Research

In an optical cavity, the bandwidth of supported frequencies and the intensity buildup are inversely proportional. Thus, higher buildup is only possible at the cost of a reduced available frequency range, and this limits possible applications such as in gravitational wave detection. To overcome this limitation, the concept of a white-light cavity was developed, in which the bandwidth of the cavity is enhanced via a mediumwith negative dispersion inside the cavity. The negative dispersion leads to a frequency-dependent phase compensation that effectively renders a wider range of frequencies resonant with the cavity. We have studied a white-light cavity medium based on four-level atoms in double-Λ configuration. This configuration is known to exhibit resonantly enhanced four-wave mixing, such that the spatiotemporal dynamics inside the medium becomes relevant. We perform a full simulation of the propagation of all fields, and find that the probe field dispersion is in addition changed by a coherent field which is generated within the medium via four-wave mixing. Counterintuitively, this in-medium dynamics leads to a further enhancement of the cavity bandwidth.

Media with a negative index of refraction promise a multitude of fascinating applications. Negative refraction can be achieved by coupling both the magnetic and the electric field component of a probe beam to the medium. The challenge is thus to provide a medium with sufficient electric andvmagnetic response at the same frequency. So far, this could be achieved with metamaterials, but theoretical proposals suggest that dense atomic gases could also exhibit a negative index of refraction. We have shown how the dense gas approach can be used to obtain lossless negative refraction in an active, i.e., amplifying dens gas of metastable Neon atoms. A weak incoherent pumping field can be facilitated to reduce absorption or to transfer the system to an active state in a controlled fashion. We have also studied mechanisms to amplify the magnetic response of the atomic medium. The parametric enhancement of magnetic response in chiral media by one inverse power of the finestructure constant could be traced back to the closed-loop structure of the atomic medium, and we could in addition identify a non-parametric enhancement by almost another factor of the inverse finestructure constant.

A particular class of laser-prepared media are so-called closed-loop media, where the applied laser field form a loop, see figure. These systems are known to exhibit a rich spectrum of features, and they are sensitive to the relative phase of the applied fields. They have, however, the unusual property that they do not evolve into a time-independent steady-state unless a particular condition, the so-called multiphoton resonance condition, is fulfilled by the laser detunings. If one of the applied fields is a probe field pulse with a finite frequency width, then this condition cannot be fulfilled for all spectral components of the probe pulse, and a time-dependent analysis is required. To do so, we apply a Floquet decomposition to the equations of motion and identify the different scattering processes contributing to the medium response. We find that the response oscillating in phase with the probe field is not dependent on the relative field phase. The phase dependence arises from a scattering of the coupling fields into the probe field mode at a frequency which in general differs from the probe field frequency. In particular for short pulses with a large frequency width, inducing a closed loop interaction contour may lead to a distortion of the pulse shape via this phase-sensitive scattering. Apart from these general results, we demonstrate that both the closed loop and a corresponding nonclosed loop configuration allow for sub- and superluminal light propagation with small absorption or even gain, where one of the coupling field Rabi frequencies acts as a control parameter that enables one to switch between sub- and superluminal light propagation.

In this project, we consider mechanical effects of the matter-light interaction, with the emphasis on laser cooling. Especially the cooling of trapped ions to the ground state of the trapping potential is a crucial step in the preparation of the medium for many current experiments. Laser cooling usually relies on the fact that the trapped particle acquires a change in momentum during an interaction with a photon. This of course also holds for the spontaneous emission, which for cold particles induces a motion similar to a random walk due to the statistical distribution of the emission directions, and thus gives rise to a finite cooling limit. In order to circumvent this, we propose a scheme where - in addition to the cooling laser field - other coupling laser fields are used to design the absorption spectrum of the atom such that unwanted transitions leading to a heating of the system are suppressed. The scheme makes use of double electromagnetically induced transparency (EIT) in order to allow for a complete suppression of the cooling laser field absorption at certain frequencies. Then, on average and in leading order of the so-called Lamb-Dicke expansion, the trapped particle is only excited together with a decrease in the motional quantum number, such that no unwanted spontaneous emissions can occur. Therefore, the scheme allows to efficiently cool the system to the motional ground state with almost complete ground state occupation. The extension to multiple-EIT allows to cool at different trap frequencies simultaneously. This is of interest e.g. for setups with different axial and radial trap frequencies or for the cooling of ion strings.