Attosecond MetrologyA method for attosecond pulse characterisation

Adam Wyatt1

Ian Walmsley1

Laura Corner1

A. MonmayrantJohn Tisch et al2

Eric Cormier3

Louis F. DiMauro4

1Clarendon Laboratory, University of Oxford2Blackett Laboratory, Imperial College3Centre Lasers Intenses et Applications, Universite Bordeaux4Brookhaven National Laboratory

Eadweard Muybridge’s Horse in Motion

Outline

• Motivation

• Analytic Representation of Optical Pulses

• SPIDER Generalisation & Implementation

• High Harmonic Generation

• XUV SPIDER Variants

• Research Tasks

• Conclusion

Overview of presentation

MotivationSuccess of Femtoscience – applications for Attoscience

Pump Probe Experiments – Advanced Flash Photography

• To capture event, need to have flash shorter than event.

• If exact detail of probe pulse known, pulse only needs to be comparable in duration to event in interest.

Success of Femtoscience Applications for Attoscience

• Tracking molecular motion in chemical reactions (femtochemistry)

• Detection & control of coherent processes.

• Micromachining

• Nobel Prizes!!!

• Tracking electronic motion

• Surface science

• ????

MethodsCurrent pulse characterisation methods

Non-Interferometric Interferometric

Tomographic Chronocyclic Tomography2:Reconstructs 2D density function from 1D data sets.

SPIDER4:Reconstructs spectral phase from 1D data set using a direct (non iterative) algorithm.

Spectrographic FROG3:Reconstructs pulse shape from 2D data set using iterative algorithm.

Different classes of characterisation methods and some examples1:

1I. A. Walmsley & V. Wong, J Opt Soc Am B,13(11), 1996

2M. Beck et el, Opt Lett, 18(23), 1993

3R. Trebino et el, Rev Sci Inst, 68(9), 1997

4L. Gallmann et el, Opt Lett, 24(18), 1999

2 Beam Interferometery

2 2

1 201,02 01,02 01 02

1 201 02

1 01 2 02 01 02

; , ; ;

2 ; ;

cos ; ;

I t E E

E E

t t

2 001,02 01,0

2

2 022 1 01 01; ; ,,; , E E tt tI 1 01 01; ,E t

Generalised Interferometer Spectrum

Spectrometer

Beamsplitter

~2/(t01-t02)

I()

2 02 02; ,E t

Carrier Frequency

dc ac aci iI I I e I e

1 2

2 2

1 2

1 2

dc

ac

I E E

I E E e

ad acccFT I I t I tI t tt t

How to extract the phase information

~2/

I()

t

I tFourier Transform

2 1 argfilter iIFT I e

filter

ac i

I H t I t

FT I e

What is needed for SPIDER

0

0 0

0 0 0

0 01

0

2

1n

n n

2

T

Spectral Shear

T

t

2/T

Nyquist:

Noise:

2

T

Sampling Interval

Classic SPIDERExperimental Set-up

XUV SPIDER

01 02

01 02

02

Generating the shear in the XUV

31 33 35 37n

(n)

31 33 35 37

2 1n n n

Example

• 30fs driving pulses at 800nm ~ 209 x1012 rad s-1

• 13nm corresponds to n = 61• 1nm bandwidth at 13nm t = 275as• Shear at driving freq. = / 61 = 1.2nm

XUV SPIDERSPIDER method for HHG radiation

SEA-XUV SPIDERSEA-SPIDER method for HHG radiation

Fourier Transform

2 2' '

' 2 2( , ) ', ',kx kx x

i ii x L LI x E x E x e e

SPIDER Adv.Comparisons of different techniques

SPIDER Pros: SPIDER Cons:

• Simple, direct inversion algorithm.

• Possible real time diagnostics.

• Possible spatial information.

• Higher SNR (photoelectron spectrometer)

→ Increased accuracy

→ Single shot capability

• Pulse train statistics.

• Self-consistency checks.

• Split driving pulses – lower intensity lower harmonic number.

• Need to generate two identical, spectrally sheared pulses of high intensity & stability.

Different SPIDER Adv.Comparison of XUV-SPIDER and SEA-SPIDER

XUV-SPIDER SEA XUV-SPIDER

Pros: • Rapid Update rates – real time diagnostics.

• Pulses see same section of gas.

• No intensity limit on driving pulses.

• Lower resolution for spectrometer.

• Measure spectral phase at different spatial co-ordinates.

Cons: • Maximum intensity of driving pulses due to ionisation. Maximum harmonic number.

• High resolution spectrometer required.

• Average over spatial phase.

• More data – slower update rates.

• Pulses see different gas densities.

Simulated ResultsSimulated HHG data and XUV SPIDER reconstruction

31 33 35 37-15

-10

-5

0

5

10

Original and Reconstruction of Phase Of Harmonics

Harmonic Order

Pha

se /r

ad

Rescaled Harmonic Spectrum

Phase from 800nm driving pulse

Phase from 800.5nm driving pulse

Reconstructed Phase

-10 -8 -6 -4 -2 0 2 4 6 8 100

1

2

3

4

5Temporal Profiles of Attosecond Pulse Trains

time /fs

Inte

nsity

/arb

. uni

ts

Fourier Transform Limited (FTL)

Simulated Profile

Reconstructed Profile

31 33 35 37-15

-10

-5

0

5

10Original and Reconstruction of Phase Of Harmonics

Harmonic Order

Pha

se /r

ad

-10 -8 -6 -4 -2 0 2 4 6 8 100

1

2

3

4

5Temporal Profiles of Attosecond Pulse Trains

time /fs

Inte

nsity

/arb

. uni

ts

Rescaled Harmonic Spectrum

Phase from 800nm driving pulse

Phase from 802.5nm driving pulse

Reconstructed Phase

Fourier Transform Limited (FTL)

Simulated Profile

Reconstructed Profile

Generating the carrier frequencyCan do – need to improve!

Fringes 2D Fourier Transform

Generating the shearSome ideas still to be tested

Bi-Mirror / Knife edge 4-f Knife edge / Full puls shaping

AOPDF Pulse Shaping

Hard – too large bandwidthLow power output (high losses)

Easily implementedLimitations

Osc. AOPDF Amp.

HCFHHGMetrology

Research TasksWhat to do?

• Simulate HHG with two driving pulses directly (c.f. combing spectra from individual pulse

simulations).

• Find optimal shear and time delay for typical noise parameters.

• Test XUV-SPIDER for shorter pulses (5 fs) via simulation.

• Test how different driving pulses can be.

• Test generating shear

Conclusions

• Applications & motivation for Attoscience. Success of femtoscience.

• SPIDER technique. What is needed (carrier frequency & time delay) Conventional implementation.

• XUV SPIDER. How to create shear via HHG.

• Pros & Cons of SPIDER. Lots of good points, limited by creating sheared pulses.

• Still more to do

• Promising outlook!

What have we learnt?