Direct two-photon resonant excitation and absolute frequency measurement of cesium transitions using a femtosecond comb Vela Mbele ∗‡ , Jason E. Stalnaker ∗ , Vladislav Gerginov † , Tara M. Fortier ∗ , Scott A. Diddams ∗ , Leo Hollberg ∗§ and Carol E. Tanner † ∗ National Institute of Standards and Technology, 325 Broadway, MS847, Boulder, CO 80305, USA † Department of Physics, University of Notre Dame, Notre Dame, IN 46556-5670, USA ‡ National Metrology Institute of South Africa, P.O. Box 395, Pretoria, 0001, and School of Physics, University of the Witwatersrand, Private Bag 3, Wits, 2050, GAUTENG, RSA § hollberg@boulder.nist.gov Abstract— We measure the optical transition frequencies of the 6s 2 S 1/2 → 8s 2 S 1/2 , 9s 2 S 1/2 , and 7d 2 D 3/2,5/2 transitions in a 133 Cs vapor cell, with an uncertainty < 100 kHz using a femtosecond laser frequency comb. Recently, optical frequency metrology using stabilized mode-locked lasers has resulted in the development of new spectroscopic techniques and improved frequency measure- ments (see e.g. Stowe et al. [1]). While many of those experiments have been performed with rather complicated ap- paratus including MOT’s and precision atomic beam sources, we report a relatively simple experimental approach using a frequency comb, to excite a multitude of two-photon tran- sitions in a cesium vapor cell at room temperature. Similar experiments have also recently been reported by Fendel et al. [2]. Using this simple apparatus we obtain absolute transition frequencies with uncertainties < 100 kHz, and make marked improvements on the measurement of hyperfine coupling constants. Our apparatus, shown in Fig. 1, is comprised of ×2 f-to-2f Interferometer ν 2ν Filter 550 nm HR (650 – 1070 nm) Solid State 8W pump Synthesizer H-Maser f rep f o Mode Locked Ti:Sapphire Laser PMT Cs Cell Filter 1 Filter 2 Filter 455 nm Cs Setup Computer AOM PMT Fig. 1. The experimental set-up for the cesium two-photon comb spectroscopy. a cesium vapor cell and a mode-locked femtosecond ring 852 nm 894 nm 698 nm 672 nm 794 nm 761 nm 659 nm 697 nm 455 nm 7d 2 D 3/2 7d 2 D 5/2 6p 2 P 3/2 6p 2 P 1/2 7p 2 P 3/2 6s 2 S 1/2 8s 2 S 1/2 9s 2 S 1/2 Fig. 2. The energy level diagram (not drawn to scale) shows the excitation schemes used in this work. Solid lines indicate electric dipole excitation steps, and the dashed and broken lines the decay and the observed fluorescence, respectively. Not shown is the hyperfine splitting of the energy levels. cavity laser based on a Ti:sapphire crystal pumped by a frequency doubled Nd : YVO 4 , at 532 nm. Each of the ∼ 10 5 , discrete, optical modes in the octave spanning output radiation of the laser is well described by ν n = nf rep + f 0 [3], where f rep and f 0 are the repetition rate of the laser and the carrier envelope offset frequencies, and n an integer mode identifier. The laser offset frequency is stabilized using the well known self-referencing technique employing a standard f -2f interferometer [4]. The repetition rate of this laser, which is about 1 GHz, is stabilized to a synthesized RF signal using a piezo-electric transducer to control the cavity length. The synthesizer is referenced to a hydrogen maser, allowing for the frequency of each comb mode to be determined with fractional uncertainty ∼ 2 ×10 -13 . Light from the laser, spanning 600 to 1000 nm, is split by a non-polarizing beam-splitting cube. The beams are then counter-propagated and focussed by f = 15 cm lenses to, the cesium vapor cell. Figure 2 summarizes the atomic levels studied in this work, showing the corresponding excitation pathways, and the fluorescence detection channel 147 MC1.3 9:45 AM – 10:00 AM U.S. Government work not protected by U.S. Copyright