Molding the flow of light: photonic bands in optical lattices

Abstract

The topic of this research project are optical lattices and their application for creating new physical systems with uncharted features. In particular, novel lattice geometries yielding the promise of an almost unlimited malleability of their photonic band structure will be explored. Optical lattices are cold and dilute atomic gases structured into periodic arrangements by means of the spatially modulated dipole force exerted by a set of overlapping laser beams. Optical lattices are characterized by an intrinsically perfect periodicity, a lattice constant compatible with optical wavelengths, and realtime tunability.Because of their periodic structure, optical lattices bear many analogies with crystalline or metallic solids. However, they also share far-reaching analogies with other periodic materials, such as photonic crystals, which represent a class of dielectric materials characterized by spatially periodic variations of the refraction index. Photonic crystals yield the promise of many technological applications in the domains of thin-film optics, of all-optical telecommunication as well as quantum communication, or of metamaterials for superlenses and cloaking devices. Despite impressive progress made in fabricating photonic crystals, they suffer from fundamental difficulties in providing the required fidelity over long ranges due to fluctuations in the position and size of the building blocks. This disorder disturbs those properties of photonic crystals based on global interference: It reduces the Bragg reflectivity, extinguishes the transmitted light, and ultimately destroys the photonic band gap.Our goal is to demonstrate that it is possible to realize photonic band gap structures in optical lattices, and that these represent a viable alternative to photonic crystals. The essential feature of photonic band gaps is to allow for an almost unlimited tailoring of the density of electromagnetic modes. The accessibility of optical lattices to real time manipulations, distinguishes them from photonic crystals, whose structure is fixed by their design. The hope to be able one day to confine light around localized elements perturbing the periodic order feeds speculations on the feasibility of photonic registers, switches or even computers.While until now only simple cubic optical lattices have been studied, the realization of omnidirectional band gaps is conditioned to the availability of particular lattice structures, such as diamond lattices. This is only possible with atomic species with specific level structures, such as is the case for ytterbium or strontium. Our experimental efforts will thus start with the allocation of an ultracold strontium gas. The gas will be cooled to quantum degeneracy, loaded into an optical lattice, and then brought into a Mott insulating state, which is characterized by perfectly well-defined occupation number of the lattice sites. After this has been achieved in a standard cubic lattice, a diamond-shaped lattice geometry will be tested, and signatures for photonic band gaps will be quested. (AU)