59
Atoms in strong laser fields Johan Mauritsson
Atomsinstronglaserfields
JohanMauritsson
The life of an electron JOHANMAURITSSON,LUND,SWEDEN
h4p://www.atomic.physics.lu.se/research/a4osecond-xuv-spectroscopy/research/
Laser–ma4erinteracAon
Atoms
Electrons Ions
Photons
Laser–ma4erinteracAonIondetector
+Vacc
Ground
Field-freeFlighttube
Detector
Laser–ma4erinteracAonElectronoriondetector
EppinkandParker,Rev.Sci.Instr.,68,3477(1997)Vrakking,Rev.Sci.Instrum.,72,4084(2001)
ExtracAon
DetecAon
Electrons
Ions
2Dprojec*on
3Dreconstruc*on
VelocityMapImagingSpectrometer(VMIS)
-VelocityMapImagingSpectrometer
ATIinArgon
ExcitaAonvs.ionizaAon
g
f
Igef ∝2
rE
g
Ige ∝2
rEε
ε
2
21 mvE
E
kin
kin
=
+= φω!Photoelectriceffect
SelecAonrules
onpolarizaticircular 1onpolarizatilinear 0
1
±=Δ
=Δ
±=Δ
mml
s
p
p
s d
Helium Argon
Sphericalharmonics(Ylm)andangulardistribuAons
( )
φ
φ
φ
θπ
θθπ
θπ
θπ
θπ
π
i
i
i
e
e
e
222,2
1,2
20,2
1,0
0,1
0,0
sin3215
Y
cossin818
Y
1cos3165
Y
sin83
Y
cos43
Y
41
Y
±±
±±
±±
⋅=
⋅=
−=
⋅=
=
=
∓
∓
θ0=l
1=l
2=l
0=m
0=m
0=m
1=m
1=m 2=m
HeliumionizedbyashortXUVpulse
Sphericalharmonics
h4p://en.wikipedia.org/wiki/Spherical_harmonics
Dyelasers
Ti:sapphirelasers
High-orderharmonics
100as
1fs
10fs
100fs
1ps
10ps
1970 1980 1990 2000
Pulsedu
raNo
nWavelength>1fs
Femtosecondbarrier
LASERSShorterpulsesandhigherintensiAes
LASERSShorterpulsesandhigherintensiAes
Intensity
W/cm
2
1010
1015
1020
1025
Q-switching
Mode-locking
CPA
1970 1980 1990 2000
MulA-photonionizaAon
g
f
22Igeiief ∝rErE
i
(thisshouldactuallybeasumoverallpossibleintermediatestates)
??? 4 pkin IE −= ω!
ATI
FirstATI
(1979)
Freeelectroninlaserfield
0=kinE Ekin = ??????=kinE
I II III
Freeelectroninlaserfield
Whatistheenergyoftheelectrononcethefieldhaspassed?
Freeelectroninlaserfield
0=kinE 0=kinE???=kinE
I II III
WiggleenergyPonderomoAveenergy
Whatistheenergyofanelectronwiggelinginalaserfield?
PonderomoAveenergy
( ) ( )
( ) ( ) ( )
( ) ( ) ( )
( )
( ) ( ) 2
20
22
2
20
22
P
0
0
0
4cos
221
U :Energy
cos1
:Velocity
sin :onAccelerati
:Force
sin :fieldLaser
ωε
ωωε
ωω
ε
ωε
ωε
met
metvm
tme
tmetE
meta
tmateEtF
ttE
===
−=−=
=−=
=
YouhavetoprovidetheelectronwithsufficientenergyToovercomeIpandUp
Wheredoestheenergycomefrom?
Intensity
Energy
-Ip
0
kinEkinE
???
Shortvs.longpulses
m 1µ~
1. TheelectronhasAmetoleavethefocalvolumebeforethelaserpulsehaspassed2. TheelectrondoesnothaveAmetoleavethefocalvolumebeforethelaserpulsehaspassed
PonderomoAveshiflongpulse
I=2.21012W/cm2
I=1.11013W/cm2
Intensity
Energy
-Ip
0
kinE
PonderomoAveshifshortpulse
1000Ph
otoe
lectronen
ergy
IR-Harmonicdelay(fs)200
Intensity
Energy
-Ip
0
kinE
Howistheelectronionized?
TunnelionizaAonThestrengthofthelaseriscomparabletotheintraatomicforces
Tunnelingwavepackets
• BornnearthepeakoftheIRcycle• BornoutsidepotenAalwell• IniAalvelocity~0• TemporalwidthdependsonIRintensity• Periodicprocess
RepeAAonoftheprocess
TheionizaAonismaximizedeveryAmethelaserfieldismaximized,i.e.twiceperlasercycle
Electron“born”inalaserfieldThefinalelectronenergydependsonwhenduring
thelasercycletheelectronisionized
( ) ( ) ( ) ( )
( ) ( )
( ) ( ) ( )[ ]
( ) ( )[ ]
( ) ( )[ ]( )
( ) ( ) PdriftTT
drift
UEtme
mtvmE
tette
tte
ttet
tedtdtF
tttEttE
+≡+==
−=
−=
−=
+−−
=
−==
∂
∂−==
222
2
0
00
000
0
0
222
coscos
coscos
sin
,sin
Ap
ApAA
pp
p
A
ωωωε
ωωωε
ωε
ωε
Electrontrajectories
Electrontrajectories
pdri<
( ) ( )tet drift App −=
Streaking
h4p://www.cchem.berkeley.edu/leonegrp/research/GasPhaseA4o.html
Linearvs.circularpolarizaAon
Linearvs.circularpolarizaAon
Linearvs.circularpolarizaAon
( ) ( ) ( )[ ]
( ) ( ) ( )[ ]
PdriftP UEU
tytxt
tytxt
22
2
2
0
2
0
12
12
sinˆcosˆ1
cosˆsinˆ1
ξξξ
ωξωξω
ε
ωξωξ
ε
+≤≤
+
++
=
−+
=
A
E
Pdrift
Pdrift
UEUE
=
≤≤ 20Linear:
Circular:
ExpectedresultsAbovethresholdionizaNon DoubleionizaNon
2Up
He+He2+N
umbe
rofe
lectrons
Num
bero
fion
sIntensity
Photoelectronenergy
ObtainedresultsAbovethresholdionizaNon
2Up
Num
bero
felectrons
Photoelectronenergy
DoubleionizaNon
He+
Num
bero
fion
sIntensity
He2+
10Up
ObtainedresultsPhotons:oddharmonicsdetected
Anextendedplateau
ThreepuzzlingobservaAons
• HighenergyATI(upto10UP)• DoubleionizaAonatlowintensiAes• High-orderharmonics
III Theelectroninteractswiththeatomand:- resca9er- knockasecondelectronout- emitaccumulatedenergyasaphoton
II Thefreeelectronisacceleratedbythefield,andmayreturntotheatomiccore
TheelectrontunnelsthroughthedistortedCoulombbarrierI
Threestepmodel
[email protected],1994(1993)
[email protected],1599(1993)
ThesamemodelcanbeusedtoexplainallthreeobservaAon
High kinetic energy electrons
Non-sequential ionization
High-order harmonic generation
HighenergyATI
AbovethresholdionizaNon
2Up
Num
bero
felectrons
10Up
AnelectronthatscaZersandchangesdirecNon180degreescanpickupaddiNonalenergyduringthenexthalf-cycle.
RescaZeredelectrons
Non-sequenAalionizaAon
Sequ
entia
l
He
He+
He++
Electrondynamicsinalaserfield
Elec
tric
fiel
d-1
0
1
Time (IR cycles)0 1 2
Electrontrajectories
-GeneraConofa@osecondpulses
Photonemission-aninterferenceeffect
Sameelectron!
InterferenceleadstoanoscillaNngdipolei.e.photonemission
High-OrderHarmonicGeneraAon
ElectrondynamicsThereturnenergydependsontheAmeofionizaAon.Emax(electron)=3.17UPEmax(photon)=IP+3.17UP
Atom
FieldElectrons
APT–modelockedfslaseranalogy
Time
Frequency
Plateau
Cut-off
Singleharmonic~fewfs
~fs
~as
Severalsynchronizedharmonics~fewasTwoa4osecondpulsesperIRcycle
Whyonlyoddharmonics?BothoddandevenATIpeaks!
Frequency
TimeTime
Frequency
CanwegenerateonlyonepulseperIRcycle?
Breakthesymmetryandchangetheperiodicity
StrongIR
M. D. Perry et al., Phys. Rev. A 48, R4051 (1993) H. Eichmann et al., Phys. Rev. A 51, R3414 (1995) I. J. Kim et al., Phys. Rev. Lett. 94, 243901 (2005)
IonizaAonattheseAmesleadstothesame
electrontrajectories
IonizaAonattheseAmesleadstodifferent
electrontrajectories
+ =
X momentum
Y m
omen
tum
τ = 0 asAngulardistribuAons,singlepulse
Helium
PolarizaA
on
direcAon
θMomentum
AnelectroninastrongIRlaserfield
Itatani et al., Phys. Rev. Lett. 88, 173903 (2002)
Δp=-eA(t)
Thequantumstroboscope
ConvenNonalstroboscope: nscalingQuantumstroboscope: n2scaling+interference
-Toincreasethesignalstrength
Thequantumstroboscope-Argonandalaserfield
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