Ultrafast Dynamics in Solid Plasmas Using

Doppler Spectrometry and Giant magnetic Pulses

Amit D. Lad

Ultrashort Pulse High Intensity Laser Laboratory (UPHILL)

Tata Institute of Fundamental Research,

Mumbai – 400005

1www. tifr.res.in/~uphill

2

CollaboratorsS. Mondal, V. Narayanan, Gourab Chatterjee,

Prashant Singh, S. N. Ahmed,

Tata Institute of Fundamental Research, Mumbai, India

P. P. Rajeev and A. RobinsonCentral Laser Facility, Rutherford-Appleton Laboratory, U. K.

J. PasleyDepartment of Physics, University of York, Heslington, U. K.

S. Sengupta, A. Das, and P. K. KawInstitute of Plasma Research, , Bhat, Gandhinagar, India

W. M. Wang, Z. M. ShengInstitute of Physics, CAS and SJT University, P. R. China

R. Rajeev, M. Krishnamurthy, and G. Ravindra Kumar

Attending ICUIL 2010

Intensity : 1019 W/cm2

Laser τ : 30 X 10-15 s

Plasma T : 102 / 105 eV

Plasma Velocity : 107 -108 cm/s

Plasma

Laser

Target

ncr

Laser

Scattered Light

Scattered Light

HeatTransport

X-raysFastParticles

Laser Plasma Interaction

3

Topic 1

Dynamics of plasma critical surface

Topic 2

Hot electron propagation inside dense plasma

Topic 1

Dynamics of plasma critical surface

Topic 2

Hot electron propagation inside dense plasma

6

Motivation

6

To Estimate the Plasma Expansion Velocity and

thereby the Instantaneous Plasma Profile

Plasma motion occurs at very high velocity

(> 107 cm/sec)

So plasma profile changes rapidly

This implies, plasma conditions change significantly during laser interactions

Ultrafast Plasma Dynamics

7

Probe(Time Delayedw. r. t. Pump :

0 to 30 ps)

Probe Pulse Experiences Doppler Shift

Pump-Probe Experiment

Spectrometer

Capturing Plasma Motion “as it happens”

8

Target :Aluminium

P-polarized Laser Pump

400 nm, 80 fs Spot : 60 µm~1012 W/cm2

800 nm, 30 fsSpot : 17 μm

5 x 1018 W/cm2

UV-Visible High

Resolution Spectrometer

Delay Stage

Target

5% BS

Off-Axis Parabolic Mirror for Focusing

Bending Mirror

2ω Crystal

PUMP Laser (800 nm)

Probe Laser400 nm

50% BS

UV-Visible Spectrometer

Doppler –Shift Experimental

Set Up

9

Pump Laser (800 nm)

750 775 800 825 850

Inte

nsi

ty (

arb

. un

its)

Wavelength (nm)

34 nm

Fundamental(ω)

390 395 400 405 410 415

Inte

nsi

ty (

arb

. un

its)

Wavelength (nm)

3 nm

Second Harmonic (2ω)

For 400 nm : Δλ = 3 nm at 80 fsFor 800 nm : Δλ = 34 nm at 30 fs

Laser Pulse Shape

10

Sharp 2ω profile makes it easier to see small

spectral changes

TIFR Expt.: Mondal et al., PRL 105, 105002 (2010)

Doppler Shift

11

Target :Aluminium

P-polarized Laser Pump

Target :Aluminium

1017 W/cm2

800 nm2 ps

Spectrometer

~1012 W/cm2

400 nm80 fs

3x1018 W/cm2

800 nm30 fs

Spectrometer

Kalashnikov, PRL 73, 260 (1994).

0 to 30 ps

398 399 400 401 402 403 404

30 ps0 ps

Nor

m. I

nten

sity

(a.

u.)

Wavelength (nm)

4 ps

12

Time Delayed Spectra

Target : Aluminium

Target : Aluminium

13

Time Delayed Spectra

399.5 400.0 400.5 401.0 401.5 402.0 402.5

30 ps0 ps

Nor

m. I

nten

sity

(a.

u.)

Wavelength (nm)

4 ps

To Observe Small Shifts it is Better to Observe Differences

i.e.

Time Delayed Probe Spectrum – Reference Probe Spectrum

14

15

If the time delayed spectrum is red-shifted with respect to zero time delayed spectrum :

subtracted spectrum (later spectrum - zero time delay spectrum) will show minima followed by maxima

397 398 399 400 401 402 403 404 405

Dif

fere

nce

(a. u

.)

Wavelength (nm)

4-0

16

If the time delayed spectrum is blue-shifted with respect to zero time delayed spectrum :

subtracted spectrum (later spectrum-zero time delay spectrum) will show maxima followed by minima

397 398 399 400 401 402 403 404 405

Dif

fere

nce

(a. u

.)

Wavelength (nm)

30-0

Dynamics Over Time Scale of 30 ps

396 398 400 402 404

396 398 400 402 404

Wavelength (nm)

4-0

Dif

fere

nce

(a.

u.)

2-0

8-0

16-0

19-0

22-0

26-0

30-0

Mondal et al., PRL 105, 105002 (2010)

Blue-shift

Critical Surface is Expanding towards

the probe beam

Red-shift

Critical Surface is Receding from the probe beam

Reversal of difference

probe spectra (from red to blue shift)

PumpProbe

0 3015

t (ps)

18

Dynamics Over Time Scale of 30 ps

Why Red Shift ???

The pump laser launches a compression wave into

front surface plasma

At early times compression wave forces the critical surface into the target

19

Doppler Shift

Why Blue Shift ???

At later times a compression wave has propagated into

a region of overdense plasma

Critical surface of the probe sits in the region that is undergoing rarefaction,

thus critical surface is moving into the vacuum and towards the laser 20

Doppler Shift

Pump:800 nm,

3 x 1018 W/cm2

Probe:400 nm

Target : Aluminium

Red-shift

Blue

-shi

ft

0 5 10 15 20 25 30-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

(

nm

)

Delay Time (ps)

A polynomial fit

21

Doppler Shift in Reflected Probe Spectra

Mondal et al., PRL 105, 105002 (2010)

Vexpansion = 0.5v (λ/Δλ) (cos θ)

Critical surface moves (expanding)AWAY from the target

Critical surface move INTO the target

0 5 10 15 20 25 30-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Ex

pa

nsi

on

Vel

oci

ty

X 1

07(c

m/s

ec)

Time (ps)

22

Velocity and Acceleration from Doppler Shift

0 5 10 15 20 25 30

-6

-4

-2

0

2

4

Time (ps)

Acc

eler

atio

n X

101

8 (c

m/s

ec2 )

Velocity AccelerationInstantaneous

Mondal et al., PRL 105, 105002 (2010)

Pump:λ = 800 nm

Probe:λ = 400 nm

Target : Aluminium

Red-shift

Blue

-shi

ft

24

Doppler Shift in Reflected Probe Spectra

0 5 10 15 20 25 30-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

(

nm

)

Delay Time (ps)

Δλ=2 x crit λ probe

c

Mondal et al., PRL 105, 105002 (2010)

TOPIC 2

HOT Electrons Transport -------

GIANT magnetic fields

Probe(Time Delayedw. r. t. Pump)

Pump-Probe Experiment

To Polarimeter

Polarimetry

Target Al coated glass P-polarized

Laser Pump

400 nm

800 nm, 30 fs

Hot electron currents,Giant magnetic fields,

Plasma motion…….

TIFR + IPRPhys. Rev. Lett. 89 225002 (2002), PRE (2006); POP (2009).

Principle: Probe polarization changes due to magnetic field created by pump

Probe(Time Delayedw. r. t. Pump)

Pump-Probe Experiment

To Polarimeter

Polarimetry

Target :100 µm thin Fused Silica

P-polarized Laser Pump

800 nm

800 nm, 30 fs

Hot electron currents,Giant magnetic fields,

Plasma motion…….

Principle: Probe polarization changes due to magnetic field created by pump

DetectorsPD: Integrated CCD: Spatial resolution

28

Measured Magnetic Field of Relativistic Electrons

Giant, Ultrashort Magnetic Pulse !

Target Front Target Back

0 1 2 3 4 5 6 7 8

0

10

20

30

40

50

60

70

B (

MG

)

Time (ps)0 20 40 60 80 100

0

2

4

6

8

10

12

14

B (M

G)

Time (ps)

5 x 1018 W cm-2

Aluminium coated glass

2 x 1018 W cm-2

100 µm Fused silica

Mondal et al., (manuscript under

preparation)

‘Hot electron’ currents and ‘Cold return’ currents interact with each other

Currents become unstable (Weibel instability- B dependent)

Electron beam breaks up into filaments

Magnetic field gets localized and inhomogeneous

Direct Evidence?

Relativistic Electron Transport

30

Measured Magnetic Field of Relativistic Electrons

Time AND Space Resolved (Polarigram): Target Front

0.2 ps 0.9 ps 1.1 ps 1.5 ps

2.5 ps 3.2 ps 4.1 ps 5.0 ps

5.5 ps 6.0 ps 6.5 ps7.0 ps

Front

Mondal et al., (manuscript under preparation)

MG

31

Measured Magnetic Field of Relativistic Electrons

Time AND Space Resolved (Polarigram): Target BACKBack

2.8 ps 5.5 ps 8.3 ps

Time Delay=11.1 ps ps

13.9 ps 16.6 ps

33.3 ps 49.9 ps 52.7 psMondal et al., (manuscript under preparation)

MG

32

Magnetic Field

Front Back

First direct observation of filamentation and inhomogeneity! (TIFR expts; 2008-2009, manuscript in prep.)

33

• We report the first ever pump-probe dynamics of the critical surface of solid density plasma produced by relativistic intensity, femtosecond lasers

• Spatial and temporal profile of magnetic field is captured simultaneously for the first time.

• Evolution of electron filamentation captured

• First measurements of magnetic field at the back of the target.

Conclusions

34

Thank you !!!

Tata Institute of Fundamental Research

Ultrashort Pulse High Intensity Laser Laboratory

T5 SPECS

Wavelength = 800 nmMaximum Energy = 1 JPulse width = 30 fsContrast >= 10-6

Repetition Rate = 10 Hz Existing Laser35

20 TW

Dynamics by Doppler Shift – Earlier Experiments

36

Target :Aluminium

I =1017 W/cm2

800 nm2 ps

Spectrometer

Main Results :

The pump self-reflection was used

to measure its spectral shift

No dynamics captured after the

intense laser pulse disappears

Kalashnikov, PRL 73, 260 (1994).

398 400 402 404

30 ps 26 ps

22 ps 19 ps 16 ps

8 ps 4 ps 2 ps 0 ps

Nor

m. I

nten

sity

(a.

u.)

Wavelength (nm)

Target : AluminiumPump:800 nm,3 x 1018 W/cm2

Probe:λ = 400 nm

Visual Guide

37

Dynamics Over 30 ps

38

• Ocean Optics Spectrometer (HR 2000)

• Used for data acquisition

Range

Resolution :0.5 Å

350 nm 445 nm

λ

Single Shot Spectrometer

Measuring B by Polarimetry

Faraday Effect: (B // k) The linearly polarized light gets rotated. Difference in phase accumulation between LCP and RCP.

Cotton-Mouton Effect: (B k) Linearly polarized light gains ellipticity, Reason: Difference in refractive index for component of Electric field parallel and perpendicular to magnetic field.

= (n+-n-) kz

Principle: Probe polarization changes due to magnetic field created by pump

40

Hot electron TransportGeneration and damping of B

Bc

Jc

dt

dBhot

2

2

4

• Hot electrons Jhot stream into bulk• Return plasma currents compensate• The electrical resistivity -1 limits buildup and determines decay of magnetic field.

Plasma layerSolid

Laser

Current loops

Hot e-

Cold e-

Source Diffusion

Probe

PumpBS l/4

Analyzer

PD2

PD3

Target

PD1

k-k

B

Probe

Interaction Area

Measuring Giant Magnetic Fields

Principle: Probe polarization changes due to magnetic field created by pump

Pump-Probe Polarimetry

Probe Spot size ~60 μm

Pump Spot size ~17 μm

Pu

mp

Probe

Spatial Matching of Two Beams

Target Pu

mp

Probe

Beams are hitting a new target spot every time

80 fs(264 μm)

t

30 fs(99 μm)

Before temporal matching

t

After temporal matching

-0.5 0.0 0.5 1.0 1.5 2.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivit

yTime (ps)

Probe ahead of Pump

Probe after the Pump

Pump and Probe arrive at the same time

Temporal Matching of Two Beams

Probe ahead of pumpReflects from Metal

No plasma contribution as yetPump Probe OverlappedTime = 0Partly reflected from plasma

Now probe reflected from plasmaformed by the pumpStudying evolution of plasma

-0.5 0.0 0.5 1.0 1.5 2.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivit

y

Time (ps)

Probe ahead of Pump

Probe after the Pump

Pump and Probe arrive at the same time

Pump-Probe Technique