Ulrich Hohenester


14.01.2020 : A late Christmas present

Arrays of plasmonic nanoparticles sustain lattice resonances which can be tailored through nanoparticle geometry and spacing. By changing the wavelength and propagation direction of an incoming light beam, these resonances can be easily mapped out in experiment. Of particular interest is the point close to the center of the Brilloun zone where two resonances come spectrally close to each other (see bottom of figure on the left) which can be associated with upper and lower polariton branches. As Veronika Tretnak experimentally observed during her PhD thesis, depending on the size of the nanoparticles it can be either the lower or upper polariton branch that is optically bright.

To understand why this is the case Veronika consulted the literature, and although similar findings were reported by several groups she could not find a simple explanation for the effect. This is the point where I entered the discussion and where the whole misery started. Although my simulations gave results in agreement with the experimental data, we were completely unable to find a simple explanation. I remember many discussions with the nanooptics group which I could follow up to some point but got lost sooner or later. Maybe it was my bad influence or the nature of the problem itself that the confusion spread among my collaborators and most of our discussions ended in despair.

The whole story would not have come to a gratifying end without the heroic persistance of Andreas Hohenau who could finally trace back the effect to the reflection phases of the plasmonic nanoparticles. The paper which has now been published in J. Phys. Chem. C resolves the problem that puzzled us for so long, and as a final present for not giving up the Editor decided to accept our manuscript on December 24.

08.01.2020 : Back on the track


Oops, my news section came to a complete halt in the last two years. There is no particular reason for that, at some point I stopped updating the page and somehow got used to it. However, there are now exciting news so I should probably bring it back to life. At the beginning of this year my first textbook "Nano and Quantum Optics" has been published with Springer.
It is meant as a modern primer on the rapidly developing field of quantum nano optics which investigates the optical properties of nanosized materials.The essentials of both classical and quantum optics are presented before embarking through a stimulating selection of further topics, such as various plasmonic phenomena, thermal effects, open quantum systems, and photon noise. As a goodie enclosed please find a movie of the above image of 25 oscillating dipoles located on a ring, as well as a promise that I will bring my news section back on the track in 2020 ...

15.01.2018 : Keeping up with my heroes

mgocubeskiing1Mark Stockman, one of the pioneers in the field of plasmonics and definitely one of my heroes, usually starts his talks at conferences by showing a picture of himself in full and sportive action on the slope. At the end of last year I and my nanoplasmonics colleagues Andi Trügler and Jo Krenn spent a beautiful day in the Styrian mountains together with the head of our institute Uli Fölsche. Uli was kind enough to take a short movie of my skiing skills. As one can see, my style is more conservative and old school in comparison to Mark's dynamic approach. Nevertheless, seeing the video makes me wonder whether I shouldn't leave office more often to explore winter's wilderness instead ...

08.01.2018 : Dr. Dario Knebl

Congratulations to Dario Knebl who passed his final exams today !!  Dario's thesis is concerned with gap plasmonics and can be approximately separated into two parts. The first part deals with the simulation of electron energy loss spectroscopy (EELS) experiments of two silver nanocubes connected through a molecular tunnel junction. As described in more detail in the news entry of January 28, 2016 we tried to model a celebrated experiment of the Nijhus group using our combined expertise of electrodynamics and ab-initio simulations, and failed. In the paper, and now in full detail in Dario's thesis, we discuss why we believe that our results are correct and a reinterpretation of the experiments might be needed.

The second part of Dario's thesis tried to combine electrodynamics and ab-initio simulations for a simplified Jellium model. Unfortunately, this part was less successful and although we identified the critical aspects of our approach there remains a lot to be done to bring things to an end. Nevertheless, at some point it was time for Dario to write up and to bring his PhD study to an end. With Dario I now lose my last PhD student (the situation will improve hopefully soon) and the probably fastest runner I have ever had. In any case, I wish Dario all the best for the future and hope that despite the ups and downs of his thesis he had a great time in my group ...

08.01.2018 : Mathieu's passion for modes

Congratulations to Mathieu Kociak for his recent Nature Physics paper on self-hybridization of plasmonic modes !! Mathieu is one of the pioneers of mapping of plasmonic nanoparticles using electron energy loss spectroscopy (EELS), and for many years he has been among the most critical but at the same time also most enthusiastic users of our MNPBEM toolbox.

The motivation for his recent paper is mainly due to the renewed interest of mode descriptions in non-Hermitian systems, but it also goes back a little bit to an old debate whether the second plasmon mode of a nanotriangle should be assigned to a quadrupole or hexapole. A while ago we investigated the problem using our classification scheme in terms of edge and film plasmons, and studied the morphing of a disk into a triangle. In this paper we also introduced a perturbation scheme for plasmon modes, similar to related schemes in quantum mechanics, which we used to support our interpretation. To be honest, even at the time we submitted our paper I thought that this scheme was a nice gadget but would be never used again seriously by anybody else.

Now Mathieu has proven me wrong and has gone much deeper into the subject than I would have ever dreamed of. In a sense I feel bad because it gives me the impression that I have left something unexplored which I should have considered more carefully. On the other hand, it is great to see that science belongs to the community as a whole, and things we have done in the past can evolve and flourish. And Mathieu is such a nice guy that I feel really happy for him and, once again, congatulate him for this success !!

22.12.2017 : Forschungspreis Land Steiermark 2017

mgocubeOn december 15 I have been awarded the research prize of the province Styria (Forschungspreis des Landes Steiermark 2017) for our paper on EELS mapping of a single MgO nanocube, which has appeared earlier this year in Nature. The picture on the left shows me together with the rector of our university and Heidrun Zettelbauer, a historian who won the Erzherzog-Johann prize. A press release in German can be found here. Of course I feel deeply honoured and wish to thank those who contributed to our paper, in particular Andi Trügler for our longstanding and successful collaboration, and Maureen Lagos and Phil Batson for sharing their beautiful experimental data with us.

The university of Graz also produced a short promotion video describing my research in more detail. Shooting the video was quite an experience, it took about four hours and I tried to perform as relaxed as possible, altogether the film team did a great job. Describing plasmonics for the public is a tough job, and we finally opted against the notoriously omnipresent plasmonics pregncy test and for the medieval glass windows. Enjoy the video and click if you want to know how to get rid fo all the money related to the prize.

07.07.2017 : Good vibrations and enjoy the summer

It has been a busy but successful year so far, and I am looking forward to vacations without physics, bureaucracy and teaching. In short, it is time to go off-line.

I wish everyone nice summer vacations and good vibrations. If you click the link to the left (don't be afraid, there is no pishing or hacking here) you will see the cover page of Nature advertising our MgO cube paper (see News entry of 23.03.2017). It's great being on the cover of Nature and even greater to achieve such a cool and chilling appraisal ...

26.06.2017 : Plasmon fields in 3D

mgocubeThis manuscript is around for almost too long, but now it has finally appeared in Nature Communications. In the paper we show that the photonic environment of plasmonic particles can be tomographacially reconstructed using the solution of an inverse problem. The methodoly has been developed by Toni Hörl, who is the first author of the paper, during his PhD thesis, experiments were performed by Georg Haberfehlner in the group of Gerald Kothleitner (FELMI). As always the collaboration has been extremely pleasant and efficient.

I have shown the beautiful and impressive results at a number of conferences while the paper was still under review. There was nothing wrong with the referee reports, which can be even inspected at the Nature Communications homepage (in case you wondered how we a treated by our referees), but for what reason ever each step in the refereeing process took extremely long and at some point I started to doubt whether the paper would ever appear. So finally here it is, and as always it feels really, really great to hold a Nature paper in hands. Thanks to all who have contributed to this success, and I hope that at some point we will find time to properly celebrate ...

23.06.2017 : Kleeblattlauf 2017

mgocubeAt the end of each and every academic year there is the same procedure, the classical Kleeblattlauf which is a relay race where a team of four has to finish in less than one hour. Fortunately the running distance is not too long, 2200 and 1800 meters for the 35+ men and women, respectively, but unfortunately it goes up the Rosenberg hill and then down again. In addition, tradition requests that weather is humid, temperatures are high, and before, after, or during the race a thunderstorm is mandatory.

In the second year in a row I joined the directors' team of our institute, with the head Uli Fölsche and his deputies Claudia Haagen, me, and Martin Sterrer. As can be seen in the picture on the left, starting the race is more joyful than ending it. Altogether we performed very well and ranked among the top ten, which is partially attributed to the shorter distance we were allowed to run in comparison to the students and our younger colleagues. As for me it was a first fitness test after turning fifty at the beginning of the same week, and with 09:24 I was at least only one second slower than the year before ...

01.05.2017 : Gerhard Unger

On May 1st Gerhard Unger joined our group for a couple of months as a Postdoc. Gerhard is a mathematician who has specialized in the last couple of years on the boundary element method (BEM) for the solution of Maxwell's equations and on the computation of so-called resonance modes. We hope that he will explain us all the things we always wanted to know about BEM but never dared to ask, and look forward to numerous joint investigations --  Welcome !!

23.03.2017 : Vibrations mapped by an electron beam

mgocubeMagnesium oxide (MgO) is a solid mineral held together by ionic bonding. In collaboration with Maureen Lagos and Phil Batson from Rutgers university we have now demonstrated that the bulk and surface vibrations of a MgO nanocube can be observed with atomic resolution in an electron microscope: a swift electron (with a velocity of half the speed of light) passes by or through the nanocube and has a certain chance to excite a quantized vibration mode (phonon) thereby loosing a tiny fraction of its kinetic energy. By spectrally analyzing the energy loss and raster scanning the electron beam over the sample, one obtains detailed information about the phonon modes of the nanocube. What might appear somewhat technical, in fact constitutes a major breakthrough in the field of electron energy loss spectroscopy (EELS) and has been honoured by publication in the prestigious Nature journal.

As pointed out by Christian Colliex in the News & Views perspective, the energy resolution of about 10 meV reached in experiment in combination with the atomic spatial resolution can be considered as a world record in EELS that opens a new window to the observation of phonons in nano structures. In analogy to
the controlled manipulation of electrons and photons, which have led to the fields of electronics and photonics that have revolutionized our everyday life, the control of phonons might lead to sound and heat revolutions in phononics. Our work demonstrates that EELS with the new generation of electron microscopes provides an ideal measurement device for phononics.

Our part to the game has been the simulation of the EELS maps and spectra. Taking over our expertise from plasmonics to phononics has been relatively easy for the surface phonon modes. For the bulk phonon losses things have been considerably more difficult and there remain a lot of follow-up questions that we will hopefully explore in the future. To be honest, publication in the Nature journal has never been on my radar and now that it has worked out it has hit me by surprise. In fact so much that I feel totally speechless and have to realize with despair
that this is probably the most descriptive and boring News entry I have ever written (apologies !!), so I guess it's better to stop here and to start celebrating ...

09.03.2017 : Biggg

The new version of the MNPBEM17 toolbox for the simulation of plasmonic nanoparticles is finally out. There is also an accompanying paper. In the following I briefly discuss what is new and then discuss the current statuts of the toolbox.

The new toolbox version introduces a number of features that should significantly speed up simulations for large (biggg) nanoparticles with several thousand to ten thousand boundary elements. We introduce two novel concepts, namely hierarchical matrices for compression and faster matrix manipulations, and iterative BEM solvers. This combindation should allow using significantly less memory, say ten percent in comparison to the standard approach, and the simulations can be about a factor of ten faster, where details depend on the actual nanoparticle geometries. The new features have been implemented in a C++ library which is accessd in Matlab through MEX files.

All developments have been around for a while, but it took me some efforts to bring them into shape for a stable release. In particular errors in the MEX files (unfortunately there is a lot that can go wrong there, at least with my programming skills) regularly caused a Matlab crash, so I am happy that the programs seem to run smoothly at the moment. As always, I owe a biggg thanks to Andi Trügler who keeps running multiple simulations at every hour of the day, and thus is usually the first one to spot errors. To be honest, I was expecting a larger speed-up for the simulations, so I would consider the present implementation as a first step with potential for further improvements.

Where do we stand with the toolbox? We are happy that with about 1000 downloads in the last year it seems to be well accepted by the plasmonics community, and this certainly gives us motivation to continue with code development (although we usually enjoy more fiddling around with physics). A lot remains to be done, including quantum plasmonics, a Galerkin scheme, or quasinormal modes, to name just a few of our ideas. What will be done in the future is unclear, so just let us know if we should even put something different on our agenda.

03.03.2017 : Academic gown

modenaOn March 2-3 I have been part of a PhD jury at the University of Modena. In the Italian system a PhD work lasts for exactly three years, and at the end there is a joint final exam consisting of a 30 minutes presentation and a brief discussion for each candidate. The evaluation is based on two external referee reports and the assessment of the supervisor, as well as the performance of the candidate in the exam. Finding appropriate questions for all 12 candidates of this year graduating in "Physics and Nanoscience" has been a hard job. Nevertheless, I have truly enjoyed listening to the interesting presentations and I have been particularly impressed by the great performance of most candidates. Physics in Italy has a great tradition, which probably goes back to Enrico Fermi, and the subject still seems to attract the brightest of the students. The exam ended with a prolumgation of the grading, which gave me the possibility for a first encounter with academic gowns. As one can see in the picture, I still feel somewhat unrelaxed in the new stylish outfit ...

16.02.2017 : So Austrian ...


Nothing fantastic happening at the moment, but great news hopefully approaching soon. So it's time to report about the small things in life, which are often even more pleasing. Like the Austrian habit of enjoying winter where it can be enjoyed best, the mountains. The picture above (taken by my son Moritz) shows a delegation of our physics institute at the summit of the Bruderkogel, with Jo Krenn on the left, me in the middle, and the dean of our faculty Christof Gattringer on the right. Is it worth skipping a whole day of hard work for an exhausting trip to the sunny winter scenery? The answer can be already guessed from our faces. Sometimes physics has to come second ...

19.01.2017 : Happy new year


Happy new year 2017 !!  Winter term has entered its busy phase, with all the lectures ending in the next two weeks and my time table packed with too many exams. So it's good to remember the quiet days of christmas vacations where I had time to enjoy hicking in the Styrian mountains together with my brother Erhard. As one can see on the picture above, we are still in good shape and look pretty cool ...

27.10.2016 : Dr. Robert Schütky

rschuetkyThe start of winter term has been unexpectedly busy, as usual, so this news is no longer super new: On September 30 Robert Schütky passed his final PhD exam with distinction. Robert's thesis has been concerned with the theoretical description of fragmented and excited Bose-Einstein condensates. In the framework of the multi-configurational time dependent Hartree method for bosons, with the acronym MCTDHB that is even more complicated than that of our famous toolbox, the dynamics is described in terms of orbitals, accounting for the real-space part of the correlated many-boson wavefunction, and an atom-number wavefunction that describes how the bosons are distributed among these orbitals. Unfortunately, the exponential growth of the Hilbert space forbids the consideration of too many orbitals in simulations, where typically everything more than four orbitals has to be considered as too much.

For quite a while I had the idea to introduce an approximate description scheme for the atom number dynamics using density matrices. The time evolution of density matrices is governed by Heisenberg's equations of motion, and once a suitable tuncation is introduced at the level of higher-order correlations it is relatively easy to derive a closed set of equations. Thus life could be beautiful if it was not for this drawback: the resulting nonlinear equations turn out to be intrinsically unstable. Robert was working hard to get things under control, and although he succeeded in some cases by exploiting additional conservation laws the whole endeavour did not turn into a success story. I recently learned that there might be a way out of this miserably situation, so while for me things are not yet entirely finished for Robert it was time to write up what he had done at some point. He delivered a great thesis and demonstrated in his final defense (with Norbert Mauser from Vienna as examiner) that he turned into a true expert of this research field.

Robert belongs to a group of bright and successful students who all started in the same year. At the beginning of their studies they had the ambitious plan of building an atomic-force microscope from scratch, a struggle that woked out fine, as far as I am informed, but had a somewhat unexpected side effect: all of the students opted for theoretical phyiscs in the later part of their studies. Julia Danzer specialized in quantum chromodynamics and currently works as a Firnberg awardee at the Wegener Center in Graz, Gabi Jaritz finished her master thesis in my group a while ago, Markus Huber works as a senior Postdoc in theoretical particle physics, and Andi Trügler has become the backbone of my (now shrinking) research group. Starting October 2016 Robert is now working at the Kirchliche Pädagogische Hochschule (KPH) in the field of teachers' education. Being a gifted lecturer and a great physicist, I am sure that he will perform excellently in this new environment, and I wish Robert all the best for his future !!

28.09.2016 : Ghosts of Berlin

Summer term 2015 I spent on sabbatical at the Humboldt university Berlin, where I was generously hosted by Claudia Draxl. My plans were to combine plasmonics with first principles calculations, a topic of great importance for gap plasmonics as well as as for the description of light-matter interaction at the nanoscale. Although I worked hard, the progress of my work was slower than expected: it appears that while getting older we become more efficient in organizing work but less effective in doing the actual everyday work.

So it finally took more than a year to get our paper on quantum plasmonics accepted in PRB. I should say that the paper has been around for quite a while, but unfortunately it had a somewhat complicated refereeing process and finally ended up hidden somewhere at the bottom of my desk, and it finally needed a strenuous effort to recompile everything and to resubmit the manuscript to PRB where it was smoothly accepted. Altogether I am very happy about this work which sets the ground for future ab-initio studies in the field of plasmonics. Many sincere thanks to my co-author Claudia for the pleasant collaboration, her patience during the ripening process of the paper, and finally for making possible the beautiful stay in Berlin. 

19.09.2016 : NFO-14

japanFrom september 4-8 I attended the NFO-14 conference in Hamamatsu, Japan (see group photo). The topic of the conference is near field optics, aiming at light concentration and manipulation at the nanoscale. What might sound somewhat technical and boring turns out to be an extremely vivid and lively subject with many applications, ranging from nanotechnology over sensorics to life sciences. In particular with the 2014 Nobel prize in chemistry for nanoscopy this field has received  renewed impetus and, according to the closing comments of Jean-Jacques Greffet, the NFO conference series has escaped from becoming a club of old gentlemen meeting every second year. Indeed, it has been a great conference showing many directions for future developments, and I had the opportunity to present our plasmonic EELS studies in an invited talk.

After the conference I took the opportunity to spend a few days in Japan. Together with Andi Trügler,  Markus Krug, and Martin Belitsch (here in Tokyo), who were kind enough to accompany me on this trip, we spent three beautiful days in Kyoto before heading to mount Fuji, whom we climbed after the end of the official climbing season in horrible conditions with strong rain, however, with the advantage of being almost alone on the summit of this stunning mountain. Altogether it took us five hours to go from station five to the summit and back again, so we had enough time to relax in an Onsen afterwards. Finally, we stayed for two days in Tokyo, breathing in the spirit of this huge and amazing city. 

02.09.2016 : A sensitive tip

krugThe extreme sensitivity of plasmonic nanoparticles to changes in the local dielectric environment renders them ideal for sensor applications, such as the notoriously cited plasmon-based clinical pregnancy test. Quite generally, the stronger the field confinement the better the sensitivity. This calls for strategies to (i) design particles with extreme nearfield enhancements, and to (ii) guarantee that the analytes to be detected indeed bind to the hot spots. Related problems have been investigated for quite a while in the nanooptics group by Verena Leitgeb, Andreas Hohenau, and Joachim Krenn.

These activities have now led to an investigation where the sensitivity is mapped directly in real space by raster scanning a fiber tip over a gold nanodisk and simultaneously measuring the scattered light intensity. In a paper that has just been accepted for publication in Nanoscale we now demonstrate that the interpretation of the tip-induced intensity and spectral changes is more involved than one might naively expect, and builds on constructive or destructive interference of light emission from the tip and the nanoparticle. The hero of the trade is Markus Krug who not only performed most of the experiments but additionally had to negotiate and decide between the different requests from the numerous co-authors.

As always it has been a pleasure collaborating with the nanooptics group, not only with the experienced but also with the younger team members, in this case Markus Krug, Gernot Schaffernak, Martin Belitsch, and Marija Gašparic, who have inhaled the entire group spirit already after a few years of PhD work. This spirit includes the full, truly, and fully truly understanding of the basic physics underlying the experimental findings, partially based on our simulations. As always Andi Trügler has been extremely patient in performing the requested simulations. My main contribution, in addition to drafting the theory part of the paper, has been to understand and interpret the simulation results. Disentangling the role of the tip and nanoparticle in the coupled tip-nanoparticle system has been quite tricky, and we have benefited from the versatility of our BEM approach which allows to turn on and off specific interaction and back-reaction channels in order to evaluate their respective importance. Altogether this is a beautiful paper that also shows that a mere agreement between experiment and simulations does not mean that one understands the system, but one should rather continue to dig deeper underground ...

19.07.2016 : So simple, so beautiful

Franz Schmidt seems to have a true passion and enthusiasm for the simple things. For quite some time now he has used the electron microscope and electron energy loss spectroscopy (EELS) to investigate systems that looked simple at first sight but turned out to be much richer at the end. First came the plasmonic eigenmodes of silver nanodisks, including our mythical "dark breathing mode", which was later classified in terms of edge and film modes. Next came the universal dispersion relation for edge modes. And now we go square and investigate couplings of edge modes in silver nanocuboids.

In a paper that just has been accepted for publication in Nano Letters we show that the plasmon modes of such nanocuboids can be intuitively understood in terms of couplings between modes confined to the nanocuboid edges. In my view it is a beautiful and extremely clear work, and I thank Franz, Hari, and the other members of the nanooptics group for bringing me on board of this paper. I don't know what Franz will investigate next -- at some point probably even Franz will run out of still simpler systems -- however, maybe his investigations will become even more successful once he goes for complexity.

11.07.2016 : Dr. Jürgen Waxenegger

JuergenOn July 11th Jürgen Waxenegger passed his final PhD exam with distinction. Jürgen's thesis has been concerned with plasmonic simulations including substrates and stratified media, which are now part of our toolbox published in Computer Physics Communications. As described previously in this News section (23.03.2015), the methodology underlying our approach is rather technical and requires maneuvering through the complex plane in order to solve Sommerfeld-type integrals, and  Jürgen had a hard time to survive in the battlefield of equations. His thesis gives an impression about the fact that this was not a fun project but real work !!

Jürgen's PhD work started quite badly in 2013 when he had a horrible ski accident during the Nano and Photonics workshop in Mauterndorf. He broke his leg which needed complicated surgery, and it took Jürgen almost a year to bring things back to normal. In a sense I feel a little guilty as we were skiing together and Jürgen, who is not such an experienced skier as others in my group (see news entry about Toni below), was trying hard to keep pace with the others. In the end his accident looked completely harmless but turned out to be the opposite -- as so often it is a surprisingly fine line that separates luck from bad luck.

Jürgen has been an incredibly serious and persistent worker who has probably spent more time at university than almost all of our permanent staff members. He has a certain passion for gossip and various news, including my news section that he seems to check very regularly even if nothing is happening for quite some time. Altogether he has always been at the social center of our group, as can be seen in the picture that was taken after his final exam. Now it is time to celebrate (apologies for the casual dressing of the examiners, however, it was a really hot day). Jürgen will now join the bank sector and I wish him all the best for whatever the future will bring !!

13.06.2016 : Happy birthday

Happy birthday to James Clerk Maxwell on the occasion of his 185th birthday !!  Where would we be without his magic four equations ...

02.05.2016 : Dr. Anton Hörl

In the first week of May Toni Hörl passed his final PhD exam with distinction. For many years Toni has been the backbone of the tomography activities in my group, investigating the question: How can one extract the three dimensional plasmonic fields from a collection of rotated EELS maps?  In his PhD thesis he gives a detailed account of this problem and discusses our present viewpoint that we have also published in a series of research papers.

Toni is a silent but persistent worker who has been in my group since the beginning of his master studies. It took me a while to realize how effective he is working. In our first meetings I was never sure whether he really understood what I was trying to explain, and this at first sight rather sceptic and cautious perception has remained over the years. However, the next day or so Toni would show up and it became clear that he not only understood things perfectly well but that he continued to ponder on them and usually came up with great ideas of how to improve them or what to do next. I must say that I totally enjoyed working together with him, and I have been continuously fascinated by his persistence and innovation.

Besides physics Toni is a gifted snowboarder and skier, in fact he is one of these Alpin persons who grew up with snow and who can easily handle all conditions from powder to breakable crust. I remember a few situations where I was close to despair while Toni was not even showing any sign of effort. The last time this happened was on a crossing of the Tote Gebirge, which we did together with Andi Trügler and my son Moritz in early April. For a whole day I followed Toni who was leading the last stage over 35 km from Tauplitz to Wurzeralm. Trying to keep pace with my younger colleagues probably describes best my position in the group and in science, and this will remain my destiny. For the younger ones there still remains the world to be explored, and for this I wish Toni all the best !!

17.03.2016 : Biexcitons in graphene nanoribbons

grapheneGraphene is a magic material that has attracted enormeous interest in recent years. Being a semimetal without any bandgap, it is less attractive for optical and optoelectronic applications, such as lasers, LEDs, or photo voltaic devices. To render graphene suitable for these applications one has to open a gap, which can be achieved by cutting thin slices out of the 2d graphene sheets, so called graphene nanoribbons (GNR).

In a paper that has just appeared in Nature Communications we now demonstrate that in GNRs two optically excited electron-hole pairs form a bound state --a biexciton-- with a surprisingly high binding energy. As often in nanoscience, the true heroes of the trade are those who produce the material, in our case chemists from the group of Klaus Müllen in Mainz, the most cited German chemist. They produced GNRs with exceptional optical properties. Femtosecond pump-probe experiments were carried out in Milano, by Giancarlo Soavi from the group of  Giulio Cerullo, who observed a stimulated emission peak attributed to the formation of the biexciton. My contribution was to perform in collaboration with my two Italian partners Deborah Prezzi and Elisa Molinari simulations that confirmed the high value of the biexciton binding energy.

My sincere thanks go to Elisa Molinari, my former boss during my Postdoc years in Modena, for her support during all the years, her friendship, and for bringing me on board of this illustrious team of authors. Elisa was always keen on collaborating with experimentalists. “The most-cited paper are those in collaboration with experimentalists”, she used to say, and “those papers are good for your CV”. She always promised that “you simply have to run a simulation to get a joint publication”, a point which is believed in the beginning but which was of course never really true. With hindsight I must say it was her who tought me most of the things that brought me forward in my career.
The present paper is somewhat special, as my job was to recycle programs that a former master student of mine, David Kammerlander, wrote almost ten year ago. I am not an overly organized worker, and I am proud that I (i) found the programs, (ii) was able to decipher them after all the years and (iii) could introduce the modifications that were necessary to adapt our quantum Monte Carlo simulations from carbon nanotubes to GNRs. Naturally, things turned out to be more difficult than I was expecting, but that's a different story. Altogether it was a fun project in a field that does not belong to my main expertise, and I would like to thank all co-authors, in particular Giancarlo and Deborah for their tremendous efforts in revising the manuscript. Now there is time to celebrate the rare moments of publication in one of the Nature journals ...

28.01.2016 : Plasmon or not plasmon ?

doublecubeIn a paper that has just been accepted for publication in Physical Review B we now investigate gap plasmons in a silver nanocube dimer. When two plasmonic nanoparticles are separated by a gap in the sub-nanometer range, electrons can tunnel from one particle to the other one. This usually leads to the emergence of a charge transfer plasmon (CTP) which has lower energy than the bonding plasmon mode (dipole moments of the individual particles aligned in parallel) because the charge neutrality of the individual particles is broken, and the whole dimer acts as a single huge dipole.

In a recent paper by Tan et al. [Science 343, 1496 (2014)] the authors showed that tunneling can be increased by introducing a molecular tunneling layer between the particles, as shown in the above image. Using electron energy loss spectroscopy (EELS) the authors reported the emergence of a low-energy peak in the EEL spectra which they interpreted as a CTP. This paper triggered a lot of interest, also for us: together with Peter Puschnig, a leading expert of density functional theory (DFT) calculations of molecular layers, and Jo Krenn from the nanooptics group we performed a systematic simulation study of molecular tunneling and its impact on the plasmonic EEL spectra. The hero of the trade is Dario Knebl who worked extremely hard with both the DFT and EELS simulations.

To make a (very) long story short, we could neither reproduce the simulation results of Tan et al. nor we could support their interpretation of the low energy peak as a CTP. Contradicting a Science paper is usually not a good idea, and we spent much more time than we would have otherwise done to verify our results. The paper now reports what we obtained with our simualtion approach and why we believe that the Tan paper calls for a re-interpretation. Refereeing was rather rough, however, to my surprise not because the reviewers didn't believe our results but because they found them not particularly interesting. So here is finally our work that still excites me a lot, and I hope that it will receive better reception from the plasmonics community.

28.01.2016 : Highly cited

As of September/October 2015, the paper describing our Matlab toolbox MNPBEM for the simulation of plasmonic nanoparticles has received according to the web of Science enough citations to place it in the top 1% of its academic field based on a highly cited threshold for the field and publication year. We are happy that the toolbox continues to serve the plasmonics community as a viable simulation device.

12.01.2016 : Dr. Georg Jäger

Georg JägerJust before Christmas, on December 16, 2015, Georg Jäger passed his final PhD exam with distinction. Georg has worked in my group for quite a while and has become the master of optimal quantum control with Bose-Einstein condensates. I always liked his relaxed approach towards physics, although Georg can also work hard and persistently if required. Within the last couple of years it has not always been easy to keep our optimal quantum control activities alive, and Georg has done a marvelous job in taking care of our collaborations with the groups of Jörg Schmiedmayer (TU Vienna) and Christiane Koch (Kassel). For reasons that were never entirely clear to me, all publications together with him (which are now collected in his cumulative PhD thesis) turned out to be particularly simple, clean and appealing. Georg has also been particulary talented presenting his work to a broader audience, and our last paper is devoted to a Matlab based primer for optimal control theory that is currently under review. It was clear for a while that Georg would not pursue a career in academia, and I thus wish him the very best for his future wherever he will end up !!

28.11.2015 : Decennial 

Novemeber 28 is the decennial of my plasmonics activities: on November 28, 2005 my first paper on plasmonics entitled "Surface plasmon resonances of single and coupled metallic nanoparticles: A boundary element method approach" appeared in Physical Review B, which I wrote together with Joachim Krenn. As you can check in the above link, the manuscript has the somewhat programmatic file name plasmon.pdf and shows that at that time I was completely unaware of how much related stuff would follow. Since then I have invented a more refined strategy for naming the by now almost 40 plasmonics papers.

With hindsight, I am surprised how I could survive in this research field considering my initial ignorance about optics and nearfield optics. I had a hard time to learn that plasmonics is an old research field and so many things have been already investigated by the pioneers in the old days. On the other hand, plasmonics has become a fashionable field in the last decade and has attracted a large number of researchers with various scientific backgrounds, who probably share with me a similar ignorance. It has been wonderful 10 years with many exciting experimental and theoretical collaborations, and I am looking forward to all the things that are still ahead ...

10.11.2015 : Parametric squeezing amplification

blochsphereAt the moment Georg Jäger is in the process of finishing his PhD thesis, so it was time to also write up some of his investigations whose publication we delayed for much too long. In a paper that has just been accepted for publication in Phys. Rev. A we examine together with Thorsten Schumm and Jörg Schmiedmayer from the TU Vienna parametric squeezing amplification in Bose Einstein condensates (BECs) trapped and manipulated in the vicinity of atom chips.

In optical interferometry, a light beam is split into a measurement and reference beam; the measurement beam acquires some unknown phase, which is finally read out through interference with the reference beam. Such measurements are typically subject to shot noise, but the measurement sensitivity can be significantly boosted by using non-classical (for instance squeezed) light. Similarly, in atom interferometry the BEC is initially split, through transformation from a single- to double-well confinement, the BEC wavefunction acquires different phases in the measurement and reference wells in the waiting stage of the BEC interferometer, and finally one reads out this phase through condensate interference. As we show in our paper, parametric squeezing amplification allows to boost the measurement sensitivity and to bring it close to the fundamental Heisenberg limit.

What is not stated clearly in the paper is that the Schmiedmayer group tried hard to experimentally implement such squeezing protocols, but ran into difficulties with thermal excitations which introduced fast and efficient (unwanted) dephasing. Georg accompanied these experiments with detailed simulations, estimating the importance of various decisive parameters, and the paper now reports our present understanding of the subject. I hope that the next generation of experiments will benefit from our investigation, and I thank all collaborators for this pleasant collaboration, in particular Georg -- also for his patience and persistance.

10.11.2015 : Edge of the wedge

Surface plasmons in flat metallic nanoparticles can be classified as film plasmons (confined to the flat parts of the nanoparticles) and edge plasmons (confined to the edges). In a paper that has just been accepted for publication in Optics Letters we now investigate film and edge plasmons in a tapered wedge structure, using electron energy loss spectroscopy (EELS) and simulations. Edge plasmons propagating towards the tip of the wedge become focused and deliver electromagnetic energy to nanoscale volumes.

This paper is one of a few mysterious ones that Andi Trügler reported to me in an e-mail, just before leaving for vacations in Island this summer, and that amazed me because I was totally unaware of both the progress of the work and the fact that the writing of the paper was (almost) finished. One might wonder whether this ignorance qualifies me as a co-author of this beautiful piece of work. In any case, many thanks to Andi and Franz Schmidt who did a magnificant work regarding simulations (in this case really heavy simulations for the micrometer-sized wedges) and experiment, as well as to Jo Krenn for carefully supervising this work and finally compiling it into such a convincing manuscript. One of our referees stated "it is my pleasure to review such beautiful manuscript", and we can only add that the pleasure is equally on our side. It appears that Franz is currently working through the Platonic bodies of plasmonic nanoparticles, so I hope that there are still some related and comparably beautiful investigations ahead.

28.10.2015 : Juxtaposition

nanocubesSo here is a 3d reconstruction of two coupled silver nanocubes measured through electron tomography in the group of Gerald Kothleitner at the TU Graz. This nanoparticle dimer is not a real beauty but it has style and character !! In a paper that has just been accepted for publication in Nano Letters we use this 3d geometry as an input for our plasmonics simulations performed with the MNPBEM toolbox (what else). We obtained excellent agreement between the measured and simulated electron energy loss spectroscopy (EELS) maps, bringing the comparison between experiments and simulations to a truly quantitative and correlative level.

The experimental results are now around for a while, and I and my group tried hard to submit the measured EELS data to our plasmon field tomography previously developed (see news entry just below). However, we soon had to realize that such tomography would be too complicated, at least for our present simulation machinery, and we thus opted for a more conservative approach where we compare experiment and simulation on par. Inserting the precise 3d geometry and the supporting membrane into our simulation software turned out to be a tricky part, and also the EELS simulations required massive computer time. The master of the trade is Andi Trügler, who spent days and days with the simulations, never losing patience depsite the numerous requests he received for modifying (now really for the last time) the simulation parameters.

The prize for all the hard labour is the most detailed comparison between experiment and simulations I have hitherto seen in the field of plasmonics. Many thanks also to Georg Haberfehlner from the TU Graz who did a really great job regarding both measurements and data analysis. My final part was to give the paper enough spin so that it could easily roll through the refereeing process, which turned out to be not too complicated for this beautiful piece of work. Thanks to Andi, Georg, and all other collaborators for this fruitful and pleasant collaboration, and I am looking forward to continue looking closer to the magical nanoscale.

07.09.2015 : EELS tomography (reloaded)

Computer tomography allows to reconstruct 3D objects from a series of 2D projections, which is of crucial importance in various fields ranging from medicine to material science. Previously, we have shown that electron energy loss spectroscopy (EELS) measurements of plasmonic nanoparticles can be interpreted in terms of a tomography scheme, at least when the particles are small and several other assumptions hold. A similar scheme (using the same assumptions) was formulated independently by Nicoletti et al., Nature 502, 80 (2013), who also demonstrated the tomography approach experimentally. However, according to a refere "These works, although quite exciting, were leaving more questions than answers".

This somewhat unfortunate situation is partly due to the several assumptions that underlie our approach. In a paper that has just been accepted for publication in ACS Photonics, we now try to release these assumptions and go for a most general plasmon tomography scheme. This work is part of Toni Hörl's PhD thesis, and Toni worked hard to implement our initially vague ideas about how this could be done. As we show in the paper, our approach works surprisingly well, although some final challenges still remain. In any case, thanks a lot to Toni for working so hard and for making significant progress in such a difficult problem !

28.07.2015 : So thin, so bright

WS2The discovery of graphene has recently led to the quest for other quasi two-dimensional materials. One example are transition metal dichalcogenides such as molibdium or tungsten disulphide WS2. Although a plasmonics expert has given the advice that "one should not follow today’s pattern of rapid-fire testing of all the well-known conductors, doped semiconductors, or popular materials du jour (graphene, MoS2, and whatever comes next)", we have recently decided to investigate together with the group of Rudi Bratschitsch in Münster the optical properties of an atomically thin layer of WS2 coupled to metallic nanorods.

In a paper that has just been accepted for publication in ACS Photonics we show that the nanorods act as nanoantennas, exploiting the strong plasmonic nearfield enhancement, and allow for extremely efficient out-coupling of photons. The optical properties of the thin WS2 layer are indeed highly remarkable, whereas plasmonics provides as always a powerful means for further engineering the light-matter interaction.

The collaborations with Rudi's group has been pleasant but (as also in the past) quite time consuming. Johannes Kern has done a great job from the experimental side. From the theoretical side, Andi Trügler has spent a lot of time with the simulations that turned out to be significantly more complicated than initially thought. The system consisting of a thin WS2 layer on top of a glass substrate coupled to a gold nanorod turned out to be an ideal test bench for our recently developed layer-structure feature of the MNPBEM toolbox, which forced me to rewrite and correct many toolbox functions several times. Also the extremely large permittivities of the WS2 layer forced us to push our programs to their limits. Altogether I think that this is a beautiful piece of work and I thank all co-authors for this great collaboration!

16.07.2015 : Most downloaded

I have been doing a lot of research recently, with progress in several directions (and despair when things become too complicated), but there has been nothing outstanding to share in this news section. So it's time to summarize a few minor success stories.

Our MNPBEM toolbox has been for quite a while among the top downloaded articles of Computer Physics Communications (rank 20, better than some of the true bestsellers such as Libxc). And it's also doing well on our personal where the last version has been downloaded more than 300 times. There have been many positive remarks from our users such as:

I am a regular user of the toolbox and have to say frankly that I am an experimental physical chemist and generally used to be afraid of calculations. But when I was introduced to the toolbox, I was interested and excited to see the results which are in accordance with the experiments.

The toolbox is very nice and well documented. Thank you for this great job!

I am working on my Bachelor thesis and was advised to use this MATLAB toolbox.

We truly appreciate positive or any kind of feedback! There are also a number of papers where our toolbox has been used and people have worked hard to find out whether things work properly, coming to conclusions such as:

As a result, we eventually claim that the toolbox by Hohenester and Trügler provides reliable numerical solutions.

At the moment there is little progress with further developments, but let's see what the future will bring. In any case, we are extremely happy that the toolbox serves the plasmonics community as a useful simulation environment.

18.05.2015 : Plasmonics goes quantum

My plasmonics activities now last for more than a decade and (with a few exceptions) I have been always dazzled by the classic elegance of classical electrodynamics. Now time has come to turn a little quantum. Recent research efforts of several groups have focused on the quantum-classical boundary of plasmonics, and have suggested various variants of quantum corrected model.

In a paper that has just been accepted for publication in Physical Review B I present a variant of the Aizpurua-Nordlander implementation for quantum plasmonics, which models plasmon tunneling by introducing an artificial material with a tunnel conductivity inside the gap. Such tunneling leads to the formation of new plasmon modes, the so-called charge transfer plasmons. In my paper I show how to replace the artificial material through non-local boundary conditions. This approach has the advantages that it can be implemented much more easily within a boundary element method approach, as used in our MNPBEM toolbox, and that it introduces only contact resistance but no ohmic losses, as is the case for artificial materials.

In principal, time is now ripe to tackle the more interesting problem of computing the tunnel conductivities from first principles. I am currently working on the topic, however, my progress is much slower than expected and I am stuck with a number of stupid problems. Let's see what the future will bring ...

8.-12.04.2015 : DINAMO conference in El Chalten, Patagonia


From April 8-12 I attended the DINAMO conference in El Chalten, Patagonia, a remote place located very close to the beautiful Cerro Torre and Fitz Roy mountains. What brings 75 scientist to the end of the world, about 48 hours of travel away from any decent place elsewhere, what makes them stay at a hotel with practically no internet and mobile connection, what makes them share their hotel rooms with other colleagues? It is the combination of the beautiful landscape and the excitement about physics, ranging from plasmonics over optical trapping to complex media, that made this meeting a highly memorable and successful event. According to the organizers, the idea for the meeting appears to be a rather drunken story which finally brought us to a hotel bar serving as our conference room. Yet, it was a week with many stimulating discussions, almost everybody enjoying the free time without internet, and all of us were extremely happy having accepted the invitation to this crazy event.

Prior to the conference I was hiking for a week in the national park Torres del Paine located nearby in Chile. A short report in German can be found here.

23.03.2015 : The new version of the MNPBEM toolbox

The paper describing the new version of our MNPBEM toolbox has just been accepted for publication in Computer Physics Communications. The MNPBEM14 version supersedes the previous versions, and we ask all users to change to the new one which now includes simulations of plasmonic nanoparticles situated on substrates or within layer structures, as well as more refined boundary element integrations, and a more user-friendly interface.

Inclusion of substrates and layer structures was on our to-do list for a long time, actually first attempts were already included in the first version. However, we soon realized that things are more difficult than we initially thought and we decided to make our first attempts not available to the public. Honestly speaking, the whole topic is rather unsexy, there exist tons of literature on the subject and it was clear from the beginning that things are doable, but the implementation and testing would become cumbersome. Substrate effects are important for the understanding of many experiments, and at some point we were unwise enough to promise that we would implement it -- so the misery started.

I am grateful to the Austrian science fund FWF who supported our endeavour and forced us to finally deliver.
Jürgen Waxenegger investigated the problem as part of his PhD thesis, and maneuvered almost endlessly through the complex plane in order to compute the Sommerfeld integrals needed for the computation of the substrate Green functions. My initial fears became true, the whole implementation was a nightmare of bookkeeping and of combining many different steps without loosing the general picture. To make things even worse, I had meanwhile started to change several other features in the toolbox which at some point became such a messy place that I was not sure whether I would ever finish the next version. At the end, it was a hard push for several weeks to bring things back to normal.

Is it worth writing software in such a way that it can be used by the public? I am not sure. Quite generally, several Maxwell solvers such as FDTD or DDA already exist and are doing a great job. Yet, I think that our BEM approach is perfectly suited for plasmonics simulations and our Matlab implementation has the advantage that simulations can be easily performed and one can simply play around without digging to deeply into the toolbox technicalities. I realize that there are now many users (the latest version has so far been downloaded more than 160 times) and I am happy that we can give something useful back to the plasmonics community. So probably the toolbox will stay alive for several more years and this will force us to continue with support. Thus, whenever you have suggestions or think that something should be improved just let us know.

11.02.2015 : On the cover of EPJB

EPJB coverLike in 2014, the new year starts with an article featured on the journal's cover page. This time it is a paper on plexcitons that appeared in the European Physical Journal EPJB. The system we investigate is a hybrid nanoparticle (matchstick structure), which consists of a semiconductor rod attached to a metallic cap. The optical properties of the semiconductor part are characterized by excitons, these are strongly bound electron-hole pairs. The optical properties of the metallic part are governed by plasmons. The interplay between these two types of excitations leads the formation of a hybrid exciton-plasmon excitation, a so-called plexciton.

In this work we theoretically investigate the importance of various coupling mechanisms that influence the plexciton properties. On the one hand, plasmons allow for a more efficient light emission, on the other hand Ohmic losses in the metal lead to non-radiative losses that might eventually over-compensate the benefits of the plasmonic field enhancement. We develop a methodology that allows to properly model the optical properties of such hybrid nanoparticle, which was hitherto missing, and demonstrate its applicability for a prototypical hybrid structure. An article featuring our work can be found here.

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Ulrich Hohenester
Institut für Physik, Karl-Franzens Universität Graz, Austria