strong-field physics in the x-ray regime louis dimauro itamp fel workshop june 21, 2006 fundamental...
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strong-field physics in the x-ray regime
Louis DiMauroITAMP FEL workshop
June 21, 2006
• fundamental studies of intense laser-atom interactions• generation and application of attosecond pulses• ultra-fast optical engineering
LCLS parameters
• XFELs are an x-ray source of unprecedented brightness
0.11101001000
wavelength (nm)
10-1
101
103
105
107
109
1011
1013
1015
peak power (W)
NSLS
APS
HHG PlasmaLaser
Lasers
T3
ALS
LCLSTESLADUV-FEL
DUV-FEL CPA
• only viable sources are accelerator-based
…so theorists beware
what we think we are measuring may not be what we are measuring!
…and welcome to the 70’s!
Linac Coherent Light Source at SLAC
• 15 GeV e-beam, 100 meter long undulator• output: 0.8-8 keV (1.5 A), 10 GW, 1012 photons/shot, 230 fs, 120 Hz• five thrust areas slated for first operations
AMOP at the LCLS
LCLS experimental halls
• AMOS team located in the near-hall• initial experimental end-station on soft x-ray beamline (0.8-2 keV)• scientific thrust: fundamental strong-field interactions at short
wavelengths
AMOS team experimental menu
short term strategy• shine light on Roger’s wall (scientific impact)• define a prominent role in XFEL development
• atomic ionization (nonlinear physics/metrology)• x-ray scattering from aligned molecules (laser/x-ray experiments)• cluster dynamics (non-periodic imaging)
contrast: long and short wavelength strong-field physics
ponderomotive potential is everything at long wavelengths
e- in Coulomb + laser fields• electron ponderomotive energy (au):
Up = I/42
• displacement: = E/2
• PW/cm2 titanium sapphire laser:Up ~ 60 eV & ~ 50 au
for LCLS @ 102 PW/cm2 Up & are zero!
contrast: long and short wavelength strong-field physics
• laser multiphoton ionization
• x-ray multiphoton ionization
n=2
neon photoabsorption
n=1
• laser multiphoton ionization
x-ray strong field experiment
x-ray multiphoton ionization
photoionization
Auger
2-photon, 2-electron
sequential
correlated ionization
needle in the haystack: coincidence measurements
reaction microscopySchmidt-Böcking
J. Ullrich et al., JPB 30, 2917 (1997)
• detect by correlating particle-particle events
coincidence measurement
true-to-false ratio:T/F = t C f (1 C + N1)(2 C +N2)t,1,2(,,) detection coefficients
N1,2 noise counts C average count rate f repetition rate
• the 120 Hz operation of the LCLS makes coincidence experiments high risk, thus the initial AMOS end-station will not have this capability.
LCLS parameters for strong-field studies
photon energy: 800 eVnumber of photons: 1013/shotpulse energy: 1 mJpeak power: 5 GWfocused spot size: 1 mflux: 1033 cm-2 s-1
intensity: 1017 W/cm2
period: 2 asnumber of cycles:40,000ponderomotive energy: 25 meVdisplacement: 0.003 au
global survey of single atom response
Keldysh picture
optical frequencytunneling frequency = (Ip/2Up)1/2 -1
“photon description”
“dc-tunneling picture”
0.1 1 10 100
keldysh parameter,
0-7
0-5
0-3
0-
0
( )frequency au
10-9
10-7
10-5
10-3
10-1
101
field amplitude (au)
10-7
10-5
10-3
10-1
101
frequency (au)tunnel
tunnel
MPI
MPI
Ti:sap & Nd
excimer
CO2
• data compiled based on both electron and ion experiments
mir
lcls
1-photon, 1-electron ionization
consider a 1-photon K-shell transition:
K 10-18 cm2
RK = KFlcls 1015 s-1 (saturated)
tK = 1/RK = 1 fs photoionization
L-shell
neon photoabsorption
K-shell
• rapid enough to ionize more than one electron!• fast enough to compete with atomic relaxation?• is the external field defining a new time-scale?
2-photon, 2-electron ionization
2-photon, 2-electron
S.A. Novikov & A. N. Hopersky, JPB 35, L339 (2002)
consider a 2-photon KK-shell transition:
KK 10-49 cm4 s
RKK = KKF2lcls 1017 s-1 (saturated)
tKK 10 as
neon
KK
10-5
2 cm
4 s
• so RKK RK
2-photon, 2-electron ionization
2-photon, 2-electron
• are the electrons correlated?• in a strong optical field single electron dynamics dominate.
photoionization Auger photoionization
i.e. is it sequential ionization?
2-photon, 1-electron ionization
consider a 2-photon K-shell transition:• estimate 2-photon cross-section, 2
perturbative scaling laws1: 2 10-54 cm4 s2nd-order perturbation theory2: 2 10-52 cm4 s
1P. Lambropoulos and X. Tang, J. Opt. Soc. Am. B 4, 821 (1987) 2S. A. Novikov and A. N. Hopersky, JPB 33, 2287 (2000)
transition probability:P2 = 2F2lcls 0.1-1
x-ray above-threshold ionization
• estimate (2+1)-photon cross-section, 2+1
2+1 = 2cycle 10-90 cm6 s2
P2+1 = 2+1F3 lcls 10-4
• ATI should be observable
bound-continuum ATI
bound-c-c ATI
Auger ATI
distinguishable
can the LCLS get to the strong-field regime
• impose a Keldysh parameter of one = (Ip/2Up)1/2 Up 400 eV (8 eV)
@ 800 eV, intensity needed is 1021 W/cm2 (1014 W/cm2)
= 0.3 au (19 au)
• number of photons is fixed, require tighter focus and shorter pulselcls ~ 10 fs (very possible)
thus require a beam waist of 50 nm (in principle possible)
0.1 1 10 100
keldysh parameter,
0-7
0-5
0-3
0-
0
( )frequency au
10-9
10-6
10-3
100
103
field amplitude (au)
10-7
10-5
10-3
10-1
101
frequency (au)
tunnel
tunnel
MPI
MPI
lcls II
there are other sources on the horizon
Cornell Energy Recovery Linac107 photons/shot in 20 fsrepetition rate: 0.1-1 MHz
coincidence measurement
true-to-false ratio:T/F = t C f (1 C + N1)(2 C +N2)t,1,2(,,) detection coefficients
N1,2 noise counts C average count rate f repetition rate
• the 120 Hz operation of the LCLS makes coincidence experiments high risk, thus the initial AMOS end-station will not have this capability.
• the ERL does have the advantage of high duty cycle!• does it have high enough intensity for exploring multiphoton processes?
maybe?
consider a 2-photon K-shell transition:• estimate 2-photon cross-section, 2
perturbative scaling laws1: 2 10-54 cm4 s2nd-order perturbation theory2: 2 10-52 cm4 s
1P. Lambropoulos and X. Tang, J. Opt. Soc. Am. B 4, 821 (1987) 2S. A. Novikov and A. N. Hopersky, JPB 33, 2287 (2000)
ERL parameters:106/shot, 100 fs, 1A20 nm waist yields flux ~1030 cm-2 s-1 (1015 W/cm2)
transition probability:P2 10-(57)
106
107
108
109
1010
1011
1012
photons/shot
10-8
10-6
10-4
10-2
100
102
104
106
2-photon probability
two-photon ionization
2
10≅ −(52−54) cm4 s
20 nm waist100 fs, 1A
LCLS
• use near resonant enhancement of 2
• increase number of photons/shot over average power
open the possibility of temporal metrology!
ERL
2-photon, 2-electron ionization
2-photon, 2-electron
S.A. Novikov & A. N. Hopersky, JPB 35, L339 (2002)
consider a 2-photon KK-shell transition:KK 10-48 cm4 sPKK 0.1
neon
KK
10-5
2 cm
4 s
coincidence investigations:• atomic• aligned molecules• cluster dynamics