Absorption Enhancement of an Electric Quadrupole Transition of Cesium Atoms in an Evanescent Field

We have observed for the first time reflection spectra of an electric quadrupole transition for the cesium atom ($6{}^{2}S_{1/2}\mathrm{\text{−}}5{}^{2}D_{5/2}$) line at an angle of incidence from ${\theta }_c-11.9$ to ${\theta }_c+107.5\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$, where ${\theta }_c$ is the critical angle for total reflection. From a comparison with the calculated absorption in the attenuated total reflection, the oscillator strengths for $s$ and $p$ polarizations were found to increase with an increase in the angle of incidence by a factor up to 1.5 at ${\theta }_c+83.8\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$ and 2.4 at ${\theta }_c+107.5\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$, respectively, in the experiment. The dependences of the observed enhancement on the angle of incidence were in good agreement with the calculated ones for the oscillator strength of the quadrupole transition in the evanescent light.

Figure 1

(a) The experimental setup: LD, laser diode; FPI, Fabry-Pérot interferometer as a frequency marker; GTP, Glan-Thompson prism; PD, photodiode; FG, function generator; DAC, digital-analog converter; BS, beam splitter. (b) Energy level diagram of the Cs electric quadrupole transition ($6{}^{2}S_{1/2}\mathrm{\text{−}}5{}^{2}D_{5/2}$) with hyperfine structure splitting.

Figure 2

Reflection spectra obtained in the experiment (solid line). The left and right figures are for $p$ and $s$ polarizations, respectively. The dashed line in each figure is the calculated one, in which the oscillator strength is assumed to be constant. The angular detuning is shown in each figure.

Figure 3

Dependence of the signal amplitude on the angular detuning from the critical angle. The open circles and the closed triangles are the experimental results. The solid and dashed lines are the calculated results, where the oscillator strength is assumed to be constant.

Figure 4

The ratio of the signal amplitudes between the experiment and the calculation, as a function of the angular detuning from the critical angle. In the calculation, the oscillator strength is assumed to be constant. The solid and dashed lines show $g_p({\theta }_i)$ and $g_s({\theta }_i)$, respectively. In the inset $g_p({\theta }_i)$ and $g_s({\theta }_i)$ are shown up to $\delta =+814\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$, which corresponds to ${\theta }_i=\pi /2\mathrm{r}\mathrm{a}\mathrm{d}$.