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
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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
Support: DoE, NSF, JHUAPL
HEDLP  2008
Some applications of filaments
High power, femtosecond laser beams propagating through air form extremely long filaments due to nonlinear selffocusing ((3)) dynamically balanced by ionization and defocusing.
Introduction to Filamentation
0
neff = n0 + ngas + nplasma
Pcr ~ 2/8n0n2
Filament images at increasing power form extremely long filaments due to nonlinear selffocusing (
(Pcr occurs at 1.25 mJ for a 130fs pulse)
What does a filament look like?
5 mm
0.8Pcr
1.3Pcr
1.8Pcr
2.3Pcr
2.8Pcr 3.5 mJ
Delayed inertial response form extremely long filaments due to nonlinear selffocusing (
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Molecules: 78% nitrogen, 21% oxygen
“prompt” and “delayed” optical response of air constituents
Prompt electronic response
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Laser polarization





Atoms: 1% argon
Classical picture form extremely long filaments due to nonlinear selffocusing (
molecular axis
induced
dipole
moment
intense laser field
(~1013 W/cm2)
timedependent
refractive index shift
random
orientation
“some” alignment
degree of alignment
< >t : timedependent ensemble average
n0=n(random orientation)
Laser field alignment of linear gas molecules
Quantum description of rigid rotor form extremely long filaments due to nonlinear selffocusing (
even
(“rotational constant”)
(j: ≥0 integer)
where
: moment of inertia
Rotational wavepacket
An intense fs laser pulse “locks” the relative phases of the rotational states in the wavepacket
Field alignment and “revivals” of rotational wavepacket
eigenstate
Quantum revival of rotational response form extremely long filaments due to nonlinear selffocusing (
The timedelayed 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.
Singleshot 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
Extract probe (x, t) to obtain n(x, t).
Experimental setup and sample interferogram – Imagine a streak camera with 10fs resolution!
0 ps
~ 2 ps
Sample interferogram
N2O gas
250 mm
652nm
723nm
Chen, Varma, York and Milchberg, Opt. Express 15, 11341 (2007)
Rotational wavepacket of D – Imagine a streak camera with 10fs resolution!2 and H2 molecules
P=7.8 atm
I=4.4x1013 W/cm2
room temperature
Rotational quantum “wakes” in air – Imagine a streak camera with 10fs resolution!
TN2 , ¾TO2
Vg pump
vg pump
SSSI measurement showing alignment and antialignment “wake” traveling at the group velocity of the pump pulse.
2m filament – Imagine a streak camera with 10fs resolution!
f/300 focusing
Object plane
Polarizing beamsplitter
CCD
Pumpprobe filament experiment
T – Imagine a streak camera with 10fs resolution!N2 , ¾TO2
B
A
5 mm
C
D
(ps)
(ps)
8.4
8.0
8.8
8.0
8.8
8.4
Filaments are trapped/enhanced or destroyed
Both beams collinear, probe filament coincident with alignment wake of N2 and O2 in air
CCD camera saturation
Trapped filaments are ENHANCED
White light generation, filament length and spectral broadening are enhanced.
Aligning filament (left) and probing filament (right), misaligned
Conclusions alignment wake of N
Pump power scan alignment wake of N(probe=3.4Pcr)
0.68Pcr
1.12Pcr
Increasing aligning pulse energy
1.72Pcr
2.20Pcr
2.60Pcr
3.72Pcr
(ps)
Response near t=0
A
laser
A
(ps)
Spectral broadening alignment wake of N
The spatiotemporally 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
T alignment wake of N=8.2ps
T/2
3T/4
Example:
N2
T/4
nitrogen
ps
peak width ≈T / jmax(jmax+1) ~ 40 fs for N2
Molecular rotational wavepacket revivals
modelocking analogy: coherent sum of longitudinal modes
typ. spectrum
modes
pulse width ≈ (round trip time) / (# of modes)
1D spatially resolved temporal evolution of O alignment wake of N2 alignment
0.5T
0T
0.25T
x
(mm)
(fs)
0.75T
1T
1.25T
x (mm)
(ps)
High power, femtosecond laser beams that propagate through air form extremely long filaments due to nonlinear selffocusing ((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.
Introduction to Filamentation
=61.8 cm air form extremely long filaments due to nonlinear selffocusing (1
T=270 fs
T
0.301024cm3
Rotational wavepacket of H2 molecules at room temperature
Experiment:
Fourier
transform
Lineout at x=0
Calculation:
P=7.8 atm
I=4.4x1013 W/cm2
Charge density wave in N air form extremely long filaments due to nonlinear selffocusing (2 at 1 atm
“probe” pulse
vg
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Quantum beat index bucket
Experimental setup and sample interferogram air form extremely long filaments due to nonlinear selffocusing (
110 fs
high pressure
exp gas cell
1 kHz Ti:Sapphire regenerative amplifier
(up to ~8 atm)
P: pinhole
BS: beamsplitter
HWP: l/2 plate
SF4: dispersive material
~300 mJ
xenon gas cell
(12 atm)
supercontinuum
(SC)
Michelson interferometer
0 ps
~ 2 ps
Sample interferogram
N2O gas
250 mm
652nm
723nm
Experimental setup and sample interferogram air form extremely long filaments due to nonlinear selffocusing (
110 fs
high pressure
exp gas cell
1 kHz Ti:Sapphire regenerative amplifier
(up to ~8 atm)
P: pinhole
BS: beamsplitter
HWP: l/2 plate
SF4: dispersive material
~300 mJ
xenon gas cell
(12 atm)
supercontinuum
(SC)
Michelson interferometer
0 ps
~ 2 ps
Sample interferogram
N2O gas
250 mm
652nm
723nm