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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