History of CARL

In 2003 we started at the Universitaet Tuebingen an experiment to study the behavior of atoms in a high-finesse ring cavity. In such a ring cavity light can travel in two counter-propagating directions, which gives rise to a standing light wave in which atoms can be trapped. However, in a ring cavity the phase of the standing wave is free to rotate. For very high finesses, the cavity field decouples from the lasers and becoms extremely sensitive to other coupling mechanisms. For instance, the light scattered from a single atom between the counter-propagating modes can substantially shift the phase. Thus the scattering of light not only kicks the atom, but has also a backaction on the standing wave, in the sense of a phase-shift.

The shift of the optical lattice induced by scattering off an atom is obviously sensed by all the other atoms. Their motion gets thus correlated. This means that kinetic energy can be exchanged between atoms, even if they are located at distant lattice sites.

CARL detected

Our ring cavity consists of two high-reflecting curved mirrors and an input coupler. The transmissions of the mirrors depend on the (linear) polarization of the light modes and are for s-polarization on the order of a few ppm. We measure using the ring-down method a finesse of F = 135000 corresponding to a cavity linewidth of 13 kHz. The round-trip length of the ring-cavity is 8.7 cm, the beam waist at the location of the MOT is about 100 mm. The intracavity power, largely enhanced by the factor F to values around 10 W, gives rise to a deep optical potential at detunings on th order of 1 nm. The ring cavity is driven in one or two directions by a titanium-sapphire laser. The Ti:sapphire laser is locked to one of the eigenfrequencies of the ring cavity using the Pound-Drever-Hall locking technique.

The backaction of atoms on the light stored in a high-Q ring-cavity can have various signatures. One of them is the observation that moving atoms shift the phase of the standing wave by scattering photons between the modes. The scattering can be strong enough to generate a standing wave out of a travelling one. In march 2003 been have able to verify this feature by the following experiment. We loaded about 10⁶ atoms from the magneto-optical trap into our ring-cavity standing wave. If the atoms sit near the antinodes, the photon scattering from one mode into the other is balanced. After 50 ms we switched off one of the counter-propagating modes. The atoms responded to the sudden intensity imbalance of the modes by scattering photons from the full into the empty mode. The recoil transferred by the scattering process accelerated the atoms. This in turn drags the cavity-standing wave. We observed the phase evolution of the standing wave by beating two output beams corresponding to the two counter-propagating modes on a photodiode. We see that the phase of the cavity field is, indeed, dragged by the atoms. The observations are in good qualitative agreement with simulated curves based on a theory by Gangl et al. [Phys. Rev. A 61, 043405 (2000)]. The observations constitute the first unambiguous realization of a correlated atomic recoil laser (CARL) predicted by Bonifacio and coworkers in 1994 [R. Bonifacio and L. de Salvo, Nucl. Instr. Meth. Phys. Res. A 341, 360 (1994)].

The beat signal oscillations essentially reveal the dynamics of the standing wave. But it is also interesting to probe the dynamics of the atoms, i.e. their position and their velocity. The atomic density distribution can be monitored by absorption imaging, the velocity distribution by spectroscopy of recoil-induced resonances. The figure shows the displacement of the center-of-mass of the atomic cloud after 6 ms CARL dynamics. The displacement coincides with the displacement of the standing wave.

First BEC in a cavity

To study CARL in the regime of quantized motion we have set up an experiment, where atoms are Bose-Einstein condensed (BEC) before being transferred into an optical cavity. In october 2006 we were observed collective dynamics of BECs interacting with the two counterpropagating modes of the cavity. This has been the first experiment worldwide with BECs in a cavity. The high finesse of the cavity permitted us reach the strong collective coupling regime.

We observed collective atomic recoil lasing (CARL) and superradiant Rayleigh scattering with ultracold and Bose-Einstein condensed atoms in an optical ring cavity. Both phenomena are based on instabilities evoked by the collective interaction of light with cold atomic gases. Our observation clarifies the link between the two effects. The observation of superradiant behavior with thermal clouds as hot as several tens of uK proves that the phenomena are driven by the cooperative dynamics of the atoms, which is strongly enhanced by the presence of the ring cavity.

Children's corner

Here you can watch the behaviour of a cavity-field coupled to an atom. The cavity is symmetrically pumped. The red dots denote the standing wave light field and the blue dot is the atom.

Watch an atom released in a cavity standing wave.

Watch an atom pinned at a fixed position in a cavity standing wave.

Watch an atom in a cavity standing wave whose one direction is suddenly switched off.

Evaluate our ring cavity parameters.

Two atoms in a cavity standing wave start to move collectively.

Page updated 23.11.2015