Biomedical Engineering Reference
In-Depth Information
second-harmonic reference wave. An interesting consequence of the coherent nature of SHG is that
the second-harmonic signals generated in different media by radiation originating from the same light
source are mutually coherent. Because of it, both the amplitude and the phase of the second harmonic
generated by the specimen may be encoded in the form of a fringe pattern and retrieved by either optical
or numerical reconstruction of the hologram. Either way, hologram reconstruction produces 3D images
of the SHG intensity contrast (holographic SHG intensity images), but numerical reconstruction also
enables quantitative SHG phase contrast (holographic SHG phase images).
9.1.1 the Short History of Holographic SHG imaging
To the best of our knowledge, holographic SHG imaging was first reported by Ye Pu and Demetri
Psaltis (2006). At that time, Pu, a postdoctoral fellow, was working with Psaltis, director of the DARPA
Center for Optofluidic Integration at the California Institute of Technology, and the two of them were
looking for means to monitor highly dynamic systems. Monitoring highly dynamic systems poses
two big challenges. First, it requires high-speed image acquisition, which can only be provided by
nonscanning imaging techniques, and second, it needs a 3D imaging scheme that does not suffer
from heavy background scattering. It happened that digital holography of second-harmonic signals
fulfilled those needs.
In the first years of holographic SHG imaging, only two groups worldwide were active in the field. The
only other group, based in Switzerland, was led by Christian Depeursinge, an adjunct professor at the
École Polytechnique Fédérale de Lausanne (EPFL), and specialized in digital holographic microscopy.
Etienne Shaffer, at the time a doctoral student working in Depeursinge's group, had started investigating
possible application of digital holographic microscopy to nonlinearly generated signals, such as SHG.
That year, a strange turn of fate had Pu and Psaltis moving their lab to EPFL, thus concentrating all
world research on holographic SHG microscopy in one single Swiss institution.
In 2008, Pu et al . detailed the holographic SHG microscopy technique more deeply and investigated
its benefits over direct SHG imaging in terms of signal-to-noise ratio (Pu et al . , 2008). Then, Psaltis' team
dedicated a lot of efforts in the development of highly efficient nanoprobes for SHG imaging and Chia-
Lung Hsieh, a doctoral student working in Psaltis' group, demonstrated 3D imaging of these markers
by holographic SHG microscopy (Hsieh et al . , 2009). A year later, Hsieh reported successful antibody
conjugation for specific labeling (Hsieh et al . , 2010b), as well as its use for phase conjugation imaging
(Hsieh et al . , 2010a,c).
During that time, Depeursinge's team devoted its efforts to exploitation of the SHG phase signal.
(Shaffer et al . reported the first representation and interpretation of the SHG phase and, as a proof of
concept, demonstrated that it could reveal the polarization component of the focused laser illumina-
tion responsible for generation of second harmonic at a glass/air interface (Shaffer et al . , 2009). One
year later, they established the relation between the detected SHG phase, the medium refractive index
and the axial position at which second harmonic is generated. This SHG phase relation made possible
nanometer-scale, 3D tracking of SHG-emitting nanoparticles (Shaffer et al . , 2010a). At the same time,
SHG phase was also proposed as a contrast agent for imaging of label-free biological sample (Shaffer et
al . , 2010b), revealing phase-matching conditions.
Earlier that same year, Omid Masihzadeh, a doctoral student working with Randy Bartels, associate
professor at the Colorado State University, reported on the label-free holographic SHG intensity images
of biological specimen (Masihzadeh et al . , 2010).
Second-harmonic interferometry has been around for some time (Chang et al . , 1965), but its use for
imaging purposes, as in the so-called interferometric SHG microscopy (Yazdanfar et al . , 2004), is some-
what newer. Even more recent is holographic SHG imaging; its nonscanning counterpart recently made
possible by the development of both ultrafast lasers and very sensitive digital sensors. While holographic
SHG imaging has only started gathering interests and enthusiasm, it has already paved the way to other
nonlinear holographic imaging techniques (Xu et al . , 2010b). In the years to come, and as ultrafast lasers
Search WWH ::




Custom Search