GFP laser action all in a live cell (except for the resonant mirrors)

[John Griessen]

http://www.bbc.co.uk/news/science-environment-13725719

[Forrest Flanagan]

Biological laser beams are still a ways off.

On Sun, Jun 12, 2011 at 9:16 PM, John Griessen <jo...@industromatic.com> wrote:

http://www.bbc.co.uk/news/science-environment-13725719

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[Jelmer Cnossen]

Sounds cool.. I still have to read the paper but I can imagine it being a nice way to enhance the efficiency of GFP (As the photons will only travel in the direction you want them to)

This is the direct paper link: http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2011.99.html

[Forrest Flanagan]

I don't think this causes the photons only to travel in one direction, I think it just synchronizes them.

[John Griessen]

On 06/13/2011 06:15 PM, Forrest Flanagan wrote:
> I don't think this causes the photons only to travel in one direction, I think it just synchronizes them.

The external mirrors do cause most of the light to be in a narrow angle beam,
and the light amplification effect synchronizes wavefronts.

[Simon Quellen Field]

No, I think Jelmer is right.

As photons are emitted, some of them will hit one of the mirrors and bounce

back through the medium again on its way to the other mirror. Those photons

that hit one of the mirrors at 90 degrees will make many passes through the

medium, and thus become amplified.

Those amplified photons will always be at 90 degrees to the mirrors. That is

a fairly narrow beam, and gets very narrow if the mirrors are farther apart.

In order to detect the photons, the detector either needs to be inside the

optical cavity (where it will block some of the photons) or it needs to be

outside the cavity, which requires the mirror closest to the detector to be

partially silvered, so that some photons can escape to be detected.

That second setup is what Jelmer seems to have been referring to, since

the photons are travelling in the direction you want them to, i.e. towards the

detector.

Of course the direction is less important than the amplification for our purposes.

Amplification allows much higher signal to noise ratios.

Having built many dye lasers in my youth (well, maybe dozens), this looks like

something that is very amenable to hacking at home. The mirrors are cheap,

you don't need to go really small like the experimenters did, and you can use

some purple of UV LEDs or a cheap 405 nm purple laser pointer to excite the

dye (although I can't guarantee the bugs will survive high brightness purple or

UV radiation).

If you cement the half-silvered mirror to an expendable microscope objective

(say a $35 40x objective), you can aim your digital SLR down the tube where

the eyepiece used to be and snap some 18 mega-pixel images (I use a Canon

T2i for micro-photography). If the bugs aren't moving, you can integrate using a

long exposure, but the laser amplification might let you do real time video.

If someone wants to send me a culture of GFP E. coli, I will try the experiment

and post 1080p video.

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[CoryG]

I really don't see how this is a LASER, it's not coherent, it's not
polarized, and the only thing LASER-like is that you pump it (and
they're actually just pumping the chemical that makes jellyfish glow -
with a LOT of added junk in the way of the beam [the cellular material
and proteins]). Even the notion that it spits out photons in sync is
flawed since you would have a wavefront moving at the speed of light
just from the pump, which might not be a factor in a larger laser
(which will typically work off a critical number of photons being
required to traverse through the half-mirror) - but in this all that
junk that isn't contributing to the light emission will likely absorb
enough light that it can't bounce back and fourth between the mirrors
to reach that point before losing a lot of power to heat buildup from
absorbed photons.

All that said - it is kind of cool in that it might have uses in
photonic computing - having identified an organic molecule capable of
transforming one wavelength to another could be pretty useful in that
regard, if we can could get enough such molecules cataloged in a
manner that allows us to identify a group that can interact with each
other with minimal energy loss in a closed environment it could pave
the way for a processor working at the speed of light that just has to
be pumped with light that is of a wavelength other than those used for
calculations (of course keeping the molecules in the proper structure
would be a factor, but organic molecules exhibiting these effects tend
to be fairly large so that's a plus).

> <solenoidcl...@gmail.com>wrote:

>
>
>
>
>
>
>
> > I don't think this causes the photons only to travel in one direction, I
> > think it just synchronizes them.
>

> > On Mon, Jun 13, 2011 at 6:13 PM, Jelmer Cnossen <j.cnos...@gmail.com>wrote:
>
> >> Sounds cool.. I still have to read the paper but I can imagine it being a
> >> nice way to enhance the efficiency of GFP (As the photons will only travel
> >> in the direction you want them to)
> >> This is the direct paper link:

> >>http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2011...

>
> >> On Mon, Jun 13, 2011 at 11:02 PM, Forrest Flanagan <
> >> solenoidcl...@gmail.com> wrote:
>
> >>> Biological laser beams are still a ways off.
>

[Jelmer Cnossen]

On Tue, Jun 14, 2011 at 1:36 AM, Simon Quellen Field <sfi...@scitoys.com> wrote:

No, I think Jelmer is right.

As photons are emitted, some of them will hit one of the mirrors and bounce

back through the medium again on its way to the other mirror. Those photons

that hit one of the mirrors at 90 degrees will make many passes through the

medium, and thus become amplified.

Those amplified photons will always be at 90 degrees to the mirrors. That is

a fairly narrow beam, and gets very narrow if the mirrors are farther apart.

In order to detect the photons, the detector either needs to be inside the

optical cavity (where it will block some of the photons) or it needs to be

outside the cavity, which requires the mirror closest to the detector to be

partially silvered, so that some photons can escape to be detected.

That second setup is what Jelmer seems to have been referring to, since

the photons are travelling in the direction you want them to, i.e. towards the

detector.

Thanks for explaining :) 

Of course the direction is less important than the amplification for our purposes.

Amplification allows much higher signal to noise ratios.

Here is what I was thinking (As far as my basic knowledge of lasers go):

When I mentioned GFP efficiency I was mostly thinking about getting as many photons from a single GFP to the objective as possible, before it photobleaches and you lose it (or it moves and your localization has lower resolution). 

In single-molecule fluorescence microscopy this is one of big the limiting factors. 

If I understand the amplication you mentioned, this amplification involves 'pushing' a GFP (that's already excited) to give off a photon in the laser direction, instead of some random direction. In other words, the GFP still gives the same number of photons, but they are directed.

Having built many dye lasers in my youth (well, maybe dozens), this looks like

something that is very amenable to hacking at home. The mirrors are cheap,

you don't need to go really small like the experimenters did, and you can use

some purple of UV LEDs or a cheap 405 nm purple laser pointer to excite the

dye (although I can't guarantee the bugs will survive high brightness purple or

UV radiation).

If you cement the half-silvered mirror to an expendable microscope objective

(say a $35 40x objective), you can aim your digital SLR down the tube where

the eyepiece used to be and snap some 18 mega-pixel images (I use a Canon

T2i for micro-photography). If the bugs aren't moving, you can integrate using a

long exposure, but the laser amplification might let you do real time video.

If someone wants to send me a culture of GFP E. coli, I will try the experiment

and post 1080p video.

Then again, if you just want a picture of a bug, you can do it already with conventional fluorescence microscopy. So although a laser sounds fancy, I'm not yet sure what to use it for. 

Note that in the paper they are using a mammalian kidney cell with a diameter of about 15 um. It sounds likely that a single E. coli will be too small to generate enough light and get the lasing going.

[Nathan McCorkle]

On Tue, Jun 14, 2011 at 5:10 AM, CoryG <co...@geesaman.com> wrote:
> I really don't see how this is a LASER, it's not coherent, it's not

Why isn't it coherent? If the pumping light is blocked and GFP only
fluoresces at a single output freq, wouldn't it be coherent (is
coherent defined with more terms that having just a single
wavelength???)

Otherwise, yeah it seems to be a bit of a hype, but I am no
photonics/laser/physics scientist... can someone here explain better
what a laser is, and how this approaches or encompasses that
definition?

--
Nathan McCorkle
Rochester Institute of Technology
College of Science, Biotechnology/Bioinformatics

[Simon Quellen Field]

It is coherent.

I don't see why you would think it wasn't.

In any fluorescent dye, there is a lag between excitation and emission.

A photon of the right color will stimulate emission in excited molecules.

The emitted photon will be in phase with the stimulating photon.

All you need for a laser is an excited dye and an optical cavity.

This setup qualifies.

The reason it is useful in microscopy is that the stimulated photons are moving

in the direction of the sensor, instead of in a random direction. The number of

photons that hit the sensor is thus many times larger than in normal fluorescence

microscopy.

If there were 'a lot of added junk' in the way of the photons, then you wouldn't see them.

This is no different than normal fluorescence microscopy, which also relies on the

cells to be effectively transparent.

Your comment on why you think the photons are not in sync also confuses me.

The pump (say a blue LED) is emitting a lot of photons which are exciting the GFP.

That's all it is there for. The pump is not coherent, (you could use a laser, but it

isn't a requirement). The pump simply keeps hitting the molecules with blue photons,

causing them to become excited. In the setup I plan to use, the blue light comes in

parallel to and between the mirrors, so it is at right angles to the green laser photons,

and is thus easier to filter out at the sensor, since only a few scattered blue photons will

go in that direction.

The stimulated emission is in sync (same phase) with the stimulating photons that

are bouncing between the mirrors. Each time a photon bounces through the cell,

it has a chance to stimulate an excited dye molecule to emit a photon. After the

molecule emits a photon, it can be excited again, and stimulated to emit a second

photon.

Stimulated photons have the same polarization as the stimulating photon.

If spontaneous emission is much slower than stimulated emission, then the light

will end up polarized. For example, consider a spontaneously emitted photon

that starts things off. That photon will most likely miss the mirrors. So will most

of the others. But eventually, a photon is emitted perpendicular to the mirrors, and

bounces back into the medium. It stimulates an emission, so now there are two

photons, in phase, and with the same polarization, and the same direction of

travel. They bounce against a mirror and re-enter the dye, stimulating more emission,

so now we have four photons in sync, in phase, with the same polarization, and

travelling in the same direction.

You can see that in a short time, this exponential growth will mean that the laser

light will consist mostly of photons that are in phase, polarized the same, and travelling

perpendicular to the mirrors. If one in a thousand gets through the partially silvered

mirror, we detect it in the sensor, and it no longer adds to the amplification.

In normal fluorescence microscopy, the GFP molecule is excited and emits thousands

of photons before one happens to go in the right direction to be picked up by the sensor.

Thus photo bleaching is a problem. But if almost all of the emissions make it to the

sensor, because they were stimulated by photons going in that direction, then we get

much better yield before the molecule degrades.

A single molecule can be made to lase. Thus the size of E. coli is not an obstacle.

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On Tue, Jun 14, 2011 at 5:10 AM, CoryG <co...@geesaman.com> wrote:

[Simon Quellen Field]

As a laser, the E. coli have an advantage over other dye lasers.

That is because when a molecule is photo bleached, the bug creates more.

Thus it is self-renewing.

How much of an advantage this is in reality is not so apparent.

You still have to feed the bugs, and adding sugar to the medium is no easier

than adding more dye to a non-biological dye laser.

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[Nathan McCorkle]

Simon, if you are using just two mirrors, won't some photons still go
in random directions that aren't towards the sensor? Are all the
mirrors coated so they are reflective to GFP emission except the one
in the direction of the sensor???

--

[Simon Quellen Field]

The spontaneously emitted photons will most likely miss the mirrors.

That can be good, since it allows us to use a polarizing filter to let the

stimulated photons through, and block any extraneous light from the sensor.

The stimulated photons are emitted perpendicular to the mirrors, since that is

where the photons that stimulated them came from.

There are only two mirrors. One is partially silvered, and the sensor is behind that.

There is no point in using expensive dielectric mirrors, an ordinary mirror at the

back, and a simple partially coated mirror at the front will do fine. The front mirror

must still reflect a decent amount of green back into the medium, in order to get

amplification. The light will bounce back and forth many times, getting brighter

on each pass.

Spherical mirrors can be used to eliminate some of the fiddling around needed

to get them parallel.

To verify that you actually have a laser, you simply block the back mirror with

a piece of cardboard. The light should suddenly get much dimmer.

The pump light generally needs to be quite bright. This can be done with a laser

(such as a $10 405 nm 10 milliwatt laser pointer), or you can use a watt or so of

LED light focused on the sample. This is why I am curious about whether the bugs

will survive.

Most of the problems encountered in making dye lasers (which are actually fairly

simple to build for amateurs) involve wavelength tuning (not needed here) and

high gain (not needed here). Gains of 2 to 10 will make a big difference in imaging

bugs, so we don't need gains of thousands or millions.

"http://www.repairfaq.org/sam/lasercdy.htm"

"http://192.197.62.35/staff/mcsele/lasers/LasersDye.htm"

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[Nathan McCorkle]

On Tue, Jun 14, 2011 at 4:14 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> The spontaneously emitted photons will most likely miss the mirrors.
> That can be good, since it allows us to use a polarizing filter to let the
> stimulated photons through, and block any extraneous light from the sensor.
> The stimulated photons are emitted perpendicular to the mirrors, since that
> is
> where the photons that stimulated them came from.

Why are emitted photons always emitted perpendicular to the excitation
source? Wouldn't there still be some axis of rotation that the emitted
photons not be reflected toward the sensor? i.e. if you put an
L-shaped allen key into the chuck of a drill, the free end of the
allen key is perpendicular to the drill's chuck, but as the drill
rotates it doesn't necessarily point up or down, the position of the
two parallel mirrors.

[Simon Quellen Field]

The laser light is always perpendicular to the mirrors.

Have you ever been in a room that had two mirrors on the walls

opposite one another? You seem to be able to see into infinity.

The laser acts the same way. The light acts as if it were going through

a pipe that is a mile long.

The excitation source can be light from any direction.

Spontaneous emission will be in random directions. But stimulated emission

(laser light) will always be perpendicular to the mirrors, because the light

that is doing the stimulating appears to come from the back mirror, which

looks like it is a mile away, due to the millions of reflections.

What makes the light into a beam is the mirrors, not the direction of the

light from the lamp.

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[Jelmer Cnossen]

A single molecule can be made to lase. Thus the size of E. coli is not an obstacle.

I think this is a bit too optimistic. You need a decent amount of light to compensate for the losses, which is harder to do with a smaller organism. 

Moreover, if you have to put your bacteria such as E. coli into a small ~2 um microfluidic mirror setup, you will get lots of diffraction which will cause a lot of losses as well.

[Nathan McCorkle]

E. coli suck anyway, just use a hardier and larger S. cerevisiae if
size is that big of a deal... assuming they are as clear as an E. coli
or human cell.

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[Simon Quellen Field]

I plan to use mirrors I can manipulate by hand, like I do with other lasers

I build. I see no reason to have a mirror smaller than the aperture of the

microscope objective I will be using to collect the output light. In fact, a

larger mirror will be much easier to align. Moreover, I want considerable

distance between the mirrors, so that I get only laser light into the sensor.

The plan is to have the whole culture lase. I can pick out different bugs and

parts of bugs in the image later.

Anyone got some GFP yeast they can send me?

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[CoryG]

> It is coherent.
> I don't see why you would think it wasn't.

http://en.wikipedia.org/wiki/Coherence_(physics)
This qualifies in neither the time nor the spatial phase domains
without amplification sufficient enough to operate in a pulsed fashion
with a half silvered mirror or similar - in which case the cells
themselves are not lasing.

> In any fluorescent dye, there is a lag between excitation and emission.
> A photon of the right color will stimulate emission in excited molecules.
> The emitted photon will be in phase with the stimulating photon.
>
> All you need for a laser is an excited dye and an optical cavity.
> This setup qualifies.

You also need a medium (typically a plasma or crystal - in this case
cells+dye) that is able to emit more than it loses. Given the volume
of dye to the volume of a cell and extracellular junk (nutrients
solution), and the almost spherical envelope around each cell that
consists of a transparent material, though also consists of a material
with a different dielectric than the solution it sits in and the
inside of the cell, it's almost like shining a light through a bunch
of small glass spheres - it will scatter and break spatial phase
domain coherence.

I'm not sure if this would work without trying it, but if you're
interested in employing it for microscopy that results in finer detail
than fluorescent microscopy you might try hitting it with light at the
frequency to pump it without recording, then a pulse of laser light at
the same wavelength as the emissions (with this method, if it can be
made to work as a LASER, but otherwise you might try silvered/half-
silvered mirrors and the purified dye extracted from cells to focus on
getting the mirrors to handle the phase synchronization for you) with
enough resolution on the sensor to separate the brightness from the
secondary emission (safe freq) pump and the actual secondary emission
taking place - that way you might even be able to (with a fast enough
camera anyway) get a 3d image of the cell along any given plane you
scan it across (this could probably be further refined with a thin
film beam splitter designed to reflect the natural power of the
secondary pump - might even be easier to get a splitter at the right
wavelength and tune the secondary pump's amplitude to it than messing
around with the brightness on the sensor).

[Simon Quellen Field]

This qualifies in neither the time nor the spatial phase domains
without amplification sufficient enough to operate in a pulsed fashion
with a half silvered mirror or similar - in which case the cells
themselves are not lasing.

I'm finding it very difficult to parse what you are trying to say.

Whether a laser is pulsed or not has nothing to do with its coherence.

And you seem to think there is not enough amplification to lase, but you

give no reasons for this belief, so I can't understand why you would believe

that.

Lasers as small as 44 nanometers across have been built:

"http://www.networkworld.com/community/node/44443"

VCSEL lasers are routinely built that are 10 microns across.

So size is clearly not the issue.

You also claim that shining a light through a slide with cells on it will scatter

the light so much that the light is no longer coherent. But when I shine my laser

through such a slide, it is easy to see speckles on the wall, proving that the

light is still plenty coherent. It is an easy experiment to do -- most of the readers

of this page have both a slide with cells on it and a laser pointer.

The wavelength of the light is half a micron. Coincidentally, this is about the

width of an E. coli bacterium. In water, the phase differences between the bug

and the water are so small that phase contrast microscopes don't show much

extra information at all. The glass spheres you are afraid will scatter the light

are more like balloons of salt water in a glass of fresh water. They almost

disappear. You seem to be concerned about the bacterial walls having a different

dielectric constant than the water, but you are ignoring their thickness, which

is a small percentage of the wavelength of the light, and will not have much

effect.

Read the article again:

"http://www.bbc.co.uk/news/science-environment-13725719"

Some quotes from the article, with my emphasis in bold:

Flooding the resulting cells with weak blue light causes them to emit directed, green laser light.

Laser light differs from normal light in that it is of a narrow band of colours, with the light waves all oscillating together in synchrony.

The pair used green fluorescent protein (GFP) as the laser's "gain medium", where light amplification takes place.

Temporal coherence is demonstrated by the narrow bandwidth of the laser compared

to light from a similar non-laser source. A good example is comparing the spectrum

of a red LED to a red laser. I do just that on my web page:

"http://sci-toys.com/scitoys/scitoys/light/cd_spectroscope/spectroscope.html"

Compare this spectrum of a red LED:

"http://sci-toys.com/scitoys/scitoys/light/cd_spectroscope/red_led_spectrum.jpg"

to this spectrum of a red laser:

"http://sci-toys.com/scitoys/scitoys/light/cd_spectroscope/laser_diode_spectrum.jpg"

It is easy to look at the spectrum of a laser to see if it is temporally coherent.

Looking at the data from the paper, it is easy to see the very narrow pulse at

516 nm in the spectrogram:

"http://cdn2.digitaltrends.com/wp-content/uploads/2011/06/biological-laser-single-cell-green-650x264.jpg"

It appears to be 5 nm wide, which is quite narrowband.

You can also see the knee in the curve of output light vs pump light. This is laser

amplification. Ordinary fluorescence is linear.

As for spatial coherence, that is meaningless when the laser is less than a wavelength

across. Nonetheless, coherence is a feature of laser light, not a requirement. The

requirements are stimulated emission, and amplification, both of which are clearly

evident.

The original paper in Nature Photonics says:

individual cells in a high-Q microcavity produce bright, directional and narrowband laser emission, with characteristic longitudinal and transverse modes.

"http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2011.99.html"

This is a laser. You may not want to believe it, and you may want to cite Wikipedia

articles to prove that the article in Nature Photonics is wrong, but it really looks like

a laser to me. I've built homemade dye lasers, gas lasers, and solid state lasers,

and this one looks like a fun one to build. They used really tiny mirrors because they

wanted a really tiny laser they could put inside someone. I would like to do it with

mirrors that are easier to handle, and with cells that are easier to grow. Other than

that, I expect to use their methods.

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[John Griessen]

On 06/14/2011 02:37 PM, Nathan McCorkle wrote:
> Simon, if you are using just two mirrors, won't some photons still go
> in random directions that aren't towards the sensor?

Sure, but the ratio is huge. .01% in other directions. Except for the scattered stuff in
a non-optically clear animal...

At some point, stimulated emission is just another mystery of the universe.
For practical considerations, the stimulated light goes along in exactly the same direction
as triggering light.

John

[CoryG]

On Jun 15, 10:47 am, Simon Quellen Field <sfi...@scitoys.com> wrote:
> > This qualifies in neither the time nor the spatial phase domains
> > without amplification sufficient enough to operate in a pulsed fashion
> > with a half silvered mirror or similar - in which case the cells
> > themselves are not lasing.
>
> I'm finding it very difficult to parse what you are trying to say.
>
> Whether a laser is pulsed or not has nothing to do with its coherence.
> And you seem to think there is not enough amplification to lase, but you
> give no reasons for this belief, so I can't understand why you would believe
> that.

http://en.wikipedia.org/wiki/Phasor
http://en.wikipedia.org/wiki/Frequency_domain
http://en.wikipedia.org/wiki/Time-domain

While pulsing or continuous wave light sources are both capable of
coherent light, I stated using silvered mirrors might work because the
only way this qualifies as a LASER as opposed to just a florescent
molecule inside a bunch of otherwise irrelevant material (irrelevant
outside of the method of production) is if the molecules exhibiting
the effect are used as the lasing medium, not considered a complete
laser - for instance, you could take a ruby rod, pump it in the same
manner with a flash tube, and without the mirrors it is still more or
less an omni-directional light source that happens to emit more light
in 2 directions just due to the crystal shape (which could be
disregarded entirely in the shape of a sphere for instance). While
this is a cool production method, you have to account for the fact
that the materials actually emitting light only comprise a small
volume of a larger area in this fashion, while the light might be
mostly coherent (for most practical purposes you won't be measuring
the angle at the tip of the beam, which would be proportional to the
speed of light and the beam width based on the form of stimulation
[the pump] - calling it temporally coherent is not technically correct
- this is why the only truly coherent lasers are FELs - all others
require a half-silvered mirror to act as a block until there is a
critical number of photons before a synchronous release, thereby
operating in a pulsed fashion). As far as spatial coherence is
concerned, you absolutely need the mirrors or a small aperture to
filter out the light, the aperture method leaves little room outside a
FEL for Light Amplification by Stimulated Emission of Radiation so you
are again stuck with mirrors, and in the spatial domain context you
have to account for junk around the lasing medium (cellular
components, cell walls, solution, interfaces between each component) -
because even if it seems clear on a single pass of light, a photon is
likely to travel hundreds of thousands or millions of times between
those mirrors before it is able to escape - while its a bit of a
statistical issue when you can reach that critical mass (in trying to
predict the likelihood of any photon traveling through the half-
silvered mirror on a given pass) - if you can't reach that critical
number it will not lase - it will just glow - when you do it will
discharge light much like a simple LC oscillator discharges
electricity, in pulses - building up photons then releasing them when
they can break through the barrier - which also leads to the other
issue with the junk around the lasing medium past absorption without
secondary emission in the same wavelength, which is the scattering -
it might not throw it off further than desired on the first pass, but
when you're making hundreds of thousands or millions of trips back and
fourth the scattering intensifies to the point that you will very
likely lose the photons perpendicular to the path between the mirrors.

[Simon Quellen Field]

OK, so you didn't actually read the article.

That explains some of the misconceptions.

They used mirrors.

I also plan to use mirrors.

You are not quite correct in implying that lasers can't work without mirrors.

My superradiant nitrogen gas laser has no mirrors.

But GFP will not be superradiant, so we can agree that mirrors are needed.

But you seem to completely misunderstand how half-silvered mirrors work.

They don't reflect all the light until it builds up to some threshold, whereupon

they suddenly become transparent.

A partially silvered mirror reflects some portion of the light, and lets the rest

pass through. In our case, about 70 percent will be reflected, since we are

using a 70% mirror to get better gain. We could go 90% or more if we need

to.

Out of every 100 photons, 30 of them will escape. There is no critical number,

and no synchronous pulsing. That's not what temporal coherence means. Temporal

coherence simply means narrow band emission. Monochromatic light.

The light is coherent because of stimulated emission.

So somehow the photons need to get to the excited molecules in order to stimulate

the emission. The mirrors do this. The mirrors are also why the output is a beam

instead of being omnidirectional.

Any losses in the lasing medium from diffraction or refraction are just losses.

They don't multiply.

If 99% of the light passes through the bug and 1% is scattered in the medium,

then when the mirror reflects 70% of the light back through, that is doubled by

the medium, and another 1% is lost. The gain in the medium overwhelms any

losses. Most of the "loss" is through the partially silvered mirror, and I put "loss"

in quotes because that is the output we built the laser for in the first place.

In any event, I expect the scattering caused by a 1 micron thick layer of water

and E. coli will be much less than 1%, and certainly much less than what we

are already removing as output.

There is no critical mass of photons needed to get through the partially silvered

mirror. Of course you weren't actually trying to say the photons had mass, but

were using a concept borrowed from nuclear physics, but nonetheless, even one

photon has a 30% chance of getting through.

The laser is a pulsed laser for entirely different reasons.

Dye lasers need extremely bright excitation sources to get enough of the molecules

in the excited state to form a population inversion. This heats up the liquid.

To have a CW laser (continuous wave, not pulsed) dye lasers have to have the

liquid flowing pretty fast, so that the temperature stays within reason. GFP is

probably much more sensitive to heating than Rhodamine-6G or Fluorescene,

just because it is a protein. Does anyone on the list know what heat denaturing

does to it, or what its temperature tolerances are? In any event, we want to keep

the heating low to keep the bugs alive. So we pulse.

I might also want to ask the folks on the list how they think the bugs will tolerate

5 nanosecond pulses of 337.1 nanometer light from my 3 megawatt nitrogen laser.

That is what I plan to use as the illumination pump for the GFP.

It is conveniently out of the range of my cameras sensors, and the objective lens

will block a lot of it anyway, so I won't need optical filters to separate the pump

light from the GFP light.

-----

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--

[CoryG]

The 30% that pass through are the result of the magnetic moment of the
photon imparting energy to the mirror when not reflected - in this
regard the mirror acts like a capacitor, storing up charge for a
photon that can hit it at peak charge and carry it all along with it
much like a spark through a typically insulating medium. It is a
critical number of photons required in that regard as experiments have
been done proving this with single photon beams - even when the photon
hits the same spot as the previous one in an environment isolated from
other energy sources, the percentage passed remains the same and the
percentage reflected remain the same, but there is a time delay with
those photons that pass through the mirror equal to the number of
photons coming in over time relative to the number required to cause
the mirror to pass them. This has been explained pretty well in QM
via the photon's magnetic moment and is responsible for temporal
coherence of the beams (ie: storing up photons and releasing them in
sync pretty much the same way secondary emission adds to amplitude of
the host photon). I've never actually heard of a CW dye laser - I'll
have to look into it more, from my prior study of the subject the only
truly coherent CW laser is the FEL - which operates on synchrotron
radiation and doesn't require mirrors.

[Simon Quellen Field]

I don't think so.

;-)

Temporal coherence is still evident in lasers that have no mirrors,

like my nitrogen laser.

You are trying to argue that a single photon will never pass through a

mirror, no matter how thin the coating on the mirror. A single atomic

layer of silver would block it under your scenario, until some number of

successive photons suddenly caused a flood of them to be released.

And somehow the mirror would store a photon forever until enough

of its pals had come along. And this would happen with any photon,

whether or not it was from a laser, so all reflections from mirrors would

be temporally coherent.

Temporal coherence comes from the fact that the photons are emitted

when stimulated by other photons, so they are in phase. The mirror has

nothing to do with it.

There are many CW lasers. My pocket lasers are CW, as are my HeNe

lasers.

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[CoryG]

Temporal coherence is as you say a product of secondary emission,
however it is secondary emission in the mirror that allows for this in
fully coherent pulses. A single layer of silver would only block or
reflect wavelengths that are shorter than it, anything larger passes
through - what allows for half-mirrors (or beam splitters) is a thin
coating that is below the wavelength but not so relative to the
dielectric effects of multiple media the photon must traverse (ie
glass and silver or any other compounds used) - when you have photons
striking the surface they impart energy that makes the surface wobble
at the frequency of the absorbed photon, as it does so it's relative
size changes to another photon of the same wavelength and if it is at
just the right thickness to start to absorb/reflect (determined
primarily by angle and dielectric effects) it will allow the photon to
pass through taking the added energy from that wobble with it - this
is how secondary emission and most forms of quantum tunneling work as
well. Mirror-less Nitrogen lasers are NOT temporally coherent
http://en.wikipedia.org/wiki/Coherence_time - you do need a mirror
working in a pulsed fashion for that outside of a FEL working with an
electron beam near the speed of light.

On Jun 16, 10:48 am, Simon Quellen Field <sfi...@scitoys.com> wrote:
> I don't think so.
> ;-)
>
> Temporal coherence is still evident in lasers that have no mirrors,
> like my nitrogen laser.
>
> You are trying to argue that a single photon will never pass through a
> mirror, no matter how thin the coating on the mirror. A single atomic
> layer of silver would block it under your scenario, until some number of
> successive photons suddenly caused a flood of them to be released.
> And somehow the mirror would store a photon forever until enough
> of its pals had come along. And this would happen with any photon,
> whether or not it was from a laser, so all reflections from mirrors would
> be temporally coherent.
>
> Temporal coherence comes from the fact that the photons are emitted
> when stimulated by other photons, so they are in phase. The mirror has
> nothing to do with it.
>
> There are many CW lasers. My pocket lasers are CW, as are my HeNe
> lasers.
>
> -----

[Simon Quellen Field]

This is just wrong.

You do not need mirrors for temporal coherence.

It is very easy to show the spectral width of lasers without mirrors, and it is

very narrow band, like most lasers.

You keep saying things that are just wrong, and then citing Wikipedia

as a source, even though it says nothing about what you are talking about.

The tiny Wikipedia article you cite says nothing about nitrogen lasers or

mirrors to back up your claims. And Wikipedia is usually not the best source

to start with, although I admit it is very convenient. It usually requires citations

to back it up, and frequently has them. But if a statement in Wikipedia does

not have a reference, treat it as suspect.

You started out by saying the GFP laser was not coherent.

That was wrong.

You said it couldn't have gain.

That was wrong.

You said that lasers need mirrors.

That was wrong.

You said that the only laser that didn't have mirrors was a free electron laser.

That was wrong.

You say that all lasers except free electron lasers are pulsed.

That was wrong.

You make the silly claim that mirrors store photons until enough of them are

collected to let some through, and that a single photon can never pass

through a partially silvered mirror.

You have been spouting nonsense for several pages now, without citing

any references that back you up. Show us a reference that says that HeNe

lasers are not CW, or that nitrogen lasers are not narrow band.

"http://books.google.com/books?id=qP7HvuakLgEC&pg=PA814&lpg=PA814&dq=temporal+coherence+in+nitrogen+laser&source=bl&ots=X0k32YDoK6&sig=dPIYadzDK7U-CRuL3kYZc4gjYaE&hl=en&ei=2z_6TbWfJ5GosQPugtzdBQ&sa=X&oi=book_result&ct=result&resnum=9&ved=0CF8Q6AEwCA#v=onepage&q&f=false"

says "A single-frequency laser output is fully temporally coherent."

The nitrogen laser is very narrow band.

There are many people on this list that I happily accept as authorities,

and whose statements I accept without asking for citations. I am afraid

I can't extend that acceptance to what you have been saying in this thread.

It makes me wonder if I should question everything you say in other threads

as well.

People will be using this list as a source for years.

I try to be very careful about what I say here.

The Internet does not forget, and nonsense with your name on it will live a long

time, and show up when people Google you before making hiring decisions.

Question what you say, and cite references that back you up.

From "http://www.rp-photonics.com/continuous_wave_operation.html"

The first continuous-wave laser with a helium–neon laser operating at 1153 nm [1]. A version working with the now common emission wavelength of 632.8 nm was demonstrated soon after that. Later on, many other types of lasers were developed which can also be operated continuously: other gas lasers, many types of solid-state lasers (including semiconductor lasers), and dye lasers.

From "http://www.britannica.com/EBchecked/topic/135158/continuous-wave-laser"

A laser is called continuous-wave if its output is nominally constant over an interval of seconds or longer; one example is the steady red beam from a laser pointer. 

Since you like Wikipedia: "http://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb-tester"

When a photon emitted by the light source reaches a half-silvered plane mirror, it has equal chances of passing through or reflecting. 

If single photons can't pass through a half-silvered mirror, then classic experiments like

this one "http://ffden-2.phys.uaf.edu/211_fall2004.web.dir/Andrew_MacPhail/index3.html"

would not be possible. Here is another source for the same experiment:

"http://www.mat.ucm.es/catedramdeguzman/old/01historias/haciaelfuturo/Burgos090900/CenterQuantumComp/comp.html"

 

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[CoryG]

> This is just wrong.
> You do not need mirrors for temporal coherence.
> It is very easy to show the spectral width of lasers without mirrors, and it
> is
> very narrow band, like most lasers.

Right, you don't need mirrors for temporal coherence, if you use a
FEL, otherwise you do. And if it's not a FEL it lacks that without
pulsed beams - you can cite this as irrelivant if you like, but it
seems you lack a basic understanding of the wave nature of light:
http://en.wikipedia.org/wiki/Phasor

> You keep saying things that are just wrong, and then citing Wikipedia
> as a source, even though it says nothing about what you are talking about.
> The tiny Wikipedia article you cite says nothing about nitrogen lasers or
> mirrors to back up your claims. And Wikipedia is usually not the best source
> to start with, although I admit it is very convenient. It usually requires
> citations
> to back it up, and frequently has them. But if a statement in Wikipedia does
> not have a reference, treat it as suspect.

The article I cited was not specific to a particular type of LASER, it
was specific to the wave nature of a photon.

> You started out by saying the GFP laser was not coherent.
> That was wrong.

It was not wrong.

> You said it couldn't have gain.
> That was wrong.

I did not say it could not have gain, I said that in a realistic
system it would not have suitable gain to be better than any existing
method of exploiting secondary emission unless you extract the dye
rather that leave it as a small portion of a cell with a lot junk that
is entirely unrelated to the function of the LASER.

> You said that lasers need mirrors.
> That was wrong.

I actually cited one that does not.

> You said that the only laser that didn't have mirrors was a free electron
> laser.
> That was wrong.

I said the only fully coherent LASER that does not require mirrors is
the FEL - which is correct.

> You say that all lasers except free electron lasers are pulsed.
> That was wrong.

I said all fully coherent LASERs except the FEL are pulsed.

> You make the silly claim that mirrors store photons until enough of them are
> collected to let some through, and that a single photon can never pass
> through a partially silvered mirror.

I never said a single photon cannot pass through a partially silvered
mirror, I explicitly stated one could if the silver were a thickness
below the wavelength with effects of adjacent dielectric mediums and
the angle of the photon factored in (at which point it becomes a
matter of probably as to whether a single photon would pass through or
not - that statistical portion being a rough approximation itself of
the phase angle upon "impact" with the material assuming you do not
know the phase angle). And mirrors can store photons for a short
duration of time when they are in a low enough energy state - which
occurs either when placed in that state at the start of an experiment
or after they have passed a photon thereby dropping to a lower state -
the physics of it are virtually identical to that of the lasing medium
itself.

> You have been spouting nonsense for several pages now, without citing
> any references that back you up. Show us a reference that says that HeNe
> lasers are not CW, or that nitrogen lasers are not narrow band.

HeNe are not completely coherent, they have a high degree of spatial
coherence, but over a great distance even this is lacking, they are
thereby not completely coherent. Spatial, temporal and frequency are
all domains of coherence in light, frequency(narrow band) coherence
alone is not fully encompassing of coherence.

> "http://books.google.com/books?id=qP7HvuakLgEC&pg=PA814&lpg=PA814&dq=t...

> "
> says "A single-frequency laser output is fully temporally coherent."
> The nitrogen laser is very narrow band.

The source cited is so overly simplified as to make it outright wrong
- calling spectrum coherence temporal coherence. Temporal coherence
is related to but not the same as frequency coherence - a basic idea
of it is that temporal coherence is the difference in phase angle
between any two waves in a wavefront:
http://en.wikipedia.org/wiki/Coherence_(physics)

> There are many people on this list that I happily accept as authorities,
> and whose statements I accept without asking for citations. I am afraid
> I can't extend that acceptance to what you have been saying in this thread.
> It makes me wonder if I should question everything you say in other threads
> as well.

I'd be honored if you question everything I say, you should for any
source of information - especially on the internet - but in this case
you are quite wrong and I would suggest you at least study the
fundamental laws of nature that have been observed by others before
you begin categorizing them this or that. Here's a relatively
simplified but verbose source to get you started:
http://hyperphysics.phy-astr.gsu.edu/hbase/quacon.html#quacon

> People will be using this list as a source for years.
> I try to be very careful about what I say here.

Try harder.

> The Internet does not forget, and nonsense with your name on it will live a
> long
> time, and show up when people Google you before making hiring decisions.

Is that why you are arguing fallacy so strongly?

> Question what you say, and cite references that back you up.
>
> From "http://www.rp-photonics.com/continuous_wave_operation.html"
>
> > The first continuous-wave laser with a helium–neon laser operating at
> > 1153 nm [1]. A version working with the now common emission wavelength of
> > 632.8 nm was demonstrated soon after that. Later on, many other types of
> > lasers were developed which can also be operated continuously: other gas
> > lasers, many types of solid-state lasers (including semiconductor lasers),
> > and dye lasers.

I am aware of the definition of continuous wave LASERs and nothing in
the links above describing them is new to me, nor is it relevant to
the conversation.

> From "http://www.britannica.com/EBchecked/topic/135158/continuous-wave-laser
> "
>
> > A laser is called continuous-wave if its output is nominally constant over
> > an interval of seconds or longer; one example is the steady red beam from
> > a laser pointer.
>
> Since you like Wikipedia: "http://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb-tester"
>
> > When a photon emitted by the light source reaches a half-silvered plane
> > mirror, it has equal chances of passing through or reflecting.

I'm familiar with the experiment, and it is not an equal chance, it is
based on the energy state of the mirror if left alone - but in the
context of that experiment it is linked with the beam after testing -
this exploits a method if entanglement that LASERs do not naturally
exploit by splitting a relatively coherent beam, interacting with the
photons from a weak beam, and recombining all the beams. Outside of
this very specific form of virtual cavity it does not apply.

>
> If single photons can't pass through a half-silvered mirror, then classic
> experiments like

> this one "http://ffden-2.phys.uaf.edu/211_fall2004.web.dir/Andrew_MacPhail/inde...

> "
> would not be possible. Here is another source for the same experiment:

> "http://www.mat.ucm.es/catedramdeguzman/old/01historias/haciaelfuturo/...

Those experiments are why it is important to use the word "coherence"
appropriately as coherent in all domains unless you are specifying a
subset thereof.

[Simon Quellen Field]

With apologies to the group, I will be taking the conversation with CoryG private,

as it seems to be doing more harm than good.

[CoryG]

I look forward to continuing this discussion with you Simon.