Atom Optics with Diffractive Structures

Atom Optics with Diffractive Structures O. Carnal and J. Mlynek Fakultät für Physik, Universität Konstanz, Germany Experiments demonstrating the feasiblity of using nanoscale diffractive structures as optical devices for atoms herald many promising applications of atom optics. Atoms are usually thought of as hard spheres with a well-defined diameter, posi tion and velocity. But at the beginning of this century, Louis de Broglie showed theoreti cally that they possess both particle and wave characteristics: an atom with mass m and velocity V behaves like a wave with wavelength λ = h/ mv, where h is Planck’s constant. The wave nature of atoms was verified experimentally shortly afterwards. The de Broglie wavelength X is very small, being about 0.1 nm for helium atoms with the velocity of 1000 m/s typically found in a gas reservoir at room temperature: it de creases for heavier and faster atoms. An atom can therefore be thought of as a wavepacket of a characteristic size with oscillations inside the packet having a pe riod equal to the de Broglie wavelength. Depending on the experiment being perfor med, either the particle or the wave nature is predominant. The fact that atoms behave like waves implies that one can build, in principle, optical elements for atomic waves instead of light waves. Interest in optical elements to, for exam ple, focus, image and split beams of atoms in the same way as photons arises for main ly two reasons. First, an “atom microscope’’ has the potential to achieve high spatial resolution (in the nanometre region) with minimal damage to the surface under inves tigation since atoms are uncharged and can have kinetic energies below 10 meV. Se cond, an interferometer using atoms instead of light allows one to perform fundamental experiments on quantum mechanics and gravitation which were not possible before. Microscopy The resolution of a microscope is limited by the wavelength of the particles em ployed: it Is impossible to resolve structures smaller than half the wavelength (≈ 0.5 µm Olivier Carnal is on the research staff of the Physics Faculty, University of Konstanz, Post fach 5560, W-7750 Konstanz. He received his diploma in physics from the ETH Zurich in 1988 where he worked with Professor Mlynek before spending a year at the ENS, Paris. He moved to Konstance in 1990 and was awarded his Ph.D. in 1991. Dr. Carnal was awarded the Swiss Physical Society’s 1992 Balzers Prize. Professor Jürgen Mlynek has been with the University of Konstanz since 1990 after spen ding four years as an Assistant Professor at the ETH Zürich. He studied at the University of Hannover where he received his diploma (in 1976), a Ph.D. and his Habilitation (in 1984). He won the German Physical Society’s 1987 Physikpreis and 1992 Leibniz Prize. in the visible). This lower limit can be decreased dramatically using electroma gnetic waves with much shorter wave lengths such as X-rays or electron waves in an electron microscope. The maximum re solution of an electron microscope is com parable to the size of an atom (≈ 0.1-1 nm). But one has to pay a high price for this immense resolution: electrons are charged so specimens soon become charged lea ding to Image distortion. X-rays carry a very large energy so specimens are easily dama ged. The ideal solution would be a micro scope based on particles with both a low energy and a short wavelength. Atoms fulfill these conditions perfectly. For example, a helium atom with V = 1000 m/s and ≈ 0.1 nm has a kinetic energy of roughly 20 meV which is about 1000 times smaller than that of an electron, and 106 times smal ler than that of an X-ray photon with a com parable wavelength. The low energy en sures that the surface being investigated is treated very gently and it reduces the pene tration depth of atoms into the base material to about one atom layer. Only the upper most surface layer is probed with atoms and the usually less Interesting underlying struc tures do not contribute to the image. More over, by preparing atoms so that they carry an electric or magnetic moment, one can selectively probe the electronic and magne tic properties of a surface. In view of the important opportunities for atom microsco py, for surface-sensitive microprobes and even for atom lithography, there is great Interest In building imaging elements for atoms. Interferometry Another fascinating application of optical elements for atoms involves interferome ters. Such devices, also best known In clas sical optics, split an incoming light beam into two spatially separated beams and then recombine the beams. In coherent beam splitting, splitting and recombination have to be performed in such a way that the phase, i.e., the position of maxima and minima of the two waves, is preserved. Changes to the positions of the wave maxima along one path can be detected very accurately by measuring differences in the overlap of the two beams after recombination. The total intensity at the output of an inter ferometer therefore tells us something about the relative phase difference between the two paths. This property can be exploited in the following way: the phase of an atom is altered by passing the atom through a region with a different potential energy. Since the total energy of the particle is pre served, the change in potential energy causes a reduction in kinetic energy, and therefore In the atomic velocity, and that the wave maxima for one path arrives slightly later at the detector. An interferometer can in fact detect potential differences between the two paths which are 1010 times smaller than the kinetic energy of the atoms. The positions of the maxima can be chan ged, for instance, by the interaction of atoms with external fields or with other particles. Since atoms have an internal structure which does not exist in electrons or photons they feel a totally different environment. Atom interferometers therefore have a num ber of unique potential applications which will be presented later. Optical elements Optical elements for the visible domain (beam splitters, mirrors, lenses, etc.) are familiar to all of us. They ave found In photo graphic cameras, microscopes and lasers, and can be easily realised by passing light through glass substra with a refractive index not equal to one and having a special shape and polished surfaces. The construction of optical elements for neutral atoms is consi derably more difficult since atoms cannot pass through solid materials. Both Ions and electrons can be easily deflected and manipulated by electric or magnetic fields. Building optical elements for charged particles is therefore a wellestablished technology, as can be seen In the results obtained in storage rings, accele rators and electron microscopes. However, these methods do not work for neutral atoms. The situation seemed hopeless until the invention of the laser and the tremen dous developments in microfabrication over the last two decades. Together they have opened up (...truncated)