1. 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 VUV Frequency combs Ohio State University 2010 Phase-coherent synthesis of the electromagnetic spectrum

2. VUV metrology and quantum optics with frequency combs Precision test of fundamental physics Simple atoms = Tough wavelengths EUV atomic clocks? Shorter is better. (1s 2 ) 1 S (1s2s) 1 S, 3 S (1s2p) 1 P, 3 P (1snp) 1 P, 3 P 58 nm He

3. 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.

4. Frequency Comb Technology in the VUV, XUV? An infinite train of identical VUV pulses and arbitrary wavelengths in a compact setup. 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 >1 Kilowatt precision frequency comb Step 1: Ionization Three step model Step 2: Field Reversal Step 3: Recombination U r U r U r

5. Cavity-based coherent pulse buildup Frequency Domain Frequency comb Cavity modes Jones et al., Phys. Rev. A 69, 051803 (R) (2004) Time Domain Jones & Ye, Opt. Lett. 27, 1848 (2002) Cavity enhancement: Linear response Preserves coherence Power enhancement

6. Intra-cavity HHG at 100 MHz C. Gohle, et. al., Nature 436, 234 (2005). R. J. Jones et. al., Phys. Rev. Lett. 94, 193201 (2005).

7. Challenges to overcome Relatively low harmonic orders Relatively low power Power scaling? Can we maintain a linear response for the cavity? Brewster plate output couplers introduce linear and nonlinear dispersion R. J. Jones et. al., Phys. Rev. Lett. 94, 193201 (2005). C. Gohle, et. al., Nature 436, 234 (2005).

8. Low dispersion high reflector at 1070 nm Etching adds birefringence but nearly no additional loss or dispersion fundamental nearly unaffected by subwavelength grating Operation in XUV (50-150 nm) SiO2 fresnel reflection: ~45% Overall grating efficiency: ~10% Diffracted Harmonics Xenon jet HHG 70 degree Incidence Power Scalability! HHG output-coupling via a Small-Period Diffraction Grating D. C. Yost et. al. Optics letters 33, 1099 (2008)

9. XUV frequency combs with grating OC and Yb similariton frequency combs Phosphor screen (Sodium salicylate) Yb laser offered 10 W of 1070 nm light Power increase of nearly 10 4 at 60 nm Highest HHG orders ever produced at ~100 MHz rep rates Power scaling  greatly increased harmonic power 5th791113151719 10  W - 50 nW per harmonic 21 Yost, Schibli, Ye, Opt. Lett. 33, 1099 (2008) 214 nm 153nm 119 97 82 71 63 56 51 nm

10. 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 COMBS- Axel Ruehl

11. 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

12. Testing the high harmonic coherence Interfere pulses n and n+1 (7 th harmonic): demonstrate a coherence length 100,000X longer than in previous work L. Xu et al., Opt. Lett. 21, 2008 (1996).

13. VUV Coherence Tests Demonstrated pulse-to-pulse coherence at 7 th 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

14. Argon Spectroscopy  5 electric dipole transitions possibly within the 13 th 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

15. 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.

16. 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