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SEMINARIOS

Enero 2010

Miércoles 20 de Enero a las 12.00 "Semiconductor nanostructures for quantum optics and quantum information" One of the most important building blocks in solid state physics that have contributed to the magnificent revolution of quantum optics and quantum information are semiconductor nanostructures, in particular quantum dots based on III-Vs semiconductors, such as InAs quantum dots on GaAs matrixes. The ground state of a single quantum dot behaves like an almost ideal two level system. As a light source, single quantum dots offer many of the advantages of single atoms and for this reason, the are usually refered to as “artificial atoms”. They posses high optical transition oscillator strengths and narrow, discrete emission lines; but are fixed in position which makes them easy to work with and potentially useful for commercial single photon devices (emitters/detectors). Also, the high refractive index contrast between semiconductors and air, permits the fabrication of optical cavities and resonators with embedded QDs as active emitters and detectors, this makes semiconductor nanostructures ideal for the study and exploitation of cavity quantum electrodynamics. In this seminar, I will present an introduction to quantum optics and quantum information based on semiconductor quantum dots. Semiconductor quantum dots are exceptionally good for the generation of single photons on demand, both when excited optically or electrically. The generation of single photons in a turnstile device is of great importance for quantum optics and quantum information applications, such as the generation and distribution of quantum keys for information encoding. Also, QDs are a very good source of polarization entangled photons on demand. I will continue presenting some studies of cavity quantum electrodynamics employing quantum dots and photonic crystal slab nanocavities, such as the interactions between excitons and photons in the weak and strong coupling regime. I will finish this seminar presenting the latest results towards the practical realization of an in-plane single photon gun based on QDs and photonic crystal waveguides. Dr. Daniel Granados. Quantum Information Group (QIG), Toshiba Research Europ Ltd. Lugar: IMM

Lunes 25 de Enero a las 12.00 "Active plasmonics at a nano-scale" The use of electromagnetic surface waves to tailor and probe light-matter interaction at a nano-scale opens new horizons in modern nano-optics. In this talk I will discuss three recent developments in active nanoplasmonics. Undestanding exciton-plasmon-photon conversion pathways of optically excited nano-scale light emitters (CdSe quantum dots) coupled to low-Q plasmonic nanocavities (silver nanowires) enables device engineering and motivates fundamental quantum-optical applications [1,2]. Plasmonic interferometry with tilted slit-groove plasmonic microinterferometers imprinted in noble metal films represents a novel spectroscopic technique with high temporal and spatial resolution. Experiments with composite magneto-plasmonic metal/ferromagnet/metal multilayer films demonstrate the possibility of magneto-optical manipulation of surface plasmon wave vector and enable probing the electromagnetic field distribution inside a metal with nanometer depth resolution at optical frequencies [3]. Combining these experiments with ultrafast optical pump-probe techniques opens a new way to monitor transient laser-induced phenomena on solid surfaces and stimulates the development of novel ultrafast active plasmonic devices [4,5]. 1) Y. Fedutik et al., Phys. Rev. Lett. 99, 136802 (2007) 2) A. Akimov et al., Nature 450, 402 (2007) 3) V.V. Temnov et al., Nature Photonics 4 (2010) 4) K. MacDonald et al., Nature Photonics 3, 55 (2008) 5) V.V. Temnov et al., Opt. Express 17, 8423 (2009) Vasily V. Temnov. Massachusetts Institute of Technology. Lugar: IMM

Miércoles 27 de Enero a las 12.00 "Molecular Biophysics of DNA repair Nanomachines" Unrepaired DNA breaks can lead to genomic instability or cell death. They occur frequently during normal cellular metabolism and are caused, for example, by the collapse or stalling of the replication fork in response to DNA damage. Proper DNA-end processing and handling are essential for the survival of the cell and prevention of carcinogenesis. In Bacillus subtilis[1], broken DNA ends are first processed to a 3¢-ssDNA overhang terminated at a recombination hotspot (Chi) sequence. This reaction is catalysed by the AddAB helicase-nuclease; a complex molecular motor protein that unwinds the DNA duplex and degrades the nascent single-strands in a Chi-regulated manner. Single-molecule manipulation and imaging techniques offer huge potential to investigate DNA break repair reactions in new ways, providing information that is inaccessible to conventional ensemble experiments. We have used Atomic Force Microscopy (AFM) and Magnetic Tweezers (MT) to characterize the AddAB helicase and nuclease activities at the single molecule level. AFM showed that AddAB binds at DNA ends and process duplex DNA into ssDNA molecules. However, AFM images consistently showed a remaining population of apparently unprocessed dsDNA molecules. The fraction of unprocessed molecules dropped upon addition of increasing concentrations of SSB protein or larger amounts of AddAB protein. Our results are consistent with a model in which the DNA strands reanneal behind the translocating AddAB enzyme. This effect is suppressed by destabilizing the interaction between DNA strands via binding of SSB or multiple AddAB motors, or by the interaction between AddAB and Chi during translocation. Using MT, we measured the DNA translocation rate for the motor as ~500 bp/s at a force of 4 pN. During translocation we detected frequent pauses, jumps and sliding: these phenomena are currently the subject of further investigation. F. Moreno-Herrero. Department of Macromolecular Structure, Spanish National Center of Biotechnology, CSIC. Cantoblanco. Lugar: IMM

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