High-sensitivity and a high-speed nanoscale measurement is an important subject in modern industry, especially when analysis of high-speed moving nanoscale objects on a surface is required. Several objectives in this direction can be achieved by using singular beam microscopy, which we investigate experimentally for the examination of small phase steps. We discuss the challenges of rigorous modeling of experiments employing high-numerical-aperture illumination and describe experimental results performed with a medium numerical aperture of 0.55. The investigated equivalent phase step heights reached as low as 10 nm (about 1/15 rad).
Investigation of nanoscale objects is becoming increasingly important with development of modern nanotechnology
related industries. Under certain conditions, some information on the investigated object can be obtained in the forward
scattering far field when the object is scanned by a focused beam. The sensitivity of the far field based measurements
depends on a number of factors including the shape of the investigated object. In this work we present a case study
comparing far field response in scanning mode. The response sensitivity for nano-scale phase objects of different shapes
and different phase volumes under various illuminating conditions is discussed. We perform a paraxial simulation with
investigated phase objects represented as thin optical elements: free standing and as a part of a surface.
Rigorous vector analysis of high numerical aperture optical systems encounters severe difficulties. While existing
analytic methods, based on the Richards-Wolf approach, allow focusing of nearly planar incident wavefronts, these
methods break down for beams possessing considerable phase jumps, such as beams containing phase singularities. This
work was motivated by the need to analyze a recently introduced metrological application of singular beams that
demonstrated an experimental sensitivity of 20nm under a moderate numerical aperture of 0.4. One of the possibilities to
obtain even better sensitivity is by increasing the numerical aperture of the optical system. In this work we address the
issue of high numerical aperture focusing of the involved singular beams. Our solution exploits the superposition
principle to evaluate the three dimensional focal distribution of the electromagnetic field provided the illuminating
wavefront can be described as having piecewise quasi constant phase. A brief overview of singular beam microscopy is
followed by deeper discussion of the involved high numerical aperture focusing issue. Further, a few examples of
different singular beam focal field distributions are presented.
The Richards-Wolf approach to analyze tight focusing by high-numerical-aperture aplanatic optical systems was designed to treat incident waves having a planar (or negligibly curved) wavefront at the entrance pupil. In this paper we extend the approach to incorporate also wavefronts with piecewise quasi-constant phase. The evaluation of the field distribution in the vicinity of the geometrical focus is accomplished by linear superposition of the contributions from all the segments with quasi-constant phase. As an application example, a tightly focused optical field is evaluated for a -phase-step-modulated incident wavefront.
Quickly developing nanotechnology drives the industrial need for fast but sensitive nano-scale feature detection and
evaluation. In this work we bypass the diffraction limit for achieving nanoscale sensitivity by introducing optical
singularities into the illuminating beam for a modified laser scanning microscopic architecture. A good correspondence
was obtained between laboratory experiments and corresponding simulations that indicated a theoretical potential of 1nm
sensitivity under a practical signal to noise ratio of 30dB. For analysis of the experimental and simulation results, two
simple but effective algorithms were developed. A significant improvement of signal to noise ratio in the optical system
with coherent light illumination can be achieved by utilization a highly redundant data collected during experiments. Our
experimental results validate achievable sensitivity down to 20nm. The unique combination of nano-scale sensitivity
together with implementation simplicity and on-line, real-time analysis capability make Singular Beam Microscopy a
valuable industrial analytic method.
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