Branko Kolaric

In situ tuning the optical properties of a cavity by wrinkling

In this letter we propose an original, in situ, approach to tune the optical properties of an optical cavity, based on the wrinkling of compressed metal/polymer multilayer thin films. This phenomenon is conceptually described, simulated, and experimentally confirmed. The main idea is to use wrinkling to modulate the effective refractive index of the upper interface. This modulation induces a spectral shift of the cavity modes. The work presented here constitutes a first step to the development of stretchable and curved photonics.

Linear and nonlinear optical properties of colloidal photonic crystals.

Linear and Nonlinear Optical Properties of Colloidal Photonic Crystals Luis Gonz alez-Urbina, Kasper Baert, Branko Kolaric,* Javier P erez-Moreno, and Koen Clays* Department of Chemistry and INPAC Institute of Nanoscale Physics and Chemistry, K. U. Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Belgium Laboratoire Interfaces et Fluides Complexes, Centre d’Innovation et de Recherche en Mat eriaux Polym eres, Universit e de Mons, 20 Place du Parc, 7000 Mons, Belgium

Structuring of poly(DADMAC) chains in aqueous media: a comparison between bulk and free-standing film measurements

Recently, thin film balance (TFB) measurements have shown that foam films (thickness 5–120 nm) containing polyelectrolytes thin stepwise. This was connected to the ordering of the polyelectrolyte chains in the film, since pure foam films thin in a continuous way. The present paper shows disjoining pressure isotherms of films containing the polycation poly(DADMAC) and small angle neutron scattering (SANS) spectra of the corresponding aqueous solutions at different polyelectrolyte concentrations. The steps Δh in the isotherms are of the same size as the correlation length ξ in the bulk solution. This leads to the conclusion that the interactions between the polyelectrolyte chains which are responsible for their structuring in the bulk determines also the stratification behavior of polyelectrolyte/surfactant films. These findings were confirmed by the influence of further parameters like the molecular weight, the ionic strength and the degree of charge. The results of the TFB measurements and the SANS experiments show the same dependence on these parameters.

Single molecule fluorescence spectroscopy of pH sensitive oligonucleotide switches

Several authors demonstrated that an oligonucleotide based pH-sensitive construct can act as a switch between an open and a closed state by changing the pH. To validate this process, specially designed fluorescence dye-quencher substituted oligonucleotide constructs were developed to probe the switching between these two states. This paper reports on bulk and single molecule fluorescence investigations of a duplex-triplex pH sensitive oligonucleotide switch. On the bulk level, only a partial quenching of the fluorescence is observed, similarly to what is observed for other published switches and is supposed to be due to intermolecular interactions between oligonucleotide strands. On the single molecule level, each DNA-based nanometric construct shows a complete switching. These observations suggest the tendency of the DNA construct to associate at high concentration.

Angular Dependence of Fluorescence Emission from Quantum Dots inside a Photonic Crystal

The fluorescence of emitters embedded in a photonic crystal is known to be inhibited by the presence of an incomplete photonic band gap or pseudogap acting in their emission range. Here, we present a study of the angular dependence of the fluorescence emission of emitters embedded in a photonic crystal. Our results clearly show an angular dependence of the fluorescence emission, which is caused by the presence of an incomplete 3D band gap.

Influence of the polycation architecture on the oscillatoric forces of aqueous free-standing polyelectrolyte/surfactant films

In the present study, freestanding films were formed from aqueous solutions of surfactant and polyelectrolyte and they were investigated by a thin film pressure balance. The surfactant was either a nonionic alkylpolyglucoside or cationic hexadecyltrimethylammonium bromide. Linear poly(diallyl dimethylammonium chloride) (PDAD-MAC) and poly(ethylene imine) (PEI) of different degrees of branching were investigated. The resulting disjoining pressure isotherms show steps in film thickness owing to oscillatoric structural forces. With respect to the polyelectrolyte architecture the step size, Δh, shows different scaling behavior with the polyelectrolyte concentration, c. While Δh in the thickness of the films containing linear polyelectrolytes scales as c -1/2, for the branched polyelectrolytes Δh scales as c -1/3. The effect of electrostatic interactions on the film stratification was studied by changing the ionic strength or the polymer charge density. For this purpose the ratio between cationic and neutral monomers was varied in the case of the strong PDADMAC or in the case of weak PEI the pH was changed. Both increasing the ionic strength and decreasing the polymer charge density leads to a reduction in the oscillation amplitude of the disjoining pressure. At high ionic strengths or for neutral polymers the steps in the film thickness vanish.

Controlled fluorescence in a beetle’s photonic structure and its sensitivity to environmentally induced changes

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

Interactions between polyelectrolyte brushes in free-standing liquid films: influence of ionic strength

Foam films of charged amphiphilic diblock copolymers are investigated using a thin-film balance. At low polymer concentrations the disjoining pressure shows a continuous exponential decay as a function of the thickness This indicates film stabilization by electrostatic repulsion. At higher polymer concentration stratification occurs with increasing pressure. This is due to the formation of a layer of block copolymer micelles which is pressed out of the film in analogy to usual foam films containing micelles of small surfactant molecules. With increasing salt concentration the film becomes thinner as the charged part becomes more and more coiled and the surface charges more screened.

Far-field disentanglement of modes in hybrid plasmonic-photonic crystals by fluorescence nano-reporters

Abstract In this paper, the resonance modes exhibited by a hybrid nanostructure have been disentangled in the far-field owing to narrow-band fluorescence nano-reporters. Hybrid plasmonic-photonic crystals were fabricated using large (457 nm) monodisperse polystyrene spheres self-assembled into 2D photonic crystals and subsequently coated by a 30 nm thick silver layer. Such structures exhibit a complex resonance pattern, which has been elucidated owing to numerical simulations and electric near-field patterns obtained with a scattering type scanning near-field optical microscope (s-SNOM). For the sake of disentangling the resonance modes of the hybrid structure in the far-field, different types of semiconductor quantum dots (QDs), acting as nano-reporters of the local interactions, were dispersed on top of distinct structures. Depending on the relative overlap of the emission spectrum of a particular type of QDs with the resonance features of the hybrid structure, we affect their emission rate in a unique way, as a consequence of the complex interaction occurring between the plasmo-photonic modes and the excitons. Such plasmonic structures appear to be particularly relevant for fluorescence-based sensing devices.

DEVELOPMENT OF MAGNETIC MATERIALS FOR PHOTONIC APPLICATIONS

In this manuscript, the synthesis and characterization of superparamagnetic particles and their silica-coated counterparts as building blocks for magnetic photonic crystals is fully described. The advantages and disadvantages of the presented synthetic method are discussed. Preliminary results considering the presence of magnetic species within a photonic crystal are also presented. Suppression of emission of the quantum dots within photonic crystals is attributed to a decrease of the number of available photonic modes for radiative decay. The presence of materials with permanent magnetic moments within photonic crystals shows that suppression of their emission is scaled with the strength of the magnetic field.