טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentNormatov Alexander
SubjectSingular Beam Focusing and Scattering by Nanoscale
Objects
DepartmentDepartment of Electrical Engineering
Supervisors Professor Emeritus Joseph Shamir
Dr. Boris Spektor
Full Thesis textFull thesis text - English Version


Abstract

Light interaction with nanoscale objects draws intense attention in a number of research and application fields in nanotechnology. This work investigated scattering and absorption aspects of this interaction for some cases of tightly focused beams, emphasizing beams containing optical singularities. The investigations produced some interesting results, which were partly due to the sub-wavelength sized features of the tightly focused fields. The obtained scattering investigation results provide valuable clues for metrology oriented applications while the absorption investigation results also benefit biology and chemistry oriented applications.

The presence of a line phase dislocation in some of the investigated beams contradicts an assumption, commonly made for tight focusing. The suggested re-formulation allows vector evaluation of tightly focused fields for incident piecewise quasi-constant wavefronts. The comparison between fields obtained with the commonly used Debye approximation and Kirchhoff approximation exposed the presence of a focal quadratic phase for the latter case. A deeper investigation revealed unexpected quadratic phase anomalies of the tightly focused field for beams with phase dislocations.

The investigated problem of scattering of tightly focused beams by nanoscale objects required rigorous electromagnetic treatment. In this work, the evaluation of the scattered field was performed with the Source Model Technique, which proved itself fast and reliable. The high speed of numerical evaluation allows conceiving real-time optical metrology applications utilizing the a priori information about the scattering object. As an example of such application, the investigation of the far field responses of a plasmonic nanowire, scanning through tightly focused beams, was performed. The results demonstrated position sensitivity of 1nm with about 33dB signal to noise ratio. At the scan position located in the optical singularity region, where the beam power was expected to approach zero, the nanowire was observed to produce a strong scattering response. This enhanced response was attributed to the presence of an axial electric field which was connected to the generation of a dipole polarization along the beam propagation direction.

Deeper investigation of the excitation of nanowire surface plasmon resonance by tightly focused fields allowed design of nanowire cross-sectional shapes for power absorption enhancement. The shapes were designed according to a proposed physical criterion which aimed at the maximization of matching between the nanowire plasmon resonance field and the incident tightly focused field. It was shown that properly shaped and positioned silver nanowires absorbed up to 65% of the total power of incident beams.