WEIRD Fluorescence Two-Photon absorption!

Episode 54
#optics
#laser
#nonlinearoptics
Lets take a look at Two-photon absorption
In this episode, let's look at a really unusual Fluorescence phenomenon, two photon absorption!
This phenomenon is used in two photon microscopy for imaging cells buried deep in living tissue, and could be used in volumetric 3D displays and ultra dense data storage!
This was originally predicted by Maria Goeppert-Mayer in 1931 in her doctoral dissertation. Thirty years later, the invention of the laser permitted the first experimental verification of TPA.
Maria went on to become the second woman to win a Nobel Prize in physics for proposing the nuclear shell model of the atomic nucleus.
The unit for the two-photon absorption cross section is named the Goeppert Mayer (GM) unit after her.
1064 laser module： bit.ly/3ZTLKeZ
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0:00 Intro
0:18 Fluorescence
1:06 Maria Goeppert Mayer Two-photon absorption
1:42 Two-photon absorption explanation, Jablonski diagrams
2:43 Suitable Fluorescent Dyes
2:48 Rhodamine 6G
3:18 Single photon fluorescence
4:45 15kW IR Laser!
5.45 Extreme power density
6:00 Two Photon fluorescence!
6:58 Sneak peak and credits!

[@alexwang007]

I am a master's student working for Dr. Eric Johlin on making an improved version of the machine called nano scribe. We are using the two photo absorption process to do two photon lithography, except this process can make voxels that are truly 3D, and are smaller than the diffraction limit.

[@LesLaboratory]

Cool! I would be interested in reading about it!

[@dinozaurpickupline4221]

Could this be used to bring down the theoretical limit of trans fabrication?

[@endosage]

YES! I knew this was possible! Threshhold fluorescence! And yes, for volumetric displays!! WOOO!!!

[@alexwang007]

@@LesLaboratory Of course! I'll first put down some basic info: we are using a 15 kW femtosecond laser that outputs 770 nm (frequency doubled from 1550 nm fiber laser, rep rate in the megahertz). The laser is somewhat visible to the naked eye, but at enough power density (cranked up laser power+ focusing optics) it can excite fluorescent dye that are only sensitive to short wavelengths (like printer paper) ! this of course also can polymerize photoresist. The voxel is much smaller than the diffraction limit because this process is sensitive to power^4

[@alexwang007]

Some updates after some SEM imaging, just to get a sense of scale: 780 nm laser, half power, 100 um/s feedrate, focused with Leica's 100x immersion oil objective, we got a max voxel diameter of 180 nm, not bad! Development also worked without causing collapse of thin structures.

[@CineSoar]

When I was in laser field service, I used to install tunable (over the range of Ti:Sapph) ultrafast (roughly 100fs) lasers for use as illumination sources for 2-photon microscopy. There, you have a double advantage. The 2 photon interaction only happens at the precise focal plane, and the pulse of light is faster than the phonon moment, so the heat that would be generated by CW light has no time to propagate beyond the illuminated 'pixel'. This allowed for deep imaging, with high resolution and clarity, into living cells. These were some of my favorite gigs, because there was almost always a researcher, or grad student, who was eagerly awaiting 'first light', to image some sample they had prepared in advance. I saw ground breaking research realized, in real-time, within hours of signing off the new laser.

[@LesLaboratory]

Nice! The Laser I am using is very slow in comparison and heating is a problem. This is quite apparent when you view the output beam from the cuvette, it wobbles and shimmers as the heat changes the refractive index of the solution, a Schlieren effect I guess, except the light source itself is causing it! It would be awesome to see and be involved in stuff like that. There is so much research going on in the world, and undoubtedly there is some extraordinary research going on.

[@CineSoar]

@@LesLaboratory I don't miss the relentless travel. But, I did get to see a wide range of really cool research, and amazing machines and facilities along the way. Coincidentally, my office is in Schlieren.

[@graealex]

Top notch lighting now, massive jump in quality. Only thing, at 3:30 it wasn't particularly clear which lasers were which color, as the fluorescence overpowered the original laser light.

[@LesLaboratory]

Thanks! Your suggestions 😉 Agreed. The sensible thing I think for this scene would have been to reduce the brightness of the pointers. If I had nipped down the aperture the scene would be too dark.

[@graealex]

@@LesLaboratory I think it would have been enough to show the lasers working without already pointing them at the dye. I had to rewind and listen closely to what color the laser was supposed to be. But I do like the lighting, especially with the blue light in the background. I still think the intro looks a bit untidy, though. The background should have a distinct color and bokeh, and have a good separation from the foreground where you talk. But I suppose it's a workshop and not a photo studio, so maybe not easy to set up.

[@LesLaboratory]

@@graealex Thanks for your feedback, it's something I will continue to work on. Lighting and Audio are not bad now. Yeah the workshop thing is a problem, but space is extremely tight. I have some ideas on what to do about it, it will just require a little more experimentation.

[@mechadense]

Really cool demo. Thanks.

[@andymouse]

Fascinating stuff, my mind boggles at the thought of a single 'pixel' being formed....cheers RIP Calculon

[@LesLaboratory]

Thanks 😊

[@turfptax]

This is great stuff! Thank you for sharing your passion!

[@LesLaboratory]

My pleasure!

[@scootndute579]

Great video 👏

[@TheTablet314]

Very nice to see!

[@mikeselectricstuff]

Might be interesting to use a liquid lens to control the position of the focal point - add some X/Y galvos and you have a 3D display!

[@LesLaboratory]

That's a cool idea, I wonder how fast you could actuate one? I wonder whether a CD-ROM objective might give enough movement on the z-axis...

[@CineSoar]

There's been quite a bit of exploration into this, but it seems as if it may have stalled some years back. I can recall 'displays' using blocks of doped glass, with a scanner underneath. The promise of practical applications like being able to image the airspace around an airport, in 3D was touted. Not sure why it didn't gain momentum. This was probably two decades ago. It's not a dynamic display, but scanning the focal point of a laser in 3 dimension is what gives you those 3D Crystal Photos you see in the mall.

[@LesLaboratory]

@@CineSoar Yes, it seems curious that the exploration stalled. My best guesses are cost of equipment, or efficiency. A decade ago it would have been inconceivable that you could have a diode pumped Q-switched YAG for under $500. At the same time, loads of research has gone into various Dyes, Phosphors, and Quantum dots, so maybe now the tech has matured enough to make it a viable option again. There are practicalities as well, for example, in the experiment shown, you must wear safety glasses. but I suppose with care n design, and appropriate coatings, such a display could become practical.

[@hamjudo]

​@@LesLaboratoryIt can render a light saber, but not a Jedi or anything else that is opaque. Not much market for monochrome 3D displays where everything is see through, when VR goggles can do full color 3d rendering of scenes that include prisms, lenses, and anything else that doesn't cross the uncanny valley. There are a lot of uses for two photon fluorescence, but practical 3d displays aren't one of them.

[@chopper3lw]

Very Nifty! Thanks

[@hullinstruments]

So glad to see you chugging along with more incredible content. Your lab has grown sooooo much since I last saw it. I had to quit watching your channel. Sadly Taking care of him losing both of my parents back-to-back the last year or so..... kind of negated my business and therefore my lab. Now that it's all gone it's just too depressing to be reminded by what's going on and the stuff that I used to be into. And the metrology work that I used to do every day. Took me twenty years to build that workshop and laboratory and business. But it's great knowing guys like you will be out there,.... to see you continuing your insanely successful and awesome KZbin and photonics journey! Couldn't help but notice "thought emporiums" KZbin venture into dye laser territory last week. Knowing that you had to have some huge influence on him and many others. Open up so many people's minds to what is possible.... Just like your influence on my explorations. I wish you much success!👍

[@Bobbias]

That's awful. From one random internet person to another, I hope you recover from this and get a chance to continue doing what you love.

[@LesLaboratory]

Aw man! That's so sad. Don't give up on your dreams though in the end it's all we have. I don't know what help I can offer, but I can listen.

[@hullinstruments]

@@LesLaboratory thank you very much. But I had a great run. I've got to do everything I ever could have dreamed of or wanted to do. Multiple times. Can't complain. The best thing is seeing folks like you taking the time and energy to share your interests and passions with others. You're niche interest brought so much freshness and exciting developments into my lab. I had so much photonics and lab equipment for so long..... And it's not like I was necessarily bored or anything.... But you're experiments really relet that spark and showed me a huge unexplored universe. And relate that old spark and passion, and made me just as excited as anything ever had with photonics. And I know it's had that effect on many more people as it can be seen all across the internet!. Bringing new possibilities and passions alive in others. Extra stepping stones on there personal journey... That may have never been revealed otherwise. That's huge. And It's great. Rock on🤘😁🤘

[@KenKlondike]

Years ago I re-created Eric Korevars cesium chamber. This used two distinct intercepting lasers to energize the valence of an atom and discharge a visible photon. Was impressive but note that it was based on atoms not molecules. Thanks for the video

[@Muonium1]

Yet AGAIN, in a veritable endless barren desert of dull hour long lecture videos, boring whiteboard diagrams, and dry, often poorly done animations, your channel is an oasis of simply *doing and SHOWING the damn phenomenon* plainly, straightforwardly, intuitively, BRILLIANTLY. There is not a single other example of this phenomenon of 2PA demonstrated on video in existence anywhere so far as I'm aware, and I've looked. This is literally the first time I've ever seen it shown after first learning of it decades ago. It is simply nothing short of astounding to me that even today there are still so many fascinating physical phenomena like this that professors will drone on for literal hours about without ever once bothering to show the thing in operation. Sending more money your way.

[@LesLaboratory]

Thanks so much for your comments and support! Practical stuff is right up my alley, I use to spend ages poring over the amateur scientist column in Scientific American, it's such a shame it ended, but if it was re-incarnated I would hope it would contain stuff like this.

[@Muonium1]

@@LesLaboratory Oh god, don't get me started on what a travesty that once august institution has become today. Reason did a whole interview with Mike Shermer last year on the woke ideological rot that has eaten it from the inside out. The editor today spends her time frittering away any last vestiges of reputability the magazine once had by tweeting out things like "The “terrifyingly ordinary” nature of football’s violence disproportionately affects Black men" and "biological sex doesn't really exist". It would be hilarious if it weren't so maddening and sad. Things like The Amateur Scientist are gone forever, but the good news is that its spirit lives on in a new generation of dozens of "amateurs" who carry the torch for the next generation of curious people - I'm watching that happen right here!

[@LawpickingLocksmith]

Nice, thanks.

[@cambridgemart2075]

Excellent content, as always. Would love to see some of your stuff in person; as it happens, there will be another UKLEM this year in mid August, it would be great if you could make it. Or you could wait until Aurora LEM 1 happens in Northern Finland!

[@LesLaboratory]

Thanks, I keep trying but work gets in the way! What's the date in August?

[@cambridgemart2075]

@@LesLaboratory It's the 11th to 14th August

[@zachreyhelmberger894]

VERY cool!

[@Farathus]

Pretty sweet! would be interesting to measure how high of a peak intensity you really achieved here. The beam entering the objective looked quite rough compared to the Gaussian the calculator assumed. Did you have to watch out to not move the glass walls of the vile through the focus? Could be a fancy laser engraver that thing =)

[@LesLaboratory]

It's not as rough as it looks. Most of the garbage you see is because a red laser pilot beam is built into the head. Although the 1064nm beam is collimated, the red is not, and is all ver the place. I have ordered an IR pass filter to get rid of it. If you pass the focussed beam through the glass walls, you get a tiny white flash of breakdown on the surface of the glass, which is not healthy for cuvettes (you can see this in the video when I replace RG6 for Rhodamine B) this is not good for any optics! Occasionally there is a tiny white flash at the focal point in the dye solutions as well, so I may well be right at the threshold of dielectric breakdown with this laser.

[@jtcustomknives]

That’s really cool.

[@3amali1]

What a fantastic demonstration! Why such channels have only 12k subscribers! KZbin must tweak their support algorithms for the sake of humanity!

[@LesLaboratory]

Thanks! Maybe social media will pick it up and some point and help push it!

[@amanawolf9166]

Huh, interesting. I wonder how this could be used in the creation of a 3D display within a liquid medium.

[@LesLaboratory]

It can surely be done, you just need to be able to control the focal point in a 3d volume, and scan and modulate it appropriately. I suspect though that this will be full of challenges!

[@freedom_aint_free]

Amazing experiment, you could use dichroic mirrors to separate de fluorescence from the laser.

[@LesLaboratory]

In principle it might be possible to have the cuvette coated to absorb the 1064nm light making it considerably safer. It is something I will have to look into.

[@edcorns3964]

And we still don't have night-vision *lenses* as described in Philip J. Corso's 'The Day After Roswell', quarter of a century later... Why? And we still don't have *photonics* , either -- analogous to electronics, but using photons (meta-materials) rather than electrons (semiconductors/transistors) for computation, via two-(or more, in theory)-photon absorption of a number of different frequencies, where the number of used frequencies is a prime (and the larger that prime, the more *orders of magnitude* the processing is faster than with electronics... in addition to all the other advantages of photonics over electronics, like no problem with waste heat, *orders of magnitude* lower power consumption, *orders of magnitude* higher clock speeds, etc.), and those computations are performed in p-adic number systems... Again, why? Of course, I already know the answer to both of these questions (two hints: Switzerland, AGI). I'm simply thinking out loud.

[@zilipoper]

hi, can you tell me what dyes can be excited by a copper vapor laser?

[@LesLaboratory]

Any with an absorbtion curve that falls within the emission lines of Copper Vapor. Rhodamine 6G, Rhodamine B, possibly Fluorescein, as well as many IR dyes can be pumped with a Copper Vapor Laser.

[@nikolaiturcan6963]

WOW, AMAZING Subject, I can't Thank You enough...

[@LesLaboratory]

Thanks!

[@imajeenyus42]

Just tried this with my 60W galvo engraving laser and it works a treat! Really neat to see.

[@LesLaboratory]

Awesome! Can you scan an image in the liquid?

[@imajeenyus42]

@@LesLaboratory Unfortunately I've got a much longer focal length lens (about 190mm long) so you get more of a line rather than a point in the liquid! But it does scan this line around which is cool. I've put up a wee video here: kzbin.info/www/bejne/n6e5kpuYqahraJY

[@florinpetrache7834]

Very nice. But I can't wait to see what you will do with that ruby ​​bar. I would like to see how the ruby ​​laser will be.

[@LesLaboratory]

It will make an appearance at some point...

[@TheShokida]

It's amazing to see someone perform a two-photon absorption experiment. By the way, what kind of microscope objective do you use? is it for infrared or for visible.

[@LesLaboratory]

Thanks! It's just a regular 20x microscope objective. Its not coated to IR, so you have to be mindful of reflections. Appropriate safety glasses are a must!

[@TheShokida]

@@LesLaboratory thanks a lot for your prompt response! I hope to see more. Please continue with your cool experiments.👍

[@robinyilmaz1155]

Very cool! It seems a rather inefficient process (total photon energy in / total photon energy out). Why is that? Is it that most incoming photons are not used due to the low success probability? What if we surround the fluid in a dichroic mirror that reflects our laser wavelength, but transmits the converted wavelength? That way our light can only escape in the desired wavelength. (Or as heat)

[@LesLaboratory]

The efficiency is for the most part down to the low probability of TPA occurring. It may be possible to recycle the light, though I imagine it would be quite an engineering challenge. Quantum dots offer a much higher TPA cross section, so it is worth investigating those first.

[@danajohnson5993]

i found that a green laser pointer creates an orange (scattering?) beam through a container of extra virgin olive oil, though the exit beam is still green. Curious to know why this is? i presume the green pigment / dye? in the olive oil fluoresces, but not sure by what method given this video. No effect on refined olive oil.

[@LesLaboratory]

Either it is fluoresce or it could be Raman scattering. I will be doing a video on the topic.

[@GeoffryGifari]

so its the strict intensity requirement that makes this suitable for volumetric display?

[@GeoffryGifari]

hmmm if the intensity is high enough for 2-photon absorption, how much of the leftover IR light is left just passing through?

[@LesLaboratory]

The intensity needs to be high enough that the probability of two photons arriving at the same time can excite a molecule. What this means for Volumetric displays is that only the focal point satisfies those criteria an will illuminate, the beam path remains dark.

[@LesLaboratory]

Most of the IR light will pass through. I imagine that this is where one of the biggest problems lie with volumetric displays, how to dump that wasted IR...

[@joewebster903]

Yes but what about photonics absorption that induces hyperchromic changes in a molecules adsorptivity by 300 percent with a bathochromic shift

[@npeters97]

Maybe this is a dumb question, optics is far from an expertise of mine: is two-photon absorption a function only of the intensity of the incoming beam? Do polarization and/or directionality come into play at all? Could two lasers, for example, pointed at some dye with beams orthogonal to each other produce the same effect where they intersect (if the resultant flux were high enough)?

[@npeters97]

Great video, by the way. Very fascinating. Seeing the voxel of light is kind of uncanny.

[@poodook]

Yes, they can. Take a look at 3D optical data storage when you get the chance. Certain implementations employ that exact pumping geometry you describe.

[@LesLaboratory]

yes, as @poodook says below. Focusing is just easier though.

[@Zenodilodon]

I have gotten 2 photon absorption and fluorescence with Rhodamine 6G using 638nm at 700mw which gave me a yellow/green emission!

[@LesLaboratory]

Interesting, did it fluoresce only at the focal point, or along its beam path?

[@Zenodilodon]

@@LesLaboratory No focal point needed, the beam will do just fine. You will need a red blocking filter though. Ill send over an email with pictures.

[@LesLaboratory]

@@Zenodilodon cool! It sounds like anti-stokes, I'm interested to see it!

[@Zenodilodon]

@@LesLaboratory Yeah that's my thought as well on what could be causing the up conversion. I am not 100% certain, but it's the most likely mechanism of action.

[@nkdm87]

2PA has also great aplication in micromanufacturing, as it can be used for polimerisation. Thanks to the 2PA threshoild beeing quite hogh, the voxel can be sub-difraction limit sized.

[@LesLaboratory]

Yes I remember some academic papers discussing TPA resin printers. it is possible to store data in bulk materials with TPA as well.

[@nkdm87]

@@LesLaboratory the whole 'data storage' is just a theoretical explanation, that is used in dozens of scientific papers ;).

[@plusmartini]

Any chance I could use two photon absorption on air? Basically doing the same focusing of light and make nitrogen or some other element in air emit light?

[@LesLaboratory]

A Laser with a peak power of over a Megawatt or so, when focussed to a point, will ionize air. I did this about 10 years ago with a pulsed YAG Laser I repaired. This will be coming up in a future video, along with even cooler stuff!

[@plusmartini]

@@LesLaboratory awesome! Im Wondering if there are any fluorescent properties of air that would require less power?

[@lasertv3598]

Rhodamine 6G has almost no absorption at 405nm so why did it fluoresce so well with your UV pointer?

[@LesLaboratory]

Absorption depends on a number of factors including solvent used and concentration. The concentration of the Dye used here, was optimized for UV pumping. For the experiment shown it would be more appropriate to optimize for ~532nm given we expect TPA at 1064nm to occur in the solution.

[@poodook]

Maybe try to do 3 photon absorption with that IR pump laser and stilbene 420 dye?

[@LesLaboratory]

Interesting, it's worth a shot!

[@poodook]

@@LesLaboratory it’s pretty incredible to think that MGM’s prediction of the 2 photon effect preceded the laser by like 30 years. Truly brilliant scientist.

[@andrewradford3953]

That's an impressive amount of energy ftom a 2W laser to a 17kW pulse of IR laser light then focused to the equivalent of GW per square centimetre. There I was kinda impressed that my large solar array can produce a megawatt in five days..

[@robertbass4590]

OK - I admit it. The focused fluorescent point made me laugh out loud. Total nerdgasm!

[@LesLaboratory]

To be fair, so did I when I saw first light!

[@synterr]

Can't wait for such times it would be possible to just lit a dot in the air, without need for very high power laser to ionize nitrogen.

[@LesLaboratory]

It is possible to light a dot in air, however this requires high peak powers, normally on the order of around a Megawatt per pulse. That being said, Lasers are becoming less expensive.

[@SenpaiSkyy]

I was wondering about quantum dots. I wanted to know if they would lase. Just got my order of Rhodamine 6G and B in the mail.

[@LesLaboratory]

I'm pretty sure I read a paper once about Lasing quantum dots, the title was something along the lines of disordered lasing with nanoparticles. I will look it up when I get home. At least one paper I read recently said that quantum dots were cheap, so I will be looking into that too!

[@Muonium1]

anything will lase once if you pump it hard enough.

[@zaprodk]

I think there is a typo in the title "abrorption" :)

[@LesLaboratory]

Thanks, fixed! I'm a bit dense sometimes!

[@JamesChurchill]

But can you give me five megawatts by mid-May?

[@mahmoudiamir]

I didn't understand, how 2 photon absorption, resulted in one point ilumination?

[@LesLaboratory]

The light intensity need to be high enough for 2 photon absorption to occur. This only happens right at the focal point.

[@climatebabes]

Doesn't this mean you can pump up IR radiation to more valuable light? Doesn't that go against some thermodynamic law?

[@LesLaboratory]

No, the process requires two low energy photons, to create a single higher energy photon, so energy is conserved.

[@climatebabes]

​@@LesLaboratory​Yes, indeed, of course, laws of nature, but it seems to mean that you can eventualy get UV light out of a IR radiating source, which seems to be in conflict with entorpy laws..

[@LesLaboratory]

@@climatebabes in the national ignition facility, IR laser light is coverted to green light, which is then converted to UV light. Energy is conserved at each step. In the end, the intense UV is further upconverted to X-ray wavelengths. Of course the overall efficiency of the system, even when finely tuned is abysmal!

[@climatebabes]

@@LesLaboratory This is funny because as space is very cold you could thus in principle generate electricity (coupling the cascade light generation to photovoltaics) from geothermal and other 'waste' heat indefinitely, while the common understanding is that low grade heat is near worthless. This is different from a thermoelectric device if I'm not mistaken. You may also be able to use it for night vision applications.

[@realcygnus]

nifty

[@zhanzo]

Theory shown is terribly wrong. Everything should be fluorescent if that was true, and the y axis is energy in a jablonski diagram, so the emitted photon has the same energy there. See singlet-triplet transitions, and thermalization to band edge.

[@LesLaboratory]

It is true, and they are . Most glasses, organics (almost anything organic including people, animals and plants), mineral oils, metal oxides etc are in fact all fluorescent to some degree. The next time you are in a optics lab put on your safety glasses, get yourself a violet laser pointer and turn off the lights and shine it around the place. Make a list of all the things that you can't see the fluorescence from. It will be short. As I said in the video the Jablonski diagrams are simplified, no singlet states, no intersystem crossing. All I was interested in demonstrating was two photon fluorescence.