Properties of light fields near sub-micro and nano-scale structures

In this thesis various phenomena that appear if light interacts with objects having a size comparable to the wavelength are experimentally and theoretically analysed. The work is concentrated on the analysis of phase singularities in electromagnetic fields, on the excitation of plasmon-polaritons and phonon-polaritons in arbitrary shaped cylindrical objects and on the analysis of force and torque exerted on dielectric and metallic wires by light fields. Phase singularities are points in space in which the amplitude of the electromagnetic field is zero and the phase is not determined. Generally, two types of singularities (also called dislocations or vortices) exist: screw dislocations and edge dislocations. In the first part of this thesis, conditions for the appearance of edge dislocations generated by phase bars and trenches in the transmission region have been investigated. It was shown by using rigorous diffraction theory as well as scalar theory, that a smallest object size exists, which is necessary for generating a phase singularity in the far-field of the structure. As this far-field is experimentally accessible with a high-resolution interference microscope, the basic theory could be experimentally verified. Once a phase singularity appears in the diffracted field, the distance between pair-wise generated dislocations depends in good approximation linearly on the lateral object size of the bar or trench. Screw dislocations generated by computer generated holograms have been experimentally investigated using likewise the high-resolution interference microscope. It was found that in higher order Gauss Laguerre laser beams that carry multiple singularities, the dislocations do not degenerate. In the second part of the thesis, different types of small particle polaritons have been investigated. Plasmon-polaritons are an oscillation of the charge density in metallic nanoparticles excited in resonance with the frequency of the illuminating wave-field. Phonon-polaritons are a resonant oscillation of lattice vibrations in bipolar dielectric materials. Once such a polariton is excited in a small particle, a large near-field amplitude in the vicinity of the object and a large scattering cross section can be observed. The scattering cross section is the amount of scattered light into the far-field. In this thesis, the boundary element method was applied to analyse the influence of the shape and the presence of neighbouring particles on the excited plasmon and phonon spectra. By comparing the efficiency in exciting plasmon-polaritons and phonon-polaritons it was found that the phonon excitation is more efficiently due to a lower imaginary part of the dielectric constant in the relevant spectral domains, cause a lower damping and broadening of the polariton signature. In a last part of the thesis, the force and torque exerted by wave-fields on cylinders made of different materials in various size domains was analysed, mainly for cylinders with a circular and elliptical cross section. The conditions for which the particles are attracted or repelled from the optical axis as a function of the geometry have been determined. In addition, the capability of trapping theses particles was analysed. For metallic particles this behaviour was investigated as a function of the wavelength. It was shown, that for wavelengths smaller than the plasmon wavelength the particles are basically repelled from the optical axis, whereas attraction is observed for wavelength larger than the plasmon wavelength. This behaviour offers possible applications in sorting particles in accordance with their shape. A trapping of metallic particles is possible for wavelength in the infra-red spectral region, where the general behaviour is comparable to small dielectric particles.

Thèse de doctorat : Université de Neuchâtel, 2004 ; 1786