Toruń (Poland), 10-11 October 2008

Registered participants

  • dr. Javier Aizpurua, DIPC and CSIC, Donsotia-San Sebastian, Spain
  • dr. Adam Babinski, University of Warsaw, Warsaw, Poland
  • prof. Gerd Bacher, University Duisburg-Essen, Duisburg, Germany
  • prof. Garnett Bryant, National Institute of Standards and Technology, Gaithersburg, MD, United States
  • dr. Boguslaw Burak, Comef Scientific and Research Equipment, Katowice, Poland
  • dr. Juan Ignacio Climente Plasencia, Universitat Jaume I, Castello de la Plana, Spain
  • dr. Beata Derkowska, Nicolaus Copernicus University, Torun, Poland
  • dr. Ewa Dobruchowska, Technical University of Lodz, Lodz, Poland
  • prof. Marek Godlewski, Polish Academy of Sciences, Warsaw, Poland
  • prof. Alexander Govorov, Ohio University, Athens, United States
  • dr. Ireneusz Grulkowski, Optopol Technology S.A., Torun, Poland
  • prof. Marian Grynberg, University of Warsaw, Warsaw, Poland
  • prof. Wlodzimierz Jaskolski, Nicolaus Copernicus University, Torun, Poland
  • dr. Lukasz Klopotowski, Polish Academy of Sciences, Warsaw, Poland
  • dr. Piotr Kossacki, University of Warsaw, Warsaw, Poland
  • dr. Andrzej Kudelski, University of Warsaw, Warsaw, Poland
  • dr. Sebastian Mackowski, Nicolaus Copernicus University, Torun, Poland
  • prof. Andrzej Miniewicz, Wroclaw University of Technology, Wroclaw, Poland
  • dr. Wieslaw Nowak, Nicolaus Copernicus University, Torun, Poland
  • Mariusz Pawlak (PhD student), Nicolaus Copernicus University, Torun, Poland
  • Marta Pelc (PhD student), Nicolaus Copernicus University, Torun, Poland
  • Dawid Piatkowski (PhD student), Nicolaus Copernicus University, Torun, Poland
  • dr. Arkadiusz Ptak, Poznan University of Technology, Poznan, Poland
  • dr. Grzegorz Sek, Wroclaw University of Technology, Wroclaw, Poland
  • dr. Bartlomiej Szafran, AGH University of Science and Technology, Krakow, Poland
  • dr. Zbigniew Wasilewski, National Research Council of Canada, Ottawa, Canada
  • dr. Arkadiusz Wojs, University of Cambridge, Cambridge, United Kingdom

Plasmonic Interactions in Surface-Enhanced Spectroscopy and Microscopy

Javier Aizpurua

Centro de Fisica de Materiales CSIC-UPV/EHU, DIPC and CSIC
Paseo manuel Lardizabal 4, 20018 Donsotia-San Sebastian, Spain

Plasmons play a key role in surface-enhanced spectroscopy, sensing, and imaging from the visible to the infrared. We analyse the interaction of plasmons with molecular vibrations in surface-enhanced infrared absorption (SEIRA), as an excellent complementary tool to surface enhanced Raman scattering (SERS) for molecular spectroscopy. The interaction of vibrational excitations with plasmonic modes gives rise to increased vibrational signal and enhanced contrast with lineshape modification of the molecular fingerprints. The spectral modifications are due to the interference of the plasmonic and the vibrational electromagnetic fields, analogous to the Fano lineshape profiles. This effect is calculated and addressed both for the far-field signal of ODT molecules on gold nanowires, as well as for the near-field infrared signal of a molecular thin layer of PMMA on a resonant cavity in scattering-type near-field scanning optical microscopy (s-SNOM). Moreover, we present novel plasmonic structures that behave as efficient substrates to perform simultaneously SERS and SEIRA, providing a host for complementary molecular spectroscopy. In addition to the changes in spectroscopic signal and contrast, near-field optical imaging of nanoobjects is also affected by strong plasmonic interactions between the tip and the sample. We show the distortion of plasmon patterns in nanoscale optical microscopy of metallic objects, such as gold nanodisks and dimers when the tip interacts strongly with the objects, modifying the near-field plasmon patterns both in amplitude and phase. An understanding of the modified plasmon patterns is crucial to correctly interpret optical nanoimaging of metallic objects.

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Quantum dots in the InAs/GaAs wetting layer - a case study

Adam Babinski

Institute of Experimental Physics, University of Warsaw
Hoza 69, 00-012 Warsaw, Poland

Properties of self-assembled quantum dots (QDs) have been a subject of intense investigation over last decade with an InAs/GaAs system as a main object of interest. Less attention has been paid to properties of the wetting layer (WL), which is an inevitable result of the Stranski-Krastanow growth scenario.
In this presentation we report on properties of excitons localized in potential fluctuations in the WL. Recombination of excitons localized in the single fluctuations can be observed in the micro-photoluminescence measurements at low temperature at energy approx. 5 meV lower than the bandgap energy of the WL. The splitting between two linearly-polarized components of the single exciton X and biexciton 2X emission lines, which is due to an anisotropic electron-hole exchange interaction allows for their attribution to a neutral exciton and biexciton respectively. No splitting can be observed in the case of the X*, as expected for a charged exciton (two identical carriers of trion are in a singlet state). Clear photon antibunching has been observed in time-resolved measurements of the neutral exciton emission. Both the X and X* excitons split in magnetic field in two components circularly polarized in opposite directions. The effective excitonic g factor changes from dot to dot, from 1.2 to 2.
Presented results show that the potential fluctuations in the WL can be treated as shallow-potential quantum dots and they represent an option in a search for single photon sources for quantum information processing.

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Optical spectroscopy on nanostructured ferromagnet-semiconductor hybrids

Gerd Bacher

Electronic materials and nanostructures, University Duisburg-Essen
Bismarckstr. 81, 47057 Duisburg, Germany

The ability to locally define and manipulate carrier spin states in a semiconductor is one key issue in spintronics. We use fringe fields provided by tiny ferromagnets as well as micro-coils to obtain local spin control in an underlying semiconductor.
In a first series of experiments, the fringe field of microstructured Fe/Tb multilayer ferromagnets with out-of-plane magnetization was used to imprint a remanent magnetization into the magnetic ion system of a magnetic semiconductor. This results in a locally varying carrier spin polarization of up to 25% at zero external fields. We demonstrate the ability of switching the ferromagnetic state by an intense laser pulse and probe the magneto-optical response of the semiconductor. Using a current pulse travelling through a micro coil, the experiments are extended to achieve a spin control in a semiconductor on a micrometer length and a sub-microsecond time scale.
In the second series of experiments fringe fields stemming from ferromagnets with in-plane magnetization are used to locally modify the coherent spin dynamics of both, electrons and magnetic ions. We demonstrate the ability of a complete reversal of the spin orientation within the spin coherence time of magnetic ions in CdMnZnSe quantum wells with respect to a reference measurement and show the potential of locally manipulating coherent spin states in GaAs up to room temperature.

Work is done in collaboration with Y. Fan, J. Puls, F. Henneberger (HU Berlin), E. Schuster, W. Keune (U Duisburg-Essen), D. Reuter, A. Wieck, S. Fischer, U. Kunze (U Bochum), W. Maciej, T. Wojtowicz, G. Karczewski (Polish Academy of Science).

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Dynamical control of quantum dot optics

Garnett Bryant

Quantum Processes and Metrology Group, National Institute of Standards and Technology
100 Bureau Drive, 20899-1070 Gaithersburg, MD, United States

Dynamical control of the optics of quantum dots could be achieved by using external fields to induce or split level degeneracies, polarize optical transitions, modify coupling in closely spaced dots, and entangle transitions, all capabilities needed to use dots in quantum information processing. I discuss two approaches being pursued by our group at NIST to dynamically control excitons in quantum dots.
First, I discuss recent experiments carried out in our group to dress exciton-biexciton complexes in semiconductor quantum dots. The photoluminescence of a single quantum dot was measured while a resonant laser optically dressed either the exciton transition or the exciton to biexciton transition. High resolution spectroscopy reveals splittings of the linearly polarized fine-structure states that are nondegenerate in an asymmetric quantum dot. These splitting manifest as triplets or doublets and depend sensitively on the resonant laser intensity and detuning. This approach realizes complete resonant control of a multiexcitonic system in emission.
Dynamical control of self-assembled dots could also be realized by use of external strain. To understand the coupling between internal strain due to lattice mismatch and external strain imposed for control, we use atomistic tight-binding theory. External strains imposed by mechanical bends of GaAs nanomechanical resonators with embedded InAs dots are considered. A bend acts like a DC electric field, inducing Stark-like level shifts. Such applied strain can be used to mix and control the level ordering and spatial distribution of electrons and holes in closely spaced, vertically or laterally coupled dots. Results show how active control of the optics of single dots and coupled dots can be achieved.

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Holes in artificial molecules: an antibonding ground state?

Juan Ignacio Climente Plasencia

Departament de Quimica Fisica i Analitica, Universitat Jaume I
Escola Superior de Tecnologia i Ciencies Experimentals, UJI, Avda. Sos Baynat s/n, 12080 Castello de la Plana, Spain

Resonant tunneling of carriers between vertically coupled quantum dots enables the formation of hybridized, molecular-like orbitals which are important in many quantum dot-based devices, including those aiming at optically-controlled quantum information storage.[1] The differences in size and composition of quantum dots is overcome by the application of the vertical electric field, which brings the two quantum dot levels into resonance and induces either electron or hole tunneling.[2] The tunneling of electrons is now well understood[1-3], it leads to the formation of bonding molecular ground states in analogy to natural diatomic molecules. However, tunneling of holes does not have a counterpart in diatomic molecules and is less understood. In fact, previous atomistic calculations suggested a reversal of bonding and antibonding hole molecular ground states as the interdot barrier distance increases.[4-6]
In this work, we present theory and experimental observation of the formation of the antibonding hole molecular ground state. Using a 4-band k¡p approximation, the hole states are described as Luttinger spinors[7], which contain all the relevant symmetries. It is shown that the spin-orbit interaction in the valence band breaks the parity in the growth direction, mixing bonding and antibonding heavy- and light-hole components of the spinor. This mixing destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal at interdot distances of about 2 nm.[8] Clear experimental evidence of this peculiar hole behavior is found in magneto-photoluminescence experiments of double dots.[9]

[1] M. Bayer et al., Science 291, 451 (2001); E.A. Stinaff et al., ibid 311, 636 (2006).
[2] A.S. Bracker et al., Appl. Phys. Lett. 89, 233110 (2006).
[3] H.J. Krenner et al., Phys. Rev. Lett. 94, 057402 (2005); G. Ortner et al. ibid 94, 157401 (2005).
[4] W. Jaskolski, Acta Phys.Pol. A 106, 193 (2004); Phys. Rev. B 74, 195339 (2006).
[5] G. Bester et al., Phys. Rev. Lett. 93, 047401 (2004); Phys. Rev. B 71, 075325 (2005).
[6] M. Korkusinski et al., Proceedings of the 27th Int. Conf. Phys. Semicond., 685 (2005).
[7] J.M. Luttinger, and W. Kohn, Phys. Rev. 97, 869 (1955). L.Rego et al . Phys. Rev. B 55, 15694 (1997).
[8] J.I. Climente et al., Phys. Rev. B (in press).
[9] M.F. Doty et al. ArXiv:0804.3097 (2008).

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Experimental study of third order optical nonlinearities in MPcs and their host-guest systems

Beata Derkowska

Institute of Physics, Nicolaus Copernicus University
Grudziadzka 5, 87-100 Torun, Poland

Third order nonlinear optical properties of metallophthalocyanines (MPcs) thin films and solutions were investigated using degenerate four wave mixing (DFWM) method at 532nm. Third harmonic generation (THG) measurement at 1064nm performed on MPcs thin films is also discussed. We found that the chi<3> value, obtained by DFWM method, is larger than that measured via THG experiment. The variation in chi<3> values occurs due to the different resonance contributions in solution and solid state of MPcs. We focused our studies on metallophthalocyanines because the substituting different metal atoms into the ring should lead to change the third order nonlinear optical properties. We chose to investigate copper phthalocyanine, cobalt phthalocyanine and zinc phthalocyanine containing 3d transition metal atoms and magnesium phthalocyanine system, where the central metal lacks d electrons.
The concentration dependence of third order nonlinear optical susceptibilities of MPcs dissolved in tetrahydrofuran (THF) was also investigated. The study of the influence of the solution concentration on chi<3> shows the linear correlation. In the case of microscopic nonlinearity, we calculated the second order hyperpolarizability (gamma) for MPcs solutions. We found that the chi<3> and the gamma values are affected by the nature of the central metal atom.
We also studied how the replacement of peripheral substituents around the MPcs cores correlates with nonlinear optical properties. Third order nonlinear optical susceptibilities of MPcs with liquid crystal (MPcs-LC) and DNA-CTMA surfactant complex (CoPc-DNA-CTMA) measured by DFWM method were also investigated. We found that the chi<3> values of MPcs-LC and CoPc-DNA-CTMA increase in comparison with the corresponding values of MPcs. We supposed that this is caused by the change of the charge transfer effects and the dipole moments of the molecule with the change of the molecules dimension.
To develop pi-conjugated molecules with even better nonlinear optical properties, it is necessary to understand how variations in both the pi-conjugated ligands and in the other groups attached to the central atom in the molecules affect the linear and nonlinear optical properties of the molecules. This is a challenging task because simple modifications in the structures can simultaneously affect a variety of interacting charge-transfer mechanisms that contribute to the linear and nonlinear optical properties of the molecules. The long-range aim is to learn how the substituting different metal atoms into the ring and altering peripheral and axial functionalities of the phthalocyanines correlate with nonlinear optical properties, and to enhance those properties via control of the molecular structures.

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Nanomaterials for photovoltaic applications

Ewa Dobruchowska

Department of Molecular Physics, Technical University of Lodz
Zeromskiego 116, 90-924 Lodz, Poland

The most of the present world production of photovoltaic (PV) cells is based on crystalline silicon. Currently, their power conversion efficiency has reached 24%. However, the high-temperature fabrication routes to single-crystal and polycrystalline silicon are energy intensive and expensive. The search for alternative solar cells is therefore focused on thin films of organic (based on ?-conjugated polymers) and hybrid (organic-inorganic) nanocomposites that can be prepared by cheaper, solution based methods. The examples of such structures are represented by interpenetrating networks of n-type and p-type organic semiconductors that form so called bulk heterojunctions, as well as by dye-sensitized nanocrystalline inorganic semiconductor layers.
Nanocomposites with enhanced surface of p-n junction were prepared with use of so called "reticulate doping" method. The general idea behind the term "reticulate doping" is that the low molecular weight "dopant" (n-type) forms a separate crystalline phase, which penetrates a polymer (p-type) matrix or at least its surface layer. We have found, by means of surface potential decay measurements, that high degree of crystallinity of the n-type component improves its transport properties and brings an increase in photogeneration efficiency at the p-n junction.
Hybrid organic-inorganic structures, applied in dye sensitized solar cells (DSSCs), combine the best properties of organic (e.g. processability) and inorganic materials (e.g. good charge transport) properties. A research effort is currently in course of understanding the parameters that control DSSC performance to improve its stability as well as the power conversion efficiency. The improvement of efficiency was also the aim of our investigations. In order to increase the photo-current we have chosen two different ways: enhancement of the light path and improvement of absorbance in the longer wavelength region of visible light.
Additionally, we have done a step towards plastic electronics by applying completely plastic counter electrode as PEDOT:PSS on polyester substrate instead of platinized conducting glass commonly used in DSSCs. Generally, flexible electrodes present some advantages in comparison to electrodes on glass substrate e.g. lower weight, impact resistance and less form and shape limitation. However, in this case our main attention was focused on studies of behaviour of such plastic counter electrode in the presence of IÂŻ/I3ÂŻ redox couple. In order to probe chemical and electronic properties of PEDOT:PSS foil, X-ray photoelectron spectroscopy was used as a powerful tool to study the atomic composition of the surface.

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Impurity carriers interactions in semiconductor nanoparticles

Marek Godlewski

Dept. of Solid State Spectroscopy, Institute of Physics, Polish Academy of Sciences
al. Lotnikow 32/46, 02-668 Warsaw, Poland

Nanoparticles of wide band gap II-VI compounds doped with either transition metal or rare earth ions are intensively studied due to their possible use as fluorescent labels in biology and medicine. In this work we describe properties of ZnMnS and CdMnS nanoparticles. Intra-shell transitions of Mn2+ ions are parity and spin forbidden processes, i.e., the resulting light emission should be inefficient. However, in nanoparticles the 4T1 to 6A1 intra-shell transition shows a surprisingly bright photoluminescence (PL) together with a short PL decay time. We relate the observed PL enhancement and PL lifetime decrease to two spin dependent magnetic interactions between: (a) localized spins of Mn2+ ions (Mn - Mn interactions) and (b) Mn2+ ions - spins/magnetic moments of free carriers. The latter mechanism is significantly enhanced in nanoparticles.
Dynamics of spin-related relaxation processes is intensively studied, since long times of spin relaxation are required for application of diluted magnetic semiconductors (DMS) in spintronic devices. These processes can be studied with technique of optically detected magnetic resonance (ODMR), which will be shortly described. ODMR investigations of wide band gap II-Mn-VI compounds allowed us to determine efficiency of Mn-Mn and DAP-Mn spin flip interactions. However, we found that rates of Mn2+ intra-shell transitions depend only weakly on efficiency of the above interactions. We found that interactions between local spin moments of Mn2+ ions and non-equilibrium 2D electron gas dominate. Important role of these interactions in energy dissipation processes (radiative and nonradiative recombination processes in DMS samples) is evidenced.

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Optical properties of superstructures composed of nanocrystals and photosynthetic molecules: Exciton-plasmon interaction and energy transfer

Alexander Govorov

Department of Physics, Ohio University
Clippinger Labs, 45701 Athens, United States

Motivated by recent experiments on nanocrystal superstructures [1,2], we study theoretically optical properties of hybrid complexes assembled from semiconductor quantum dots (QDs), nanowires (NWs), and metal nanoparticles (NPs). The interaction between excitons in semiconductor nanocrystals (QDs or NWs) and plasmons in metal NPs leads to several effects: energy transfer between nanocrystals, electromagnetic enhancement, reduced exciton diffusion in nanowires, exciton energy shifts, and interference and non-linear phenomena [3,4]. Using kinetic equations for excitons, we model exciton transport in a nanowire and explain the origin of the blue shift of exciton emission observed in recent experiments on hybrid NW-NP assemblies [2]. We also model artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and crystalline (metal and semiconductor) nanoparticles [5]. We show that, by using superior optical properties of nanoparticles and involving energy transfer, one can strongly enhance the efficiency of light harvesting [5,6]. Interaction between a discrete state of exciton and a continuum of states can also appear in the epitaxial quantum dots [7,8]. In the regime of strong optical fields, the interaction between nanocrystals (semiconductor QDs and metal NPs) creates a non-linear Fano effect (an asymmetric peak in the total energy absorption in the non-linear regime) [4,8]. Our theory explains present experimental results and provides motivation for future experiments and applications. Potential applications of dynamical exciton-plasmon systems are in sensors and light-harvesting devices.

[1] J. Lee, A. O. Govorov, J. Dulka, and N. A. Kotov, Nano Letters 4, 2323 (2004); J. Lee, A. O. Govorov, and N. A. Kotov, Angewandte Chemie 117, 7605 (2005).
[2] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007).
[3] A. O. Govorov, G. W. Bryant, W. Zhang, T. Skeini, J. Lee, N. A. Kotov, J. M. Slocik, and R. R. Naik, Nano Letters 6, 984 (2006).
[4] W. Zhang, A. O. Govorov, and G. W. Bryant, Phys. Rev. Lett. 97, 146804 (2006).
[5] A. O. Govorov, Advanced Materials, online; A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007).
[6] S. Mackowski, S. Woermke, A. J. Maier, T. H. P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A. O. Govorov, H. Scheer, C. Braeuchle, Nano Lett. 8, 558 (2008).
[7] K. Karrai, R. J. Warburton, C. Schulhauser, A. Hoegele, B. Urbaszek, E. J. McGhee, A. O. Govorov, J. M. Garcia, B. D. Gerardot, and P.M. Petroff, Nature 427, 135 (2004).
[8] M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R.Barbour, B. D. Gerardot, R. J. Warburton, and K. Karrai, Nature 451, 311 (2008).

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Micron-resolution in-vivo human eye imaging using spectral optical coherence tomography

Ireneusz Grulkowski

Research & Development Office, Optopol Technology S.A.
Grudziadzka 5, 87-100 Torun, Poland

Optical Coherence Tomography (OCT) is a noncontact and noninvasive biomedical imaging technique that has found wide applications especially in ophthalmology. The method has prooved its usefullness in early-stage diagnosis of many eye diseases like age-related macular degeneration (AMD), macular edema, glaucoma and others.
Since the past ten years, spectral optical coherence tomography (SOCT) has been extensively developed as an alternative method to classical, Time-domain OCT (TdOCT). The new method offers higher signal-to-noise ratio at 100x shorter examination time. These features allow for a three-dimensional reconstruction of the eye structures within a few seconds and offer new possibilities in early-stage diagnosis like blood-flow analysis or particular layer segmentation.
A brief history and principle of operation of OCT devices will be presented. Results of healthy and pathological eyes obtained with SOCT device will be shown and discussed. Next-generation technologies based on OCT method will be mentioned.

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Photoluminescence Spectroscopy of Single Self-assembled CdTe Quantum Dots in Electric Fields

Lukasz Klopotowski

Institute of Physics, Polish Academy of Sciences
al. Lotnikow 32/46, 02-668 Warsaw, Poland

I will give an overview of our recent results on photoluminescence (PL) studies of single CdTe quantum dots (QDs) in electric fields.
Our studies are performed on field effect structures in which a single layer of CdTe dots is embedded between a p-doped back contact and a front Schottky gate, both separated from the QDs by barrier layers. PL is excited with a laser beam focused onto a 2 micrometer spot by a microscope objective. The signal is detected by a monochromator and CCD camera. As bias is applied, the transition lines shift according to quantum confined Stark effect. Analysis of this shift allows to evaluate the electron-hole polarizability and built-in, static dipole moment associated with the dot. These parameters give access to QD morphology and to charge distribution inside the dot.
With increasing reverse bias, the resulting electric field enables efficient hole tunneling from dots to the back contact. As a result, transition lines become broader, weaker and eventually disappear. We are able to reproduce the dependence of PL intensity and linewidth on electric field in a semiclassical approach based on WKB approximation.
Increasing forward bias results in shifting the Fermi level above the hole ground state. This in turn enables holes to tunnel from the back contact into the dots and therefore to electrically control QD charge state.
Transition energies and corresponding quantum confined Stark shifts for different charge states depend strongly on the strengths of Coulomb interactions between carriers populating the dot. A simple model neglecting exchange and correlation allows to conclude that in our dots electron-hole attraction dominates over hole-hole repulsion, which in turn is greater than the strength of electron-electron repulsion.

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Symmetry and excitation mechanisms of self-assembled CdTe/ZnTe quantum dots

Piotr Kossacki

Institute of Experimental Physics, University of Warsaw
Hoza 69, 00-681 Warsaw, Poland

A summary of our recent quantum dot (QD) spectroscopic studies will be given.
Charged and neutral quantum dot excitonic states were investigated with particular attention paid to their symmetry. Population dynamics of various QD states was studied at quasi resonant and non-resonant excitation, including capture of individual carriers and excitons, single exciton and cascade radiative recombination, spin relaxation of individual carriers as well as energy and spin transfer in coupled quantum dot pairs.
Non-classical light emission from quantum dots was examined, including emission of single photons and correlated photon pairs, as well as entanglement tests.
Experimental tools used in these studies include polarization-resolved microphotoluminescence of individual quantum dot states. Photoluminescence excitation spectra were also measured on individual QDs. A variety of time-resolved experimental methods has been used, including pulsed and cw polarization-resolved second order photon correlation measurements, PL decay time measurements, and measurements of excitation correlated spectroscopy (ECS): individual quantum dot photoluminescence excited by pairs of laser pulses with controlled temporal separation within the pair. The available temporal resolution ranged from that determined by the pulse width of the femtosecond laser (in ECS) to subnanosecond resolution of avalanche photodiode detectors.
The studied material systems are II-VI semiconductors (primarily CdTe/ZnTe QDs).
Presented results include evidence of fine structure, indication of inter-dot energy and spin transfer in strongly asymmetric QD pairs, evidence of major role of single carrier capture in non-resonant QD excitation. Long spin relaxation times of single carriers in QDs have also been demonstrated. Radiative cascade involving triplet biexcitons have been observed.

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Local analysis of surfaces with Raman spectroscopy

Andrzej Kudelski

Department of Chemistry, University of Warsaw
Pasteur 1, 02-093 Warsaw, Poland

In this talk some examples of local Raman studies of molecules adsorbed on various surfaces (including catalysts) will be presented. First, standard confocal Raman microscopic measurement will be discussed. Then, the surface-enhanced Raman scattering (SERS) effect will be described. Some basic information about the mechanism of very large increase of the efficiency of Raman scattering in the SERS effect will be given. SERS enhancement factors on various adsorption sites can differ significantly (theoretical simulations showed that, in some cases, local differences excess ten orders of magnitude). Therefore, even in measurements carried out with a Raman microscope, when the focal area is a few micrometers2, we often actually measure signal from relatively small number of molecules. Some examples of practical applications of "a few molecules SERS measurements" will be given. In the last part of the talk the combination of Raman spectroscopy and scanning probe microscopy (STM or AFM) so called tip-enhanced Raman spectroscopy (TERS) will be described. In this technique only the Raman intensities of the molecules which are very close to the tip are enhanced. Therefore, TERS has both very large spectroscopic and spatial resolution.

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Metal - Enhanced Fluorescence of Chlorophylls in Single Light - Harvesting Complexes

Sebastian Mackowski

Institute of Physics, Nicolaus Copernicus University
Grudziadzka 5, 87-100 Torun, Poland

Ensemble and single-molecule spectroscopy demonstrates for the first time that both emission and absorption of peridinin-chlorophyll-protein photosynthetic antennae can be largely enhanced through plasmonic interactions [1]. We find up to 18-fold increase of the chlorophyll fluorescence for complexes placed near a silver nanoparticle layer. This enhancement, which leaves no measurable effects on the protein structure, is observed when exciting either chlorophyll or carotenoid. The experimental findings are supported by model calculations of the electric field enhancement of both absorption and emission, which outweigh any fluorescence quenching due to the Ag nanoparticles. This result is an important step toward applying plasmonic nanostructures for controlling the optical response of complex biomolecules. Particularly appealing is the prospect of using advanced biochemical techniques to control the morphology of hybrid structures, thereby optimizing the interactions between biomolecules and metal nanoparticles. Initial results show that combination of CdTe nanocrystals with light-harvesting complexes further improves light collection efficiency by extending the spectral range and taking advantage of huge absorption coefficients typical for the nanocrystals.

[1] S. Mackowski, S. Woermke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Braeuchle, "Metal - Enhanced Fluorescence of Chlorophylls in Single Light - Harvesting Complexes", Nano Letters 8, 558-564 (2008)

In collaboration with , S. Woermke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Braeuchle

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Study of liquid crystals, photovhromic polymers and DNA-based polymers as active materials for photonic devices

Andrzej Miniewicz

Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology
Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

Photorefractive liquid crystals and photochromic materials are extensively studied because of their dynamic holographic properties (low light intensity induced reversible refractive index or absorption coefficient changes). These materials are used for optical signal processing and as diffractive or real-time holographic systems. For these applications the time scale of photo-induced phenomena and magnitude of photo-induced anisotropy (birefringence or dichroism) is of the primary importance.
We have undertaken the systematic investigations of two groups of photoactive materials: (i) dye-functionalized polymers in which molecular orientation can be controlled by polarized light by means of the coupling of optical field with the anisotropic absorption of molecules and (ii) low-molecular mass liquid crystals in which spatially modulated optical field acting through phtoconductivity via space charge electric field introduces optical anisotropy. We limit our interest to azobenzene-containing polymers and nematic liquid crystals interacting with photoconducting polymeric surface. However, the new results obtained in a novel biopolymer matrix DNA-CTMA doped with NLO active chromophores also will be reported [1].
The model compound is photochromic polymer with azobenzene-derivative covalently bonded to the polymer main chain. The interplay between cis-trans photoisomerisation process and polymer matrix interaction with photoactive molecules is a challenging problem in achieving the best performance fast (1 ms - 1 s), large and reversible refractive index changes. We show, on some examples, how chemical design of both optically nonlinear chromophores and polymeric matrix contribute to optimum performance materials. The interesting surface relief grating formation of nanometric size will also be discussed.
In another group of materials suitable for real-time holography the photoconducting polymers (PVK:TNF) are combined to act together with optically birefringent nematic liquid crystals. This unique material combination gives a new photorefractive hybrid system of excellent performance characteristics for video-type holographic retrieval.
Examples of the materials, their optical characteristics and applications in photonics will be presented and discussed.

[1] A. Miniewicz, A. Kochalska, J. Mysliwiec, A. Samoc, M. Samoc, J. Grote, Deoxyribonucleic acid-based photochromic material for fast dynamic holography, Applied Physics Letters 91, 041118-20 (2007)

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From fluorescent probes to photoactivated enzymes - atomistic simulations of bionanostrutures' dynamics

Wieslaw Nowak

Institute of Physics, Nicolaus Copernicus University
Grudziadzka 5, 87-100 Torun, Poland

To understand biological objects we need to deal with nanostructures: the photosynthesis requires nanoantennas coupled to reaction centers, nanomachines drive synthesis of ATP – the main source of energy in cells, a nanometric size rybosomes read-out information from RNA, etc. Optical spectroscopy is en excellent tool for non-invasive studies of bionanostructures. Especially useful are observations of fluorescence emitted from both endogenous (eg. GFP [1]) or exogenous fluorescent probes (eg. PRODAN [2]).
In the first part of this talk the idea of using fluorescent compounds to study biological objects of the large size will be presented. Spectral signals need interpretation. Atomistic computer simulations of molecular dynamics (MD) of large (>300 000 atoms) molecules are possible nowadays. Thus, we apply these methods to understand bionano-objects better. Applications of MD for interpretation of fluorescent nanotomography [3] experiments and spectral shifts of fluorescent TICT probes trapped in protein cavities will be discussed.
A future de novo synthesis of nanomachines will require special materials. Oligosaccharides are promising building blocks. The AFM experiments performed on single molecules help us to find out which molecular systems have desired mechanical characteristics. We will show that the details of conformational transitions may be nicely revealed by both classical [4] and quantum steered [5] dynamics.
In the last part of this contribution the example of application of computational techniques, such as the Landau-Zener model of excited state dynamics, to understand properties of widely used photoactive, biotechnological enzyme nitrile hydratase will be presented [6].

[2] A. Parusel, W. Nowak, S. Grimme, G. Koehler; J. Phys. Chem. A 102 (1998) 7149-7156.
[3] O. J. Rolinski, D.J.S. Birch; J. Chem. Phys. 116 (2002) 10411-8.
[4] G. Lee, W. Nowak, J. Jaroniec, Q. Zhang, P. E. Marszałek; Biophys. J. 87 (2004) 1456-65.
[5] Z. Lu, W. Nowak, G. Lee, P.Marszalek, W. Yang; J. Am. Chem. Soc. 126 (2004) 9033-41.
[6] K.Kubiak, W. Nowak, Bioph. J.; 94 (2008) 3824-3838.

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Excited State Absorption and its Role in Modern Spectroscopy

Dawid Piatkowski

Institute of Physics, Nicolaus Copernicus University
Grudziadzka 5, 87-100 Torun, Poland

Excited State Absorption (ESA) is well understood mechanism of two-photon absorption. This phenomenon can be characterized by the ground state absorption followed by transition between pair of the excited states. The interest in the ESA spectroscopy has been strongly connected with evolution of the laser materials. Initially, the ESA transitions, considered as detrimental for potential laser transitions, were undesired. Afterwards, absorption of the light by previously excited system became exploited to increase energy of the emitted photons, due to two-red to one-blue photon conversion (up-conversion). Although several different mechanisms of the light conversion are known, ESA still plays major role because of its simplicity and relatively high efficiency of conversion.
Excited state absorption spectroscopy is an excellent tool for probing high energy levels, especially these situated inside the conduction band. Because of strong absorption of the matrix, high lying levels of the luminescence center cannot be reached using high energy photons. Instead, they can be probed via ESA using several low energy photons at energies below absorption of the host.
Among several ESA experimental techniques, the CW phase-sensitive one will be presented. Furthermore, very unique, initial state-resolved ESA spectra, obtained for rare earths doped glasses, will be discussed and analyzed in detail as useful for prediction of new up-conversion excitation channels, which are essential for developing new materials for up-conversion lasers, up-converting phosphors, 3D displays, bio-labeling etc.

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Nanocharacterization with atomic force microscopy and spectroscopy

Arkadiusz Ptak

Institute of Physics, Poznan University of Technology
Nieszawska 13A, 60-965 Poznan, Poland

Nanostructured materials exhibit a rich variety of properties and promise exciting new advances in micromechanical, electronic, and magnetic devices. The complex structure-property relationships for these nanometer-sized materials can only be understood by employing modern microscopy and spectroscopy tools. One of the key microscopy techniques for nanocharacterization is atomic force microscopy (AFM). The term 'microscope' in the name is misleading because it implies looking, while in fact the information is gathered by "feeling" the surface with a mechanical probe.
The AFM consists of a microscale cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, solvation forces, etc. Piezoelectric elements that facilitate tiny but accurate movements on command enable the very precise scanning. The AFM can be operated in a number of modes, depending on the application. In general, possible imaging modes are divided into static (also called contact) modes and a variety of dynamic (or non-contact) modes.
Another major application of AFM, besides imaging, is force-spectroscopy, i.e. the measurement of force-distance curves. For this method, the AFM tip is extended towards and retracted from the surface as the static deflection of the cantilever is monitored as a function of piezoelectric displacement. These measurements have been used to measure nanoscale contacts, atomic bonding, Van der Waals forces, single molecule stretching and rupture forces, as well as molecular stiffness.
In my talk I will present advantages of AFM comparing to other techniques of imaging at nanometer scale as scanning electron microscopy and scanning tunneling microscopy, for instance. I will show also how to overcome the limits of classical (static) force spectroscopy by application of dynamic methods.

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Optical properties of novell epitaxial semiconductor nanostructures

Grzegorz Sek

Institute of Physics, Wroclaw University of Technology
Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland

The increasing requirements of micro- and optoelectronics followed by the intensive de-velopment of semiconductor nanostructure technology are the driving forces of the creation of new quasi-zero-dimensional objects of unusual structural or geometrical properties. There will presented here the results of spectroscopic studies (including single dot spectroscopy and enhanced sensitivity absorption-like measurements by means of modulation spec-troscopy) of epitaxially obtained nanostructures as for instance strongly asymmetric self-assembled quantum dots, like quantum dashes or quantum rods, and quantum dot quantum well tunnel structures made of III-V materials and designed for possible applications in photonics, light laser emitters in fibre-based telecommunication, solid state quantum electrodynamics experiments and quantum information processing. The information gained of the combined emission (photoluminescence, microphotoluminescence) and absorption spectra (photoreflectance, contactless electroreflectance, photoluminescence excitation) and supported by band structure calculations based on multiband model allows determining the following optical and electronic properties: (i) optical transitions and energy levels as a function of nanoobjects size, shape or contents; (ii) optically followed transition through the critical thickness for the 3D islands formation and hence the importance of the wetting layer thickness in self-assembled systems; (iii) linear polarization properties for the polarization independent light amplification; (iv) single dot emission including exciton complexes photoluminescence; (v) energy transfer and carrier losses mechanisms in quantum-dot-based laser structures.

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Optical signatures of the carrier segregation by the electric field in double quantum dots

Bartlomiej Szafran

Faculty of Physics & Applied Computer Science, AGH University of Science and Technology
al. Mickiewicza 30, 30-059 Krakow, Poland

Results of the effective mass modeling of the exciton and negative trion photoluminescence spectra for double vertically stacked quantum dots in external electric field are presented. The attention is focused on the interdot barrier thickness of 4 to 10 nm, when the tunnel coupling between the dots occurs only for the electron and not for the hole, which stays localized within a single dot. First the results for identical dots with no external electric field are presented. It is shown that the electron-hole interaction for weakly coupled dots leads to occurrence of optically inactive states in which the recombination is forbidden not by the selection rules but by the spatial separation of electrons from holes (indirect exciton states) [1]. The electric field applied to double quantum dots leads to the ground-state segregation of the carriers with electrons occupying one of the dots and the holes entering the other. Results of an exactly solvable numerical model describing the exciton dissociation by the external field are discussed [2]. The model predicted prior to the experiment that the exciton dissociation occurs through an avoided-crossing of optically active and inactive energy levels. The avoided crossings are due to mixing of the indirect and direct exciton states which occurs at the electron transfer from one dot to the other. Previous modeling in which the electron-hole interaction was neglected or treated in an approximate manner predicted cusps of the photoluminescence and not the avoided crossings, which were indeed observed experimentally [3]. Modeling of the negative trion dissociation [2] which precisely predicted the features found later [4] in a photoluminescence spectrum is also discussed. More recent results for non-perfectly aligned quantum dots, horizontal field orientation [5] and multiple stacks of dots [6] are also presented.

[1] B. Szafran, S. Bednarek, and J. Adamowski, Phys. Rev. B 64, 125301 (2001).
[2] B. Szafran, T. Chwiej, F.M. Peeters, S. Bednarek, J. Adamowski, and B. Partoens, Phys. Rev. B, 205316 (2005).
[3] H.J. Krenner et al., Phys. Rev. Lett. 94, 057402 (2005); E.A. Stinaff, et al., Science 311, 636 (2005).
[4] H.J. Krenner et al., Phys. Rev. Lett. 97, 076403 (2006).
[5] B. Szafran, F.M. Peeters, and S. Bednarek, Phys. Rev. B 75, 115303 (2007).
[6] B. Szafran, E. Barczyk, F.M. Peeters, and S. Bednarek, Phys. Rev. B, Phys. Rev. B 77, 115441 (2008).

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Recombination of free and impurity-bound 2D magneto-trions

Arkadiusz Wojs

Cavendish Laboratory, University of Cambridge
J J Thomson Avenue, CB3 0HE Cambridge, United Kingdom

Recent advances in numerical simulations of energy and recombination spectra of excitonic complexes in quasi-two-dimensional semiconductor structures in high magnetic fields will be reviewed. Discussed effects will include coupling of excitations in the plane of a quantum well and in the normal direction, response to electric field caused by asymmetric doping, binding to nearby ionized impurities, and interaction with surrounding electron or hole gas.

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