Abstracts

Commonly, the optical response of a nanostructureis obtained by using plane-wave excitation. However, a differentform of optical excitation may modify the response of thenanostructure. I present a theoretical study of the response of aspherical semiconductor quantum dot upon two different excitationforms: (1) a highly confined optical field and (2) an azimuthallypolarized laser beam. Highly confined optical fields areencountered in high-resolution near-field microscopy, andinhomogeneous laser beams, such as an azimuthally polarized beam,are used in confocal microscopy. I find that for these excitationforms, high-order multipole terms are needed to describe theoptical response of the quantum dot. I derive the selection rulesand the absorption rate corresponding to the contribution of theelectric quadruple and the magnetic dipole. For a highly confinedfield, probing electric quadrupole transitions yieldsno-improvement of the spatial resolution compared to the resolutionobtained by probing electric dipole transitions. For an azimuthallypolarized laser beam, the detection of the ratio of the electricdipole and magnetic dipole absorption rates enhances the spatialresolution which is limited by the purity of the modes of the laserbeam.
In recent experiments it was observedthat the damping of an oscillator increases as it is brought closeto the surface of a material. To address this problem, I calculatethe damping of a classical oscillator induced by theelectromagnetic field generated by thermally fluctuating currentsin the environment. The fluctuation-dissipation theorem is appliedto derive the linear-velocity damping coefficient. The theory isapplied to a particle oscillating parallel to a flat substrate andnumerical values for the damping coefIicient are evaluated forparticle and substrate materials made of silver and glass. I findthat losses are much higher for dielectric materials than formetals because of the higher resistivity. I predict thatmeasurements performed on metal films are strongly affected by theunderlying dielectric substrate and I show that the theoryreproduces existing theoretical results in the non-retarded limit.The theory provides an explanation for the observeddistance-dependent damping in shear-force microscopy. The theoryshould be of importance for the design of nanoscale mechanicalsystems and for understanding the trade-offs ofminiaturization.