The Study On Properties Of The Axially Polarized Needle-like Field Created By Reflection

As mechanical wave can vibrate parallel to its propagation direction, it is foundthat optical wave can also have a longitudinal component, especially in anasymmetrical system. Longitudinal field with many of its unique characteristicssuccessfully attracts great academic interests since then. Up till now, longitudinalfield has been studied and applied in many areas, such as particle acceleration,fluorescent imaging, nanolithography, second-harmonic generation, Ramanspectroscopy, high-density optical data storage, microscope, and optical trapping.Recently, as studies go further, people are highly interested in whether one can createa longitudinal field with long beam length, narrow lateral width, uniformity alongaxis, and high optical efficiency. The axial field of an axisymmetric optical systemsometimes is also called a longitudinal field. It is a great challenge to create aneedle-like axisymmetric field with properties mentioned above. Here we show amethod that can realize these properties all at once. The key element is a90°apex-angle concave conical mirror. By using this condenser along with a radiallypolarized incident beam of a specific field distribution, we numerically created asuper slim, uniform, pure needle-like axially polarized field. This axially polarizedfield has a length of50,000λ along the optical axis, and its lateral width stillmaintains a minimum0.36λ round spot size.In this dissertation, we first introduce the concept and discovery of longitudinalfield, then we give an outline of the studies on properties of longitudinal field thathas been established, and finally we state that the method we adopt is using a conicalmirror along with a radially polarized incident beam of a specific field distribution.By applying the integral theorem of Helmholtz and Kirchhoff we derive the integralformula at an arbitrary point within the focal region, and by using Matlab wenumerically calculate the axial field distribution of the focal region. Finally, we discuss the properties of the axial field that we obtained, and explain why it is alongitudinal field in special situation, and furthermore, how can this needle-like axialfield implicate our appreciation of non-diffractive beam, such as Bessel beam. Thelength of the axially polarized field can be extended arbitrarily in principle, while thelateral width maintains a minimum0.36λ round spot size. The length of theneedle-like field50,000λ is much longer than those obtained by other methodscarried out recently, and has increased at least by two orders of magnitude.