In recent years, optical near-field has received an ever-increasing attention owing to
its ability to localise optical signals beyond the diffraction limit. Optical near-field is
a non-propagating field existing in the close vicinity of a matter within a range less
than the wavelength of the illumination light and it carries the high spatial frequency
information showing the fine details of the matter.
An optical near-field can be generated by a near-field optical microscope with a
nano-aperture or a metal-coated fibre tip. However, common difficulties associated
with this approach, such as a fragile probe, a low throughput and signal-to-noise ratio,
and a slow response of gap controlling between the probe and the sample, make it
less applicable. Alternatively, optical near-field can be produced by total internal
reflection (TIR) occurring at the interface of a prism, which is capable of localising
the electromagnetic (EM) field in the close vicinity of the interface. However, in this
geometry, no confinement of the field can be achieved in the transverse direction,
whereas, in most applications such as optical trapping, micro-fabrication and optical
data storage, a transverse confinement of the light field is essential.
In order to achieve a transverse confinement of the light field, maintaining the
high spatial resolution of the optical near-field, and at the same time eliminating
the drawbacks associated with the conventional near-field optical microscope, a novel
near-field probe based on a high numerical aperture (NA) TIR objective combined
with annular illumination has been developed recently. In this arrangement, an
obstruction disk is inserted at the back aperture of the objective to block the light
with a convergence angle lower than the critical angle determined by the refractive
indices of the two media, resulting in a pure focused evanescent field in the second
medium.
The evanescent field produced by this method provides a useful tool for studying
light-matter interaction at the single molecule level not only because of its high
resolution but also due to its inherent merits such as no distance regulation, no heating
effect and simple experimental setup. But, the most significant advantage that makes
this method unique and superior to the other approaches in terms of producing the
optical near-field is that it allows the dynamic control of the focal field by simply
modulating the phase or amplitude or even the polarisation state of the incident beam
before it enters the objective so that complex illumination beams can be generated,
whereas in other fibre probe based approaches this goal is extremely difficult to achieve.
To make use of such a novel near-field probe, a thorough theoretical and
experimental investigation is required. A complete knowledge of the focused evanescent
field is a prerequisite for a wide range of applications including single molecule
detection, Raman spectroscopy, near-field non-linear imaging and near-field trapping.
Therefore, it is not only necessary but also urgent to exploit the focusing properties
of a focused evanescent field under complex field illumination both experimentally and
theoretically and this is the major aim of this thesis.
The complex fields, which are of particular interest in this thesis, are the radially
polarised beam and the Laguerre-Gaussian (LG) beam, because the former owns a
more compact circularly symmetric field distribution in the focal region when focused
by a high NA objective, while the latter is capable of rotating a trapped particle
by transferring the orbital angular momentum. Combining them with the focused
evanescent field is potentially able to induce novel functions in the near-field region,
which cannot be fulfilled by other near-field approaches. In this thesis, in order to
generate these two types of beams, a single liquid crystal spatial light modulator
(LCSLM) is employed to produce useful phase modulation to the incident beam.
Experimental characterisation of an evanescent focal spot is performed with
scanning near-field optical microscopy (SNOM), which is capable of providing the direct
mapping of the focused evanescent field not only because of its high spatial resolution
and its ability to detect the near-field and far-field signals simultaneously, but also due
to the motion of the piezzo-stage enables a three-dimensional characterisation of the
evanescent focal spot.
In this thesis, a SNOM system with an aluminum coated aperture probe is
implemented. The field distributions at both the interface and parallel planes with
a small distance away from the interface are obtained. To verify the applicability of
SNOM as a characterisation methodology, the field distribution in the focal region
of a high NA objective illuminated by a linearly polarised plane wave is measured
first. A focus splitting along the direction of incident polarisation is observed threedimensionally
near the interface under such a circumstance. It has been demonstrated
that the depolarisation effect plays an important role in determining the coupling
behaviour of the light into the fibre probe of SNOM. The good match between the
experimental results and theoretical predications confirms the validity of SNOM.
Theoretical investigation of a tightly focused radially polarised beam is undertaken
based on the vectorial-Debye diffraction theory because under the tight focusing of a
high NA objective, the vectorial nature of the highly localised field has to be carefully
considered in order to represent the field distribution accurately. The calculations
on the focusing properties of a radially polarised beam suggest that the longitudinal
field component in the focal region plays a dominant role in determining the overall
field distribution. Direct measurement of the focused evanescent radially polarised
beam in a three-dimensional manner near the interface is performed with SNOM. A
highly localised focal spot is achieved in the close vicinity of the coverglass. The
measured intensity distributions from SNOM show that correction of the focal spot
deformation associated with a linearly polarised beam is achieved by taking advantage
of the radially symmetric focal spot of a radially polarised beam. A smaller focal spot is
acquired due to the dominant longitudinal polarisation component in the focal region,
which possesses a more compact focal intensity distribution than that of the overall
field. The experimental results demonstrate a good agreement with the theoretical
expectations.
The fact that a radially polarised beam is capable of eliminating the focus
deformation often presented in the focal region of a high NA objective when a linearly
polarised beam is employed can be very useful in many applications, including microfabrication
using two-photon photopolymerisation technique. The theoretical study
on the two-photon point spread function (PSF) of a radially polarised beam indicates
that the focus elongation and splitting associated with a linearly polarised beam are
eliminated and the achievable lateral size of the focal spot is approximately a quarter
of the illumination wavelength, which is less than half of that under the illumination
of a linearly polarised beam. A further reductiont of the lateral size can be expected
by using annular radial beam illumination.
The investigation on the focusing properties of LG beams has also been one of
the major tasks of this thesis. Theoretical investigations of a focused evanescent LG
beam suggest that the phase shift induced by the boundary effect when a light beam
passes the interface satisfying TIR condition plays a vital role in determining the
overall shape of the total field distribution. A severe focal intensity deformation is
predicted theoretically in the case of focused evanescent LG beam illumination, which
might involve new physical phenomena when applied in the near-field trapping. Such
a focal intensity deformation is evidenced experimentally by the direct mapping result
obtained from the SNOM probe. A quantitative cross-section comparison with the
theoretical predication is conducted, which demonstrates a good agreement.
To achieve a controllable optical trap and rotation in the near-field region, complex
optical fields such as LG beams carrying orbital angular momentum, have been induced
for the manipulation of a polystyrene particle. The influence of the focal intensity
deformation on a near-field trapping has been thoroughly investigated. Rotation
motion of the particle is examined by mapping the two-dimensional (2D) transverse
trapping efficiency of the particle. Theoretical investigation reveals that a significant
tangential force component is generated on the particle when it is illuminated by a
focused evanescent LG beam. Such findings may prove useful in introducing a rotation
mechanism in near-field trapping.
The research investigations and methodologies described in this thesis provide a new
approach to characterise the near-field focal spot under complex field illumination.
It enhances the understanding of the novel near-field probe, thus opening the
pathway for numerous near-field applications including optical trapping, two-photon
excitation (photopolymerisation) and spectroscopy. The focal field rotation phenomena
demonstrated in this thesis may prove particularly beneficial in introducing a rotation
mechanism in near-field trapping using a focused evanescent field.
| Identifer | oai:union.ndltd.org:ADTP/216616 |
| Date | January 2006 |
| Creators | Jia, Baohua, n/a |
| Publisher | Swinburne University of Technology. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.swin.edu.au/), Copyright Baohua Jia |
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