(Towards) Extreme Ultraviolet Frequency Comb Spectroscopy of Helium and Helium+ Ions

VU University, Netherlands

Kjeld Eikema

€ from

ECT* 28 September – 2 november 2012 "Proton size conundrum"

Jonas Morgenweg, Itan Barmes, Tjeerd PinkertDominik Kandula, Chirstoph Gohle,

Anne Lisa Wolf, Stefan Witte

Outline

Introduction To the XUV (at 51-85 nm) with frequency combs: Two-pulse “Ramsey Comb” excitation. The next generation: “Ramsey-Fourier-Frequency Comb” First signal with two-pulse two-photon excitation in Rb Summary en Outlook

Frequency comb laser activities @ LaserLaB

XUV comb metrology, QED tests:Jonas Morgenweg, Tjeerd Pinkert, Itan Barmes, Dominik Kandula, Christoph Gohle

Coherent control: Itan Barmes, Stefan Witte

Precision spectroscopy on ions:Anne Lisa Wolf, Jonas Morgenweg,Wim Ubachs, Steven v.d. Berg

Dual-comb spectroscopy &mid-IR combs: Axel Ruehl, AlissioGambetta, Marco Marangoni et al.

Precision dissemination over fiber: Tjeerd Pinkert, Jeroen Koelemeij

Precision frequency comb calibrations, collaborations with: Wim Ubachs, Jeroen Koelemeij, Rick Bethlem, and others.Wim Vassen: He 2 3S1 - 2 1S0 transition, Science 333, 196-198 (2011)

15

51 nm = 6 PHz

100µm

Introduction QED / Ry issues?

Hydrogen

1S

2S

243 nm

243 nm

Muonic-hydrogen

1S

2S

2 keV…

6 m

C.G. Parthey et al., PRL 107, 203001 (2011)

R. Pohl et al,Nature, vol. 466, pp. 213-216 (2010).

2P

1S-2S: 2 466 061 413 187 035 (10) Hz

Comparison normal vs. muonic matter

1S

2S

Ener

gy

2x 243 nm 1S

2S

2x 60 nm

He+H H

Ener

gy

He+

6 mm

0.6 nm(2 keV)

1S

2S2P

812 nm

0.15 nm(8.2 keV)

1S

2S

2P

The 1S-2S values for H and H

Theory

Experiment

1S-2S

L1S-2S

1S-2S

B60+B7i

L(R∞)1S-2S

Rel. acc. L1S-2S

Finite size(nucl. pol.)

Rel. L(R∞)

Z2

Z≥3.7

Z4r2

Z≥6

Z2

9.869...×1012

93 856 127(348)*62 079(295)

-543(185)(40 or 15**)

3.7 ppm****

not measured650.7 ppm

2.466...×1012

7 127 887(44)1102(44)

-8(3)(2)

6.3 ppm

246606143187.035(10)162.2 ppm

H (kHz) He+ (kHz)Scaling

Test B60+B7i 25% 7% or 4%***

1S-2S Z4 83×10-31.3×10-3

H 1S-2S from C.G. Parthey et al., PRL 107, 203001 (2011)

The experimental challenge

Hydrogen

1S

2S2x 243 nm

Helium+

1S

2S

2x 60 nm

Helium

1s2

1s2s

2x 120 nm51 nm

1s5p<30 nm

Frequency comb lasers

T

Frequency Comb laser

Mode-locked laser

Nobel Prize Physics 2005

J. Hall T.W. HänschR.J. Glauber

Frequency comb lasers

frequency

Int.

0

fn = f0 + n.frep

frep = 1/Tf0 = frep x φCE / 2

time

φCE= /2φCE= 0

vg ≠ vφ

T

R. Holzwarth et al. PRL 85, 2264 (2000), D.J. Jones et al. Science 288, 635 (2000)

Frequency comb lasers as optical rulers

T

Experimentbeat note measurement;f = f0 + n frep + fbeat

Single-modelaser freq. f

Frequency Comb laser

Direct frequency comb excitation

T

Experiment

Experiment

beat note measurement;f = f0 + n frep + fbeat

Single-modelaser freq. f

Frequency Comb laser

Upconversion through high-harmonic generation

IR pulses XUV (<100 nm)

1014 W/cm2

NIR (800 nm) DUV

frequency

VUV XUV X-RAYharmonicconversion

UV

Noble gas jet

3rd 5th 7th 9th 333rd…

High-harmonic generation (HHG)

Corkum & Krausz, Nature Physics 3, 381 (2007)

IR ~1014 W/cm2

Phase coherence of HHG

Other experiments: E.g. excitation Kr continuum @ 88 nm, delays~ 100 fs – ps rangeCavalieri et al., PRL 13, 133002 (2002)A. Pirri et al. PRA 78, 043410 (2008) and more

Phase XUV ~ - IIR

Frequency comb up-conversion

IR

DUV

frequency

VUV XUV

X-RAY

harmonicconversion

XUV

UV

fn = f0 + n frep fn = p f0 + m frep

pth harmonic:Near-Infrared:

XUV comb generation methods

HHG in resonator(MPQ, JILA, Arizona, etc.)

C. Gohle et al. Nature 436, 234 2005 A. Ozawa et al., PRL 100, 253901 (2008) R.J. Jones et al. PRL 94, 193201 (2005)I.Hartl et al. Opt. Lett. 32, 2870 (2007),Etc.

A. Cingoz et al., Nature 842, 68 (2012)Argon spectroscopy at 82 nm, 3 MHz acc.

HHG after amplification(LaserLaB Amsterdam)

S. Witte et al. Science 30, 400 (2005)Zinkstok et al. PRA 73, 061801(R) (2006)T.J. Pinkert et al. OL 36, 2026 (2011)(argon 85 nm, neon 60 nm, helium 51 nm)D. Kandula et al. PRA 84, 062512 (2011)

D. Kandula et al. PRL 105, 063001 (2010)Helium spectroscopy at 51 nm, 6 MHz acc.

Frequency comb lasers – infinite pulse train

frequency

Int.

0

fn = f0 + n.frep

frep = 1/Tf0 = frep x φCE / 2

time

φCE= /2φCE= 0

vg ≠ vφ

T

Frequency comb lasers - two pulses

frequency

Int.

0

fn = f0 + n.frep

frep = 1/Tf0 = frep x φCE / 2

time

φCE= /2φCE= 0

vg ≠ vφ

T

p, kp

pump

idler

signalc(2)

fluorescence cone

seeds, ks

sp

i

i = p – s

Parametric chirped pulse amplification

BBO crystals pumped by 532 nm at intensities of 7 GW/cm2

Tuning over 700-1000 nm with little effort Bandwidth adjustable from 300 nm to 5 nm No memory effect Two comb pulses amplified by two synchronized equal pump pulses; microradian pointing sensitivity!

HHG of two pulses

IR pulses – mJ level XUV pulses

few nJ levelDivergence <2 mrad

<1014 W/cm2

f=50 cm

IXUV ~ IIR9

At the 15th harmonic to excite He

15

HHG

15

fCE ~ /3006 MHz

6.6 nsHelium

1S2

1S2S

51.6 nm

1S5P

(51 nm)

Principle and setup schematic overview

Ramsey comb excitation of helium at 51 nm

Contrast up to 60% at higher rep-rate: 50 as jitter

121 MHz

D. Kandula et al. PRL 105, 063001 (2010)D. Kandula et al. PRA 84, 062512 (2011)

He ground state measurement systematics

IR phase shifts in OPA Pulse phase front tilt Spectral/temporal phase difference between pulses Doppler shift: varying speed using He, He/Ne, He/Ar & tune angle Ionization: varying density, pulse intensity ratio Adiabatic shift in HHG shift in HHG due to excitation AC Stark shifts (IR, XUV, ioniz. l.), DC Stark (field free) Self-phase modulation (pulse ratio). Recoil shift 18.5 MHz for 5p Many more efects!:

Tests of chirp, HHG focus position, f0, out-of-centre phase, ....etc.

Theory and experiment

blue = experiments and red = theory

DrakePachucki

S. Bergeson et al.Eikema et al.

Accuracy 6 MHz (8 fold improvement)

History of the He ground state energy accuracy

Blue = experimentRed = theory

Tunable XUV Ramsey frequency comb

Argon Neon Helium

~84 nm(9th harm. in Xe)

~51 nm(15th harm. in Kr)

~60 nm(13th harm. in Kr)

T.J. Pinkert et al. OL 36, 2026 (2011)

View of the lab

HHG and excitation apparatus

Apparent two-pulse limitation: T and s

Signal = cos(tr T – s)

T

sAn error in s gives a frequency

error in tr of tr = s / T

The magic of Fourier transformation

Signal = cos(tr T – s)

T

s

FFT

tr

The magic of Fourier transformation

Signal = cos(tr T – s)

T

s

FFT

tr

T

s

2T

FFT

tr

rep

rep = 2/T

s

FC-FTS vs. FC Full-rep-rate excitation

T

s

2T

FFT

tr

rep

rep = 2/T

T 2Ttr

rep

rep = 2/T

Full rep. rate coherent addition

Two-pulse incoherent addition

s

s s s

AC-Stark shift

T

Stark

2T

FFT

tr

rep

rep = 2/T

T 2Ttr

rep

rep = 2/T

Full rep. rate coherent addition

Two-pulse incoherent addition

Stark Stark

Stark

Stark

FC-FTS features and requirements

Accuracy and resolution equivalent to full-rep rate excitation FC-FFT more easily tunable at extreme wavelengths AC-Stark shift 'free', in contrast to full-rep rate excitation With more than 1 transition: ’interference’ effects, but possible

to model. Simulations for T=160 ns show ~10 kHz accuracy

Requirements:

IR FC pulses must be amplified with constant phase and energy, ideally < /1000 and <1% energy fluctuation,irrespective of the time delay between the pulses!

The old situation with a delay line

D. Kandula et al., Opt. Express, 16, 7071-7082 (2008)

power amp.2 x 200 mJ

SHGsync.

7 ps osc.1064 nm

comb laser ~780 nm

frep=150 MHz 2x 2 mJ200 fs @ 30 Hz

Relay imaging

3-stageNOPCPA

regen amp.2 mJ

The old situation with a delay line

SHGsync.

comb laser ~780 nm

frep=125 MHz

3-stageNOPCPA

The new system

SHGsync.

comb laser ~780 nm

frep=125 MHz 2x 2 mJ 10-200 fs, T=8 ns to >10 srep. rate 300 Hz

3-stageNOPCPA

10 ps osc.1064 nm

power amp.2 x 120 mJ

'bounce' amp2 x 1 mJEOM/AOM

pulse picking & scaling

T

J. Morgenweg and K.S.E. Eikema, OL 37, 208 (2012)J. Morgenweg and K.S.E. Eikema, Las. Phys. Lett. 5, 1 (2012)

New pump laser front-end

t = 8 ns to >10 sRep. rate < 1 kHz

30 Hz rep. rate (300 Hz in prep.)

2x 1 mJ

2x 100 mJ

(Bounce) amplifier performance

(n=7; Tosc ~ 8 ns)

(n=35)

(n=160)

Amplified comb pulses - phase measurement

Single shot differential spectral interferometry

Single shot SNR sufficient for 10-20 mrad rms stability

Inaccuracy < 5mrad

BS 50 %

BS10%

OPCPA

NG

CCD Camera

Oscillator

BS 5%

PC

PC

HHG Spectr.

MZ-Interferometer

Pulse separation

Analysis

SP-FilterFiber

Phase of the amplified comb pulses

Average phase equal within 5 mrad (/1200) and pulse energy equal within 1% for a 2-pulse delay (16 ns) to 32-pulse delay (256 ns)

Laser shots

Delay

Doppler-’free’ comb excitation in Rb

CW lasers: Hansch et al., OC 11, 1 (1974)

Nanosecond pulses: Biraben et al., PRL 32, 12 (1974)

Picosecond pulses: Fendel et al., OL 32, 6 (2007)

Femtosecond pulses, and withspatial coherent control: I. Barmes et al., to be published in Nature Photonics

V-shaped phase

100µm

100µm

Transform-limited

Elimination of background by coherent control

Two-photon two-pulse Fourier-Ramsey-Comb Excitation of Rb (5S – 7S)

Inter-pulse delay T in femtoseconds

n=2; T=15.9 ns

n=4; T=31.8 ns

n=2; T=63.5 ns

Fluo

resc

ence

sig

nal

Summary and Outlook XUV frequency comb metrology demonstrated, 85-50 nm Ground state energy of helium with an accuracy of 6 MHz Fourier-Ramsey Frequency Comb Excitation with 2 pulses New system and first spectroscopy in Rb shown Few mrad variation over 256 ns; potentially <kHz XUV accuracy

Outlook: He+ in an ion trap with Be+ cooling for 1S-2S spectr., Two-photon (2*120 nm) in Helium with coherent control, Two-photon in H2 to improve ionization potential to <100 kHz Delay extension up to 100’s of s for Hz-level accuracy?

D. Kandula et al. PRL 105, 063001 (2010)D. Kandula et al. PRA 84, 062512 (2011)

T.J. Pinkert et al. Opt. Lett. 36, 2026 (2011) J. Morgenweg et al. Opt. Lett. 37, 208 (2011)

I. Barmes et al., Nature Photonics, to be published

Ions in a Paul trap

The people

Itan Barmes

Tjeerd Pinkert

JonasMorgenweg

WimUbachs

ChristophGohle

RoelZinkstok

AxelRuehl

StefanWitte

DominikKandula

AmandineRenault

Anne LisaWolf

From here on additional slides

Coherent control for Doppler-reduced excitation

Full spatial coherent control

Spatial and atom selective excitation

To be published in Nature Photonics

Enhanced signal to noise

15 kHz absoluteaccuracy

Abs. calibration2.6x betterthan previousmeasurements

Hyperfine &Isotope shiftup to 10xbetter than before

Flat phase

V-phase85Rb(3-3)

87Rb(1-1)

85Rb(2-2)

87Rb(2-2)

Full measurement series, frep=121.5 MHz

PureHe

Neseeded

Arseeded

OPA: Pump laser influence on phase

Requirements:

– Pump pulse wave fronts equal to </20– Pump pulse intensity equal within few %

Then:– Comb amplified wave fronts equal to /300

dLconversionkz ss '')0()(

k = kp – ks - ki

2-pulse ‘Ramsey’ comb principle

Phase coherent pulse excitation: Ramsey (1949), Hänsch and coworkers (1976/77), Chebotayev et al. (1976), Snadden et al. (1996), Bellini et al. (1997, 1998), ....

Time domain Frequency domain

T = 1/T

t

t

He ground state ionization energy

Theory position (Pachucki, PRA 2010)

Direct comb spectroscopy in Ca+ for a search of variation

• Dipole allowed: 4s 2S1/2-4p 2P1/2 or 3/2

• Forbidden: 4s 2S1/2-3d 2D5/2

• All wavelengths of interest accessible with frequency comb at full repetition rate: equilibrium!

Ca+ ion

Relative to: 411 042 129 776 393.2 (1.0) Hz as measured byChwalla et al, PRL 102, 023002 (2009)

Lifetime 3d 2D5/2 = 1 second

Comb excitation of the 729 nm clock transition

A.L. Wolf et al., Opt. Lett. 36, 49 (2011)