Introduction

Molecule Optics is still in its infancy although one can hope to see important achievements in experimental quantum physics, like

• Tests of fundamental quantum phenomena:
• demonstration of wave-particle duality for particles of high mass and complexity
• quantitative investigations of decoherence in complex systems
• Precise measurements of molecular properties and other physical quantities
• Making of nanostructures using interferometrical molecule lithography

We have demonstrated that structures made of light can be used to coherently control the motion of complex molecules. In particular, we showed  diffraction of the fullerenes C₆₀ and C₇₀ by a thin grating made of green laser light.
We have shown 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.

The development of devices and arrangements for the coherent manipulation of molecular matter waves are urgently needed.
Nanofabricated gratings have already been demonstrated to be useful in fullerene interferometry.
However optical, non-material manipulation techniques offer many advantages since they

have 100 % transmission

offer high flexibility (laser can be rapidly switched, shaped etc..)

cannot be blocked (no sticking to surfaces)

This indicates: optical gratings will be the elements of choice for the manipulation of even larger molecules

The setup is shown in figure 1. It resembles our previous diffraction experiment at a material grating but the grating is replaced by a standing light wave, which is constructed by retroreflecting the 515 nm line of an Ar⁺ - laser from a mirror. The power of the standing light wave can be varied between 0 W and 9,5 W by rotating the halfwave plate in front of the polarizing beam splitter. In order to increase the longitudinal coherence of the fullerene beam a velocity selector (slotted disc velocity selector, SDVS in fig. 1) has to be employed.

Fig. 1: Setup of the experiment

The standing light wave constitutes a periodic structure with a periodicity of half of the laser wavelength, i.e. 257 nm. The most probable velocity of the fullerenes amounts to 120 m/s, which corresponds to a de Broglie wavelength of  4,6 pm (4,6*10^-12 m) for C₆₀ and 4,0 pm for C₇₀. So we expect diffraction angles for these fullerenes of  18 µrad and 15 µrad, respectively. In a photon picture the observed deflection amounts to twice the photon recoil of the green laser photons.

By varying the power of the standing light wave the induced phase shift and so also the relative height of the individual diffraction orders can be varied, as shown in figure 2. For comparison also the undiffracted beamprofile is shown on top. In contrast to atoms the absorption of the 'grating' photons doesn't lead to spontaneous emission but to an internal heating of the molecule, so the absorption of n photons deflects the fullerene by n photon recoils. Twice the mean number of absorbed photons is given by the imaginary part of the mean phase shift , quoted in fig. 2. The resolution of our detector is good enough to resolve the individual diffraction peaks but the absorption of an odd number of photons fills up the minima in between and decreases the contrast.

Outlook
 

In contrast to the extremely fragile material structures used in our previous interference experiments standing light waves proved to be a promising alternative - especially for the coherent manipulation of even larger molecules: they have perfect periodicity, high transmission and cannot be blocked or destroyed by the molecules. The diffraction efficiency can be varied continously and so they are ideal candidates for beam splitters in a molecule interferometer.

Reference

Olaf Nairz, Björn Brezger, Markus Arndt, and Anton Zeilinger, Diffraction of Complex Molecules by Structures Made of Light, Phys. Rev. Lett. 87, 160401 (2001)

    © Olaf Nairz, 10/2001