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VUV Frequency combs
Phase-coherent synthesis of the electromagnetic spectrum
Dylan Yost, Arman Cingoz, Tom Allison and Jun Ye
JILA, University of Colorado Boulder
Collaboration with Axel Ruehl, Ingmar Hartl and Martin Fermann
IMRA America
Ohio State University
2010
VUV metrology and quantum optics
with frequency combs
He
• Precision test of fundamental physics
•Simple atoms = Tough wavelengths
•EUV atomic clocks?
Shorter is better.
(1snp) 1P, 3P
(1s2s) 1S, 3S
(1s2p) 1P, 3P
58 nm
(1s2) 1S
Motivation for VUV
frequency combs
• VUV monochromator (5
meters long gets frequency
resolution of 10-5).
• VUV frequency comb could
offer frequency resolution of
10-9 -10-12 at VUV/XUV
wavelengths.
• Cover the whole spectrum
and utilize automatic
calibration.
• Compact table-top source.
Frequency Comb Technology
in the VUV, XUV?
• An infinite train of identical
VUV pulses and arbitrary
wavelengths in a compact
setup.
Three step
model
Step 1: Ionization
U
E
r
• High harmonic generation for
frequency conversion (convert
1070 nm comb to XUV).
• Power requirements for XUV
comb generation: ~100 MHz
repetition rate and 10 J pulse
energy
Step 2: Field
Reversal
U
Step 3:
Recombination
U
E
r
r
>1 Kilowatt precision frequency comb
Cavity-based coherent pulse buildup
Time Domain
Jones & Ye, Opt. Lett. 27, 1848 (2002)
Tin
• Linear response
• Preserves coherence
• Power enhancement
Frequency Domain
Cavity enhancement:
4T
F
N = 2 = 4T  
L
 2π 
2
Jones et al., Phys. Rev. A 69, 051803 (R) (2004)
Cavity modes
Frequency comb
Intra-cavity HHG at 100 MHz
R. J. Jones et. al., Phys. Rev. Lett. 94, 193201 (2005).
C. Gohle, et. al., Nature 436, 234 (2005).
Challenges to overcome
•Relatively low harmonic orders
•Relatively low power
•Power scaling?
•Can we maintain a linear response for
the cavity?
C. Gohle, et. al., Nature 436, 234 (2005).
•Brewster plate output couplers
introduce linear and nonlinear dispersion
R. J. Jones et. al., Phys. Rev. Lett. 94, 193201 (2005).
HHG output-coupling via
a Small-Period Diffraction Grating
Low dispersion high reflector at 1070 nm
Etching adds birefringence but nearly no
additional loss or dispersion
fundamental nearly unaffected by
subwavelength grating
Diffracted Harmonics
Operation in XUV (50-150 nm)
SiO2 fresnel reflection: ~45%
Overall grating efficiency: ~10%
Power Scalability!
70 degree
Incidence
D. C. Yost et. al. Optics letters 33, 1099 (2008)
HHG
Xenon jet
XUV frequency combs with grating OC
and Yb similariton frequency combs
• Yb laser offered 10 W of 1070 nm light
• Power increase of nearly 104 at 60 nm
• Highest HHG orders ever produced at ~100 MHz rep
rates
• Power scaling  greatly increased harmonic power
Phosphor screen (Sodium salicylate)
Yost, Schibli, Ye, Opt. Lett. 33, 1099 (2008)
214 nm
153nm
5th
7
119
9
10 W - 50 nW per harmonic
97
11
82
13
71 63
56 51 nm
15 17 19 21
New Yb Fiber System



~120 fs pulses, 154 MHz repetition rate with 80 W
average power
Requires record-level pulse stretch/compression rates
of ~4000
Full high bandwidth control over fceo and frep
See SCALING OF YB-FIBER FREQUENCY COMBSAxel Ruehl
Preliminary Results
•
5 kW of average intracavity power with HHG output coupler
demonstrated
•
~10 times improvement in harmonic yield
•
Currently limited by mirror coatings grating
•
Higher damage coatings, new grating design and new cavity
geometry under development
Testing the high harmonic coherence
L. Xu et al., Opt. Lett. 21, 2008 (1996).
Interfere pulses n and n+1 (7th harmonic):
demonstrate a coherence length 100,000X longer than in previous work
VUV Coherence Tests
Demonstrated pulse-to-pulse
coherence at 7th harmonic.
-Coherence time lower
bound of 15 ns (20 MHz
linewidth measurement
limited by shot noise).
•Shows pulse to pulse
coherence (frequency comb),
but is not a very sensitive
test of phase noise
Argon Spectroscopy
5 electric dipole transitions possibly within the 13th Harm.
Bandwidth (2 probable)


Natural linewidths 10 – 20 MHz
Comb spectroscopy should allow for absolute frequency
determination at the MHz level


Will be the most sensitive test of frequency comb coherence
Frequency Resolved
Spectroscopy
•
Absolute Frequency Determination by optical lock of IR
frequency comb to Iodine stabilized Nd:YAG
•
Preserve coherence through HHG process by minimizing
cavity lock noise
•
Reduce Doppler widths to ~10 MHz and frequency resolved
spectroscopy through crossed beam geometry.
Outlook
Thanks to:
Axel Ruehl, Ingmar Hartl and Martin Fermann at IMRA America.
Ye group members past and present, especially R. Jason Jones,
Thomas Schibli, Kevin Moll and Mike Thorpe