1. Characterization of high intensity sub-4-fs laser pulses using spatially encoded spectral shearing interferometry 1) (Ultrafast Pulse Characterisation Tutorial)2) (SPIDER Tutorial)3) Spatially Resolved sub-4-fs Characterisation The attoboys Bring your mugs!

2. Ultrafast Pulse Characterisation? • Ultrafast pulse characterisation is a unique problem • Shorter than electronic response times • no direct time-domain measurement possible • Measuring the spectrum (easy), but gives no phase information

3. Requirements

4. Classes of Ultrafast Pulse Characterisation Methods • We consider self-referencing methods here: • (Auto-)Correlation • 1d data set for 1d field, no direct solution, ambiguities • Spectrographic • 2d data set for 1d field, iterative solution, converges to exact solution • Tomographic • 2d data for 1d field, direct algebraic solution • Interferometric • 1d data set for 1d field, , direct algebraic solution • Easily extendible: 2d data for 2d field

5. Autocorrelation • Direct measure of the RMS pulse duration: • This is exact! • But a deconvolution factor is used by most people • pulse duration = FWHM of AC divided by: • Gaussian pulse shape: 1.44 • Sech pulse shape: 1.54 (gives shorter pulses ;-)

6. Characterisation of really short pulses • Problems: • large bandwidth • Details of spectral phase important • Knowledge of detailed pulse intensity profile useful • High intensity: need to worry about pre- post-pulses • Spatial structure • Spatial chirp • Space-time-couplings

7. Spectral Interferometry • Linear and single shot technique • Phase difference of two pulses from spectrum measurement!

8. Spectral Interferometry: phase extraction • In practice done by Fourier filtering. Takeda algorithm • [Takeda M, Ina H, Kobayashi S. Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry. J. Opt. Soc. Am. 1982;72156.]

9. self-referencing Spectral Interferometry • self-referencing Spectral Interferometry == SPIDER • Spectral Phase Interferometry for Direct Electric-field Reconstruction • Self-reference by spectral shearing • Linear temporal phase modulation • upconversion with two different monochromatic frequencies

10. SPIDER phase • The phase-difference measured by SI is an approximation of the phase gradient: • Retrieval of spectral phase by integration and scaling by shear • Spectrum + spectral phase => E(t) after Fourier transform

11. SPIDER: experimental implementation • Baby-SPIDER: • temporal encoding, i.e. two pulses interfere with delay • Monochromatic ancillae by chirping a pulse • Shear set by delay tau and chirp

12. SPIDER: experimental implementation • APE-SPIDER layout

13. SPIDER for really short pulses • Really short = <10fs down to <4fs down to… • Bandwidth gets quite large (we have almost 500nm spectrum ) • Spectra can be problematic (intensity nulls) • Easily adjustable shear beneficial (jump over nulls) • Calibration of cal.-phase, , and shear becomes critical • [Birge JR, Kärtner FX. Analysis and mitigation of systematic errors in spectral shearing interferometry of pulses approaching the single-cycle limit \[Invited\]. J. Opt. Soc. Am. B. 2008;25(6):A111--A119. Available at: http://josab.osa.org/abstract.cfm?URI=josab-25-6-A111.] • ZAP-SPIDER • ZAP • Adjustable shear • Still temporal encoding • SEA-SPIDER • ZAP • Adjustable shear • Phase sampled at spectrometer resolution. • =>No oversampling required • Spatial information

14. SEA-SPIDER • Spatially Encoded Arrangement for Spectral Phase Interferometry for Direct Electric field Reconstruction [Kosik et al. Optics letters. 2005;30(3):326–328] • Spatial fringes rather than temporal fringes like in conventional SPIDER • Spectral phase is sampled at the spectrometer resolution • Allows spatial resolution along 1 axis (here along the spectrometer slit) Spectral shear Spatial carrier Typical data trace (40dB colour scale) Note the spatial / not spectral fringes

15. SEA-SPIDER setup You pay for the benefits…

16. SEA-SPIDER data • “Intuitive” fringes • The fringes follow the phase gradient • Quadratic spectral phase -> linear fringe tilt • Data shown for 7fs pulse + gvd FTL - gvd

17. SEA-F-SPIDER • Tackle ancilla preparation problem • Remember: quasi-monochromatic frequency required (Normally achieved by temporal stretching)

18. SEAF calibration • temporal stretching • Calibration error from noise • Imprecise shear calibration for broadband spectra • The shorter the pulse, the worse it gets even though higher precision is required • Shear calibration changes with pulse chirp • Upconversion frequency has to be “guessed” • Direct spectral filtering: • Very accurate shear calib (> factor 10 better) • Independent of input pulse chirp! • Direct knowledge of upconversion frequency Monte-Carlo simulation of noise dependence: Experimental calibration:

19. A supercomputer

20. Now the actual experiment…

21. The Making of a short one… • 1) ask Leo

22. The Making of a short one… • 2) go to the atto-lab: • Femtolasers CPA • Fibre • Chirped mirror compressor • Latest generation • Ultra-broad bandwidth • GVD is smooth from 500nm to 1100nm • Lower GVD oscillations • Double angle technology • 5 mirrors at 5deg • 5 mirrors at 20deg

23. Setup • Differentially pumped hollow fibre [Robinson et al. Applied Physics B 2006;85(4):525-529] • Biggest problem: getting the UV part of the spectrum detected • No blue glass filter: -> less dispersive grating -> blue detected in 1st order rather than in 2nd order • “Special” UV sensitive CCD. Windowless CCD chip.

24. Calibration • Record filter transmission spectrum as function of filter angle

25. SEA-F-SPIDER data • 1d-reconstruction in beam centre

26. Spatially resolved pulse reconstruction • The pulse is reconstructed (standard 1D) independently in every vertical beam position (i.e. along the spectrometer slit) All data is contained in a single shot trace! • The pulse has been centered temporally by it’s 1st order moment (physically sensible) • Beam can be scanned across slit to get full 2D map • Data-cube: Pulse phase and intensity for all x,y: I(x,y,omega)*e^(i*phi(x,y,omega) • For strict spatio-temporalreconstruction one needs a separate spatial shearing measurement. To relate the individual pulse phases/arrival times (SPIDER cannot measure pulse arrival time = linear phase; random unknown absolute phase becomes a linear phase after integration)

27. Literature • Further reading: • Iaconis C, Walmsley IA. Spectral phase interferometry for direct electric-field recontruction of ultrashort optical pulses. Opt. Lett. 1998;23(10):792-794 • Walmsley IA, Dorrer C. Characterization of ultrashort electromagnetic pulses. Adv. Opt. Photon. 2009;1(2):308-437. Available at: http://aop.osa.org/abstract.cfm?URI=aop-1-2-308. • Monmayrant A, Weber S, Chatel B. A newcomerʼs guide to ultrashort pulse shaping and characterization. Journal of Physics B: Atomic, Molecular and Optical Physics. 2010;43(10):103001. Available at: http://stacks.iop.org/0953-4075/43/i=10/a=103001?key=crossref.a3845bf6ff5ac8d2545d08312ebeb698. • Stibenz G, Ropers C, Lienau C, et al. Advanced methods for the characterization of few-cycle light pulses: a comparison. Applied Physics B. 2006;83(4):511-519. Available at: http://www.springerlink.com/index/10.1007/s00340-006-2190-5 • Kosik E, Radunsky A, Walmsley I, Dorrer C. Interferometric technique for measuring broadband ultrashort pulses at the sampling limit. Opt. Lett. 2005;30(3):326 • Witting T, Austin DR, Walmsley IA. Improved ancilla preparation in spectral shearing interferometry for accurate ultrafast pulse characterization. Opt. Lett. 2009;34(7):881-883. Available at: http://ol.osa.org/abstract.cfm?URI=ol-34-7-881 • Gallmann L, Steinmeyer G, Sutter DH, et al. Spatially resolved amplitude and phase characterization of femtosecond optical pulses. Optics Letters. 2001;26(2):96-98. • Zaïr A, Guandalini A, Schapper F, et al. Spatio-temporal characterization of few-cycle pulses obtained by filamentation. Opt. Express. 2007;15(9):5394-5404. Available at: http://www.opticsexpress.org/abstract.cfm?URI=oe-15-9-5394 • Dorrer C, Kosik EM, Walmsley IA. Spatio-temporal characterization of the electric field of ultrashort optical pulses using two-dimensional shearing interferometry. Applied Physics B: Lasers and Optics. 2002;74:209-217.

28. Next week: Journal Club! • http://charles.qols.ph.ic.ac.uk/~twitting/consJCDM/ • Take your pick: Femtosecond filamentation in air and higher-order nonlinearitiesHarmonic generation in metallic, GaAs-filled nanocavities in the enhanced transmission regime at visible and UV wavelengthsAttosecond chirp compensation over broadband high-order harmonics to generate near transform-limited 63 as pulsesCharacterizing ultrabroadband attosecond lasersControl of the high-order harmonics cutoff and attosecond pulse generation through the combination of a chirped fundamental laser and a subharmonic laser field (THEORY PAPER)Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe TechniqueMolecular high harmonic generation in a two-color fieldTunable ultrafast extreme ultraviolet source for time- and angle-resolved photoemission spectroscopy

29. DUMP

30. Spectral filters • Filters are commercially available ~3nm FWHM • Layertec can make 0.6nm