Biomedical Engineering Reference
In-Depth Information
(a)
PMT
(b)
PMT
TL
TL
F
F
BE
BE
BS
BS
Di-M
Di-M
M
M
M
M
M
M
M
M
MO
MO
L
L
FDC
FDC
DL
DL
S
S
L
L
MO
MO
o
BE
BE
o
*
FL
FL
F
F
BS
BS
r
r
M
M
TL
CCD
TL
CCD
SLM
SLM
FIgurE 9.15
Schematics of holographic SHG microscope for recording the complex scattered SHG field (a) and
of the phase-conjugate scanning microscope (b). Note that (a) and (b) show the same setup with different light illu-
minations for different steps of the experiment. The light not in use is blocked in the experiment and is not shown
in the figures. BS, beam splitter; L, lens; M, mirror; Di-M, dichroic mirror; BE, beam expander; FDC, frequency
doubler crystal; MO,
∞
-corrected microscope objective; TL, tube lens; S, specimen; F, filter; CCD digital sensor;
SLM phase-only reflective spatial light modulator and PMT photo-multiplier tube. Beam envelopes are represented
by uniform gray shades (dark for fundamental and pale for the second harmonic), while image formation is repre-
sented by dashed rays.
is based on characterization of wavefront distortion by use of a coherent point source emitter located
inside the turbid medium, at the position of desired focus or subject to image. In their work, a non-
centrosymmetric second-harmonic generating BaTiO
3
nanoparticle (300 nm in diameter) served as the
coherent point-like emitter.
Let us look into the details of holographic SHG imaging-based optical phase conjugation process.
In a first step, a hologram of the second harmonic emitted by the nanoparticle and subsequently scat-
tered throughout the turbid medium is recorded by its interference with an SHG reference wave, see
Figure 9.15a. Full characterization of the wavefront distortion induced by the turbid medium is pro-
vided by the reconstructed SHG amplitude and phase. The reader may have noticed that first step is
nothing but a straightforward application of imaging with a state-of-the-art holographic SHG micro-
scope, as described in Section 9.3.
Then, in a second step, an illumination wave that is the phase-conjugate of the scattered SHG emis-
sion from the nanoparticle has to be generated and projected in the turbid medium. A simple way to
generate the phase-conjugated illumination wave, at the SHG wavelength, is to shape the reference wave
