Optical spectroscopy and microscopy of metallic nanostructures

In this work metallic nanostructures are studied using optical spectroscopy and microscopy. The most striking feature of the optical properties of metallic nanostructures is their capability to support plasmons, coupled oscillations of the conduction electrons and the electromagnetic field. When a plasmon is excited, the optical field can be locally enhanced by orders of magnitude. Also scattering and absorption cross sections increase significantly. These properties open up many interesting applications as well as give information about light-matter interactions on the nanoscale. Optical microscopy is the natural tool to study the optical properties of nanostructures. The spatial resolution of conventional microscopy is limited to approximately half the wavelength of light. However, by a suitable choice of contrast mechanism, e.g., polarization, wavelength, or degree of polarization, a wealth of information can often be extracted about the nanometer scale details of a structure although they cannot be directly resolved. In this thesis, optical spectroscopy is combined with microscopy to investigate two types of metallic structures: nanoparticles and slits in metallic thin films. A new method of optical microscopy is developed to make it possible to detect and spectroscopically study individual gold nanoparticles smaller than 10 nm in diameter. The plasmon resonances of such particles are researched using the developed technique. The light transmission properties of narrow slits fabricated in gold thin films are also investigated using methods of optical microscopy. It is shown that the transmission spectrum exhibits resonances whose properties sensitively depend on the dimensions of the structure. Furthermore, the influence of an external disturbance on the transmittance spectrum is studied, an important question for applications in sensing and optical switching. Results of numerical calculations are compared with the experimental data and a good agreement is found. The central element of any microscopy system is the focusing optic. In this thesis the focusing of partially polarized light is studied theoretically. It is observed that, strikingly, light may become locally fully polarized even when the incident optical field is unpolarized. This counter intuitive effect is confirmed by experiments

Enhancing the optical excitation efficiency of a single self-assembled quantum dot with a plasmonic nanoantenna

We demonstrate how the controlled positioning of a plasmonic nanoparticle modifies the photoluminescence of a single epitaxial GaAs quantum dot. The antenna particle leads to an increase of the luminescence intensity by about a factor of eight. Spectrally and temporally resolved photoluminescence measurements prove an increase of the quantum dot's excitation rate. The combination of stable epitaxial quantum emitters and plasmonic nanostructures promises to be highly beneficial for nanoscience and quantum optics.Comment: 5 pages, 4 figure

Electrospun Electroluminescent CsPbBr3 Fibers as Flexible Perovskite Networks for Light‐Emitting Application

Thin-film perovskite light-emitting diodes have gained increasing attention in the last 6 years. With the possibility to process the emitting layer from solution, the way for 1D morphology of the semiconductor for flexible devices is paved. Herein, for the first time single-step fabrication of CsPbBr3@PVP nanofibers in a customized electrospinning process performed under ambient conditions from a water-based precursor solution is reported. The water-based approach allows the incorporation of a conductive polymer into the compound fiber by blending the perovskite precursor ink with commercially available PEDOT:PSS dispersion. The results demonstrate electrospun fiber mats which are stable at ambient conditions for at least 5 months and can be utilized in electroluminescence devices. Photoluminescence studies on the perovskite fibers reveal a blueshift of the emission peak compared to thin films possibly due to the generation of nanocrystals of ≈12 nm by in situ nanocrystal pinning as confirmed by transmission electron microscopy. A proof-of-concept electrically pumped light-emitting device is built with the obtained fiber mat. The perovskite nanofibers offer promising applications in flexible and stretchable optoelectronics.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Universität zu Köln http://dx.doi.org/10.13039/501100008001Peer Reviewe

Optinen lähikenttämikroskopia: Mittakärkien valmistus ja testimittauksia

Tämän työn tavoitteena on kehittää optisessa lähikenttämikroskopiassa (SNOM) käytettävien kuitukärkien valmistusprosessi ja käyttää valmistettuja kärkiä testimittauksissa. Testimittausten tavoitteena on osoittaa mittakärkien toimivuus ja esittää kuinka valon aallonpituutta pienempiä rakenteita voidaan tutkia SNOM:lla. Työ tehdään Teknillisen korkeakoulun Materiaalifysiikan laboratoriossa käyttäen mikropipettien vetolaitetta ja Optoelektroniikan laboratorion tyhjöhöyrystyslaitetta. Testimittaukset suoritetaan Materiaalifysiikan laboratoriossa olevalla kaupallisella lähikenttämikroskoopilla. Mikropipettien vetolaitteella vedetään yksimuotoisesta optisesta kuidusta teräviä kärkiä, jotka sen jälkeen päällystetään sivuilta alumiinilla käyttäen tyhjöhöyrystystä. Sopivassa höyrystysgeometriassa kärkeen saadaan jäämään aallonpituutta pienempi aukko, jota voidaan käyttää mittakärkenä siirtämään korkearesoluutioista tietoa tutkittavan pinnan läheisyydestä kaukokenttään. Työssä tutkitaan mahdollisuuksia tarkkaan hallita kuitukärjen muotoa; tämä todetaan erittäin vaikeaksi, jopa mahdottomaksi. Tyhjöhöyrystyksessä kokeillaan kahta erilaista lähdettä ja todetaan, että tehokkaampi elektronisuihkuun perustuva laite ei sovellu mittakärkien valmistamiseen. Heikompitehoisen, resistiiviseen kuumennuksen perustuvan lähteen kanssa toimiva pyörivä kuitupidin suunnitellaan. Lisäksi määritetään sopivat höyrystysparametrit resistiivisen lähteen käyttöön. Valmistettuja mittakärkiä käytetään testimittauksiin kahdella eri näytteellä. Ensimmäinen näyte on metalloitu pintaprofiilihila, jota tutkitaan kolmella erityyppisellä mittakärjellä. Mittaukset suoritetaan sekä itsetehdyillä että myös kaupallisilla päällystetyillä mittakärjillä ja itsetehdyillä päällystämättömillä kärjillä. Päällystetyillä mittakärjillä tehdyt mittaukset osoittavat, että sekä kaupallisilla että itsetehdyillä kärjillä on sama resoluutio. Mittaukset päällystämättömillä kuiduilla kuitenkin viittaavat siihen, että suurin osa päällystetyillä kuiduilla mitatusta rakenteesta on pinnan topografian aiheuttamaa harhaa. Toisena testinäytteenä käytetään GaAs-AlAs kvanttikaivorakennetta, jossa pitäisi esiintyä ainoastaan optinen rakenne ilman topografiaa. Testimittausten mukaan rakenteen pinta ei kuitenkaan ole tässä tasainen. Mittausten perusteella voidaan jälleen todeta, että itsetehtyjen ja kaupallisten mittakärkien tuloksissa ei juuri ole eroja. Lisäksi näillä mittauksilla voidaan osoittaa mahdollisuus tutkia aallonpituutta pienempiä rakenteita suurella tarkkuudella käyttäen optista lähikenttämikroskopiaa

Validering av frågor avseende nedvarvning och återhämtning : Samband mellan salivkortisol och subjektiva skattningar

Validation of unwinding and recuperation questions : Correlations between salivary cortisol and subjective ratings The aim of this study was to validate subjective ratings of questions on unwinding and recuperation and to investigate the relationships with cortisol output. Participants were 25 white-collar workers employed at a Swedish government authority. Linear regressions and repeated measures of ANOVA were used to calculate mean levels of salivary cortisol as related to recovery/recuperation. The results show significant main effects of cortisol as related to subjective ratings (p < 0.01) and a significant interaction effect (p < 0.05) between cortisol and sex. The study indicates that high levels of morning cortisol are significantly related to failure to recuperate. Results are discussed in terms of validity, prediction, sex differences, selection and generalization. Key words: Unwinding, recuperation, validity, allostatic load model, salivary cortisol, health, subjective ratings, method.Syftet var att validera subjektiva skattningar avseende nedvarvning och återhämtning genom att undersöka sambandet med kortisolutsöndring som antas utgöra en indikator för återhämtning. Undersökningsdeltagarna utgjordes av 25 högre tjänstemän anställda vid en statlig myndighet. Med linjär regressionsanalys och variansanalys för beroendemätningar analyserades kortisolnivåer i saliv relaterat till subjektiva skattningar av återhämtning. Resultaten visade att höga nivåer av morgonkortisol var förknippat med bristande nedvarvning och återhämtning. Signifikanta huvudeffekter (p < 0,01) av kortisolnivå relaterat till subjektiva skattningar samt interaktionseffekter av kön (p < 0,05) erhölls. Resultaten diskuteras i termer av validitet, prediktionsförmåga, könsskillnader, felkällor och generaliserbarhet

Tunable longitudinal modes in extended silver nanoparticle assemblies

Nanostructured materials with tunable properties are of great interest for a wide range of applications. The self-assembly of simple nanoparticle building blocks could provide an inexpensive means to achieve this goal. Here, we generate extended anisotropic silver nanoparticle assemblies in solution using controlled amounts of one of three inexpensive, widely available, and environmentally benign short ditopic ligands: cysteamine, dithiothreitol and cysteine in aqueous solution. The self-assembly of our extended structures is enforced by hydrogen bonding. Varying the ligand concentration modulates the extent and density of these unprecedented anisotropic structures. Our results show a correlation between the chain nature of the assembly and the generation of spectral anisotropy. Deuterating the ligand further enhances the anisotropic signal by triggering more compact aggregates and reveals the importance of solvent interactions in assembly size and morphology. Spectral and morphological evolutions of the AgNPs assemblies are followed via UV–visible spectroscopy and transmission electron microscopy (TEM). Spectroscopic measurements are compared to calculations of the absorption spectra of randomly assembled silver chains and aggregates based on the discrete dipole approximation. The models support the experimental findings and reveal the importance of aggregate size and shape as well as particle polarizability in the plasmon coupling between nanoparticles

Large-area enhancement of far-field fluorescence intensity using planar nanostructures

While nanoscale local enhancement of fluorescence is a well-studied phenomenon, enhancement of far-field radiation of large-area samples using nanostructures has been explored much less. In this work, we demonstrate planar nanostructures with exceptionally high far-field intensity enhancement factors and in addition show that the enhancement can be made controllable via the polarization state of the pump light. We also find that, counterintuitively, the Purcell effect plays only a small part in the observed far-field enhancement. The results of these studies can be applied, for example, in thin-film optical emitters, fluorescence sensors, and wide-field fluorescence microscopy.Peer reviewe

Imaging and Steering Unidirectional Emission from Nanoantenna Array Metasurfaces

In radiofrequency antenna engineering, the array factor made long-distance communication with steerable transmission and receiving possible. At optical frequencies, low-loss signal transmission via free space by using nano antennas is still in its infancy. Here, we suggest applying the array factor to the optical frequency regime by shaping the radiation pattern of plasmonic metasurfaces featuring nano antenna arrays. We arrange dipolar gold nanoantennas operating at 785 nm wavelength in wavelength-sized arrays and control the phase that drives the antenna elements. We obtain collimated and unidirectional radiation from this metasurface upon illumination with circularly polarized light, which is not prone to major losses as in common plasmonic waveguide structures. We furthermore demonstrate switching the unidirectional emission to opposite directions with additional beamsteering by modifying the array factor. Our experiment corroborates the evidence for spin orbit coupling between the helicity of light and suitably designed plasmonic metasurfaces, which can exhibit the spin-Hall effect for light