 Okay well thank you very much the organizers for for this invitation and for their hard work to bring us all together here. I have the feeling that it's gonna be a really nice conference. I'm going to talk about a project that we started last summer and that concerns collective, here so that I see what I'm talking about, collective effects in viruses which we think they hold major roles in in assembly, in form and stability and as such they carry an integral part of the physics of virus function and also new virus-based technologies in which we are interested. Here is an example of a mechanical kind of illustration of collective properties and that's one of my favorite toys, perhaps some of you too play with this. You have magnetic beads that make a Nycosahedral T equals 3 structure and if you want to solve this puzzle you start to add beads one by one and you have choices of where do you want to start. Here I start with a hexamer, yeah the pentamers are in blue, I start with a hexamer and I form a let's say let's call it an intermediate of assembly and what you may notice is that it looks a lot like just a hemisphere plucked out of the original structure but so we start here with a pentamer, it has a five-fold symmetry, it has a jagged edge where I'm adding particles. Now here these magnetic beads were put together by starting at the two-fold symmetry axis and you see that the line, the edge, the growth edge here is not as jagged as this one. So what does that tell me? It tells me that regardless the interaction potential between the particles which remains the same between the two intermediates I'm going to affect the growth mode due to the fact that as I add particles I'm straining the intermediate mechanically. This type of stress that I add as I grow my puzzle is a collective effect. The line tension as Rob would call it I guess it depends, its jaggedness depends not only on the interaction potential between particles but what how we grew this and what how much curvature strain you have there. In other words how many intrinsic defects if you want versus normal hexagonal lattice points you have in your capsid as you grow it. So that's an example of mechanical collective effect that I think many people here are interested in and like Roya and Martin Castellanova is gonna give us a talk I think later in the week about it but what I would like to talk about is this idea of coherence of interactions that leads to new properties and I'm going to talk about something very different from mechanics. I'm going to talk about super radiance about a photonic type of mechanism or application and so that we get everybody I saw how many people were reluctant in saying that they are physicists didn't know whether they should you know raise their hand or not which is a good thing I think it shows that this is a nice heterogeneous crowd but so that we have everybody on the same page and in terms of at least in terms of what I'm going to talk about I would like to point out of the phenomenon a phenomenon that in my mind is reminiscent of what I'm going to talk about and this you know the westerners perhaps is are not very used with this kind of social behavior but this is if they would like to learn more about it they can oh sorry they can read about this paper in this paper about it what you know in Eastern Europe we had this we had sometimes to listen to demagogues talking and although they were very boring we had to please them by showing enthusiasm at the end of their talks and so how would we do that you clap your hands but you just can clap only that hard right however if you start clapping at unison the contrast between the sound that you produce and then the sort of the no sound is so great that the enthusiasm is apparent and so this is what happens these guys actually put a microphone in a conference room and I do not know who was talking it's actually not relevant but they put a microphone in a conference room and he's here you have wide nose of noise of people clapping their hands incoherently and then suddenly you know listening to their neighbors they start to build up this unison clapping yeah and and they get what I will call during my talk super radians without using more power you use coherence to create higher contrast okay so this is the idea here that the new properties arise when there is a well-defined phase relationship among the microscopic constituents of a material in our case we are going to talk about capsids and there is a good well-defined phase relationship especially if the capsids are symmetric so the premise is that if you if your constituents have a quantum character molecules do tend to have that character and they are located in a spatially coherent pattern you have quantum symmetry enforced selection roles in other terms when you do transitions between the states of these symmetrically placed constituents you are going to end up starting from spatial coherence you are going to end up with collective temporal coherence in other words well in the more formal way Fermi put this well Dirac actually 20 years before Fermi in terms in terms of his of the Fermi golden rule this is this is the rate of changing from the initial state to the final state and you have here the symmetries of the initial state the final state encoded in this scalar product in which also you have the Hamiltonian of the perturbation that is going to be in my case the electronic the electromagnetic field which defines the whole symmetry of the problem and the symmetry of the solution has to be to have the symmetry of of the Hamiltonian and the result then is that new dynamic properties starting with this premise new dynamic properties so some of which are impossible to create in systems near the thermodynamic limit are going to result okay so what examples do do I can can I can I tell you we have this great example from photosynthesis where energy transport at mesoscopic scales can be much more efficient in quantum networks so here we have a very simplified schematic of what happens in photosynthesis where you have light harvesting complexes that are at the heart of the process in which you have often a symmetric arrangement in a ring-like structure of chromophores that are self-assembled now what happens there they are so tight together that when the light comes in a photon gets absorbed and they are so tight to together these dies or chromophores that their electronic wave functions know of each other and so when the photon gets absorbed it creates like ripples if you want in the electronic wave function of the entire chromophore ring and those ripples of energy are called excitons now excitons in quantum mechanics can travel not only like particles let's say following a trajectory but they can be in two places at the same time and that's what makes the transport of excitons very efficient so if that happens you can transfer this energy from the photon from far away to the reaction center here where the actual chemistry that is important for photosynthesis happens yeah so another example or sort of thing to keep in mind is that if you have any independent oscillators and this is also a classical kind of picture if you have any independent oscillators that their collective response are going is going to be in terms of signal to noise ratio limited by square root of n but if you have a coherent ensemble of oscillators the signal to noise ratio when you measure things is going to be proportional with n and therefore you can switch between states for example with less energy this is why we are interested because if we can make let's say molecular photonic circuits that are quantum coherent in nature it will take less switching energy now let's get a little bit more concrete our question here is whether the structural fidelity of self-assembled virus like particles which one is depicted here in our case we like to play with metal nanoparticles inside the virus shell that is grown on the surface of the metal nanoparticle so no RNA here just a big blob of metal but the structural fidelity of the capsid the question is whether it can be harnessed to control dynamics in the way I told you from symmetry to temporal coherence and control this way the dynamics of collective excitation of energy conversion so I would come with a photon I would have a bunch of chromophores on the capsid placed at identical symmetric locations and the chromophores would then absorb in a collective fashion and it would discharge their energy in collective fashion to another instead of a reaction center instead of a bunch of molecules that can harvest that energy into separations molecular separations you would have a plasmon that is gold particle or metal particle will have a collective ex oscillation of free electrons that is called the plasmon that can be coupled with that high in well electrons these electrons that are undergoing the plasmon oscillation they are hot and therefore you can create conditions for chemical reactions look at if we look at the scales of what I just mentioned in terms of the the chromophores organized in light harvesting complexes this is a hundred nanometers sorry this is 10 nanometers this is 10 nanometers bar here you can see that the rings of chromophores that we would form are comparable with the rings of chlorophyll chromophores that form in bacterial chlorophyll so chances are chances are that we are going to look at the same or a similar type of physics now there are some fundamental phenomena of interest here like enhanced optical absorption something that I will talk about here it's called a decay super radians this unison clapping of hands if you want then there is enhanced plasmon coupling and which would produce hot electrons for photocatalysis and the consequence of the super radians behavior which is going to be at the heart of this talk is that we could create in principle deep sub wavelength coherent light sources which could be useful for photonics biomedical applications in this very simplified cartoons you're going to find out we are going to cover it and tell you that one one symptom of our or sort of fingerprint of this collective behavior is that that I call super radians is the fact that the light is going to be be emitted instead of being emitted isotropically like from a bunch of dipoles that emit independently here the light is going to be emitted directionally so imagine that you have something like a surgical surgical scalpel that goes not these are optical devices that where you should actually ultrafast pulses of light the tissue now what if we these ultrafast pulse pulses of light going on the tissue you could detect these virus like particles that are super radiant because they shoot the light right back at you yeah that would be the sort of the far fetched application that that we would be targeting with such concentrated sources of light so let's take a look at a little at the phenomena there are a variety of coupling mechanisms here that affect optical absorption optical absorption is not going to going to be similar or the same with the optical observe absorption of a bunch of independent chromophores that are disordered in a small volume you are going to have you could have strong coupling like when the chromophores are really close together you can create excitonic states like those ripples in the electronic wave functions that I I mentioned and you could have weak dipole dipole coupling like in the Forrester energy transfer known as the Forrester energy transfer in which the chromophores will organize the the phases of their oscillations but in you know in a way that is very different from overlapping of electronic wave functions one interesting thing about the the two possibilities here which are differentiated in terms of the distance between chromophores or the distance between nearest neighbors is something that Greg Schultz mentioned in one of his papers he noted that for photos photosynthesis in order to have collective absorption is better to couple several molecules through weak dipole-dipole interactions that are weakly screened because the space between the molecules then extend a many electron system through space you don't want to have pi conjugated bonds that make that ring what you want is to have chromophores that self-assembled in order to get the right optical absorption that's one sort of quote that's stimulated our attention an interesting also difference in terms of optical absorption is what happens when you add a metal nanoparticle at the center of your chromophore array because then you have every dipole is going to have an image in the metal and if you have tail to head sort of orientation which means when the dipole is perpendicular to the surface you are going to have an enhancement because you have two dipoles you can see the mirror image yeah you have an enhancement of the absorption if the dipoles are parallel you are going to have a suppression so in order to see any type of effect we have to control the location and orientation of these dipoles on the surface of our virus have to be mindful about these kind of strong distance and orientation dependent dependent effects so going back to just an array of chromophores that is organized into let's say a 30 nanometer kind of particle so a very sub-waveling particle we are all familiar with fluorescence and its isotropic emission when the chromophores are in solution and they are far away from each other emission is isotropic a detector that can see the emission after pulsed excitation would show a single exponential decay characterized by this exponential lifetime if you have a volume that contains the detectors and they are at much shorter distances between them than the wavelength then the prediction is and this is Robert Dicke who did it for its first time in 1954 that you are going to get a coherent spontaneous radiation process by which the emission is going to be anisotropic in the direction of the pump beam and the detector will see instead of an exponential like this it will see a pulse the emission will be in a pulsed fashion with a duration of that pulse being about the spontaneous lifetime over and the number of chromophores that makes the ensemble and so this was observed for ideal gases and I'm going to point out why it was observed for gas phase but also later on in quantum dot and another type of solid state chromophores but it was never done well the way we are trying to do it there are characteristic differences between normal fluorescence and super radians and I will point a few here that we are going to track down during this presentation one is that fluorescence emission as you know is that it has the intensity proportional with the number of emitters that you have in the beam while the super radian emission in the limiting case the best case scenario will have its intensity proportional with n square at high density of chromophores you expect quenching and not only expect that you are going to see it unless you put those chromophores the right way in super radian emission you are going to see reduced to no quenching one of the interesting things about the super radian emission is that burst of energy is actually going to contain a hundred percent you can have hundred percent efficiency of conversion between pump beam and emitted light so there is reduced to no quenching then is here you have mono exponential decay with us this is the spontaneous lifetime and there in super radian emission you have a burst emission that as I said is shortened by the number of chromophores that you are using on a particle moreover if you only have an array of chromophores it's going to emit either in free space or it will decay non-radiatively by collision and collisional decay however if you put a nanoparticle in and the nanoparticle has these electromagnetic modes as I said called plasmons you can couple instead of free space and shoot the light out as fluorescent emission you can couple with the oscillations here and you have relaxed selection rules with respect to coupling to photons so you might end up with a lot more energy that goes into the plasmon and then from the plasmon back into to another chromophore that would emit light and we don't know yet what the pattern of emission from here would be it was interestingly it just took off in 2010 with this theoretical considerations that that describe what I just said there is Tigran Shakbazian who published a paper in 2010 and since then there is quite a bit of theory but still is only theory no experiments for the plasmonic decay effect so super radians and the symmetry has not been included in any of these L the quantum number yeah I don't know about it so with with particle with super I see m13 is much larger than the wavelength though yes I know the diameter is less than wavelength the length is much longer than the wavelength because because Zvonimir Dudjitz is them with normal optical microscope it's a micro long so we are going to talk here about much smaller scales you have when you have nanoparticles you have a resonator and you have a gain medium that comes from the chromophores and the two ingredients give us the this what we ask the question whether lasing conditions even could be met at this scale those are the conditions for a laser the first claim was done by Noginov in 2009 of such a particle that that emits coherent radiation but the way they made it was with a nanoparticle encased in a died off shell once again the symmetry is not present in this case so our objectives are to establish conditions or to organize chromophores in a manner leading to cooperative photophysics at room temperature that's important and determine whether the optical characteristics emerging from cooperative emission can be further enhanced by coupling to surface plasmids and these are the works that I know off that looked at the type of viruses we are looking at labeled with dyes none of them treated the dynamics the cooperative dynamics as far as I know of an ensemble of chromophores attached the first is from Nicole Steinman's group the second is from the second and the third which are dealing with a synthesis mostly are from the Matt Francis group and one very exciting in recent contribution is from Frank Sainsbury and George Lomonossoff who showed that you can actually produce GFP so genetically modified viruses in plants that have GFP's arrays instead of chromophores that well it makes us think if we could design such particles or whether we could actually establish protocols for obtaining them from obtaining these super radiant particles in plants so the current experiments in our lab consist of synthesis by post-assembly virus chemical labeling by covalent conjugation of dyes you have the photophysical properties that we go through I will go through in terms of time resolved and steady state spectroscopy the spectroscopic tuning of nanoparticles we have accomplished that but I'm not going to talk about here today as well as the nanoparticle directed self-assembly and then structural properties thermodynamic stability and mechanical stability we will carry out we are I'm not going to talk about much today but we are carrying out as usual so we will be looking at these two hallmarks shortening of radiative lifetime as a function of n and to the intensity as a function of n the number of chromophores the anisotropic emission we would like to look at it we don't have the means right now you have to do it in near field so very close to the nanoparticle surface but anyway there is theoretical considerations that show that it should be quite directional and that's an important from the point of view of brightness that's an important feature now you have to we have two things that to keep in mind about what challenges are down the road because those challenges include two phase breaking processes that otherwise would would be detrimental in in in seeing super super radiance the first one is non-radiative population relaxation time described by the characteristic time t1 so you could have quenching by non-radiative energy transfer for instance the second is polarization relaxation time when the dipoles are going to change orientation before the super radiance happens and so these two are captured by the cooperative frequency condition so one over frequency you have time and there are there is a bunch of different parameters here that that characteristic time has to be less than the characteristic time for the two population relaxations so if you want to make the characteristic time for super radiance small you have to use a small index of refraction or you have to use a strong dipole moment here the or you have to have populate a large population inversion density and therefore we use we would use pulse pumping for that now the type of system that we are going to use is our workhorse the bromosaic virus a t equals 3 30 nanometer diameter capsid that can be self-assembled around nanoparticles and keep its structure the same it and it can be conjugated by established techniques with with chromophores these established techniques are depicted here in this cartoon we have selected the dye by looking at many dyes that was just an empirical process so we are going to work with Oregon green which is this one here these are the particles they are conjugated post by Malay mid coupling they are conjugated post assembly they look very nice and what's more perhaps very important is the fact that they are more stable than the actual virus so what does that mean it means I think that the chromophore participates or is involved in at the assembly interface in an active way which would mean that the particles the chromophores are actually lodged in some crevices there that doesn't don't allow them to fluctuate so that's why we were really interested in this chromophore this is the steady-state fluorescence as a function of the number of chromophores that we put in on the we tune the number of chromophores statistically okay so it's a statistical approach we just use kinetics you we start with a free dye here this is the excitation spectrum this is the emission spectrum free dye once you put the chromophores in you can see that the chromophores have a shifted the dye has a shifted the dye on bromosaic virus has a shifted spectrum the red shifted spectrum which means that again it's in an environment that that changes from the polar environment of water to something something different which is probably the protein the protein as we increase the number of chromophores that shift doesn't change much yeah which means that we don't get this excitonic coupling I was talking about in the beginning we only have probably dipole dipole coupling there is some fluorescence quenching in steady-state it's not as drastic as we see from many other chromophores so that that's another reason we chose Oregon green keep in mind this is steady-state that means one photon at a time is absorbed by the this collective of chromophores and we have then delta n the inversion population density approximately zero so we don't expect to see here super radiance it's in water yeah everything is in water sorry yeah unless we would lower the temperature so that the 2d phasing processes are very long then we take pulse pumping with pulse pumping we can look at average the average lifetime we look by two methods one is a gated detection the other one is time-correlated single photon detection they give us the same result as the number of chromophores per virus it decays that could very well be because of non-radiative fluorescence quenching but let's take a look at the intensity average counts per particle look what happens with respect to this there's a tail that goes up as you increase the number of chromophores per particle that the fluorescence starts to shoot up these then we ask well is there some weird statistical effect can we do it on a single molecule base on a single particle basis and here we have fluorescence lifetime imaging where we look at with a confocal microscope and a picosecond laser we look at each particle and we determine the intensity and the lifetime from a single virus particle we increase the number of chromophores from left to right and here here you have intensity mapped as gray levels here you have lifetime mapped as color levels blue means short green and red means long and you see it changing as an ensemble without many fluctuations there are fluctuations you can you can actually quantify them we can quantify them as histograms what's really interesting is when we start to hit the hundred chromophore per particle threshold we have very very bright particles single particles and they stand out so those are worth to study we don't have the time resolution actually to measure exactly what the birth time is for those particles but you may we have asked ourselves is there a correlation between intensity and lifetime because that's what the super radians emission would predict and so we plotted the lifetime this is for three ensembles of particles that have different number of chromophores 50 80 and 100 and we plotted it as a function of the total this is the intensity per particle and so we assume the limited limiting case where tau the lifetime is tau not over n and the intensity is proportional with n square and then if you combine these two you get that the tau as intensity the lifetime as intensity should be a constant over the square root of the intensity minus the background intensity which we we found experimentally so this is not a fit parameter it's just this unit unit conversion constant that we use for this dotted line which looks promising it passes quite nicely through the through the data so what this conclusion the conclusion from here is that lifetime shortening does not come from non radiative quenching if it did you would see like these particles here for these particles here that's that's what I think happens but in our case for most particles the brightest ones have the shortest lifetimes yes yeah I'm actually done thank you so some conclusions we found that on a 30 nanometer capsid if you put 150 chromophores of the Oregon greens you are going to get interesting photophysical properties we don't know how these chromophores are placed on the capsid we know that they strongly interact with the capsid we see suppression of self quenching we see pulse pumping bright and the brightest particles have shortest lifetimes it could be perhaps not super agents but amplified stimulated emission we are looking into that we are very much yet in search of theoretical collaborators because this is a this is a sign I have seen at the in the town of Aspen at another it's actually a street sign that says pretty pictures won't solve anything this is what I just created for you but I think that rigorous theoretical analysis of our data would be much much better and I would like to acknowledge my group especially artists which my and you know it's that cova who have produced this data last in the short time frame of less than a year a funding from a eager from NSF and from the Department of Defense would like to thank you for your attention and I would like to take any questions you may have