Transcript PPT - IIT Kanpur

New Peaks
Scales
Nonlinear Optics
G. Ravindra Kumar
Tata Institute of Fundamental Research
Mumbai
[email protected]
R R Dasari Distinguished Lecture Series, 28 Feb 2005, IIT Kanpur
Light – Matter Interaction
Normally, Induced Dipole Reradiation
(electronic response)
,k,E
´,k´,E´
1. Optical interactions depend on the
Electric field in the light wave.
2. Valence/outer `bound’ electrons that respond to this field.
But,
3. Does this idea work when you go to high light Intensities?
NO!
What is this talk all about?
Ipeak=1017 W cm-2
50 mJ, 100 fs
(0.05J, 10-13s)
E = 1010 V cm-1
20-100 m
`Peak’ power
0.5 x1012 W
ekeV-MeV
Ions Zq+
Light
X-rays/-rays
Light pulse - Spatial Packet (Length approx. 65 micrometers !)
less than the breadth of human hair!
Intense Light Fields
Extremely large E fields generated
by short pulse, high energy lasers
Comparison with the intra-matter Coulomb field
Hydrogen atom - 1s electron
1/ 2
 0 
2
E ~ 109 V/cm
 E  1016W / cm 2
Intensity I  2
 0 
Current Highest Intensity – 1021 W/ cm2 !
(about 1012 V/cm)
Let us look at the protagonists………
Light first……..
The LaseRevolution
Small step for Maiman
Giant Leap for Laserkind!
Bringing the stars down
to earth!!
Nd:YAG
FI
Ti:sapphire
Ar+ Pump
Ti:S Osc.
90 fs
200 ps
100 fs
A glance at the laser …
Next,
Matter ……..and what happens to it?
Matter under extreme conditions
single
atom
I = 1016 W cm-2
E ~ 109 V/cm
High intensity
Photoeletric Effect
+
Rapid ionization of valence electrons
q
V    E.x
x
Tunnelling
Over the barrier
1014 - 1015 W cm-2
> 1015 W cm-2
Each atom loses at least one electron. Some can lose as many as 6 !
Energy Scales involved
Photon energy - 1.5 ev
Ionization energy (typ.) – 10 -100 eV
You see that photon energy does
not matter!
Intense, Femtosecond Light - Matter Interaction
broad features
Matter intrinsically unstable,
ionization (multiple) inevitable
`Intensity’ of light matters,
not the wavelength (photon energy)
Highly transient interaction,
`impulse’ excitation (d - function like?)
Structure and dynamics completely coupled
`dynamic’ structure?!
Creation of new states of matter
The coulomb binding field becomes perturbative,
not the light field!
Light oscillates electrons !
E(t)cos t
eE
Vosc =
m
10-100 nm
eE
m 2
100-1000 lattice spacings in a solid
Ponderomotive energy
Acceleration of the ionized electron in the laser
field
2
2 2
e E 
UP 
16 2 me
e - electronic charge
E - electric field in the light wave
 - wavelength of the laser
me - electronic mass
Acceleration
1017 g !!!
E = 2.75  108 V/cm (1013 W/cm2)
UP = 1.1 eV for  = 1.06 m
> 100 eV for  = 10.6 m
UP > 106 eV for  = 1.06 m & 1019 W/cm2
Each electron interacts with 106 photons !!
Energy Scales involved ( again…..)
Photon energy - 1.5
ev (~ 100 eV)
Ionization energy (typ.) – 101
-102 eV
>> Photon energy
Energy given to the electron
>>>>>>>> both the above!
Intense Laser - Solid Interaction
Ionization much more than in the single
particle case ( U92+ possible!)
Why?
Density effects:
additional mechanisms e.g. collisional ionization
(particle effect), collective absorption (wave process)
High density, high energy plasma formation
Extremely complex dynamics
Plasma formation in a solid
Initial ionization of valence electrons by light field
Acceleration of ionized electrons by light
(Oscillation)
Repetitive
processes
Collisional absorption
Collisions of these individual
electrons with other particles
`Inverse bremsstrahlung’
Resonance Absorption
Excitation of a plasma
wave (Collective effect)
Damping of plasma wave
Hot,dense plasma
POLARIZATION DEPENDENT ABSORPTION IN PLASMAS
Resonance Absorption (> 1015 W cm-2)
P-polarized light at oblique angle of
incidence, exciting a plasma wave.
‘Hot’ electrons (‘Fast’ electrons)
WHY study Hot electrons?
Important for Fast Ignition Fusion
Emitters of very hard X-ray pulses
Where do `hot’ electrons go?
Input Laser pulse
300fs
1.2 ps after laser pulse
Gremillet et al., PRL 83 (1999) 5015
3 ps after laser pulse
Different perspectives!!
• Coupling of laser light – reflectivity
•Time resolved studies
•Magnetic field generation
•Generation of X-ray Pulses
•Electron and Ion emission
Hot electrons emit bremsstrahlung
o Picosecond
Femtosecond duration,
Very hard x-ray pulses
100
Copper
Counts
16
10
-2
5x10 W cm
100 fs, 100 nm
T = 40 keV
1
S. Banerjee et al,
SPIE, 3886(2000) 596
100
Energy (KeV)
1000
Femtosecond, Hard X-ray Pulses !
• p-polarized light is used
throughout
• surface topography should have
had detrimental effects as “some
‘p’ becomes ‘s’
•Roughness causes ENHANCED
emission
• necessity for an additional
mechanism
Counts
80
60
30
15
40
40
80
120
E (keV)
20
0
60
80
100
120
Energy (keV)
1000
Counts
• bremsstrahlung emission from
polished and unpolished targets
at
1 x 1016 Wcm-2
Counts
100
100
rough
smooth
10
1
0
10
20
30
40
50
Angle of Incidence
P. P. Rajeev et al.
Phys. Rev. A , 65, 052903(2002)
Physics In ULTRA-INTENSE Light Fields
Matter totally
ionized
Large charge densities ( > 1024 cm-3 )
Energetic electrons ( 103 - 106 eV ) Sun
Nonequilibrium dynamics - violently driven systems
Non-Maxwellian particle distributions
Gigantic magnitudes
Magnetic fields 109 G Electric field 1010 V cm-1
Pressure 109 barsTemperature 108 K ( for e- )
Relativistic and QED effects
multiphoton Compton scattering, pair production
Nuclear excitation and fusion
Laboratory Astrophysics
Zero to Megagauss in Picoseconds!
Sandhu et al,
Phys.Rev.Lett. 89
(2002) 225002
“Megagauss in picoseconds”
Physics News Update #614 dated Nov 20, 2002
(American Institute of Physics, NY)
Why study Laser generated magnetic
fields?
• Largest available terrestrially
• Magnetic fields mirror electron dynamics
They also control them!
(specially fast/relativistic electrons)
• Understanding them important for Laser Fusion
• Potential applications in futuristic information
storage, isotope separation, MCD etc…
How to measure B ?
Direct Methods:
• Induction Probes
• Magnetization/Demagnetization
Elegant Method
• Modification of polarization state of laser light (non-contact,
highly sensitive)
Laser Pump
Hot electron jets
B
Target
Probe
Setup
TIFR + IPR
Giant Magnetic Pulse !
Sandhu et al,
Phys.Rev.Lett. 89
(2002) 225002
Magnetic Field (MG)
30
B(t) P-pol pump
Model for B(t)
25
20
15
10
5
0
-3 -2 -1
0
1
2
3 4 5 6
Time (ps)
7
8
9
10 11
Magnetic field pulse profile for p- polarized pump at 1016 W cm-2
Generation and damping of B



dB c  
c2
2

  J ho t 
 B
dt

4
• Hot electrons Jhot
stream into bulk
Source
• Return plasma currents compensate
• The electrical resistivity -1 limits buildup and
•determines decay of magnetic field.
Current
loops
Cold e-
Laser
Hot eSolid
Diffusion
Plasma layer
Phenomenological Modeling
Evolution equation
:
dB/dt = S(t) - B/ ,
Source due to the
fast electron currents
Assuming exponential source:
Representation of the
magnetic diffusion term
S(t) = S0 exp(-t/t0)
S0
B(t ) 
[exp( t / t0 )  exp( t /  )]
(1 /   1 / t0 )
Resistive decay
of B from plasma
return currents
GOOD FIT with: S0 = 53.7 MG/ps, t0 = 0.7 ps,  = 5.6 ps.
Natural decay of the hot esource produced by the RA.
(Model used by IPR collaborators)
Sandhu et al,
Phys.Rev.Lett. 89
(2002) 225002
TIFR-IPR
30
B(t) P-pol pump
Model for B(t)
Magnetic Field (MG)
25
20
15
10
5
0
-1
0
1
2
3
4
5
6
7
8
9
10
11
Time (ps)
GOOD FIT to data : S0 = 53.7 MG/ps, t0 = 0.7 ps,  = 5.6 ps.
Energy budget for the given laser input:
At 1016 W /cm2
IB absorption ~ 10%
Resonance Absorption ~ 30-40%
The rest is not coupled !
Plasmas reflect light very well… (40-50%)
The reflected light carries information about the plasma
(density, scale length, temperature….)
However, there lies the problem…
how do you couple more light in?
It is indeed possible to couple upto 90% of incident light!!
HOW?
We address this now!
A `Small’ StepTowards
Efficient Xray emitters….
Small is bountiful !
Metal Nanoparticle coated Targets
 bulk
 p2
 1
 (  i )
 nano   bulk
 nano
vF

b
l
  bulk (1  )
b
linear absorption
• coated on optically flat Cu disk by high
pressure dc sputtering
•basic optical characterization by linear
reflectivity
• permittivity changes with size – different
plasmon resonances – different
absorption ranges – different colored
particles
90
Nanoparticle
Polished
60
Drude fits
30
0
300 400 500 600 700 800 900
 (nm)
Enhanced Hard Xray emission from
metal nanoplasmas
• using spherical and
ellipsoidal nanoparticles
(b ~ 15 nm)
• 3-4 fold enhancement
in the x-ray yield at 10o
incidence
• an enhanced intensity
~ 1.4Iin
• explains the extra hot
e- component
P. P. Rajeev et al., Phys.Rev.Lett. (2003)
Surface Plasmons
Def: Electromagnetic surface waves (‘p’) which exist at the
interface between 2 media whose  have opposite signs.
Ez ~ e
E
kz
Hy
kx
dielectric (>0)
+++ --- +++ --- +++ ---
metal ( <0)
Surface plasma oscillations:
.
fluctuations of the charge on a metal boundary
kz z
Nanotricks yield Megafluxes
60
• 13-fold enhancement using ellipsoidal
particles at 45o at
6 x 1014 W cm-2
Polished
30
13-fold
Enhancement!
20
10
• Very good agreement with the model
0
20
• Almost an order of magnitude increase
in the effective intensity using ellipsoidal
particles
40
60
80
100
120
Ag
100
Au
80
1
120
Energy (keV)
140
L
• explains the observed temperature and
yield
Spherical
40
Counts
• spherical particles continue to give 3-4
fold enhancements
Elliptical
50
Cu
60
40
20
P. P. Rajeev et al., Phys. Rev. Lett. (2003);
Optics Letters (2004)
0
0
5
10
15
(a/b)
20
25
Concept of fast ignition
Petawatt laser created intense
fluxes of MeV
Electrons are guided by a
carbon fibre plasma
Plasma photonics !
Nature (23 Dec 2004)
Nature (23 Dec 2004)
Conclusions
Intense, Ultrashort light interaction with
matter – Exciting scientific frontier!
Picosecond, Megagauss (5 ps, 27 MG)
magnetic pulses demonstrated in
femtosecond laser produced plasmas.
• Enhanced, femtosecond x-ray emission
•Guiding of intense fluxes of MeV electrons
Thanks to………..
Acknowledgements…..
Aditya Dharmadhikari
Pushan Ayyub
P. Taneja
P.K.Kaw
Sudip Sengupta
Amita Das
(IPR)
Earlier Collaboration
S. Banerjee, L.C. Tribedi, R. Issac
P.D. Gupta, P.A.Naik
and others (CAT)
A brief,
yet intense ,
affair with light !