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Exciton transport in most organic materials is based on an incoherent hopping process between neighboring molecules. This process is very slow, setting a limit to the performance of organic optoelectronic devices. In this Article, we overcome the incoherent exciton transport by strongly coupling localized singlet excitations in a tetracene crystal to confined light modes in an array of plasmonic nanoparticles. We image the transport of the resulting exciton–polaritons in Fourier space at various distances from the excitation to directly probe their propagation length as a function of the exciton to photon fraction. Exciton–polaritons with an exciton fraction of 50% show a propagation length of 4.4 μm, which is an increase by 2 orders of magnitude compared to the singlet exciton diffusion length. This remarkable increase has been qualitatively confirmed with both finite-difference time-domain simulations and atomistic multiscale molecular dynamics simulations. Furthermore, we observe that the propagation length is modified when the dipole moment of the exciton transition is either parallel or perpendicular to the cavity field, which opens a new avenue for controlling the anisotropy of the exciton flow in organic crystals. The enhanced exciton–polariton transport reported here may contribute to the development of organic devices with lower recombination losses and improved performance. peerReviewed

Plasmonic nanostructures are widely utilized in surface-enhanced Raman spectroscopy (SERS) from ultraviolet to near-infrared applications. Periodic nanoplasmonic systems such as plasmonic gratings are of great interest as SERS-active substrates due to their strong polarization dependence and ease of fabrication. In this work, we modelled a silver grating that manifests a subradiant plasmonic resonance as a dip in its reflectivity with significant near-field enhancement only for transverse-magnetic (TM) polarization of light. We investigated the role of its fill factor, commonly defined as a ratio between the width of the grating groove and the grating period, on the SERS enhancement. We designed multiple gratings having different fill factors using finite-difference time-domain (FDTD) simulations to incorporate different degrees of spectral detunings in their reflection dips from our Raman excitation (488 nm). Our numerical studies suggested that by tuning the spectral position of the optical resonance of thegrating, via modifying their fill factor, we could optimize the achievable SERS enhancement. Moreover, by changing the polarization of the excitation light from transverse-magnetic to transverse-electric, we can disable the optical resonance of the gratings resulting in negligible SERS performance. To verify this, we fabricated and optically characterized the modelled gratings and ensured the presence of the desired detunings in their optical responses. Our Raman analysis on riboflavin confirmed that the higher overlap between the grating resonance and the intended Raman excitation yields stronger Raman enhancement only for TM polarized light. Our findings provide insight on the development of fabrication-friendly plasmonic gratings for optimal intensification of the Raman signal with an extra degree of control through the polarization of the excitation light. This feature enables studying Raman signal of exactly the same molecules with and without electromagnetic SERS enhancements, just by changing the polarization of the excitation, and thereby permits detailed studies on the selection rules and the chemical enhancements possibly involved in SERS.

openaire: EC/H2020/838996/EU//RealNanoPlasmon Funding Information: We acknowledge financial support from the Swedish Research Council (VR Miljö, Grant No: 2016-06059), the Knut and Alice Wallenberg Foundation (Grant No: 2019.0140), the Polish National Science Center (projects 2019/34/E/ST3/00359 and 2019/35/B/ST5/02477). T.P.R. acknowledges support from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 838996 and support from the Academy of Finland under the Grant No. 332429. T.J.A. acknowledges support from the Project HPC-EUROPA3 (INFRAIA-2016-1-730897), with the support of the EC Research Innovation Action under the H2020 Programme. Funding Information: The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC, C3SE, and PDC partially funded by the Swedish Research Council through Grant Agreement No. 2018-05973 and by the Interdisciplinary Centre for Mathematical and Computational Modelling at University of Warsaw (Grant #G55-6). Publisher Copyright: © 2022 The Authors. Published by American Chemical Society. Ultrastrong coupling (USC) is a distinct regime of light-matter interaction in which the coupling strength is comparable to the resonance energy of the cavity or emitter. In the USC regime, common approximations to quantum optical Hamiltonians, such as the rotating wave approximation, break down as the ground state of the coupled system gains photonic character due to admixing of vacuum states with higher excited states, leading to ground-state energy changes. USC is usually achieved by collective coherent coupling of many quantum emitters to a single mode cavity, whereas USC with a single molecule remains challenging. Here, we show by time-dependent density functional theory (TDDFT) calculations that a single organic molecule can reach USC with a plasmonic dimer, consisting of a few hundred atoms. In this context, we discuss the capacity of TDDFT to represent strong coupling and its connection to the quantum optical Hamiltonian. We find that USC leads to appreciable ground-state energy modifications accounting for a non-negligible part of the total interaction energy, comparable to kBT at room temperature.

In the quest to built novel metamaterials with unique optical properties, three-dimensional assemblies of metal clusters and nanoparticles are gathering significant attention. Organized geometries, such as tetrahedra and icosahedra, can be built, for example, by using DNA strands or virus capsids as templates. Here we use the jellium model and time-dependent density functional theory to study the plasmonic resonances in different arrangements of eight-electron clusters from the electronic perspective. A charge transfer ratio index based on the induced transition densities is used to quantify the charge transfer nature of the excitations at different energies. We vary the size, shape, and intercluster separation, building systems of four-cluster tetrahedra, 12-cluster icosahedra and cuboctahedra, and 20-cluster dodecahedra. All the studied systems are found to have charge transfer plasmon-type excitations at low energies. Analysis of the electron-hole transitions contributing to the transition dipole moment is further used to characterize these excitations, showing that they have significant screening contributions unlike the higher-energy excitations. The understanding gained for the optical response of these simple model systems can help in interpreting the properties of real, complex cluster systems. peerReviewed

In this thesis, the potential applications of DNA self-assembled structures were explored in both nanoelectronics and plasmonics. The works can be divided into two parts: electrical characterization of unmodified multilayered DNA origami and DNA-gold-nanoparticle conjugates after they were trapped between gold nanoelectrodes by dielectrophoresis, and the development of a novel fabrication method using DNA origami as a template for smooth, high resolution metallic nanostructures as well as optical characterization of them. One of the biggest challenges in self-assembled nanoelectronic devices is to connect them to macroscopic circuits. Dielectrophoretic (DEP) trapping has been used extensively in manipulation of micro- and nanoscale objects in solution. We have demonstrated this technique by trapping four structurally distinct multilayered DNA origami between gold nanoelectrodes by DEP and electrically characterized some of the trapped structures at high relative humidity. Most of the samples showed insulating behavior in both DC I-V measurement and AC impedance spectroscopy. In the other experiment, an assembly of three gold nanoparticles (AuNPs) conjugated with a triple-cross-over-tile (TX-tile) structure were designed, synthesized, and trapped by DEP. At the beginning no current was observed, but after a few chemical gold growth steps, Coulomb blockade behavior was observed from the liquid helium temperature up to the room temperature. Although no gated measurement was carried out, the random switching at low temperature measurements highly resembled a similar behavior of single electron transistor (SET). The second half of this thesis is focused on the development of a DNA-assisted lithography (DALI) method, in which DNA origami was used to mask the growth of SiO2 on Si chips in order to generate a stencil mask with openings of the DNA origami shape. Then the stencil was used in conventional microfabrication processes to deposit metallic nanostructures with almost the same shape as DNA origami on different substrates. Three different DNA origami were used to fabricate metallic structures with various optical properties on sapphire substrates. The localized surface plasmon resonance (LSPR) of Seeman tile and a bowtie antenna was characterized by a dark-field microscope. The surface enhanced Raman spectroscopy (SERS) of two different marker molecules on gold bowtie antennas was characterized too. Finally, the chiral double-L samples landed on a surface with different orientation combinations showed distinct circular dichroism (CD) spectra. In addition, a method to deposit DNA origami on unmodified surface with large area by spray coating technique was reported.

Grafeeni on ilmestynyt tieteen rintamalle kovalla ryminällä vasta viimeisen 13 vuoden aikana, mutta on jo nyt yksi mielenkiintoisimmista tutkimuskohteista, jolla voisi olla lukuisia sovelluksia tulevaisuudessa. Grafeeni on ilmestynyt myös plasmoniikan tutkimuskohteeksi sen lukuisten ainutlaatuisten sähköisten ja optisten ominaisuuksien takia ja sen ominaisuuksien muokattavuuden helppouden takia. Grafeeni voisi mahdollisesti korvata yleisesti käytössä olleet jalot metallit kuten kullan ja hopean pintaplasmonien tuotossa ja tutkimuksessa ja voisi johtaa uudenlaiseen valon kontrollointiin nanoskaalassa ja optiikan ja elektroniikan yhdistämiseen tässä pienessä mittakaavassa. Tässä työssä tavoitteena oli "hole-mask colloidal"-litografiamenetelmän muokkaaminen ja tarvittavien valmistusvaiheiden tutkiminen ja kehitys. Tavoitteena oli saada tehtyä näytteitä, joilla olisi 200 nm kokoisia grafeeninanokiekkoja sopivalla etäisyydellä toisistaan ja jotka kattaisivat suurimman osan näytteen pinnasta. Näitä grafeeninanokiekkoja tutkittaisiin sitten infrapunaspektroskopialla. Tavoitteena olisi saada näkyviin pintaplasmoneista johtuvia resonansseja spektreissä sekä näiden voimakkuuden ja aallonpituuden muuttaminen sähköisellä seostamisella eli tässä tapauksessa tuomalla eri vahvuista hilajännitettä grafeenikiekoille. Grafeeninanokiekkojen valmistus onnistui kohtuullisen hyvin ja varsin luotettava valmistusmenetelmä saatiin kehitettyä, vaikkakin jonkinlaista epäpuhtautta näytteisiin jäikin todennäköisesti asetonipuhdistuksesta johtuen. Pintaplasmonimittaukset eivät sen sijaan onnistuneet lainkaan eikä plasmoniresonansseja saatu näkyviin missään näytteissä. Tälle voisi useitakin syitä löytyä, mutta mahdollisesti käytetty grafeeni ei ollut kovin hyvää näitä mittauksia ajatellen tai käytetty spektrometri ei ollut tarpeeksi hyvä näin ohuiden näytteiden mittaamiseen. Mittauksia olisi voitu myös parantaa, jos näytteiden pinta-alasta suurempi osa olisi sisältänyt grafeeninanokiekkoja. Graphene has emerged as a promising candidate to replace noble metals as a plasmonic material due to its unique properties and tunability. Graphene plasmonics offer possibilities of controlling light in nanoscale devices and merging optics with electronics. The goal of this thesis was to modify a hole-mask colloidal lithography method to find out a fabrication method that could be used to manufacture samples of a randomly organized array of 200 nm sized graphene nanodisks with some distance between them and with a good amount of the sample covered by the nanodisks. These graphene nanodisks would then be studied for their surface plasmon properties by measuring their infrared spectra with a Fourier-transform infrared (FTIR) spectroscope. The energies and strengths of these surface plasmons in graphene nanodisks can be tuned by electrical doping which was studied as well. The manufacturing of graphene nanodisks was rather successful. A fairly reliable method to produce nanodisks was successfully developed with arrangement and amount of nanodisks just as desired but with some impurities still left in the samples, probably due to the final removal and cleaning with acetone. However, the surface plasmon measurements were a failure and no plasmons could be seen in any of the samples which could be due to multiple reasons such as the graphene not being good enough for measurements or the spectroscope not being able to reliably measure these extremely thin samples. The measurements could have been improved by an even better coverage of the graphene nanodisks in the samples.

Kasvava uusiutuvan energian tarve ajaa tiedeyhteisöä etsimään uusia vihreitä energian lähteitä. Osana tätä tavoitetta, WISC-projekti pyrkii yhdistämään infrapunaa keräävän aurinkopaneelin normaaliin ikkunaan. Tässä tutkimuksessa me tutkimme sisääntulevan valon kytkeytymistä kallistuneesta kultananokartiosta laattamaisen aaltojohteen ohjattuihin aaltojohdemoodeihin; tavoitteena on esittää epäsymmetristä aaltojohteeseen kytkeytymistä. Tutkimus suoritettiin simuloimalla Comsolissa käyttäen kolmiulotteista äärellisten elementtien menetelmää. Ulospäin suuntautuva aaltojohteeseen sironneen valon energiavuo mitattiin 2.1 aallonpituuden päässä nanokartiosta. Tulokset viittaavat kahden samanaikaisen dipolaarisen plasmoniresonanssin olemassaoloon nanokartion sisällä ja että ne kykenevät sirottamaan valoa epäsymmetrisesti aaltojohdemoodeihin kapealla aallonpituuksien ja taittokulmien kaistaleella. Havaittu parhain mahdollinen tilanne viittaa siihen, että nanokartio kykenee sirottamaan valoa kallistusksen mukaiseen suuntaan 8.5 kertaa enemmän kuin kallistuken vastaiseen suuntaan. Nanopartikkelin muotoa ja kokoa voisi vielä optimoida pidemmälle jotta kahden dipolin välinen spektrien päällekkäisyys paranisi, mahdollisesti johtaen suurempaan epäsymmetriaan valon kytkennässä. Tulokset ovat tärkeä askel WISC-ikkunan takaisinsironnan minimoimisessa. The increasing global need for green energy drives the science community to find new green energy sources. As a part of that effort, the WISC-project aims to integrate an infrared-collecting solar panel into a normal window. In this study, we investigated the coupling of incident light into the guided modes of a slab waveguide by scattering from a tilted gold nanocone, with the goal of demonstrating asymmetric waveguide coupling. The study was done using three-dimensional finite element method simulations in Comsol. The outward energy flow of the scattered incident light into the waveguide modes was measured at a distance of 2.1 wavelengths from the nanocone. The results indicate the existence of two simultaneous dipolar particle plasmon resonances within the nanocone, and that they are capable of scattering light asymmetrically into the waveguide modes for a narrow range of wavelengths and particle tilt angles. The discovered best case scenario indicated that the nanocone can couple scattered light into the tilt direction 8.5 times more than into the opposing direction. The nanoparticle shape and size could be further optimized to achieve a better spectral overlap for the two dipoles, possibly leading to even higher coupling asymmetry. The results are an important step towards minimizing the back-scattering in a WISC-window.