Good afternoon, everybody. So this is Michelle Wright from Leiden, and he's the head of the single molecule optics group there.
He's one of the pioneers of single molecule spectroscopy. And we have a joint project with him here at the Clarendon in the mid eighties.
Michelle to the realisation that it should be possible to optically detect a single molecule.
And a few years later, in 1990, he indeed began the first research to detect the first signal from one molecule.
Last year, the Nobel Prize in Chemistry was awarded to Betzig her another for the development of super fluorescence signatures of
fluorescence microscopy and the Nobel Committee description of the scientific background clearly underlined the groundbreaking,
significant Michels experiment as the basis for the super resolution technique.
Today, we should apply single molecule spectroscopy to molecular photo physics, and we are here to talk about that today.
And he uses the technique to explore the spatial and temporal heterogeneity of complex, condensed matter.
He has been honoured recently by the Netherlands National Scientists Society Physical Prize in 2016.
So Michel, with you. Well, good afternoon, everyone, and thank you very much, Robert, for these kind words of introduction.
It's a great pleasure and honour for me to be invited to the high places of physics, not for the first time.
I actually realised it was almost 20 years ago here in Oxford for a golden conference at Queens College.
And so I'm happy to be the advocate. All right.
So this is my title is a bit complicated, but the main thing that you have to remember out of this is a world signal.
So we'll work every single little thing, single molecule, single plasmonic gold.
Nanoparticles, mostly gold. I will explain why in a moment.
In the past, we also work with single nanocrystals.
You can think of single conjugated polymer molecules, for example, etc. This is the current composition of the group six students.
Three of them will finish this year. So it is much less big group than you might imagine.
And two post-docs, mostly because they're all up and. But of course, more people have been involved in the results.
I'm going to explain today and then as the talk proceeds.
All right. So this is a brief outline of the different subjects.
So the audience not so big. So please, I encourage you to stop me any time and ask your questions because of the number of subjects.
I have little relation with one of those except the technique that we use.
And so it's better to ask your questions as a subject just as been discussed.
And if there is no timing of it, we skip all of all the other subjects.
So we're going to look at, first of all, low temperature experiments, because several of you here are involved in interest in these results.
We should use some examples that we can move to Plasmonic Plasmonic and see, first of all,
how to detect little things not by the fluorescence but by other techniques, and then how to use plasmons for that.
And you can have the sensing of just the refractive index of a molecule.
Another small thing, all the fluorescence enhancement,
which are slightly different ways of benefiting from this very high local field that you have around the metallic structure of supersonic structures.
And then as a conclusion of the two of them, to explain a little bit about the nano bubbles that we have been studying recently
around the heated nanoparticles and maybe on some possible applications of this.
All right, so let me introduce our technique, single molecule spectroscopy.
So how does it work?
So what we want to do is look at single small things, single molecules with far field optics, not with scanning problem across copies.
That's too difficult for us, not with near-field optics, which is very similar, but just with four filters.
And then immediately the problem you encounter is the like diffraction limit light being to be focussed down to less than about Micron,
which is you can at best select one cubic micron of sample and one cubic micron.
You have one molecule that you have millions or billions of molecules depending on which molecule sizes you are interested in.
So how to distinguish one molecule among a billion of other molecules?
Well, the way to do that is to make this molecule particular,
to make this molecule specific all to find the right signal that this molecule will give
you and not the rest of the solvent or matrix molecules that you're not interested in.
And one good way of and by no means the only way, but one very efficient way of doing that is to look at fluorescence.
To work with a fluorescence. Confocal microscopy is a very old technique that has been used a lot in the biological sciences in biochemistry.
The principle of this technique is very simple as just a fluorescent six like the same kind of fluorescence you have in this go way you will
like your eye does not detect but excites luminescence of fluorescence from dyes that you will necrosis this and that you can see very,
very well on a dark background. So basically you come with a laser and excitation laser that is focussed by suitable optics,
sometimes used to different optics or excitation connection.
It's much more convenient and easy to use the same optics to excite and collect your focus beam, excite all the molecules in full volume,
and you collect the emitted fluorescence in the inverse pathway and send these fruit and filters to a photon counting detector.
Usually an average photodiode beam is good enough. Photons deployed in the old days when the optics were was not that well advanced.
So it's important at this point to realise how a particular fluorescence,
I mean not every technique allow you to detect one molecule in about a million or billion other molecules.
The reason why this works is that for us it's an electronic, resonant process.
So the molecules that we absorb light will do this much more efficiently.
The levels, its electronic levels are resonant with this light than if they are much shorter waves and much higher energies.
And the rejection coefficient of the technique, that is how much background signal from the wrong molecule is going to reject,
which in the case of fluorescence amounts to about 10 to 12 is so good because for us it is resonant.
So basically you can reject, for instance,
the romance cost of a single and filter out just random scattering to select one for the signal processing of one molecule.
Because romance is not there isn't was for this resonant.
If you would like to do a signal through the detection of signal spectroscopy with infrared absorption, for example,
this would be much more difficult because the bigger the signal to pick on ratio,
the resonance ratio you having infrared absorption is almost a thousand on maybe 10,000 doing the best case.
And this kind of coefficients will never enable you to go to the ten to a nine or 10 to 12,
which is necessary to see a single molecule among a billion of longer markers.
All right. So with this very simple introduction, then we look at the results later in the program,
see other techniques that are either related to fluorescence or completely different from the from that, but all rely on optical resonance.
Okay. So now you have yo your one molecule among a million other molecules and that means you have
to dilute yourself or your first molecules are diluted are extremely low concentration,
familial, nonporous and simple. What you can do to distinguish molecules, just scan your excitation samples and move it around.
Russell Scale. And every time you hit the process of molecule or single process, you go up and see a single and this is coded as a colour.
You see examples of single molecules, you see the size of this box is Diffraction Limited.
That goes to the perspective of your microscope.
And you also can distinguish that several of these spots are not actually homogeneous spots,
but they are lines and that means that the molecules are blinking them is a signal,
even though the laser excitation is continuous, continuous wave looks like continuously.
The first and second wave is not coming. Continuous you can focus on for a while, then it goes off and goes on again because of.
And this on one hand is annoying because of course during the black doc intervals we have no signal.
So we use information on the Morgan. On the other hand, this is very useful and powerful way of identifying seal molecules.
You see, for example, this very bright spot is probably a group of the moon dust, impurity with many, many molecules that all meet at the same time.
And then you have this very, very bright and stable spot, no blinking.
No. On the other hand, on this molecule, for example, this is why can we know?
Because the signal, if we put our laser on the spot, the signal will go on and off in one step, one step blinking.
And that allows this allows us to be sure that this is a single molecule because
two or more molecules contribute to the signal will give intermediate steps.
And also this allows you to get additional information, for example, super resolution, by identifying these such really spots,
being sources, you may find the central spot with much higher accuracy that you can have on the on the points function.
And so this allows you to locate the centroid of the spot much better and to do super resolution because we think that was the Nobel Prize of 2014,
which we wanted to bet to have another for the developing.
This imaging technique is extremely powerful in biological sites.
All right. So remember, this is bringing us up the at the same time, of course, and a blessing.
All right. And there is another way you can distinguish molecules if you can have at your disposal of an additional dimension.
So X, Y, Z, the two components of position in space are obvious ways of distinguishing different molecules.
Another one is frequency, if by any, because we can assign different frequencies, optical frequencies in this case to different molecules.
You just need to scan your excitation frequency to distinguish what the molecules in the spectrum.
That is what is done here. That's actually your original work from 1990.
Well, you have this molecule, fantasy red absorbing molecule in a transparent matrix, part of it in actually absorbing the EUV.
That is completely irrelevant for these experiments. When we scanned the frequency of our laser, we find enhancements.
So this is sort of a frequency image of our sample.
And so a distinguishing molecule of the focus of position X, Y, Z.
We distinguish discrete distinguish them are the function of excitation frequency.
And the line you see here is a lifetime limited line.
Is this an X from at a very low temperature took and the lifetime is actually to the line which is actually lifetime limited.
So that means that the photons which are delivered by these single molecules on the single photon line are absolutely indistinguishable,
you know, just a language just determined by the lifetime of the emission.
All right. So let's discuss some recent expenses we did with these short lines.
And this will probably sound familiar to many of you here, but this would be the first part of the audit.
All right. So one of the first things we we decided to do was to try to use the same molecules to probe how charges are moving and material.
And in order to do that, we decided to take a more eco crystal policy, because at that time,
we could have published very fascinating results, which turn out to be fraud, to be fake.
Later on this, we didn't know the time. And so we wanted to have sort of a local probe of these fascinating facts we've seen on Michael's.
So this child transport that was started actually in 2000, two or three, more recently, the work has been pursued by Nicole for heart.
We're going to finish this later on this year.
And it just shows you an example of the recent results of Nicole on this molecule that some of you know very well is called.
Nobody saw their name, Michael, that resulted in earnings right at around 800 nanometre in this matrix product called benzene.
And what you see here is a single molecule line are very stable, very stable and narrow.
These are the lifetime unit. As for us as well as we can determine and cannot be 100% sure that there is actually
a new spectrum if you look at living in less than 10% or 20% of disclosure.
So it's mostly like the individual least. And what you hear is the saturation examples of these.
It's like it was really very, very nice and stable. And the symbols are easy to manufacture.
Actually, I have to go back a few years now to go to the experiments by these few people who later on move to think of an economy.
And it was hoped by two post-doc Klemens Hoffmann, who was not working for us,
I mean Regensburg in Germany and he was working for some time in in Moscow.
And all this work was done in collaboration with Stockhausen, give it from the Institute of Physics of the Polish Academy of Sciences in Warsaw.
So this is another example of the kind of images you can get in the frequency spectrum.
You see when scanning the laser is working on the delays or you see lines and when you have many lines of code, you can't distinguish them very well.
You can go to the wings or change your sample and take a more diluted sample and the cumulative result of a single molecule.
And we have also done with collaboration with colleagues in France,
some dynamic simulation to understand how this DVT they've been so carrying molecule is embedded in the interesting crystal
and we find these two special embedding sites that we assigned to the spectroscopic sites that you see in the spectrum,
the red side and the main side, their orientations.
Energies Well, good agreement between simulations and experiments.
All right. This just to show the kind of system we are working with, keep in mind that in these experiments, everything is very stable.
The molecule is not wiggling too much. It is a good fit between the molecule shape and the whole.
The work has to live in the to go forward. So we move in the lattice.
And actually, in this case, this DVT is replacing three host molecules with three interesting molecules in most cases,
but it's not the same three for the two sides.
Right now you have to to to to know that interesting is a conjugate molecule and interesting crystals you can propagate you can
conduct electricity by host so you can double this material from gold with the holes and these holes can move in the letters,
even at low temperature by tunnelling. And so what we try to do here is see, well, I assume you can see a single charge.
They don't know who is it w the result and we are not able to see single charges yet, but this is a project we want to continue in the next few years.
All right. So we made such a 3D structure and this field of action, this type structure in which we inject holes of a source,
which is a gold electrode, and we collect the charge with the drain and we can choose from the potential with with a gate record.
And you see an example here. The first thing you see when you're playing affixes to a such a symbol is that we have a dog effect.
So what you see here is the spectrum so far that is your voltage.
You see strikingly different single molecule lines when you vary the voltage either positively or negatively.
You said there is a shift and the shift is roughly quadratic. The reason for that is that this molecule and the site around it are central symmetric.
So it doesn't matter for the frequency whether you blow feeling one other direction,
it should be symmetrical and that's what you see in most cases, or on very small deviations.
But it's a weak perturbation. But in some cases, these molecule for that molecule is my point is not working very well, but it's working.
So this molecule has a very strong slope. This one is even larger.
Some molecules somehow have a minor dog effect that shouldn't be really with the profile Y
because the central symmetry is broken and we're seeing this breaking of symmetry to the charge.
Injection is inside the material. So building holds, injecting the material back in the vicinity of the molecule break the symmetry.
We create a very extremely strong electric field look and feel.
And then our added electric field has a linear component that we observe here.
So this allows us to probe into the way Chelsea's objective was very indirect and difficult compared to Probe Microscopist Kelvin from Microscope,
for example. But it's a very different way of contrasting these films.
Even more interesting to look at the dynamics. And this is a complicated plot.
I didn't want to take this week to complete it. But what I can stress is this is a time at which we stopped.
We went for associate voltage of 50 volt to zero and we saw some molecules feel this change of feel of a very long time.
So the time is minutes. So this about one hour. And what you see is of these molecules relaxing up cells very quickly and then more and more slowly.
So what's happening here, we think, is that the charge distribution is now relaxing then into a new equilibrium.
And there are first processes which are behaving as well as very slow process.
And if we wait even longer, we see that this line is still relaxing very slowly.
So a very non-accidental relaxation, which for us is the reflection of the distribution of turning barriers,
because this is not the material in which holes,
tunnels or barriers, and you have small barriers, first relaxation as well as very broad and high barriers and therefore longer excitation.
You also see some noise. So this is another example.
And here it is not photon noise. It's completely it is correlated.
And you see this noise which tells us something about the way the characteristic in which this the discrete character of this, the distribution.
So by looking more closely of this whole and maybe changing the geometrical symbol, what we hope is to go to see more charges in the future.
Okay. But you can also this is because you see some dynamics.
We ask ourselves the question, what happens if instead of applying a step size to the voltage, we apply a an AC voltage and oscillating voltage.
So how will this slowly selection system of charge respond to an AC field?
And you should do this experiment with changing the density of the voltage.
Would you expect this of the line is just shifting the spectrum with the spectrum
so that it is shifting and you get more intensity of the turning points.
But there is not oscillating dependence of the frequency of the switch of time.
Well, the molecule stays long at the turning points. On the second little curve you get more intensity and you have a split of intensity.
That's the real measurement, exactly as you would expect.
The surprise, however, came when we scanned not the intensity of the voltage, but the frequency of the voltage.
So AC voltage, low frequency typically from some tens of hertz to some hundreds of kilohertz.
And what we saw is this, and most of them expect everything to be very fast.
But what's happened is that in some cases you find these resonances.
So something resonates with the frequency of the voltage and you find resonance is all over the place from that's a megahertz, which is.
But what we could apply with our voltage source all the way down to a few tens of those extremely low frequencies.
So what is the charge injection? That's what we saw. The beam.
This kept us busy several years until we understood that what we see actually here is just all just local oscillator motors, mechanical ventilation.
And when we just happen to have an electrical excitation, which is in resonance with this mechanical mode,
is to localise filaments which are defects on the crystal that we don't really know.
Then we have these resonances and there are many interesting features of the time to go into that.
But this gave us the idea of because we couldn't control these local modes, let us try to simulate them.
So let's try to replace this vocal mode by something that we know how to make and how to control.
And we did this with with a device that many of us know that's just a tuning for.
That's an oscillator which you can drive with an AC voltage in exactly which frequency.
That is how it regulates tune, the amplitude of the oscillation of the problems with the voltage intensity.
And this was published two years ago in physical review letters.
So the work was done by Peter Navarro, that sort of, you know,
and the postdoc you should chance of fiddle is now back in Mexico at the University of Mexico City.
And you she's back in China and an assistant professor position.
Okay. So what they did is just take it during fall that you can drive with an applied voltage
and pasted or attached to it in a single and try to increase always DVT molecules.
And what you expect to see is this when the doing oscillating, there is stress in the crystal.
If the stress is extensive, you get the four level of an increase of frequency, a blue shift of the resonance.
When you press an increase or decrease,
ratchet that you get and you expect therefore the line of the molecule to accompany the tuning forks oscillations in the spectrum.
That's actually what they found.
These are the kind of resonances that you see are very similar to the ones that you observe with localised systems that we didn't know what they were.
So these comparisons are very close. You also see an harmonicity just we saw in the case of these natural two level systems.
And so what we were after in this case is to try to demonstrate how sensitive this nano microphone we call it is not on microphone,
because a micro so small how sensitive it is. So on the numbers we we obtain.
So it's on the off peak or metal based world of hertz.
There's not and it's nothing compared to the to the legal detection sensitivity.
But the big advantage is legal is four km large on like all but our molecules is only a fraction of another rather larger.
And that means we have a detector which is extremely sensitive but also extremely small.
So we can place very close to an acoustic source and still be able to measure very small displacements with a significant accuracy.
So this is, of course, of high relevance to electro optical mechanical devices, when to push them into the quantum limb.
What you see here in this case is a trace of the applied voltage, plus some background.
So this is what unfortunately not the thermal background that you could expect, of course, is there.
But we are still always amazed with the somewhat background which is here is just a
noise from our Christ that is just with both that keeps the vacuum ones increases.
So we are not after that when we expose what do you want to do it better?
And then of course you have to switch off the beam and do better measurements.
All right. So this is for the first priority and seemingly just pixels could go to all any questions or remarks
or about this part before I'll switch to the rest because I would not go back to load material.
So let me switch now. So so far just to stress fluorescence and for good reason too.
But of course it's also very interesting to see other signals and fluorescence and
this is one technique that we have proposed actually is a very old technique,
actually, but we just translated it into a microscopic version back in 2002 already in my cooking module from a group in Moldova,
and this was later improved by Muniz and colleagues.
This technique is just relying on the change of index or refraction around the heated particles of so-called thermo lens.
So if you have if you have a homogeneous medium, you know, there is regular scattering.
But this is very weak effect. If you introduce a small index or a fraction, there will be scattering that was responsible for the for the blue sky.
If you make now this index, that fraction change.
A consequence of a change of temperature. And this you can do by looking at absorbing materials.
You can have something similar to the to the Mirage effect.
If you heat the black surface and a blocking surface in some other, it would be a and phoenix of pressure.
And this will modify the propagation of light rays in the parallel direction to the surface.
Very, very sensitive. So this is basically the technique we use and we use that with these set up.
So the work was done in our group recently by these two people.
And I was working for a field station, you know, in Germany and most of I almost was who was post-doc research before linking
as Rice University and now he's back in Turkey on non scientific position.
So the technique we do is as follows We have two beams.
So this is a crosstalk between two beams that we are exploiting so that Jesus Christ we effect.
It's an equal effect in which we have one beam which is there to be absorbed to heat the particle.
So the particles to the focus of this will be and then we have a probing,
which is the the red beam in this case that we focus almost on the same point experimentally.
It's very hard to achieve exactly the same point. And you should achieve a slightly different focus for the red beam and the for the green, actually.
But this is, you know, just an experimental detail.
And what happens is that the the green beam by hitting the particle is going to modulate through moderate intensity of the agreement,
modulate the index of pressure, and therefore the scatter intensity.
And we bear the signature of this because of scattering in the frames of a scattered field by this index of refraction lens and incoming field.
And this diffraction will give you a signal, which, of course we scale now, like the volume, like the poor rigidity of this.
So it's not like a normal scattering smoke engulfing the square of the the volume, which is important like in radio scattering,
but it's the definitive because there is interference in the frames, accommodating the effect between the incoming field and the sky that we want.
So I don't want to discuss the signal to noise ratio. And this is very interesting that we have no time.
And what you find with this technique is that it's a dark background technique because it's very is not
soft and there is no thermal of scattering because you do modulate the frequency of the motivation.
But it has a very high dynamics. It is a large scale of sensitivity, then in 2 to 3 in the same scale.
And for example, here what we see are 20 nanometre gold on the volumes,
which are the big dots and the same type of five nanometre, which are about 200, 2000 times weaker.
We see them on the on the same scale. So that means that there is a very, very high dynamic range for this technique.
It is not very sensitive. I should stress that the the best sensitivity that you can detect with this technique was in the one house band,
which is about, let's say, another world. So this is nothing compared to the presence which would be distant because it's much less sensitive.
But the bigger the edges that works for anything that absorbs.
So you need anything that emits that caresses, any absorber will give you photons almost because with just the heat and the heat is down.
It's even possible to apply this technique to single molecules, although it was very difficult.
So this was a big achievement. I must offer five years ago or six, and what you see is a single molecules of these, so very special.
They were very photosensitive photo resistant. So we can actually it was very hard to bleach this destroy these molecules.
These were the signals that to use oxygen.
So if you use nitrogen, they are basically for forever.
You can also compare for us and see if you can measure them to appropriate signals.
What you see as the measurements for gold nanoparticles in those fields.
And what you see here is the the ratio of the absorption of the luminescence signal to the appropriate signal.
You can deduce the quantum use of luminescence for the spot, which is extremely small number.
Then to my system to 1027,
but it's independent of size that something that was been known before they examined the York and Feldman in Munich Bay and several measurements.
But this is done on single particles and you see that this quantum is almost not dependent on the size of the particles.
Okay. Though more recently we moved from spherical gold nanoparticles to gold.
None of us. So now this is this.
This appears small to the plasmonic aspect. So we'll have to introduce this.
The subject was introduced in the group. I mentioned them before, so let me first discuss the plasmonic aspect.
So a plasma normally is a magnetic wave that propagates at the interface between it's a metal on an absorbing material and a dielectric.
So typically a wave property at the metal vacuum interface.
However, in the case of a smooth, very small particle,
the plasma is even simpler than this because of course you have to to satisfy this boundary condition the wave is propagating around the particle.
And if the size of the particle is much less than the wavelength of the plasma, then what you expect is that you have a dipole excitation,
so you have positive charge on one side and negative charge on the other side.
So just a dipole excitation, these charges division,
the particle just resembles dipole where you shift the positive charges or rather the
negative charge of the electrons with respect to the immobile ions at optical frequencies,
the ions are much heavier being much heavier than the electrons won't respond.
And therefore we have a dipole. So clearly the frequency will be in the optical domain because the mass of the electrons is set to low.
And basically it's just the frequency of resonance would be a ratio of racial frequencies,
which depends on the Coulomb field within positive and negative charges and the mass the effectiveness of the electrons.
You can also see that this frequency is actually not depending on the number of electrons that are provided.
You can suppose the particles will be small enough to be able to have this dipole excitation.
The size of the particle is not really of any matter.
However, the shape of the particle is if you take known an elongated particle,
you'll see that for a given number of charges of or given mass of the electrons,
the Coulomb field between positive and negative charge can be made arbitrarily weaker and weaker by thinking more and more in
obligated particle to take another way and extremely low becomes negligible before the shock value becomes extremely small.
And that means that you shift the frequency down to more to the right of the need for it.
So these are very nice feature because it allows us now you to consider this by tuning the aspect ratio of the
particles to tune the frequency and to that this frequency to any source or emitter that we care to observe.
So these are a very nice degree of freedom for us to observe plasmonic effects of the frequency that we are interested in.
Okay. Some are very few words of description of of these particles that these team images recalled because they
saw it with the moment in Australia where you see that they are of extremely high lattice qualities,
single crystals and basically can have in two different shapes so that these are the shape we are most interested in.
Single crystals with long axis along one of the one z was your direction of the gold can be
crystal on this other multi twin between the crystals all grown under different conditions.
We usually don't use them. That is important because first of all, the way that the shape of the appearance of this vessel is very well defined.
And also, as you see, that the crystal is of very high quality.
You have very little scattering of the electrons by defects or impurities.
So it means that the plasma installation is of the best possible quality.
It's only four months that will limit the lifetime of the Plasmonic solution.
Okay. So this is an example of the optical spectra of this of these nanorods.
So this is an example you see here, three different nanorods in a T in the image.
And here we see the same area in the optical image.
And those are two spectra, two example spectra. So this spectral and of course strongly polarised along axes.
Remember the two charges of the Ellipse, which of course is strongly polarised on the long axis?
Overall, the perpendicular transverse plasmon is a completely different frequency of the integrity here.
It's much, much weaker and you see that this resonance is also extreme shock.
That means that you will have enhanced interaction with the local field.
So you have very good interaction with the local field, the tip of the rods where the well the charge is.
So it is concentrated charge area is where the field will be.
So the two ends of the lot will be the place where the new field is strongest.
So what to do with that? So one example, this is the work of Peter Islam.
One example is this Plasmonic Plasmonic sensing plus one is sensing that we want to be able to detect a single protein molecule.
And of course there is a trade off too to, to, to strike between having a very small,
very large particle, whereas particle can be measured with a high signal to noise ratio.
However, a molecule will be very small compared to this particle.
So the effect we expect is going to be small together, large, single, with a small variation.
On the other hand, you can go to very small particles, then you have a better ratio.
But the micro is a very big for the basis you can hope to see something.
So we could find a good optimum with these kind of rods which are rather inundated with of about ten nanometre length, about 14 on the metal too.
This gave us a good a good compromise between the two demands.
So what has been done by Peter? This is now assistant professor in the Technical University.
I would mention some of his work at the end and the calculations have been done by a middle fellow who is assistant professor in Elizabeth in both of.
So the idea is very similar to the one you saw before with this acoustic detect of of acoustic waves at low temperature.
But now we're doing that at room temperature with the plasma.
So what we do is put the laser on the length of the plasma frequency.
If a molecule comes and sticks to the plasma and to the to the plasmonic particles,
and we have a shift of the plasma from the dust down to the solid like him.
And if you can measure the, for example, the absorption density,
the force almost equal or the scattering intensity with high accuracy, we see a change of the signal when the particle comes.
And. He's the example of this.
This is the example of a trace that that pit acquired with no protein in the solution so nothing happens is very stable.
Plasmonic trace.
If you have low concentrations, one event, one one group comes in binds and these are traceable several molecules by and sometimes even unbind.
And eventually you get more and more molecules at the end of the world and you can follow this.
So what Peter now is doing is doing the same experiment, not with one rod,
but with several tens of rods, an example of one of these maps with arrays of minerals.
This published in nano letters this year and you can now do these six were not looking one bit in the many tens of
different worlds and see at the same time of many parallel events of binding of protein molecules on these plus one.
Another thing you can do is fluorescence enhancement,
and it's also a nice combination of a medical property for residence with plasmonic enhancement for another world.
This work was done essentially by these two gentlemen.
So I suppose looking good and knowing the group of your conference in Belgium and some
others is back in India and is looking at DeNardo and I think has a very good start there.
All right, so what is done now? We try to adapt our goals down the road to the two day and the day we took this one,
which normally is not a good choice to look at because this molecule does not reverse very efficiently.
The quantum is about a few percent, 2%. So it would be possible to see single molecules, but it's very, very difficult if mean definitely not.
And the other conditions that we you'll see very simple.
But if you use a single nothing wrong that has a spectrum here, that's the yellow shaded spectrum.
Both has the spectrum which overlaps significantly the visitations because of the light and the fluorescent spectrum of the then you see very
strong signals and is shown here you see an example or trace where you see your signal and then suddenly there is a burst of resonance.
So what happens there is we could imagine this diffusing and molecule diffusing entering the near
field and giving this not actually what happens is much too fast on this timescale these seconds you.
So what happens is that molecules come bind to the glass or face.
I should show you the sample. So these molecules are swimming in solution diffusion through the near field.
This is only a few tenths of none of this is very, very small. The different time is extremely short.
But what happens is the more scum stick to the glass and eventually reach or detach sometimes.
And was that molecules are fixed here. There are enough for us as photons for us to observe these bursts.
And this gives us some information on the molecules and which is here for this correlation function.
So we have continued to investigating these kind of signals more and more recently,
this theoretical consideration of what's happened, what's happening there.
So the for instance enhancement photon actually comprises of two components.
There is an excitation enhancement factor and radiative enhancement factors plus
some factors corresponding to the molecule photo physics inside the molecule.
But we have no time to go into.
So in the proper conditions, the enhancements that we can expect for low quantum those is just a product of the excitation enhancement.
Yes, because of course, if you are in a new field, there is a stronger field.
It's more likely to excite the molecule in the near field of your far field.
Not only that, the non world also acts as an antenna.
So it's an antenna for reception but also for emission.
And because the plasma is not too far from the spectrum of light, there is also the radiative enhancement and these two factors encoded.
Not all can sum up or multiply up to something thousand.
That's what we measured, or even 10,000 called. Into theory, we have the proper overlap of spectrum.
So this is something that we think is potentially very interesting in chemistry,
not only to detect dyes with low quantum yields and those that do normally don't see to the single molecule level,
but also some emitters that you normally don't expect to be able to detect at all, like,
for example, bentonite complexes, which may be extremely interesting because of the high stability.
So this is something that we are exploring at the moment in our group.
So let me skip this. No. And I like to finish.
I think I have first you taught me, then this is for you.
So let me try to finish with this new subject.
Actually, we we started two years ago.
I mentioned already in this photo thermal experiments that there is heating and heating and normally you try to avoid it,
especially when we did the optical trapping experiments.
The heating may be a large one. On the other hand, this may be useful.
And what we have been exploring in here is what are the conditions under which a steam bubble or nano bubble can form around the heated part?
But if you really heat seriously the fluid around the particle, if it is in the food may change phase and this has potentially,
you know, to two sides, first of all, a very fundamental side.
So one is how does a phase transition change when you go to smaller and smaller size?
I mean, at some point you hit the somewhat dynamic limit and.
How do you approach it? So this is the face of resistance.
It's a smoother, more continuous when you go to small size. How small the size have to be.
And of course, we are still in the range of sometimes development as opposed to very microscopic.
But it's a first step in the direction that you want to go further, to explore further, to move to this very fundamental question.
On the other hand, there is a very applied side to this problem, and this is in the foot of some of cancer therapy.
There has been a recent publication in Nature Nanotechnology of some some weeks ago, I think,
in which people have measured or proven that by creating another bubbles you can kill cancer cells.
Not only that, you know, but on the metastasising cells that are only by targeting the cancer cells with golden particles.
And this effect can be, as you would expect, I mean, hitting maybe a problem, but hitting with a very few particles is not sufficient.
But you can also use some waves, acoustic shock waves that can be produced by your laser by post later.
So that's the applied site. So so we we are, you know, fundamental physicists also in this and we are interested in in the basic side.
But it's also nice if it can be useful for possibly. All right.
So this this one has been published here a few weeks ago, last year.
It's mostly the work of the how so lady is finishing his thesis now and most of his work has been on this plus one against the wolves.
But he has a successor, Thomas Newlands, who actually was a this of centre here in Oxford and the group of curious companies.
So what happens if you try to boil water in a really small town?
So you know what happens in a normal pen at room pressure when you bury what?
I have to go through 100 stations in order to get bubbles and water, boys.
But if you will. So I think that we have a very small container, actually, that you have to create a bubble which and closes the particle.
And if you look at this picture this you realise that there is such a thing as plus one pressure inside the bubble.
And because the radius of the boat was very small is le plus pressure, which is like one of the radius is very large.
It can easily amount to zero atmospheres and it also know what happens if you
increase the pressure of your boiling water when the body temperature goes up.
That's a pressure cooker. That's what we cook food faster at, high pressure and that room pressure.
That's why you can cook at the top of Mount Everest. All right.
So this is what you expect. And this calculation is drawn from the list tables of the the water properties.
What you see here is that in order to boil water, a smaller and smaller case,
this is the radius of particle 20 down with the foil you go all the way in Stokes 373 up to room pressure.
Now you have to go all the way to 530 or so Kelvin, which is something a 200 something degree.
And if you go small enough, you go above the critical temperature of water, which means that you cannot boiling one.
You go from liquid to gas without any water under normal conditions.
So this is actually a problem I would show you in any minute. This temperature already to about 300 searchers is very high for the surface go that and
so what happens then you go to this temperature goal that I was already able to migrate
on the surface of a particle that's especially bad in the area and the contact between
the particle in the glass because that means that the gold atoms in prone to move,
the particles changing. So the experiments you're going to do are not real reproducible.
That's why we prefer to do these experiments with at least to understand what was going on at least 20 years ago.
So what you see here is the same covers which contain contain both at about 50 degrees Celsius.
And that means that the temperature has much lower.
And in case of water, that means that we can neglect this rearrangement of cold atoms.
And we go to if we look at the the heating power, we can keep the particle for a long, long time without any changes of its properties.
And this is a temperature dependence of the finish of distance. While we are expecting, but also at low power,
you see that the particle temperature is constant inside because the quality of all heat connectivity is very high.
And then you have the standard one of our so we do this on day one of our dependence outside the particle which follows from the Brussels law.
And at some point you hit this curve. And that's the last time when you have a stable water,
liquid water or liquid containing these guys or the particle when you go higher and create a vapour show
in the steady state conditions and you have this jump of temperature and you see that in from this point,
the temperature increases very quickly. This is because the heat conductivity of the steam is very low because it's a gas.
So it also explains, you know, if you hit a bit too much higher than the threshold, you should hit too much lower the temperature.
But it will go up to this point where the particle is prone to change and to change shape and and the contact area is going to evolve.
So we we need to we need to learn how to control the power of the of the beam well enough so that we don't go to this regime.
So this is what an experiment was for to small detection gives users as just a foot or something on the function of of hitting power.
So increase specific and then suddenly something happens.
This is welcome. But this thing is not very political for the reasons I mentioned.
So when we moved pertained we expected to see the same and actually what we did that was not detected with rocking
liquidity for to someone we just Darity plugged in the photodiode signal into the acquisition electronics.
And what we saw is this this is the signal to this the detector current as a function of time.
And the time timescale is very, very short.
This 100 microsecond. And you see this noise is actually not noise.
If you expend blood on the screen, you see this is a succession of very brief pulses.
So what happens there is exactly what you can do in your kitchen when you put your bed of the fire.
We have bubbles, bump, bump, bump up. This explosion of bubbles is explosive boiling, except that this exploding building is extremely fast,
typically one microsecond between two different explosions. The explosion is a very brief event.
You have an integration here. This lasts about some tens of nanoseconds.
That's nearly the resolution of our detector. It's a little bit longer so we can deconstruct somehow and get some information.
But actually, we need a better detector to really see this profile. So this is a very, very first event.
How do we understand that?
So roughly our models, although we need to of course a better surgical modelling that this qualitative explanation I'm going to give you.
So what happens is that when the particle is heating, we get a layer of hot water around the heated particle and with it is first of presentation.
So we go immediately to the gas phase without the nucleation of it.
And as in many first order transitions, you have to go across the transition line before this nucleation event happens.
You have to wait on the thermal fluctuation for this nucleation to happen.
And when this happens, then you have this excess energy stored in hot water that will be able to be restored to the system.
And this is a reduction of very quickly to then to the steady state that it should have accepted.
And you see this, these are unstable.
So this instability is actually not something that that happens as a run away, but it is very actually very periodic.
This a histogram of the times between events. So in this case, very close to half of microseconds with a very small, let's say, a deviation.
So another variation of form from this one, and there is no memory.
So it seems that the system really forgets about the performance problem when you prepare the next one.
So it's completely run it. This, we really have to understand better.
That's the the first to see the first few facts that you can see.
Even more interesting. You can also see that in some cases, especially when you go to lower power,
when the time between experience rather one microsecond instead of half of microsecond,
in some case you see an additional peak that comes in the other and very remarkably if you integrate the signal.
So the second peak is very sharp to them is that the second peak has a very well-defined delay with the original explosion.
So this was really a puzzle for us until we realised, we realised that of course there are some ways released on these explosions.
It is a very violent process and there risk transfer.
It's a dynamic transfer to a shock wave of your presidential waggle or an acoustic wave that propagates around the particle.
As soon as the explosion takes rates and the sound waves can reflect, reflect on this interface, actually, that's the small structure you see there.
You see this little bump here that's exactly at the distance of the time delay that
you expect for propagation through the glass slide in which the focus is resting.
But the addition blast slider is this emotion oil. And we know the thickness because this is just a built in property of the objective.
And then the other from this which is a heavy glass, which is a massive plus and this there can be reflections here.
And what you see here is just the reflection.
This red arrow is just a reflection of some weight on this area, on this interface between the oil and the glass.
And sometimes you can even see the second reflection, which corresponds to a double reflection in this oil.
And so, again, this shows that it is possible in this way to release film, which is the material.
And the next thing we like to do is not only to release on the waves, but also to detect them.
So you can imagine this bubble, if we can maintain these bubbles, can disturb the bubble in a steady state, not exploding, but stable.
And we can do that. Then this compressible system will be very sensitive to changes of the external pressure.
So we can use this as a game, as a micro or nano microphone or able to translate or to to try and use an optical signal.
So in the acoustic signal into an optical signal. And so you can hope I mean, dream that you could do, for example,
an experiment by using these particles of small microphones and detecting this way, you know, the sun, but could provide to a body, for example.
All right. So with this, I think I came to my conclusion and in a very nutshell, so we've seen it's one of the low temperature.
We benefit from this very narrow, very sharp rise.
And this we want to continue in the future. And Plasmonic properties are extremely interesting for a whole range of experiments I just mentioned here,
the experiments we did with the Nanorods, they have the advantage of that.
We can make ourselves the Nanorods. Of course,
the variety of plasmonic particles is huge and you can even imagine much larger enhancements with different
assemblies of nanorods or even more sophisticated structures like my husband in London or many other colleagues.
So there is indeed a lot to do with combining plasmonic structure of metal particles,
and these are graphically made structures with molecules and small emitters.
So with this would like to thank you for your attention.