We study the rescattering of photoemitted electrons at aluminium needle tips. For various laser intensities we measure electron energy spectra to identify telltale features of strong-field rescattering, in particular a plateau with near-constant count rate and a high-energy cutoff. This rescattering process is used to investigate the geometry-dependent enhanced optical near-field at the apex of the aluminium tip. A large near-field enhancement, extracted from the spectral data, is supported by 3D finite-difference time-domain simulations. A systematic theoretical investigation of the near-field enhancement at aluminium tips shows a strong dependence on tip opening angle and radius of curvature.

Providing phase stable laser light is important to extend the interrogation time of optical clocks towards many seconds and thus achieve small statistical uncertainties. We report a laser system providing more than 50 µW phase-stabilized UV light at 267.4 nm for an aluminium ion optical clock. The light is generated by frequency-quadrupling a fibre laser at 1069.6 nm in two cascaded non-linear crystals, both in single-pass configuration. In the first stage, a 10 mm long PPLN waveguide crystal converts 1 W fundamental light to more than 0.2 W at 534.8 nm. In the following 50 mm long DKDP crystal, more than 50 µW of light at 267.4 nm are generated. An upper limit for the passive short-term phase stability has been measured by a beat-node measurement with an existing phase-stabilized quadrupling system employing the same source laser. The resulting fractional frequency instability of less than 5×10−17 after 1 s supports lifetime-limited probing of the 27Al+ clock transition, given a sufficiently stable laser source. A further improved stability of the fourth harmonic light is expected through interferometric path length stabilisation of the pump light by back-reflecting it through the entire setup and correcting for frequency deviations. The in-loop error signal indicates an electronically limited instability of 1 × 10−18 at 1 s.

<p>Optical atomic clocks achieve fractional systematic and statistical frequency uncertainties on the order of 10<sup>−18</sup>. This enables novel applications, such as height measurements in relativistic geodesy with ∼ 1 cm resolution for earth monitoring. Towards this goal, we set up a transportable clock based on the <sup>1</sup>S<sub>0</sub> → <sup>3</sup>P<sub>0</sub> transition in <sup>27</sup>Al<sup>+</sup>. A co-trapped <sup>40</sup>Ca<sup>+</sup> ion allows state detection and cooling via quantum logic spectroscopy and sympathetic cooling.<br>We unveil the design and the current status of the transportable apparatus and review the recent development of the laser systems. In particular, we present the clock laser setup emitting at 267.4 nm based on single-pass frequency-quadrupling which allows phase stabilization of the complete path. Furthermore, we show the performance of the fundamental frequency to reach a fractional frequency uncertainty of ~ 10<sup>−16</sup> at 1 s.</p>

No abstract