Optical trapping absorbing particles

In our experiments using an optical tweezers set-up, only slightly modified compared to the conventional system described above, we have shown that using phase singular fields we can not only trap absorbing particles but also set them into rotation. This demonstrates the transfer of the angular momentum from the light beam to the particles. 

Similar to H+ implantation, optimized MgO thin films have been implanted with 1.5 MeV Li+ ions for various fluences (lO MO ions cm ). Irradiation of crystalline MgO with energetic metal ions produces stable vacancies and interstitials in the anion sublattice. Elastic collisions with energetic particles also produce cation vacancies, but these defects do not survive because the cation interstitials quickly recombine with the vacancies. Optical absorption bands can monitor these defects induced by ion implantation. Similar observations have been already done on MgO crystals after neutron irradiation (Kappers et al. 1970, Monge et al. 2000). In crystalline MgO material irradiated with Li+ ions, the well-known defects are (1) oxygen vacancies (primarily the one-electron F center), (2) oxygen divacancies Fj, (3) V and V centers (cation vacancies that have trapped one or two holes, respectively) produced by the capture of holes by existing vacancies, and (4) an unidentified defect that absorbs at 2.16 eV (572 nm) (Gonzales et al. 1991). 

When these particles deposit on a surface, the resulting film is very porous and can be used as an optical radiation trap, e.g. black gold IR radiation bolometer films, germanium film solar absorber coatings, low secondary electron emission surfaces, and porous electrode films. The particles themselves are used for various powder metallurgical processes, such as low pressure, low temperature sintering. 

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