S. Varma, Y.-H. Chen, and H. M. Milchberg

1
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
Support DoE, NSF, JHU-APL
HEDLP - 2008
2
Some applications of filaments

• directed energy
• triggering and guiding of lightening
• remote detection LIDAR, LIBS
• directed, remote THz generation

3
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
4
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)

5
prompt and delayed optical response of air
constituents
Prompt electronic response





Laser polarization
-
-
-
-
-
Atoms 1 argon
6
Laser field alignment of linear gas molecules
7
Field alignment and revivals of rotational
wavepacket
eigenstate
8
Quantum revival of rotational response
The time-delayed nonlinear response is composed
of many quantized rotational excitations which
coherently beat.
t Tbeat
t 0
We can expect the index of refraction to be
maximally disturbed at each beat.
9
Single-shot Supercontinuum Spectral
Interferometry (SSSI) Imagine a streak camera
with 10fs resolution!
A pump pulse generates transient refractive
index ?n (r, t)
x
Imaging lens
Pump pulse
z
Probe Ref.
Probe Ref.
Imaging spectrometer
CCD
medium
y

• Probe and Ref.
• Temporally stretched (chirp) for long temporal
  field of view ( 2 ps).
• 100 nm bandwidth supercontinuum gives 10 fs
  resolution.

Extract ??probe (x, t) to obtain n(x, t).
10
Experimental setup and sample interferogram
0 ps
2 ps
Sample interferogram
N2O gas
250 mm
723nm
652nm
Chen, Varma, York and Milchberg, Opt. Express 15,
11341 (2007)
11
Rotational wavepacket of D2 and H2 molecules
P7.8 atm I4.4x1013 W/cm2 room temperature
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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.
13
Pump-probe filament experiment
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Filaments are trapped/enhanced or destroyed
15
Trapped filaments are ENHANCED
White light generation, filament length and
spectral broadening are enhanced.
Aligning filament (left) and probing filament
(right), misaligned
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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...

17
Response near t0
A
laser
A
(ps)
18
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
Alignment v. delay
B
D
A
C
E
C
E
A
D
B
19
Molecular rotational wavepacket revivals
mode-locking analogy coherent sum of
longitudinal modes
typ. spectrum
modes
pulse width (round trip time) / ( of modes)
20
1D spatially resolved temporal evolution of O2
alignment

• pump peak intensity
• 2.7x1013 W/cm2

0.5T
0T
0.25T

• 5.1 atm O2 at room temperature

• T11.6 ps

x (mm)
(fs)
0.75T
1T
1.25T
x (mm)
(ps)
21
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.

22
Rotational wavepacket of H2 molecules at room
temperature
Experiment
Fourier transform
Lineout at x0
Calculation

• The pump intensity bandwidth (2.5x1013 s-1) is
  even less adequate than in D2 to populate j2 and
  j0 states.
• Weaker rotational wavepacket amplitude.

P7.8 atm I4.4x1013 W/cm2
23
Charge density wave in N2 at 1 atm

• Filament ionization fraction 10-3 ? 2x1016
  cm?3
• 0.5 ponderomotive charge separation at
  enhanced intensity 5x1014 W/cm2 over 50-100 fs
  alignment transient ? ?Ne 1014 cm-3 ? E 0.75
  MV/cm
• Many meters of propagation

probe pulse
vg
--
Quantum beat index bucket
24
Experimental setup and sample interferogram
110 fs
high pressure exp gas cell
1 kHz TiSapphire regenerative amplifier
(up to 8 atm)
P pinhole BS beamsplitter HWP l/2 plate SF4
dispersive material
300 mJ
xenon gas cell
(1-2 atm)
supercontinuum (SC)
Michelson interferometer

• Optical Kerr effect (c(3)) and the molecular
  rotational response in the gas induce spectral
  phase shift and amplitude modulation on the
  interferogram.

0 ps
2 ps
Sample interferogram
N2O gas

• Both spectral phase and amplitude information are
  required to extract the temporal phase
  (refractive index).

250 mm
723nm
652nm
25
Experimental setup and sample interferogram
110 fs
high pressure exp gas cell
1 kHz TiSapphire regenerative amplifier
(up to 8 atm)
P pinhole BS beamsplitter HWP l/2 plate SF4
dispersive material
300 mJ
xenon gas cell
(1-2 atm)
supercontinuum (SC)
Michelson interferometer

• Optical Kerr effect (c(3)) and the molecular
  rotational response in the gas induce spectral
  phase shift and amplitude modulation on the
  interferogram.

0 ps
2 ps
Sample interferogram
N2O gas

• Both spectral phase and amplitude information are
  required to extract the temporal phase
  (refractive index).

250 mm
723nm
652nm