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Register a new userDetermination of Atom-Surface van der Waals Potentials from Transmission-Grating Diffraction Intensities
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Molecular beams of rare gas atoms and D_2 have been diffracted from 100 nm period SiN_x transmission gratings. The relative intensities of the diffraction peaks out to the 8th order depend on the diffracting particle and are interpreted in terms of effective slit widths. These differences have been analyzed by a new theory which accounts for the long-range van der Waals -C_3/l^3 interaction of the particles with the walls of the grating bars. The values of the C_3 constant for two different gratings are in good agreement and the results exhibit the expected linear dependence on the dipole polarizability.
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Contemporary experiments in cavity quantum electrodynamics (cavity QED) with gas-phase neutral atoms rely increasingly on laser cooling and optical, magneto-optical or magnetostatic trapping methods to provide atomic localization with sub-micron uncertainty. Difficult to achieve in free space, this goal is further frustrated by atom-surface interactions if the desired atomic placement approaches within several hundred nanometers of a solid surface, as can be the case in setups incorporating monolithic dielectric optical resonators such as microspheres, microtoroids, microdisks or photonic crystal defect cavities. Typically in such scenarios, the smallest atom-surface separation at which the van der Waals interaction can be neglected is taken to be the optimal localization point for associated trapping schemes, but this sort of conservative strategy generally compromises the achievable cavity QED coupling strength. Here we suggest a new approach to the design of optical dipole traps for atom confinement near surfaces that exploits strong surface interactions, rather than avoiding them, and present the results of a numerical study based on $^{39}$K atoms and indium tin oxide (ITO). Our theoretical framework points to the possibility of utilizing nanopatterning methods to engineer novel modifications of atom-surface interactions.
I revisit the problem of the interaction between two dissimilar atoms with one atom in an excited state, recently addressed by the authors of Refs.[1-3], and for which precedent approaches have given conflicting results. In the first place, I discuss to what extent Refs.[1], [2] and [3] provide equivalent results. I show that the phase-shift rate of the two-atom wave function computed in Ref.[1], the van der Waals potential of the excited atom in Ref.[2] and the level shift of the excited atom in Ref.[3] possess equivalent expressions in the quasistationary approximation. In addition, I show that the level shift of the ground state atom computed in Ref.[3] is equivalent to its van der Waals potential. A diagrammatic representation of all those quantities is provided. The equivalences among them are however not generic. In particular, it is found that for the case of the interaction between two identical atoms excited, the phase-shift rate and the van der Waals potentials differ. Concerning the conflicting results of previous approaches in regards to the spatial oscillation of the interactions, I conclude in agreement with Refs.[1,3] that they refer to different physical quantities. The impacts of free-space dissipation and finite excitation rates on the dynamics of the potentials are analyzed. In contrast to Ref.[3], the oscillatory versus monotonic spatial forms of the potentials of each atom are found not to be related to the reversible versus irreversible nature of the excitation transfer involved.
In inhomogeneous dielectric media the divergence of the electromagnetic stress is related to the gradients of varepsilon and mu, which is a consequence of Maxwells equations. Investigating spherically symmetric media we show that this seemingly universal relationship is violated for electromagnetic vacuum forces such as the generalized van der Waals and Casimir forces. The stress needs to acquire an additional anomalous pressure. The anomaly is a result of renormalization, the need to subtract infinities in the stress for getting a finite, physical force. The anomalous pressure appears in the stress in media like dark energy appears in the energy-momentum tensor in general relativity. We propose and analyse an experiment to probe the van der Waals anomaly with ultracold atoms. The experiment may not only test an unusual phenomenon of quantum forces, but also an analogue of dark energy, shedding light where nothing is known empirically.
We study the repulsive van der Waals interaction of cold rubidium $70S_{1/2}$ Rydberg atoms by analysis of time-delayed pair correlation functions. After excitation, Rydberg atoms are allowed to accelerate under the influence of the van der Waals force. Their positions are then measured using a single-atom imaging technique. From the average pair correlation function of the atom positions we obtain the initial atom-pair separation and the terminal velocity, which yield the van der Waals interaction coefficient $C_{6}$. The measured $C_{6}$ value agrees well with calculations. The experimental method has been validated by simulations. The data hint at anisotropy in the overall expansion, caused by the shape of the excitation volume. Our measurement implies that the interacting entities are individual Rydberg atoms, not groups of atoms that coherently share a Rydberg excitation.
We theoretically address grazing incidence fast atom diffraction (GIFAD) for H atoms impinging on a LiF(001) surface. Our model combines a description of the H-LiF(001) interaction obtained from Density Functional Theory calculations with a semi-quantum treatment of the dynamics. We analyze simulated diffraction patterns in terms of the incidence channel, the impact energy associated with the motion normal to the surface, and the relevance of Van der Waals (VdW) interactions. We then contrast our simulations with experimental patterns for different incidence conditions. Our most important finding is that, for normal energies lower than 0.5 eV and incidence along the <100> channel, the inclusion of Van der Waals interactions in our potential energy surface yields a greatly improved accord between simulations and experiments. This agreement strongly suggests a non-negligible role of Van der Waals interactions in H/LiF(001) GIFAD in the low-to-intermediate normal energy regime.
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