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Register a new userVan der Waals enhancement of optical atom potentials via resonant coupling to surface polaritons
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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.
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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.
The universal aspects of atom-dimer elastic collisions are investigated within the framework of Faddeev equations. The two-body interactions between the neutral atoms are approximated by the separable potential approach. Our analysis considers a pure van der Waals potential tail as well as soft-core van der Waals interactions permitting us in this manner to address the universally general features of atom-dimer resonant spectra. In particular, we show that the atom-dimer resonances are solely associated with the {it excited} Efimov states. Furthermore, the positions of the corresponding resonances for a soft-core potentials with more than 5 bound states are in good agreement with the corresponding results from an infinitely deep pure van der Waals tail potential.
yperbolic polaritons in van der Waals materials recently attract a lot of attention, owing to their strong electromagnetic field confinement, ultraslow group velocities and long lifetimes. Typically, volume confined hyperbolic polaritons (HPs) are studied. Here we show the first near-field optical images of hyperbolic surface polarities, HSPs, which are confined and guided at the edges of thin flakes of a vdW material. To that end, we applied scattering-type scanning near-field optical microscopy (s-SNOM) for launching and real-space nanoimaging of hyperbolic surface phonon polariton modes on a hexagonal boron nitride, h-BN, flake. Our imaging data reveal that the fundamental HSP mode exhibits stronger field confinement, smaller group velocities and nearly identical lifetimes, as compared to the fundamental HP mode of the same h-BN flake. Our experimental data, corroborated by theory, establish a solid basis for future studies and applications of HPs and HSPs in vdW materials.
Electromagnetic field confinement is crucial for nanophotonic technologies, since it allows for enhancing light-matter interactions, thus enabling light manipulation in deep sub-wavelength scales. In the terahertz (THz) spectral range, radiation confinement is conventionally achieved with specially designed metallic structures - such as antennas or nanoslits - with large footprints due to the rather long wavelengths of THz radiation. In this context, phonon polaritons - light coupled to lattice vibrations - in van der Waals (vdW) crystals have emerged as a promising solution for controlling light beyond the diffraction limit, as they feature extreme field confinements and low optical losses. However, experimental demonstration of nanoscale-confined phonon polaritons at THz frequencies has so far remained elusive. Here, we provide it by employing scattering-type scanning near-field optical microscopy (s-SNOM) combined with a free-electron laser (FEL) to reveal a range of low-loss polaritonic excitations at frequencies from 8 to 12 THz in the vdW semiconductor ${alpha}-MoO_3$. We visualize THz polaritons with i) in-plane hyperbolic dispersion, ii) extreme nanoscale field confinement (below ${lambda}_o/75$) and iii) long polariton lifetimes, with a lower limit of > 2 ps.
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.
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