Attosecond Metrology A method for attosecond pulse characterisation Adam Wyatt 1 Ian Walmsley 1 Laura Corner 1 A. Monmayrant John Tisch et al 2 Eric Cormier 3 Louis F. DiMauro 4 1 Clarendon Laboratory, University of Oxford 2 Blackett Laboratory, Imperial College 3 Centre Lasers Intenses et Applications, Universite Bordeaux 4 Brookhaven 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

Motivation Success 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 FemtoscienceApplications for Attoscience Tracking molecular motion in chemical reactions (femtochemistry) Detection & control of coherent processes. Micromachining Nobel Prizes!!! Tracking electronic motion Surface science ????

What’s the problem Why is it hard to characterise ultrashort pulses Duration much shorter than detector response. DetectorResponse Time ElectronicPicoseconds Streak Cameras500 femtoseconds Examples:

What are EM pulses? Superposition of CW EM Fields

What are EM pulses? Superposition of Few CW EM Fields – Pulse Trains

What are EM pulses? Superposition of CW EM Fields – Isolated Pulses

Characterising EM pulses Real Field and Analytic Field Equations Complex Temporal Electric FieldComplex Spectral Electric Field

Pulse Characterisation What completely defines a pulse Direction  basic tools Colour  spectrometer Arrival time  interferometer Direct measurementsIndirect measurements Duration  ? Shape  ? Pulse much shorter than detector response! Electric field envelopeCarrier frequency Pulse arrival timePulse spectrum Fourier Transform Absolute phase (CEO)Dispersive phase Analytic Electric Field Equations (In Angular Frequency and Time)

Methods Current pulse characterisation methods Non-InterferometricInterferometric Tomographic Chronocyclic Tomography 2 : Reconstructs 2D density function from 1D data sets. SPIDER 4 : Reconstructs spectral phase from 1D data set using a direct (non iterative) algorithm. Spectrographic FROG 3 : Reconstructs pulse shape from 2D data set using iterative algorithm. Different classes of characterisation methods and some examples 1 : 1 I. A. Walmsley & V. Wong, J Opt Soc Am B, 13(11), M. Beck et el, Opt Lett, 18(23), R. Trebino et el, Rev Sci Inst, 68(9), L. Gallmann et el, Opt Lett, 24(18), 1999

2 Beam Interferometery Generalised Interferometer Spectrum Spectrometer Beamsplitter ~2  /(t 01 -t 02 )  I(  )

Carrier Frequency How to extract the phase information ~2  /   I(  )    Fourier Transform

What is needed for SPIDER Spectral Shear   T t 2  /T  Nyquist: Noise: Sampling Interval 

Classic SPIDER Experimental Set-up

High Harmonic Generation The very basics 1.Laser distorts atomic potential. 2.Electron wavepacket tunnels through barrier. 3.Laser causes electron to oscillate back & forth. 4.Collisions with atom causes re-ionisation. 5.High energy photons emitted. Laser Atom Electron High energy photon

XUV SPIDER Generating the shear in the XUV   31   33   35   37   n  (n)     Example 30fs driving pulses at 800nm   ~ 209 x10 12 rad s -1 13nm corresponds to  n = 61 1nm bandwidth at 13nm   t = 275as Shear at driving freq. =  / 61   = 1.2nm

XUV SPIDER SPIDER method for HHG radiation

SEA-XUV SPIDER SEA-SPIDER method for HHG radiation Fourier Transform

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-SPIDERSEA 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 Results Simulated HHG data and XUV SPIDER reconstruction Original and Reconstruction of Phase Of Harmonics Harmonic Order Phase /rad Rescaled Harmonic Spectrum Phase from 800nm driving pulse Phase from 800.5nm driving pulse Reconstructed Phase Temporal Profiles of Attosecond Pulse Trains time /fs Intensity /arb. units Fourier Transform Limited (FTL) Simulated Profile Reconstructed Profile Original and Reconstruction of Phase Of Harmonics Harmonic Order Phase /rad Temporal Profiles of Attosecond Pulse Trains time /fs Intensity /arb. units 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 frequency Can do – need to improve! Fringes2D Fourier Transform

Generating the shear Some ideas still to be tested Bi-Mirror / Knife edge4-f Knife edge / Full puls shaping AOPDF Pulse Shaping Hard – too large bandwidth Low power output (high losses) Easily implemented Limitations Osc.AOPDFAmp. HCFHHGMetrology

Research Tasks Shown different driving pulses created sheared harmonics by numerically solving TDSE 1 for single 30 fs pulse HHG.  Frequencies must not differ by more than 2.5nm at 800nm. Shown max driving intensity ~ 1.7 x10 14 Wcm -2. Shown can reconstruct across harmonics given signal with fringes. Shown SEA SPIDER also feasible for experimental parameters. What’s been done? 1 Data from simulations provided by collaborator E. Cormier, University of Bordeaux, France.

Research Tasks What 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?