Optical lattices - crystalline structures bound by light

Theodor W. Hänsch

D. R. Meacher

In an optical lattice the interaction of an atomic gas with a spatially modulated light field is exploited to damp the atomic motion and then to trap the slowly moving atoms on a regular array of points associated with particular local states of the light field. The result of this process is the formation, from the initially cold but disordered gas, of a structure reminiscent of those found in solid-state crystalline materials. The study of these novel, optically-bound media necessitates the introduction to the description of the atom-light interaction of concepts more usually encountered in solid-state physics and promises to shed new light on the nature of atom-light interactions. Additionally, it provides a new means for the precise and highly parallel manipulation of atoms and a novel test-bed for some fundamental problems in physics.

Matthias Weidemüller, Axel Görlitz, Theodor W. Hänsch +1 more

We explore Bragg diffraction from atomic lattices bound by light as a diagnostic tool for studying properties of optical lattices not accessible so far. A weak laser beam at a wavelength of about half the wavelength of the lattice field is diffracted from the (100) and (130) lattice planes of a Rb optical lattice. The observation of well-defined Bragg spots confirms the long-range order in optical lattices. From the acceptance angle for Bragg diffraction we deduce the range over which crystalline order is preserved. The comparison of two Bragg spots diffracted from different lattice planes allows us to directly measure the position spread of the atomic wave packets oscillating in the light-induced potential wells. By combining conventional probe transmission spectroscopy with Bragg diffraction we study the motion of atoms that are deeply bound inside the potential wells. We show experimentally and theoretically how backaction of the bound atoms on the trapping field influences the lattice constant. We develop a model for the modification of the refractive index by the highly ordered atomic medium. Based on first-order scattering theory the model explicitly includes the influence of atomic localization on the resulting lattice constant. By measuring the change of the Bragg angle, we observe a decrease of the lattice plane separation for increasing atomic density. We report experimental evidence for the correction to the refractive index due to the finite position spread of the atoms.

A. Smerzi, A. Trombettoni

M. Inguscio, L. Fallani

This chapter discusses the physics of ultracold atoms trapped in optical lattices, i.e., ordered arrays of microscopic traps produced by the interference of counterpropagating laser beams. The motion of the atoms in these ‘crystals of light’ can be described in terms of a periodic potential, similar to the one experienced by the electrons of an ideal crystalline solid. The chapter focuses on the physics of quantum transport in optical lattices, which both provides a testing ground for ideal solid-state physics and constitutes an important resource for the determination of fundamental constants (e.g., the fine structure constant) and for the use of ultracold atoms as very precise sensors of forces (e.g., gravity).