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


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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

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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, JHU-APL

HEDLP - 2008


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Some applications of filaments

  • directed energy

  • triggering and guiding of lightening

  • remote detection: LIDAR, LIBS

  • directed, remote THz generation


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

Introduction to Filamentation

 0

neff = n0 + ngas + nplasma

Pcr ~ 2/8n0n2


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Filament images at increasing power

(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


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Delayed inertial response

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

Molecules: 78% nitrogen, 21% oxygen

“prompt” and “delayed” optical response of air constituents

Prompt electronic response

+

+

+

+

+

Laser polarization

-

-

-

-

-

Atoms: 1% argon


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Classical picture

molecular axis

induced

dipole

moment

  • 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)

time-dependent

refractive index shift

random

orientation

“some” alignment

degree of alignment

< >t : time-dependent ensemble average

n0=n(random orientation)

Laser field alignment of linear gas molecules


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Quantum description of rigid rotor

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


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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.


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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).


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Experimental setup and sample interferogram

0 ps

~ 2 ps

Sample interferogram

N2O gas

250 mm

652nm

723nm

Chen, Varma, York and Milchberg, Opt. Express 15, 11341 (2007)


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Rotational wavepacket of D2 and H2 molecules

P=7.8 atm

I=4.4x1013 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.


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2m filament

f/300 focusing

Object plane

Polarizing beamsplitter

CCD

Pump-probe filament experiment


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TN2 , 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


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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


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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...


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Pump power scan(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)


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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


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T=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

mode-locking analogy: coherent sum of longitudinal modes

typ. spectrum

modes

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


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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

  • T=11.6 ps

x

(mm)

(fs)

0.75T

1T

1.25T

x (mm)

(ps)


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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.

Introduction to Filamentation


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=61.8 cm1

T=270 fs

T

0.3010-24cm3

Rotational wavepacket of H2 molecules at room temperature

Experiment:

Fourier

transform

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

I=4.4x1013 W/cm2


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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


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Experimental setup and sample interferogram

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

(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

652nm

723nm


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Experimental setup and sample interferogram

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

(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

652nm

723nm


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