Djenan Ganic

Focusing of doughnut laser beams by a high numerical-aperture objective in free space

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.

Generation of doughnut laser beams by use of a liquid-crystal cell with a conversion efficiency near 100%

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.

Exact radiation trapping force calculation based on vectorial diffraction theory

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.

Trapping force and optical lifting under focused evanescent wave illumination

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.

Optical trapping force with annular and doughnut laser beams based on vectorial diffraction

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.

Multi-level optical data storage in a photobleaching polymer using two-photon excitation under continuous wave illumination

A multi-level recording method is implemented in a photobleaching polymer using two-photon excitation under continuous wave illumination. It is experimentally shown that information encoded with ten levels of darkness can be successfully stored. This multi-level bit optical data storage method is of importance in increasing the storage density of a recording material and the speed of data access.

Effect of numerical aperture on the spectral splitting feature near phase singularities of focused waves

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.

Three-dimensional evanescent wave scattering by dielectric particles

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.

Reduced effects of spherical aberration on penetration depth under two-photon excitation

We compare the effects of spherical aberration on the penetration depth of single-photon and two-photon excitation for instances in which the aberration is caused by the refractive-index mismatch when a beam is focused through an interface. It is shown both theoretically and experimentally that two-photon fluorescence imaging experiences less spherical aberration and can thus propagate to a deeper depth within a thick medium.

Near-field imaging by a micro-particle: a model for conversion of evanescent photons into propagating photons

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.