Biomedical Engineering Reference
In-Depth Information
9.7.1 Summary
The most distinctive advantage of holographic SHG imaging certainly is the simultaneous retrieval of
both the SHG amplitude and the phase, instead of only its intensity. This feature makes digital holog-
raphy stand apart from most, if not all, other imaging techniques and normally so, since detectors are
generally sensitive only to light intensity. The unique nature of digital holography makes possible to
retrieve multiple different contrast images from the same intensity-only hologram.
The numerical hologram reconstruction process—more specifically the numerical field propagation
algorithms—makes possible numerical focusing of the images. Numerical focusing is especially useful
to compensate drifts of focus over long-term experiments or in an unstable environment. Also, it makes
possible to bring in focus different sections of a specimen separated by distances exceeding the depth
of the field of the imaging system, which makes possible to assess the axial position of SHG scatterers.
Holographic SHG imaging, based on off-axis digital holography, is a nonscanning, single-shot image
acquisition technique. As such, it is in principle limited in speed only by the camera frame rate and
available SHG signal, which depends in a nonsaturating manner on the peak power density of the laser
source. In other words, holographic SHG imaging is especially suited for real-time imaging, and has the
potential to truly exploit the instantaneous response time of SHG.
9.7.2 outlook and Perspectives
To this day, nonscanning holographic SHG cannot compete with scanning SHG microscopy in terms of
sensitivity. Holographic SHG offers coherent amplification, low sensitivity to shot noise and very high-
phase SNR, but, in the end, it all comes down to a matter of available light sources and detectors. It is actu-
ally not possible to deliver peak powers comparable to those offered by scanning microscopes over very
large field of views. It is no wonder that the increasing enthusiasm around holographic SHG microscopy
is fueled by the recent developments of both ultrafast lasers and electron-multiplying digital cameras.
Acknowledgments
The authors would like to thank Francesco S. Pavone and Paul J. Campagnola for their invitation to con-
tribute to this topic. In addition, the authors would also like to acknowledge all members of Professor
Christian Depeursinge's group, especially Nicolas Pavillon, as well as Chia-Lung Hsieh, Pierre Marquet,
and Randy Bartels, for their respective contributions.
References
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