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Why do you think there is a population inversion? In my opinion, this seems rather unlikely given the spill of the depletion beam to the centre and that saturation effects in the excitation beam should reduce resolution -so ground state depletion should be avoided.

I think this introduction of the term 'laser" to the discussion of STED is neither correct nor warranted. Just because there is stimulated emission, this does not imply lasing.  I suggest there is no lasing in the excited volume as the fraction of exited molecules is <0.5. -i.e. there is no population inversion. Of course, it would be possible to achieve this but it would not be desirable.

My 2c

Cheers Mark


On 2/09/2012, at 10:13 PM, "Chen, De (NIH/NCI) [C]" <[log in to unmask]> wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
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> 
> Hi Peng:
> 
> For the two energy levels related to STED depletion, there is population inversion exists. So this kind of lasing is best termed the amplified spontaneous emission. 
> 
> De
> 
> ________________________________________
> From: Peng Xi [[log in to unmask]]
> Sent: Saturday, September 01, 2012 9:31 PM
> To: [log in to unmask]
> Subject: Re: STED detection
> 
> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
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> 
> Dear Guy,
>    If you consider the pump loses one photon, while the probe earns
> one photon, you know what I meant by amplification. Of course without
> population reversion constantly (the pump is so low), and absence of
> the reflective cavity, it is not a normal laser that we are familiar
> with. :)
> Peng
> 
> On Sun, Sep 2, 2012 at 8:49 AM, Guy Cox <[log in to unmask]> wrote:
>> Really it isn't a mini-laser because there is no amplification - it's only when the emitted photon gives rise to further stimulated emission that you get amplification, and laser action.
>> 
>> Fu-Jen Kao, in Taiwan, has done a lot of work on stimulated emisssion microscopy - it's useful because you know in what direction the emitted photon will be travelling.  (This is nothing  to do with STED, he's detecting the emitted photons, not spontaneous fluorescence).
>> 
>>                                                              Guy
>> 
>> -----Original Message-----
>> From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Peng Xi
>> Sent: Sunday, 2 September 2012 9:47 AM
>> To: [log in to unmask]
>> Subject: Re: STED detection
>> 
>> *****
>> To join, leave or search the confocal microscopy listserv, go to:
>> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
>> *****
>> 
>> Dear Arne,
>>     Here "mini laser" referes to the stimulated emission depletion process happening at the peripherial of the excited fluorescence PSF.
>> It is indeed light amplification if taking the excitation laser as the pump source; and sometimes it is called pump-probe process.
>>    "Multi-molecule" is a word I created in contrast to single molecule, since the ON state, spontaneous emission molecules in STED is 50-80 diameter, thus certainly contains more than one molecule.
>> 
>> Hope that helps,
>> Peng Xi
>> Ph. D.    Associate Professor
>> Dept. of Biomedical Engineering, College of Engineering Peking University, Beijing, China
>> Email: [log in to unmask]
>> http://xipeng.wordpress.com
>> 
>> On Sun, Sep 2, 2012 at 4:03 AM, Seitz Arne <[log in to unmask]> wrote:
>>> *****
>>> To join, leave or search the confocal microscopy listserv, go to:
>>> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
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>>> 
>>> What exactly is a "mini laser" and a "multi-molecule"?
>>> 
>>> Cheers Arne
>>> 
>>> -----Original Message-----
>>> From: Confocal Microscopy List
>>> [mailto:[log in to unmask]] On Behalf Of Peng Xi
>>> Sent: samedi 1 septembre 2012 01:55
>>> To: [log in to unmask]
>>> Subject: Re: STED detection
>>> 
>>> *****
>>> To join, leave or search the confocal microscopy listserv, go to:
>>> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
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>>> 
>>> Dear Alan,
>>>     I'd like to quote Paul Silven's solution on single molecule localization microscopy: "The trick to get around this problem was to realize that a SINGLE molecule's position can be located arbitrarily well. It's much like a mountain peak, which can be located to within a few yards, even though the mountain itself may be a a mile wide."
>>>     Once you understand that in a wide-field microscopy, you can determine the position of a single molecule extremely well, you can bring it to a scanning imaging mode. Take sample scan first because laser scan is equivalent. All you need to do is to set your scan step equals to sufficient sampling, so that you can accurately fit the Gaussian. The Nyquist-Shannon always holds true.
>>>     Now come to STED, where you use mini laser and dichroic to have
>>> somehow a multi-molecule with size of 50-80nm. By   scanning you get
>>> the high-resolution image.
>>> 
>>> 
>>> Sincerely,
>>> Peng Xi
>>> Ph. D.    Associate Professor
>>> Dept. of Biomedical Engineering, College of Engineering Peking
>>> University, Beijing, China
>>> Email: [log in to unmask]
>>> http://xipeng.wordpress.com
>>> 
>>> On Fri, Aug 31, 2012 at 8:22 PM, Alan Smith <[log in to unmask]> wrote:
>>>> *****
>>>> To join, leave or search the confocal microscopy listserv, go to:
>>>> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
>>>> *****
>>>> 
>>>> Dear list,
>>>> 
>>>> Firstly, sorry if this is not confocally enough for this forum, but I
>>>> hope someone would know. I was wondering if anyone could help with
>>>> some a confusion I have about STED detection.
>>>> 
>>>> I understand how the illumination psf is formed in STED using a
>>>> depletion beam. However, if the emission is detected is collected in
>>>> epi-detection, why is the resolution not determined by the diffraction limit for the objective.
>>>> 
>>>> As an example, if a single molecule was producing fluorescence, the
>>>> image will be an airy disk determined by the NA of the detecting
>>>> objective and the wavelength of light. Despite the fluorescence
>>>> solely coming from a single molecule.
>>>> 
>>>> I know that for MP excitation the psf is solely determined by the
>>>> illumination psf, but again, I am a little unsure as to the reason for this.
>>>> Plus STED can obviously be CW also.
>>>> 
>>>> Any help or suggestions would be brilliant!
>>>> Much appreciated.
>>>> Alan Smith
>> 

Mark  B. Cannell Ph.D. FRSNZ
Professor of Cardiac Cell Biology
School of Physiology&  Pharmacology
Medical Sciences Building
University of Bristol
Bristol
BS8 1TD UK

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