17 Microsphere-Assisted Interference Microscopy
463
in the reference arm. Finally, an application for the characterization of femtosecond
LASER induced nanoripples in stainless steel, spaced by a few 100 nanometres was
demonstrated [158] (Fig. 17.16).
17.4 Conclusion
Label-free super-resolution imaging techniques are of growing interest in biology
for subcellular visualization and in industry for micro- and nanomaterial characterization. Several techniques have been developed in recent years in order to overcome
the resolution limit imposed by the diffraction of light which results in the fundamental best values of lateral and axial resolutions as half of the wavelength and the
wavelength, respectively. Indeed, a diffraction-limited imaging system, working in
the visible range and having a numerical aperture of 1.0, is thus not able to visualize
an object having a lateral feature size smaller than around 300 nm and an axial feature
size smaller than around 600 nm.
Nowadays, the main full-field super-resolution techniques allow a resolution of
up to 100 nm to be reached, compared with the scanning super-resolution techniques
which can achieve a few tens of nanometres. By placing a glass microsphere within
a classical white light microscope on (or above) the sample makes it possible to
visualize sub-diffraction-limited object structures of the sample in 2D. The principle consists of collecting the super-resolution image generated by the microsphere,
allowing a lateral resolution of below 100 nm to be achieved in air. Microsphereassisted microscopy distinguishes itself from the other super-resolution techniques
by its easy-to-implement capacity and its low-cost aspect. While the results from
experimental measurements and simulations have clearly demonstrated the effect of
super resolution, with a lateral resolution limit of λ/6 being claimed, the physical
principle behind the technique remains unclear to date. The strongest contender for
an explanation of the phenomenon is that the microsphere acts as an optical element
that transforms the evanescent waves into propagating waves. But then it is also
important to consider the geometrical and optical parameters of the imaging system
due to their influence on the performance and image formation.
More recently, microsphere-assisted microscopy was further enhanced by introducing a reference arm, so as to superimpose an interference pattern on the superresolution image and to access the phase of the wavefront. Microsphere-assisted
interference microscopy was then implemented using digital holography, coherence
scanning interferometry and phase-stepping interferometry so as to be able to reconstruct the surface topography of an object sub-wavelength in size. This new 3D
imaging technique appears to be extremely promising for nanometrology and subcellular imaging.
Acknowledgements The author would like to thank the members of IPP research team of the
ICube Laboratory (Strasbourg, France) for their contributions, specially A. Leong-Hoi for her PhD
thesis work on super-resolution optical imaging and P. Twardowski for his useful discussions and
463
in the reference arm. Finally, an application for the characterization of femtosecond
LASER induced nanoripples in stainless steel, spaced by a few 100 nanometres was
demonstrated [158] (Fig. 17.16).
17.4 Conclusion
Label-free super-resolution imaging techniques are of growing interest in biology
for subcellular visualization and in industry for micro- and nanomaterial characterization. Several techniques have been developed in recent years in order to overcome
the resolution limit imposed by the diffraction of light which results in the fundamental best values of lateral and axial resolutions as half of the wavelength and the
wavelength, respectively. Indeed, a diffraction-limited imaging system, working in
the visible range and having a numerical aperture of 1.0, is thus not able to visualize
an object having a lateral feature size smaller than around 300 nm and an axial feature
size smaller than around 600 nm.
Nowadays, the main full-field super-resolution techniques allow a resolution of
up to 100 nm to be reached, compared with the scanning super-resolution techniques
which can achieve a few tens of nanometres. By placing a glass microsphere within
a classical white light microscope on (or above) the sample makes it possible to
visualize sub-diffraction-limited object structures of the sample in 2D. The principle consists of collecting the super-resolution image generated by the microsphere,
allowing a lateral resolution of below 100 nm to be achieved in air. Microsphereassisted microscopy distinguishes itself from the other super-resolution techniques
by its easy-to-implement capacity and its low-cost aspect. While the results from
experimental measurements and simulations have clearly demonstrated the effect of
super resolution, with a lateral resolution limit of λ/6 being claimed, the physical
principle behind the technique remains unclear to date. The strongest contender for
an explanation of the phenomenon is that the microsphere acts as an optical element
that transforms the evanescent waves into propagating waves. But then it is also
important to consider the geometrical and optical parameters of the imaging system
due to their influence on the performance and image formation.
More recently, microsphere-assisted microscopy was further enhanced by introducing a reference arm, so as to superimpose an interference pattern on the superresolution image and to access the phase of the wavefront. Microsphere-assisted
interference microscopy was then implemented using digital holography, coherence
scanning interferometry and phase-stepping interferometry so as to be able to reconstruct the surface topography of an object sub-wavelength in size. This new 3D
imaging technique appears to be extremely promising for nanometrology and subcellular imaging.
Acknowledgements The author would like to thank the members of IPP research team of the
ICube Laboratory (Strasbourg, France) for their contributions, specially A. Leong-Hoi for her PhD
thesis work on super-resolution optical imaging and P. Twardowski for his useful discussions and