An application is presented for the automatic identification of lost or undiscovered archaeological sites in Egypt using a shape detection technique on satellite Earth observation (EO) imagery superimposed in a geographic information system (GIS) environment. The implementation of a shape detection algorithm that employs an operator matched to the shape of buried archaeological structures is described. The implementation is based upon the optimal edge-based shape detection approach described by [1]. The algorithm is applied to EO images and the results are presented.

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.

Optical trapping techniques have become an important and irreplaceable tool in many research disciplines for reaching non-invasively into the microscopic world and to manipulate, cut, assemble and transform micro-objects with nanometer precision and sub-micrometer resolution. Further advances in optical trapping techniques promise to bridge the gap and bring together the macroscopic world and experimental techniques and applications of microsystems in areas of physics, chemistry and biology. In order to understand the optical trapping process and to improve and tailor experimental techniques and applications in a variety of scientific disciplines, an accurate knowledge of trapping forces exerted on particles and their dependency on environmental and morphological factors is of crucial importance. Furthermore, the recent trend in novel laser trapping experiments sees the use of complex laser beams in trapping arrangements for achieving more controllable laser trapping techniques. Focusing of such beams with a high numerical aperture (NA) objective required for efficient trapping leads to a complicated amplitude, phase and polarisation distributions of an electromagnetic field in the focal region. Current optical trapping models based on ray optics theory and the Gaussian beam approximation are inadequate to deal with such a focal complexity. Novel applications of the laser trapping such as the particle-trapped

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.

We present a physical model for the conversion of the 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. The model is also applicable for determination of the near-field force exerted on small particles situated in an evanescent field. 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.

In modern optical microscopy, a high numerical-aperture objective plays an important rule for high-resolution imaging and laser trapping. However, a high numerical-aperture objective results in a depolarisation process [1]. As a result, a linear polarised beam is depolarised into three components of the electromagnetic wave in the focal region. One of the components is along the direction of the beam propagation, i.e. along the axial direction. The contribution of the longitudinal component can be enhanced if the incident illumination over the lens aperture is centrally obstructed. As a result, the focal spot in free space is split into two peaks with a separation determined by the size of the obstruction [2] (see Fig. 1). (b) (a) Fig. 1 The focal shapes of a high numerical-aperture objective obstructed by an opaque disk. The normalized size of the obstruction is ε: (a) ε = 0; (b) ε = 0.5. Another feature caused by the longitudinal component is that 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 [3]. If the phase of the field over the lens aperture is also apodised, the focal shape of a high numerical-aperture objective becomes more complicated. For example, when the phase over the lens aperture possesses a helical structure, i.e. when the illumination on the lens has a screw optical singularity, the doughnut focal shape, which occurs for a low numericalaperture objective, does not necessarily exist when the numerical aperture is large [4]. In this presentation, the new features mentioned above will be discussed. [1] Min Gu, Advanced Optical Imaging Theory, Springer Verlag, 2000. [2] J. W. M. Chon, X. Gan, Min Gu , Splitting of the focal spot of a high numerical-aperture objective in free space , Appl. Phys. Lett. 81 (2002), 1576-1578. [3] D. Ganic, James Chon, Min Gu, Effect of numerical aperture on anomalous behaviour of spectra near phase singularities of focused waves, Appl. Phys. Lett., (2003), in press. [4] D. Ganic, X. Gan, Min Gu, Focusing of doughnut laser beams by a high numericalaperture objective in free space, submitted to Appl. Phys. Lett., (2003).

We present a novel technique for producing a doughnut laser beam by use of a liquid-crystal cell. It is demonstrated that the liquid-crystal cell exhibits an efficiency in energy conversion near 100%. One of the main advantages of this method is its capability of dynamic switching between a Gaussian mode and a doughnut mode of different topological charges. The liquid-crystal cell is also dynamically tunable over the visible and near-infrared wavelength range. These advantages make the device appealing for laser trapping methods used in single-molecule biomechanics and for optical guiding of cold atoms.

Summary In this paper we present a mathematical three-dimensional model for numerical calculations of near-field Mie scattering. We use this model to calculate scattered electromagnetic field distribution in various planes around dielectric particle of different sizes under S and P polarized illumination. Model is also applied to parametrically study morphology dependent resonance effects associated with near-field Mie scattering by dielectric particles.