Absolute frequency measurement of the magnesium intercombination transition 1 S 0 We report on a frequency measurement of the (3s 2 ) 1 S0 → (3s3p) 3 P1 clock transition of 24 Mg on a thermal atomic beam. The intercombination transition has been referenced to a portable primary Cs frequency standard with the help of a femtosecond fiber laser frequency comb. The achieved uncertainty is 2.5 × 10 −12 which corresponds to an increase in accuracy of six orders of magnitude compared to previous results. The measured frequency value permits the calculation of several other optical transitions from 1 S0 to the 3 PJ -level system for 24 Mg, 25 Mg and 26 Mg. We describe in detail the components of our optical frequency standard like the stabilized spectroscopy laser, the atomic beam apparatus used for Ramsey-Bordé interferometry and the frequency comb generator and discuss the uncertainty contributions to our measurement including the first and second order Doppler effect. An upper limit of 3 × 10 −13 in one second for the short term instability of our optical frequency standard was determined by comparison with a GPS disciplined quartz oscillator.
Temperatures below the Doppler limit of 1.9 mK have been observed in magnesium. The strong cooling transition was modified by a coherent two-color excitation exploiting the longer lifetime of an upper level. We developed a theoretical model to describe the light forces and cooling originating from the induced quantum interference in a three-level system. Time-of-flight measurements verified temperatures of 500 K in a onedimensional ͑1D͒ molasses in accordance with our theoretical model. By implementing this scheme in a 3D magneto-optical trap with a single ir beam, temperatures as low as 1 mK could be realized. For ideal conditions we extrapolate to temperatures of 50 K. With cooling times of about 1 ms, a fast and efficient cooling scheme was realized, particularly attractive for optical frequency standards.
Doppler cooling on narrow transitions has become a crucial technique for preparing ultracold samples of alkaline-earth-metal and alkaline-earth-metal-like atoms. For lighter species, such as calcium and magnesium, this technique relies on artificial broadening ͑quenching͒ of the upper level of the narrow line. We report on quenching experiments on a 24 Mg atomic beam. The branching ratio of the ͑3s4s͒ 1 S 0 state was determined to be  = ͑1.33± 0.53͒ ϫ 10 −5 from the measured quenching efficiency. The branching ratio combined with the known linewidth of this state yields a transition rate for ͑3s3p͒ 3 P 1 → ͑3s4s͒ 1 S 0 of ⌫ 23 = 283± 114 s −1 , i.e., one order of magnitude smaller than estimated from semiempirical data. We have applied different numerical approaches, including ab initio relativistic many-body calculations, to compute the transition probabilities of the ͑3s3p͒ 3 P 1 → ͑3s4s͒ 1 S 0 and ͑3s3p͒ 1 P 1 → ͑3s4s͒ 1 S 0 transitions. The results are in good agreement with our experimental observation. With the measured branching ratio, we expect a transfer efficiency of Dopplercooled atoms into a quench magneto-optical trap ͑QuenchMOT͒ of approximately 1% for our experimental parameters. According to our simulations, the transfer efficiency can be increased by one order of magnitude for lower ensemble temperatures as recently demonstrated by two-photon cooling in our uv MOT.
The generation of watt-level cw narrow-linewidth sources at specific deep UV wavelengths corresponding to atomic cooling transitions usually employs external cavityenhanced second-harmonic generation (SHG) of moderate-power visible lasers in birefringent materials. In this work, we investigate a novel approach to cw deep-UV generation by employing the low-loss BBO in a monolithic walkoff-compensating structure [Zondy et al, J. Opt. Soc. Am. B 20 (2003) 1675] to simultaneously enhance the effective nonlinear coefficient while minimizing the UV beam ellipticity under tight focusing. As a preliminary step to cavity-enhanced operation, and in order to apprehend the design difficulties stemming from the extremely low acceptance angle of BBO, we investigate and analyze the single-pass performance of a L c = 8 mm monolithic walk-off compensating structure made of 2 optically-contacted BBO plates cut for type-I critically phase-matched SHG of a cw λ = 570.4nm dye laser. As compared with a bulk crystal of identical length, a sharp UV efficiency enhancement factor of 1.65 has been evidenced with the tandem structure, but at ∼ −1nm from the targeted fundamental wavelength, highlighting the sensitivity of this technique when applied to a highly birefringent material such as BBO. Solutions to angle cut residual errors are identified so as to match accurately more complex periodic-tandem structure performance to any target UV wavelength, opening the prospect for high-power, good beam quality deep UV cw laser sources for atom cooling and trapping.
A novel laser sensor for position measurements of technical solid-state surfaces is proposed. An external Fabry-Perot laser cavity is assembled by use of an antireflection-coated laser diode together with the technical surface. Mode locking results from pumping the laser diode synchronously to the mode spacing of the cavity. The laser cavity length, i.e., the distance to the measurement object, is determined by evaluation of the modulation transfer function of the cavity by means of a phase-locked loop. The mode-locking external-cavity laser sensor incorporates a resonance effect that results in highly resolving position and displacement measurements. More than a factor-of-10 higher resolution than with conventional nonresonant sensing principles is achieved. Results of the displacement measurements of various technical surfaces are reported. Experimental and theoretical investigations are in good agreement.
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