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 1 www. tifr.res.in/~uphill
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
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)
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
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
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