=270 fs




 Lineout at x=0                             Calculation:
                                                               The pump intensity
                                                               bandwidth (~2.5x1013 s-1)
                                                               is even less adequate
                                                               than in D2 to populate j=2
                                                               and j=0 states.

                                                               Weaker rotational
                                                               wavepacket amplitude.


P=7.8 atm
                    H2  0.3010-24 cm3                  T
I=4.4x1013 W/cm2
         Charge density wave in N2 at 1 atm



                          • Filament ionization fraction ~10-3  2x1016
                          cm3
             vg

                          • ~0.5% ponderomotive charge separation at
                          enhanced intensity ~5x1014 W/cm2 over 50-
                          100 fs alignment transient  Ne~ 1014 cm-3 
Quantum beat index bucket E~ 0.75 MV/cm


                          • Many meters of propagation
                      Experimental setup and sample interferogram

            110 fs
1 kHz Ti:Sapphire                                               high pressure
regenerative amplifier                                           exp gas cell (up to ~8 atm)

                       ~300 mJ                                           P: pinhole
 xenon gas cell                                                          BS: beamsplitter
                                                                         HWP: /2 plate
(1-2 atm)                                                                SF4: dispersive material
            supercontinuum
                       (SC)


                       Michelson
                     interferometer                                     Optical Kerr effect ((3))
                                                                        and the molecular rotational
0 ps                           Sample interferogram            ~ 2 ps   response in the gas induce
                                                                        spectral phase shift and
  N2O gas                                                               amplitude modulation on
                                                                        the interferogram.



                                                      250 mm
                                                                        Both spectral phase and
                                                                        amplitude information are
                                                                        required to extract the
                                                                        temporal phase (refractive
                                                                        index).
652nm                                                          723nm
                      Experimental setup and sample interferogram

            110 fs
1 kHz Ti:Sapphire                                               high pressure
regenerative amplifier                                           exp gas cell (up to ~8 atm)

                       ~300 mJ                                           P: pinhole
 xenon gas cell                                                          BS: beamsplitter
                                                                         HWP: /2 plate
(1-2 atm)                                                                SF4: dispersive material
            supercontinuum
                       (SC)


                       Michelson
                     interferometer                                     Optical Kerr effect ((3))
                                                                        and the molecular rotational
0 ps                           Sample interferogram            ~ 2 ps   response in the gas induce
                                                                        spectral phase shift and
  N2O gas                                                               amplitude modulation on
                                                                        the interferogram.



                                                      250 mm
                                                                        Both spectral phase and
                                                                        amplitude information are
                                                                        required to extract the
                                                                        temporal phase (refractive
                                                                        index).
652nm                                                          723nm

				
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posted:9/23/2011
language:English
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