Optical dipole traps

Fundamental Tests of Quantum Mechanics, Edward S. Fry and Thomas ffUther Wave-Particle Duality in an Atom Inter-ferometer, Stephen Durr and Gerhard Rempe Atom Holography, Fujio Shimizu Optical Dipole Traps for Neutral Atoms,... 

Grimm, R., Weidemiiller, M., and Ovchinnikov, Yu. B. (2000). Optical dipole traps for neutral atoms. In Advances in atomic, molecular, and optical physics (ed. B. Bederson and H. Walther), vol. 42, pp. 95-170. Academic Press, San Diego. Grynberg, G., and Courtois, J. (1994). Proposal for a magneto-optical lattice for trapping atoms in nearly-dark states. Europhysics Letters, 27, 41-46. 

Levitation of Cs atoms in the evanescent field has been demonstrated in Ref. [Hammes 2002], where the the optical dipole potential created by 1 W-laser has been utilized for trapping the atoms far from the surface. The exponential profile of the potential decreases at a half-wave length A/,/w,/2 250 nm. At these distances the optical field compensates for the long-range attraction induced by the electrostatic polarization and the Casimir-Polder potential... 

Fig. 1. Scheme of the capture of electrons in a polar matrix, (a) Orientation of solvent dipole molecules around an electron (b) potential well for et (du and dlt the ground and the excited levels of an electron in a trap). The arrows indicate the optical transitions of the trapped electron. 

The inclusion of impurity atoms in MgO is much more interesting from a chemical point of view when alkali metals are used to replace Mg ions. In fact, this results in trapped-hole centers. The MVO pairs have been extensively studied in the bulk of alkaline-earth oxides by optical studies, EPR and ENDOR measurements [185,186] as well as by embedded cluster calculations [187]. The LiVO ions create an effective dipole which polarizes the surrounding lattice, with the two ions moving toward each other. The presence of an O radical, however, is most interesting when one is dealing with surface properties. This center in fact is very reactive and is the subject of the next paragraph. 

A particularly important variant of the optical force, interparticle forces, turns out to be crucial for SERS. This effect is similar to the attractive van der Waals force between small particles, which is due to interactions between spontaneously fluctuating dipoles, but the optical interaction is due to coupling between the actual particle dipoles induced by the trapping laser. Due to the interparticle optical forces, metal nanoparticles aggregate in an optical tweezers and produce hotspots, i.e., particle junctions with intense local fields for SERS. Raman probes can be excited either by the trapping laser or, preferably, by a separate low power beam that does not disturb the trapping. 

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