Diffraction of Complex Molecules by Structures Made of Light

We demonstrate that structures made of light can be used to coherently control the motion of complex molecules. In particular, we show diffraction of the fullerenes C60 and C70 at a thin grating based on a standing light wave. We prove experimentally that the principles of this effect, well known from atom optics, can be successfully extended to massive and large molecules which are internally in a thermodynamic mixed state and which do not exhibit narrow optical resonances. Our results will be important for the observation of quantum interference with even larger and more complex objects. PACS numbers: 03.75. –b, 39.20. +q, 33.80. –b, 42.50.Vk The great success of atom optics has stimulated the question whether it is possible to extend the methods developed in this field to more complex and massive quantum objects. In this Letter we demonstrate for the first time the coherent control of the molecular motion using optical structures for macromolecules. One possible manipulation technique for molecules has been demonstrated earlier with the use of material nanostructures. They have, for example, successfully been used in atom interferometry [1], in the determination of the bond length of a helium dimer [2], and more recently in interference experiments with the fullerenes C60 and C70 [3,4]. The use of solid nanostructures has the advantage of being universal and largely independent of the detailed internal character of the diffracted object. However, since the structure dimensions have to be of the order of 100 nm, material devices are extremely fragile and can be blocked or destroyed by the molecules. In contrast to that, diffraction structures made of light are promising alternatives: The periodicity can be perfect, the transmission is high, and one can realize different types of gratings. Light may act as a real or imaginary index of refraction for matter waves and thus form a phase grating or an absorption grating. For atoms, amplitude gratings can be based on various effects which lead to an effective spatially periodic depletion of relevant states of the atomic beam [5,6]. For example, the extraction of particles using ionization in a standing wave is conceivable. Phase gratings based on the nondissipative dipole force were demonstrated for the diffraction of atomic beams both in the thin grating or Raman-Nath regime [7] and in the thick grating or Bragg regime [8]. They have been successfully implemented to build up complete MachZehnder interferometers [9,10] and they find applications in atom lithography [11,12] or the manipulation of BoseEinstein condensates [13]. Combinations of absorptive and phase structures can also be used for complex blazed gratings [14]. In spite of the great success in atom optics, light gratings have not yet been applied to larger molecules. This is mainly due to significant differences between atoms and molecules which have to be taken into account in a practical realization: The optical linewidths of large molecules are typically of the order of 100 THz instead of a few MHz in the atomic regime. In the case of fullerenes the electrical polarizability, mediating the coupling to the optical field, remains constant within a factor of 2 throughout the whole frequency range from dc to UV. The high complexity of these molecules leads to non-negligible absorption from the ultraviolet well into the visible wavelength region and for the realization of a pure phase grating one would think that absorption should be excluded. This reasoning is based on the experience from atom optics that absorption is usually followed by spontaneous emission, which carries which-path information into the environment. In the following we will demonstrate that the principles of light gratings can actually be successfully carried over to fullerenes, which are internally in a thermodynamic mixed state. Even in the presence of absorption the fullerenes may maintain coherence between states of equal energy since the absorbed quanta are trapped in the molecules. A schematic of the setup is shown in Fig. 1. Essential parts of the setup are already described in [3] and an in-depth characterization of the detector is given in [4]. A ceramic oven containing the fullerene powder is heated