5th Institute of Physics

News From The Lab

Our broad research in the field of atomic physics  not only to gain fundamental knowledge, but also to aim for technological application such as a single photon source or a high performance gas sensor  requires a versatile apparatus of equipment and different laser sources, with wavelengths ranging from deep blue up to the infrared.

Scientists from the 5th Physical Institute have now exploited the availability of these tools in the lab to investigate the phenomenon of optical bistability in a driven ensemble of Rydberg atoms. Two independent experiments with thermal vapors of rubidium and cesium allow new insight into the underlying mechanisms causing such nonlinear behavior. Due to the different properties of the two atomic species, we conclude that the large polarizability of Rydberg states in combination with electric fields of spontaneously ionized Rydberg atoms is the relevant interaction mechanism.

In the case of rubidium, we directly measure the electric field in a bistable situation via two independent EIT-spectroscopies: one addresses the 85Rb isotope in the vapor and drives the optical bistability, while the second is tuned to the 87RB isotope, to simultaneously measure the electric field. In the part with cesium, we make use of the changing sign of the polarizability for different l states. Furthermore, we are able to directly apply electric fields via a specially designed microwave circuitry around the vapor cell.

The combination of both these experiments allows us not only to rule out the prevailing assumption that dipole-dipole interactions are the cause of the observed phenomenon, but also to evince our hypothesis of a charge-induced bistability. However, further theoretical and experimental investigations are necessary in order to refine the microscopic model for the bistability observed in thermal vapor, and to establish a link to superradiance, a phenomenon which is observed in very similar conditions.

Two-species spectroscopy setup

Figure 1: Setup of the rubidium experiment: Two pairs of lasers drive the two isotopes in the same volume.

Driving the bistability also affects the EIT spectroscopy on the other isotope

Figure 2: Rubidium measurements. (a) Example of a two-dimensional map showing the EIT_FP traces against both detunings for Ω477/2π=14 MHz. (b) Cuts along the dashed lines in panel (a). (c) Dependence of the fitted maximum in every row in panel (a) for different Rabi frequencies Ω477 . (d) Center frequency and amplitude of the curves in panel (c), determined via a Gaussian fit (uncertainty within marker size). The center frequency (blue dots) represents the shift in the EITOB scheme, while the amplitude (red squares) illustrates the shift in the EITFP scheme.

RF-modulation in cesium

Figure 3: Setup of the cesium experiment with the microwave circuitry.

Different l-states and external fields alter the bistability significantly

Figure 4: Cesium measurements. The transmission spectra are significantly altered by changing the l-state from 23D to 28S (a, b), or external RF-modulation with increasing frequency (c - h).

More details about this topic can be found in our recently published paper:

D. Weller, A. Urvoy, A. Rico, R. Löw, and H. Kübler: Phys. Rev. A 94, 063820 – Published 8 December 2016

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