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

LCLS is great but not perfect

e-

undulator B-fieldw ~ cm

SASE FEL

• lasing builds up from noise

…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.

AMOS end-station: single-shot measurements

courtesy of John Bozek

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

x-ray-laser metrology

photoionization

incoherent x-rays(250-400 eV)

M

LAuger

M

L

Auger electron~ 200 eV

laser1012 W/cm2

1012 W/cm2

Auger