[spr] question

Sara Abrahamsson

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\nHi, I am a student at UCSF, and stumbled on your list while searching\nfor truth... I would like to pose a question to your spr if you will\nallow me.\n\nIs the fluorescence light emitted from GFP coherent?\n\n----------------------------------------------------------------------\nSara Abrahamsson\nDept. of Physiology, Division of Bioengineering\nUniversity of California at San Francisco, Box 2240\n600 - 16th Street San Francisco, CA 94143-2240\nUSA\n\nPhone: +1(415)514-1366, Fax: +1(415)514-4242\nMobile: +1(415)359-5805\nE-mail: saraabr@msg.ucsf.edu\n----------------------------------------------------------------------\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>Hi, I am a student at UCSF, and stumbled on your list while searching
for truth... I would like to pose a question to your spr if you will
allow me.

Is the fluorescence light emitted from GFP coherent?

----------------------------------------------------------------------
Sara Abrahamsson
Dept. of Physiology, Division of Bioengineering
University of California at San Francisco, Box 2240
600 - 16th Street San Francisco, [itex]CA 94143-2240[/itex]
USA

Phone: [itex]+1(415)514-1366,[/itex] Fax: [itex]+1(415)514-4242[/itex]
Mobile: [itex]+1(415)359-5805[/itex]
E-mail: saraabr@msg.ucsf.edu
----------------------------------------------------------------------

Uncle Al

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Sara Abrahamsson wrote:\n>\n> Hi, I am a student at UCSF, and stumbled on your list while searching\n> for truth... I would like to pose a question to your spr if you will\n> allow me.\n>\n> Is the fluorescence light emitted from GFP coherent?\n\nWhy should it be coherent? In principle you could use it as laser\ndye, even as a one pass superradiant laser, but the conditions for\nlasing are vastly different from looking at a faint green glow under a\nmicroscope - that little thing about population inversion, for\nexample.\n\nIn principle one might get coherent emission by stacking the emitters\nin a periodic crystal lattice, like cornflower blue (alternate\nstacking of cyanidin and caffeic acid) or thylakoids in\nphotosynthesis. One imagines exploiting a stromal structure to couple\ncoherent emission would require a planar, extremely anisotropic\nfluorophore to support both stacking and directed primary emission.\nIsn\'t aquorin more like a beachball than a frisbee?\n\nDo you see laser speckle or diffraction fringes?\n\n--\nUncle Al\nhttp://www.mazepath.com/uncleal/\n(Toxic URL! Unsafe for children and most mammals)\nhttp://www.mazepath.com/uncleal/qz.pdf\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>Sara Abrahamsson wrote:
>
> Hi, I am a student at UCSF, and stumbled on your list while searching
> for truth... I would like to pose a question to your spr if you will
> allow me.
>
> Is the fluorescence light emitted from GFP coherent?

Why should it be coherent? In principle you could use it as laser
dye, even as a one pass superradiant laser, but the conditions for
lasing are vastly different from looking at a faint green glow under a
microscope - that little thing about population inversion, for
example.

In principle one might get coherent emission by stacking the emitters
in a periodic crystal lattice, like cornflower blue (alternate
stacking of cyanidin and caffeic acid) or thylakoids in
photosynthesis. One imagines exploiting a stromal structure to couple
coherent emission would require a planar, extremely anisotropic
fluorophore to support both stacking and directed primary emission.
Isn't aquorin more like a beachball than a frisbee?

Do you see laser speckle or diffraction fringes?

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

Andy Resnick

Sara Abrahamsson wrote:

'Hi, I am a student at UCSF, and stumbled on your list while searching
for truth... I would like to pose a question to your spr if you will
allow me.

Is the fluorescence light emitted from GFP coherent?'

GFP is green fluorescent protein, the best-known of a bunch of
jellyfish-derived fluorophores (CFP, YFP, EGFP, etc. etc. are others).

One should be careful in making assumptions about the coherence of a
(coherently stimulated) fluorophore. Some versions of GFP have an
emission bandwidth of approximately 50 nm; at a mean wavelength of 500
nm, this corresponds to a (temporal) coherence length of ~ (500/50)*500
nm = 5000 nm, which is rather short. Emission filters can increase this
length to (500/2)*500 = 0.125 mm without too much difficulty, but there
is a loss of intensity. So a sea of GFP can exhibit temporal coherence
fairly readily.

In terms of spatial coherence, it depends on the size of the emitter.
For a single GFP molecule, the spatial coherence is rather large (but
the emission rate is not constant), while for collections of GFP-tagged
proteins, all excited by the same (presumably coherent) exciter, the
spatial coherence will scale inversely as the are of the source.
Specific numbers require more information about your setup, but it is
definitely possible for the GFP to exhibit spatial coherence.