Posted by
Steffen Dietzel on
URL: http://confocal-microscopy-list.275.s1.nabble.com/BioRad-MRC-600-scan-generator-card-tp2232860p2246323.html
Dear all,
I am trying to get a better understanding of Higher Harmonic Generation.
From what I have read and experienced so far,
the forward second Harmonic Generation (SHG)
signal is in most cases stronger than the backward signal.
Is there a theory or investigaton about the
3D-distribution, i.e. what "forward" and
particularly "backward" acutally mean? I found
one paper for forward SHG (and THG) that explains
that "forward" is acutally not exactly forward
but the SHG signal is distributed as a hollow
cone, with nothing at the center (Moreaux et al.,
2001,
http://www.ncbi.nlm.nih.gov/pubmed/11222317). But
I didn't find anything for the backward signal.
I heard and read several opinions, some of which are mutually exclusive.
- backward SHG is just forward SHG signal which is scattered back.
- Some objects produce more backward SHG signal
than others (relative to the forward signal)
- "backward" is not exactly backward but goes
away to the side, at some angle to the optical
axis (hollow cone, as for forward)
- Forward Third Harmonic Generation (THG) signal
is distributed also as a hollow cone, but tighter (in the Moreaux-Paper)
- THG is not oriented, goes in all directions equally.
I'd be glad if people could comment on these points.
I guess good reviews on these subjects would also
help. The problem is that many of such articles
use tech speak which might be ok for physicists
but partly incomprehensible for others if they
use stuff like "cross-section", "dipoles" or
"vector electric field" without explaining them.
More of academic interest: I found a statement
that, at first, SHG is produced equally towards
all sides (or at least more directions) but then,
in a second step, wave interference nihilates it
except for the forward direction. However, if
there is destructive interference of light, the
energy must stay somewhere. Is the statement that
no energy deposition occurs in the sample thus
really true? (Assuming that there is no regular
absorbtion and autofluorescence).
Another one out of academic interest: Articles
often write something like "Higher harmonic
generation, including SHG and THG" - Is there
anything but these two? If we could get a >1600
nm laser, would we start to see Fourth Harmonic Generation?
Also, has anybody an idea for good SHG/THG test
slides with reproducible signals? Inspired by
papers of Guy Cox, I have tried microtome
sections of fresh potatoes which contain a lot of
SHG signal-generating starch granules, but the
granules vary a lot in size and signal. (I still
have to see whether I can find the equivalent of
an 'unidentified moss species from a Sydney wall'
in a Munich winter :-) ). Urea crystals do not
work well with water dipping objectives. Collagen
matrix sort of worked if we stayed above the
minimum laser power to generate a signal and
below the point where we fry the matrix, the
corridor is not too wide. No ideas for THG tests so far.
Thanks for any help
Steffen
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Steffen Dietzel, PD Dr. rer. nat
Ludwig-Maximilians-Universität München
Walter-Brendel-Zentrum für experimentelle Medizin (WBex)
Head of light microscopy
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