D. Piatkowski

Fluorescence enhancement of light-harvesting complex 2 from purple bacteria coupled to spherical gold nanoparticles

The influence of plasmon excitations in spherical gold nanoparticles on the optical properties of a light-harvesting complex 2 (LH2) from the purple bacteria Rhodopseudomonas palustris has been studied. Systematic analysis is facilitated by controlling the thickness of a silica layer between Au nanoparticles and LH2 complexes. Fluorescence of LH2 complexes features substantial increase when these complexes are separated by 12 nm from the gold nanoparticles. At shorter distances, non-radiative quenching leads to a decrease of fluorescence emission. The enhancement of fluorescence originates predominantly from an increase of absorption of pigments comprising the LH2 complex.

Radiation Channels Close to a Plasmonic Nanowire Visualized by Back Focal Plane Imaging

We investigated the angular radiation patterns, a key characteristic of an emitting system, from individual silver nanowires decorated with rare earth ion-doped nanocrystals. Back focal plane radiation patterns of the nanocrystal photoluminescence after local two-photon excitation can be described by two emission channels: excitation of propagating surface plasmons in the nanowire followed by leakage radiation and direct dipolar emission observed also in the absence of the nanowire. Theoretical modeling reproduces the observed radiation patterns which strongly depend on the position of excitation along the nanowire. Our analysis allows us to estimate the branching ratio into both emission channels and to determine the diameter-dependent surface plasmon quasi-momentum, important parameters of emitter-plasmon structures.

Tip enhancement of upconversion photoluminescence from rare earth ion doped nanocrystals.

We present tip-enhanced upconversion photoluminescence (PL) images of Er(3+)- and Yb(3+)-doped NaYF4 nanocrystals on glass substrates with subdiffraction spatial resolution. Tip-sample distance dependent measurements clearly demonstrate the near-field origin of the image contrast. Time-resolved PL measurements show that the tip increases the spontaneous emission rate of the two emission channels of Er(3+) in the visible region. Very efficient enhancement of upconversion PL is discussed in the context of the two-photon nature of the excitation process and homoenergy transfer between the ions within the nanocrystals. Comparison between different nanocrystals and tips shows a strong influence of the tip shape on the image contrast that becomes particularly relevant for the larger dimensions of the investigated nanocrystals.

Excited state absorption spectroscopy of ZBLAN:Ho3+ glass—experiment and simulation

The excited state absorption (ESA) spectra of ZBLAN glass activated by trivalent holmium ions have been measured in a wide spectral range (550–1750 nm) and simulated using such theoretical tools as the Judd–Ofelt formalism and McCumber theory of stimulated emission. We also propose a systematic approach for prediction of various types of up-conversion mechanisms in a given type of material. Experimental results on ESA up-conversion processes in ZBLAN:Ho3+ under red and infrared laser excitation, which confirm theoretical analysis, are presented. The optical linewidths were studied using high resolution laser spectroscopy at low temperatures and the existence of different crystallographic sites for Ho3+ ions was revealed.

Microwave-assisted synthesis and single particle spectroscopy of infrared down- and visible up-conversion in Er3+ and Yb3+ co-doped fluoride nanowires

We report a comprehensive study on the influence of solvent and organic ligands on the up-converted emission characteristics of NaYF4 nanowires. We have modified the hydrothermal reaction procedure, and used a “microwave flash heating method” to obtain 1.5 μm long and 100 nm wide up-converting NaYF4 nanowires. As-synthesized particles were dispersed in cyclohexane, and a ligand removal procedure has been performed leading to nanowires soluble in water and heavy water. The infrared emission of Er3+ ions was quenched through coupling to the vibrational modes of water molecules and the values of slopes of double logarithmic plots of power dependencies were changed. Besides the spectroscopic characterization of colloidal solutions of NaYF4 up-converting nanowires, we have also performed single particle studies of steady-state emission and luminescence kinetics. In the case of solvent-free separated particles, the ligand molecules provided the main nonradiative depopulation channels, and efficiently shortened the luminescence lifetimes. The output luminescence color also varied for colloidal solutions and single particle studies, which can be of importance for imaging and single particle tracking applications.

Polarization control of metal-enhanced fluorescence in hybrid assemblies of photosynthetic complexes and gold nanorods

Fluorescence imaging of hybrid nanostructures composed of a bacterial light-harvesting complex LH2 and Au nanorods with controlled coupling strength is employed to study the spectral dependence of the plasmon-induced fluorescence enhancement. Perfect matching of the plasmon resonances in the nanorods with the absorption bands of the LH2 complexes facilitates a direct comparison of the enhancement factors for longitudinal and transverse plasmon frequencies of the nanorods. We find that the fluorescence enhancement due to excitation of longitudinal resonance can be up to five-fold stronger than for the transverse one. We attribute this result, which is important for designing plasmonic functional systems, to a very different distribution of the enhancement of the electric field due to the excitation of the two characteristic plasmon modes in nanorods.

Competition between two types of anti-Stokes emission in Ho3+-activated ZBLAN glass

A competition between two anti-Stokes emissions has been observed and interpreted in ZBLAN glass activated by Ho(3+) ions. The first anti-Stokes emission intensity was seen to increase with temperature, whereas another, upconverted emission, was seen to decrease under the same conditions. Both observed tendencies are believed to be caused by the same effect: the multiphonon anti-Stokes excitation of the state responsible for the first emission. Analysis of the kinetics and fits of the theoretical model to experimental data are presented.

Extending light-harvesting of poly(3-hexylthiophene) through efficient energy transfer from infra-red absorbing nanocrystals: Single nanoparticle study

We report on single nanocrystal fluorescence microscopy of blends composed of colloidal up-converting NaYF4 nanocrystals doped with rare-earth ions embedded in poly(3-hexylthiophene) (P3HT) polymer. By probing both steady-state and time-resolved fluorescence properties of individual nanocrystals excited with infra-red 980 nm laser, we demonstrate that upon up-conversion to the visible spectral range, the energy is efficiently transferred from the nanocrystals to P3HT. From the analysis of fluorescence lifetimes, the energy transfer efficiency for 550 nm emission of the nanocrystals was estimated to be 60%. This observation renders the up-converting nanocrystals as potential structures for improving light-harvesting efficiency of polymers in the near-infrared spectral region.

Enhanced up-conversion in nanocrystals coupled to silver nanowires

We studied physical properties of hybrid nanostructures consisting of α-NaYF4:Tm3+/Yb3+ nanocrystals coupled to single silver nanowires. By using confocal fluorescence microscope we observed much higher intensity of the upconversion emission form nanocrystals placed in the vicinity of nanowires as compared to isolated ones. At the same time Fluorescence Life-time Imaging Microscopy demonstrates shortening of the fluorescence decays times for metal-coupled nanocrystals. We conclude that plasmon-enhanced emission rates are mainly responsible for enhanced up-conversion efficiency.

Confocal microscopy of plasmonic hybrid nanostructures

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP