1. Frequency comb lasers on the move to extreme wavelengths Kjeld Eikema LCVU day 17 Dec. 2008 Christoph Gohle, Dominik Kandula, Tjeerd Pinkert Anne Lisa Wolf, Steven van den Berg (NMI) Wim Ubachs, Wim Hogervorst

2. Outline  Introduction frequency comb lasers  Goals  First comb excitation of Ca ions in a trap  First comb excitation of Helium, at 51 nm  Summary & outlook

3. Addition of waves to pulses

4. Modelocked laser ~ frequency comb More than a million lasing modes can work together to form < 5fs pulses Laser resonator

5. Time and frequency picture frequency Int. 0 time Equivalent !!!

6. Frequency comb structure frequency Int. 0 R. Holzwarth et al. PRL 85, 2264 (2000), D.J. Jones et al. Science 288, 635 (2000) f n = f 0 + n. f rep f rep = c / 2L f 0 = f rep x  φ CE / 2  Controlled repetition rate + Phase relationship between the pulses = Frequency comb time φ CE =  /2 φ CE = 0 v g ≠ v φ

7. Time & frequency consequences Very precise pulse control  Attosecond timing resolution revolution  Reproducible phase coherent optical pulses  Absolute long distance measurement Very precise frequency ruler for THz-PHz  Optical frequency calibration revolution  Optical atomic clocks  ‘Fingerprinting’ parallel spectroscopy

8. Ti:Sa Frequency comb implementation LCVU 2 Ti:Sa based comb lasers 1 Fiber-based comb laser (Menlo Systems) f rep : 60-300 MHz nJ pulse energy 500 – 2000 nm

9. Fiber frequency comb LCVU 600-900 nm & 1100-2000 nm, f rep = 250 MHz Project of Valdas Maslinskas

10. Measuring with frequency combs frequency Int. 0 f0f0 frfr Atomic clock  f VIS-NIR comb 12.45239828 MHz CW ultra- stable laser Precision spectroscopy 456 243 268 532 146.7 Hz Counter

11. Goal 1: trace attosecond electron dynamics Attosecond dynamics in mK molecular ions Tryptofan complexes (Trp Ala 2 and Trp Leu 2 ) Remacle & Levine PNAS 103, 6793 (2006)

12. Tools for attosecond dynamics studies  Powerful and precisely controlled electromagnetic waves with sub-femtosecond resolution  Pump-probe via VIS-UV – XUV phase coherent pulses or via re-collision of electron wavepackets (HHG!)  Cold and localized molecules: molecular ions in a trap, sympathetically cooled by atomic ions to mK temperature  Many other things we probably did not think about...

13. Goal 2: XUV precision test of QED 1S 2S Hydrogen 2x 243 nm 1S-2S: 2 466 061 102 474 851 (34) Hz Lamb shift 1S: 8172.840(22) MHz M. Fischer et al., PRL 92, 230802 (2004) Uranium 91+ Lamb shift 1S = 460.2 (4.6) eV 1S 1/2 2P 3/2 138 576 (6) eV A. Gumberidze et al., PRL 94, 223001 (2005) ‏ Higher-order QED scales with >= Z 4

14. Tests in Helium and Helium+ ions HydrogenHeliumHelium+Lithium 2+ 1S 2S 243 nm 1S 2 1S2S 120 nm 1S 2S 60 nm 1S 2S 30 nm 51 nm 1S5P

15. Searching for a change of alpha Comparison lab UV spectra vs. IR telescope absorption lines in quasar spectra (looking back billions of years)

16. Direct UV frequency comb excitation Frequency comb laser @ 788 nm 2 nd Harmonic generation Excitation @ 394 nm tt  22 5 ns 10 5 modes 5 ns 190 MHz Time domain Frequency domain f n =f 0 + n f r f m = 2 f 0 + m f r

17. Ca + ions Direct frequency comb spec. of Ca + Paul trap Freq. comb pulses @ 394 nm

18. Ions in a trap Calcium ion trap & Anne Lisa Wolf

19. The setup

22. Direct frequency comb signal of Ca +

23. Mode number determination for Ca + 200 times more accurate than before Corrections for light-shifts etc. included Accuracy: 0.5 MHz

24. Helium groundstate status Energy of He 1s 2 1 S 0 (+5945204000 MHz) -400 -200 0 Theory: Drake & Yan Can. J. of Phys. 86, 45 (2008) MHz Eikema et al. PRL 71 (1993) PRA 55, 1866 (1997) Bergeson et al. PRL 80, 3475 (1998) Measurements with nanosecond duration VUV (120 nm) or XUV (58.4 nm) single pulses

25. HHG: High-Harmonic-Generation IR pulses – mJ levelXUV pulses – nJ level 10 14 W/cm 2 lens/mirror Required for 1s 2 – 1s5p transtion: 51.56 nm gas jet

26. Hoge-harmonische generatie Model van P. Corkum, PRL 1994

27. High-harmonic generation T Train of x-ray bursts Generation of 170 attosecond pulses: Lopez-Martens et al, PRL 94, 033001 (2005) ~ 5 fs

28. Problem: single pulse frequency chirp ‘Clean’ or ‘Fourier limited’ well defined central frequency ‘Chirped’ Unclear frequency due to amplification & harmonic conversion

29. Solution: two or more pulses  time frequency 

30. Direct UV frequency comb excitation Frequency comb laser @ 788 nm 2 nd Harmonic generation Excitation @ 394 nm tt  22 5 ns 10 5 modes 5 ns 190 MHz Time domain Frequency domain f n =f 0 + n f r f m = 2 f 0 + m f r

31. Direct XUV frequency comb excitation Frequency comb laser @ 773 nm Two-pulse amplifier 15 th Harmonic generation Excitation @ 51 nm ttt  15  7 ns 10 6 modes 10 4 modes 7 ns 150 MHz Time domain Frequency domain f n =f 0 + n f r f m =15 f 0 + m f r

32. Frequency comb up-conversion IR DUV frequency VUVXUVX-RAY harmonic conversion IR pulses with CE controlXUV 10 14 W/cm 2 UV Xe or Ar jet

33. How accurate does it have to be?  timefrequency @ 51 nm  !  CE  ~ /100  CE  ~ 0  =20 MHz  T = 6.6 ns

34. 2 identical pulses Accuracy required  CE  ~ /200 ! Delay 6.6 ns, accuracy ~ 10 attoseconds (10∙10 -18 s) While amplifying more than 10 000 000 x at 10 GW/cm 2 power levels, over the full 6 mm beam diameter !

35. Amplification of the comb with a NOPCPA ss pp ii The good: High gain (=short path length) Broad bandwidth (non-collinear) No thermal effects Phase preserved of signal (but...) The tricky: Picosecond pump laser Synchronization < 1 ps Angle sensitivity  p  k p pump idler signal  (2) fluorescence cone seed  s  k s  

36. Setup for He spectroscopy at 51 nm regen amp. 2 mJ power amp. 2 x 200 mJ SHG sync. 7 ps osc. 1064 nm comb laser ~780 nm f rep =150 MHz NOPCPA HHG 51 nm pulses, 6.6 ns apart 1s5p 1 P 1 1s 2 1 S 0 Helium 51 nm 2x 2 mJ 200 fs Relay imaging

37. Phase measurement setup BS 50% BS 5% NOPCPA Pump laser BS NG Oscillator PC2 PC1 G BS BS 5% D fiber Computer CCD Camera PBS

38. Phase accuracy between 2 pulses At the moment for pulses 6.6 ns apart: Interferometric measurement to 1/600 th of IR ! Phase flatness across the IR beam: ~ 1/200 th of IR Many tricks involved to control and measure this! Christoph Gohle Dominik Kandula

39. Photos of the lab

41. XUV comb generation and He excitation HHG Spherical grating XUV detector Krypton jet XUV Beam Monochromator slit 2 times >2 mJ, 200 fs Helium beam 90 o ! IR ionization beam

42. HHHG: Holey-High-Harmonic-Generation IR pulses – mJ levelXUV pulses – nJ level 10 14 W/cm 2 f=50 cm

43. Harmonic generation double pulses In Krypton @ ~772nm 7 911 13 15 Harm. :  (nm) 7 : 110 9 : 86 11 : 70 13 : 60 15 : 51

44. HHG with double NOPCPA pulses 15 th harmonic in Krypton ~ 51 nm nJ pulses Fundamental at 795 nm

45. First helium 1s5p signal at 51 nm... comb laser repetition rate scan He-ion signal FFT

46. Better Helium 1s5p signal at 51 nm 150 MHz Total scan of pulse delay < 0.5 fs ! 8 attoseconds change per scan step

47. Photo He experiment

48. Xe two-pulse 125 nm excitation  Short delay for mode identification, long delay for accuracy  Decreasing contrast is caused by the residual 10 MHz Doppler width  Best fringe position fitting result: 40 kHz  Phase errors reduce to <1 MHz at longer delays 13 times better than CW/ns pulsed experiments. Zinkstok et al., PRA 73, 061801(R) (2006)

49. Summary  First demonstration of direct frequency comb excitation of calcium ions in a trap at UV wavelengths  First XUV frequency comb excitation of helium: on route to MHz precision In preparation:  Calibration He ground state, two-photon excitation etc.  Sympathetic ion cooling of atomic and molecular ions  Attosecond electron dynamics in biomolecules  Extension of XUV combs for helium+ ions @ 51 nm