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

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

Addition of waves to pulses

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

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

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

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

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

Fiber frequency comb LCVU nm & nm, f rep = 250 MHz Project of Valdas Maslinskas

Measuring with frequency combs frequency Int. 0 f0f0 frfr Atomic clock  f VIS-NIR comb MHz CW ultra- stable laser Precision spectroscopy Hz Counter

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)

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...

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

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

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

Direct UV frequency comb excitation Frequency comb 788 nm 2 nd Harmonic generation 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

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

Ions in a trap Calcium ion trap & Anne Lisa Wolf

The setup

Direct frequency comb signal of Ca +

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

Helium groundstate status Energy of He 1s 2 1 S 0 ( MHz) 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

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

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

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

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

Solution: two or more pulses  time frequency 

Direct UV frequency comb excitation Frequency comb 788 nm 2 nd Harmonic generation 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

Direct XUV frequency comb excitation Frequency comb 773 nm Two-pulse amplifier 15 th Harmonic generation 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

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

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

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

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  

Setup for He spectroscopy at 51 nm regen amp. 2 mJ power amp. 2 x 200 mJ SHG sync. 7 ps osc 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

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

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

Photos of the lab

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

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

Harmonic generation double pulses In ~772nm Harm. :  (nm) 7 : : : : : 51

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

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

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

Photo He experiment

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, (R) (2006)

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+ 51 nm