The inadequacy of the optical trapping model based on ray optics in the case of describing the optical trapping performance of annular and doughnut laser beams is discussed. The inadequacy originates from neglecting the complex focused field distributions of such beams, such as polarization and phase, and thus leads to erroneous predictions of trapping force. Instead, the optical trapping model based on the vectorial diffraction theory, which considers the exact field distributions of a beam in the focal region, needs to be employed for the determination of the trapping force exerted on small particles. The theoretical predictions of such a trapping model agree with the experimentally measured results.

A physical model is presented to understand and calculate trapping force exerted on a dielectric micro-particle under focused evanescent wave illumination. This model is based on our recent vectorial diffraction model by a high numerical aperture objective operating under the total internal condition. As a result, trapping force in a focused evanescent spot generated by both plane wave (TEM00) and doughnut beam (TEM*01) illumination is calculated, showing an agreement with the measured results. It is also revealed by this model that unlike optical trapping in the far-field region, optical axial trapping force in an evanescent focal spot increases linearly with the size of a trapped particle. This prediction shows that it is possible to overcome the force of gravity to lift a polystyrene particle of up to 800 nm in radius with a laser beam of power 10 microW.

In this letter we present a physical model, both theoretically and experimentally, which describes the mechanism for the conversion of evanescent photons into propagating photons detectable by an imaging system. The conversion mechanism consists of two physical processes, near-field Mie scattering enhanced by morphology dependant resonance and vectorial diffraction. For dielectric probe particles, these two processes lead to the formation of an interference-like pattern in the far-field of a collecting objective. The detailed knowledge of the far-field structure of converted evanescent photons is extremely important for designing novel detection systems. This model should find broad applications in near-field imaging, optical nanometry and near-field metrology.

There has been an interest to understand the trapping performance produced by a laser beam with a complex wavefront structure because the current methods for calculating trapping force ignore the effect of diffraction by a vectorial electromagnetic wave. In this letter, we present a method for determining radiation trapping force on a micro-particle, based on the vectorial diffraction theory and the Maxwell stress tensor approach. This exact method enables one to deal with not only complex apodization, phase, and polarization structures of trapping laser beams but also the effect of spherical aberration present in the trapping system.

We report on, in this letter, a phenomenon that the central zerointensity point of a doughnut beam, caused by phase singularity, disappears in the focus, when such a beam is focused by a high numerical-aperture objective in free space. In addition, the focal shape of the doughnut beam of a given topological charge exhibits the increased ring intensity in the direction orthogonal to the incident polarization state and an elongation in the polarization direction. These phenomena are caused by the effect of depolarization, associated with a high numerical-aperture objective, and become pronounced by the use of a central obstruction in the objective aperture.

We demonstrate that because of the depolarization effect associated with a high-numerical-aperture lens, the recently predicted spectral splitting phenomenon near phase singularities of focused waves [G. Gbur, T. D. Visser, and E. Wolf, Phys. Rev. Lett. 88, 013901 (2002)] disappears when the numerical aperture is higher than critical values that are different between the incident polarization direction and the axial direction.