http://confocal-microscopy-list.275.s1.nabble.com/BioRad-MRC-600-scan-generator-card-tp2232860p2246440.html
tell you what we use as a consistent sample for forward scatter SHG. We
> 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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