$5S\text{−}5D$ two-photon transition in rubidium vapor at high densities

The rubidium $5S\text{−}5D$ Doppler-free two-photon transition has been observed in a thermal vapor cell by detecting the infrared radiation emitted during the decay from the $5D$ excited level to the $5P$ intermediate level. Different to the usual detection scheme based on blue light emission this approach does not suffer from reabsorption and still works at high densities. This not only allows for a dramatically enhanced detectable emission rate but also for investigating collisional effects such as collisional broadening and collisional energy transfer. Furthermore, we observe a yet unknown temperature dependence of the resonance linewidth for transitions into the states of the $5D_{\text{5/2}}$ level. Consequences for the design of an optical Rb-frequency standard are discussed.

Figure 1

Two-photon spectroscopy of the $5D$ states of rubidium. Simplified scheme of the relevant levels. The two fine-structure components of the $5D$ level are separated by an energy of $h90\mathrm{GHz}$ and can both be excited with our laser setup ($h$ is Planck's constant). The states of the $5D$ level decay either via $6P$ to the ground state emitting blue light or directly to the $5P$ level emitting infrared light. Wavelengths and decay rates are taken from [10].

Figure 2

Experimental setup. It differs from the standard setup only by the filter set in front of the photomultiplier. For details see text.

Figure 3

Spectrum of the $5D_{\text{5/2}}$ level and the $5D_{\text{3/2}}$ level detected with the blue filter. “2x” laser detuning refers to the difference between twice the laser frequency and the Bohr frequency of the $5S\text{−}5D$ energy difference. The solid black line is a fit with four individual Voigt profiles. The temperature corresponds to the densities where the blue emission has a maximum. At these densities the resonance lines are already broadened in parts by collisions.

Figure 4

Light power emitted by the atoms into the solid angle of detection at various atom densities. The laser drives transitions from $5S_{\text{1/2}}$, $F=2$ to $5D_{\text{5/2}}$, $F=4$ and to $5D_{\text{3/2}}$, $F=2$. The wavelengths in brackets denote the filters used for detecting the emission. The third infrared curve labeled “coll.trans.” is recorded by exciting the atoms to the $5D_{\text{5/2}}$, $F=4$ state and detecting the infrared emission at 762 nm. It demonstrates the effect of fine-structure changing collisions. The solid lines of the infrared data are the results of the model described in Secs. 4 and 5. The solid lines of the blue emission are derived from a reabsorption model described in the Appendices for completeness.

Figure 5

Full width at half maximum of the resonances vs density for the resonances of the $5D_{\text{5/2}}$ multiplet with $F=4$, 3 and the resonance of the $5D_{\text{3/2}}$ multiplet with $F=2$. The solid lines are obtained by fitting Eq. (2) to the experimental data.

Figure 6

Ratio of the observed emitted power at 762 and 776 nm for optical excitation into $5D_{\text{5/2}}$, $F=3$ and 4. The solid lines are fits according to Eq. (15).