Abstract

By utilizing absorption imaging we have observed a controllable, radian-level optical phase shift of scattered light for an isolated atomic ion in free space. A schematic of the experimental apparatus is shown on the left hand side of Fig. 1. A single ¹⁷⁴Yb+ ion is trapped in ultra-high vacuum using a double-needle radio-frequency (RF) quadrupole Paul trap. Laser light at 369.5 nm, near resonance of the ion transition is weakly focused onto the ion to provide an illumination field. The light transmitted past the ion is reimaged onto a cooled CCD camera with a magnification of 585 using a high NA phase Fresnel lens with almost diffraction limited performance [1]. Our data consist of background-subtracted, normalised absorption images obtained for different laser detunings and focusing planes. From these spatial interferograms (as shown in Fig. 1 right hand side for two exemplary detunings) we are able to isolate the scattered part from the illumination part of the light field. The experimental setup and the data taking process are described in more detail in [2]. To provide additional laser cooling, an auxiliary 369.5 nm laser beam, detuned 200 MHz from atomic resonance, is applied perpendicular to the optical axis (not shown in the figure). This allowed us to access the blue side of the transition without losing the ion. Figure 2 shows the phase shift of the scattered wave as a function of the laser detuning. Each data point was obtained from a spatial interferogram as shown in Fig. 1. The measured phase shift is in good agreement with semiclassical theory were the ion is modelled as a damped harmonic oscillator driven by the laser field.

© 2013 IEEE