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Colloquia Fall 2012-2013

 
 

The Physics Colloquia are designed to address a non-specialist, broad audience and introduce topics of contemporary research through lectures by leading experts. We warmly invite all members of the student body, including undergraduates enrolled in any programme.

Each colloquium will be held in Electrical and Electronics Engineering building theater EE01 (Click here to see the map, click here for the campus map and building codes)

Place: EE01

Time:16:00-17:00 Wednesdays (Check below schedule for the dates)

Schedule

October 10, 2012

Event Poster

Invitee: Raşit Turan
Center for Solar Energy Research and Applications (GÜNAM) and Department of Physics, Middle East Technical University, Dumlupinar Blvd., No: 1, 06800, Ankara, Turkey

Host: Oğuz Gülseren

Plasmonics and Photonics for an efficient light trapping in photovoltaic devices

ABSTRACT — Absorption of light by a solar cell can be improved significantly by light trapping structures formed on the front or back surface of the device. In particular, thin crystalline and amorphous solar cells are expected to benefit from the improved light absorption in a region closer to the surface of the cell. Recently, we have shown that vertically aligned silicon (Si) nanowires formed on flat (100) Si wafer surface by metal assisted etching can effectively be used for this purpose. In addition, periodic photonic and plasmonic structures formed in the vicinity of the p-n junction can contribute to the light trapping. In this presentation, we summarize these new approaches for light managements towards more efficient energy harvesting from solar radiation. We also  present successful demonstration of nanowire based solar cells developed at GÜNAM facilities, and our recent results on the plasmonic structures based on Ag nanoparticles.

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October 17, 2012

Event Poster

Invitee: David Coker
Director, Complex Adaptive Systems Laboratory (CASL); Director, Atlantic Center for Atomistic Modeling (ACAM); SFI Stokes Professor of Nano BioPhysics, School of Physics, University College Dublin;Boston University, Massachusetts, USA 

Host: Balazs Hetényi

Energy transfer and charge separation dynamics in natural and artificial nano-structured light harvesting systems

ABSTRACT — What are the fundamental, molecular-level differences between the artificial photosynthetic systems, composed of composite and hybrid nano structured materials, that are currently being explored in laboratories around the world for applications in artificial photosynthesis, and the nano technologies that nature has evolved to solve the problems of interconnecting the molecular-scale light harvesting, excitation energy transport and transformation processes that underlie the highly successful Global Natural Photosynthetic systems?

This presentation will focus on the lessons that have been learned recently from ultrafast nonlinear spectroscopy studies and model theoretical calculations [1-4] that suggest that in natural biological systems quantum coherent dynamics, dissipation, and dephasing must be balanced in each of the interconnected nanoscale components to achieve optimal functioning. Understanding how these factors influence the performance of natural photosynthetic machinery may help bridge the nano technology gap and enable design of optimal artificial photosynthetic systems.

References:
[1] “Theoretical Study of Coherent Excitation Energy Transfer in Cryptophyte Phycocyanin 645 at Physiological Temperature”, by P. Huo and D.F. Coker, J. Phys. Chem. Letts. 2, 825-833 (2011).
[2] “Iterative linearized density matrix propagation for modeling coherent excitation energy transfer in photosynthetic light harvesting”, by P. Huo and D.F. Coker, J. Chem. Phys. 133, 184108 (2010).
[3] “Efficient energy transfer in light-harvesting systems, III: The influence of the eighth bacteriochlorophyll on dynamics and efficiency in FMO”, by J.M. Moix, J. Wu, P. Huo, D.F. Coker and J. Cao, J. Phys. Chem. Letts. 2, 3045-3052 (2011).
[4] “Partial linearized density matrix dynamics for dissipative, non-adiabatic quantum evolution”, by P. Huo and D.F. Coker, J. Chem. Phys. 135, 201101 (2011).

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October 31, 2012

Event Poster

Invitee: John Pendry
Imperial College

Host: Ekmel Özbay

The Science of Invisibility

ABSTRACT — Electromagnetism encompasses much of modern technology. Its influence rests on our ability to deploy materials that can control the component electric and magnetic fields. A new class of materials has created some extraordinary possibilities such as a negative refractive index, and lenses whose resolution is limited only by the precision with which we can manufacture them. Cloaks have been designed and built that hide objects within them, but remain completely invisible to external observers. The new materials, named metamaterials, have properties determined as much by their internal physical structure as by their chemical composition and the radical new properties to which they give access promise to transform our ability to control much of the electromagnetic spectrum.

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November 14, 2012

Event Poster

Invitee: Clemens Bechinger
Universität Stuttgart, 2. Physikalisches Institut, Stuttgart

Host: Giovanni Volpe

Observation of kinks and antikinks in colloidal monolayers driven across periodic and quasiperiodic surfaces

ABSTRACT — Friction between solids is responsible for many phenomena like earthquakes, wear or crack propagation. Unlike macroscopic objects which only touch locally due to their surface roughness, spatially extended contacts form between atomically flat surfaces. They are described by the Frenkel-Kontorova model which considers a monolayer of interacting particles on a periodic substrate potential. In addition to the well-known slip-stick motion such models also predict the formation of kinks and antikinks which largely reduce the friction between the monolayer and the substrate. Here, we report the direct observation of kinks and antikinks in a two-dimensional colloidal crystal which is driven across different types of ordered substrates. We show that the frictional properties only depend on the number and density of such excitations which propagate through the monolayer along the direction of the applied force. In addition, we also observe kinks on quasicrystalline surfaces which demonstrates that they are not limited to periodic substrates but also occur under more general conditions.

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November 21, 2012

Event Poster

CANCELLED Invitee: Romain Quidant
ICREA Professor Plasmon nano-optics group ICFO – The Institute of Photonic Sciences

Host: Giovanni Volpe

Plasmon nano-optics: When nano gets bright and hot

ABSTRACT — TBA

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November 28, 2012

Event Poster

Invitee: Andrea Ferrari
University of Cambridge, Engineering Department, Cambridge CB3 OFA, UK

Host: Ömer İlday

Graphene Photonics and Optoelectronics

ABSTRACT — The richness of optical and electronic properties of graphene attracts enormous interest. So far, the main focus has been on fundamental physics and electronic devices. However, it has also great potential in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, the absence of a bandgap can be beneficial, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability[1]. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers [1]. Despite being a single atom thick, graphene can be optically visualized [2]. Its transmittance can be expressed in terms of the fine structure constant [3]. The linear dispersion of the Dirac electrons enables broadband applications. Saturable absorption is observed as a consequence of Pauli blocking [4,5]. Chemical and physical treatments enable luminescence [1,6]. Graphene-polymer composites prepared using wet chemistry [4-6] can be integrated in a fiber laser cavity, to generate ultrafast pulses, and enable broadband tunability [4,5]. Graphene’s suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit/s [7]. However, the low responsivity of graphene-based photodetectors compared with traditional III-V-based ones is a potential drawback. By combining graphene with plasmonic nanostructures, the efficiency of graphene-based photodetectors can be increased by up to 20 times, because of efficient field concentration in the area of a p-n junction [8]. Additionally, wavelength and polarization selectivity can be achieved by employing nanostructures of different geometries [8]. Light-graphene interaction can be tailored by using microcavities [9]. Photodetection of far-infrared radiation (from hundreds of GHz to a few THz) is important for a variety of potential applications, ranging from medical diagnostics to process control, and homeland security. THz radiation penetrates numerous commonly used dielectric materials, otherwise opaque for visible and mid-IR light. At the same time, it allows spectroscopic identification of hazardous substances and compounds, through their characteristic molecular fingerprints. In this spectral region, due to the unavoidable doping, Pauli blocking does not allow detection exploiting the common photon-induced creation of charge carriers. Efficient THz detection in graphene can be achieved exploiting the oscillating fields in a graphene field effect transistor [10]. This enables high-sensitivity, room temperature, large-area operation, not limited to a specific region of the THz range [10].

References:
1. F. Bonaccorso et al. Nature Photon. 4, 611 (2010)
2. C. Casiraghi et al. Nano Lett. 7, 2711 (2007).
3. R. R. Nair et al. Science 320, 1308 (2008).
4. T. Hasan, et al. Adv. Mat. 21,3874 (2009)
5. Z. Sun et al. ACS Nano 4, 803 (2010); Nano Research 3, 653 (2010)
6. T. Gokus et al. ACS Nano 3, 3963 (2009)
7. T. Mueller et al. Nature Photon. 4, 297 (2010)
8. T.J. Echtermeyer et al. Nature Commun. 2, 458 (2011)
9. M. Engel et al. Nature Commun 3, 306 (2012)
10. L. Vicarelli et al. Nature Materials (2012)

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December 12, 2012

Event Poster

Invitee: Niek van Hulst
ICFO

Host: Coşkun Kocabaş

Antennas for Light

ABSTRACT — An optical nano-antenna is the high frequency analogue to the well-known radio-antennas is [1]. Such resonant plasmonic nanostructures allow the control of optical fields at the nanometer scale: super- focussing, local field enhancement, increased radiative rates and angular direction of light emission. The optical antennas have dimensions of typically 20-200 nm, while efficient interaction with active materials (molecules, quantum dots, ..) takes place in the near field, at distances 1 – 10 nm. Clearly nano-control in fabrication and operation is crucial.

First I will address resonant optical nanoantennas positioned at the end of a metal-coated tapered glass fibre near-field probe, thus acting as scanning probes [2]. Direct mapping of the antenna field with single fluorescent beads and molecules reveals a spatial localization of 25-50 nm, demonstrating the importance of such antennas for nanometer resolution optical microscopy [3]. The resonance shows that the antenna is indeed equivalent to its radio frequency dipole analogue.

Next I turn to surface antennas, which are more suitable for large scale fabrication. A quantum dot is placed on the antenna such that it drives the resonance exactly at a point of high mode density. The resulting quantum-dot luminescence is fully emitted in the antenna mode: strongly polarized and with a characteristic Hertz dipole pattern. The directionality emission of the quantum dot is steered by a Yagi-design [4] or even turned into a non-dipolar emission by through multipolar antenna modes.

Finally, antennas are ideal to bring ultrafast photonics to the nanoscale through their support of high-bandwidth excitation, i.e. tuneability [5]. Here, plasmonic antennas are engineered to realize two sought-after applications of ultrafast plasmonics: sub-wavelength phase shaping, and ultrafast hotspot switching. A hotspot switch at sub-100 fs time scale is shown applying only quadratic chirp to the excitation field. This simple, reproducible and scalable approach promises to transform ultrafast plasmonics into a straightforward tool for use in fields as diverse as room temperature quantum optics, nanoscale solid state physics or quantum biology [6].

References
[1]. Lukas Novotny and Niek F. van Hulst, Antennas for Light. Nature Photonics 5, 83-90 (2011),
[2]. Lars Neumann, Yuanjie Pang, Amel Houyou, Mathieu Juan, Reuven Gordon, Niek F. van Hulst, NanoLetters 11, 355-360 (2011)
[3]. T.H. Taminiau, F.D. Stefani, F.B. Segerink, N.F. van Hulst, Nature Photonics, 2, 234 (2008); NanoLetters 7, 28-33 (2007).
[4]. Alberto G. Curto, Giorgio Volpe, Tim H. Taminiau, Mark P. Kreuzer, Romain Quidant, Niek F. van Hulst, Science 329, 930-933 (2010).
[5]. Daan Brinks, Richard Hildner, Fernando D. Stefani and Niek F. van Hulst, Optics Express 19, 26486 (2011).
[6]. Daan Brinks, Fernando D. Stefani, Richard Hildner, Tim H. Taminiau, Niek F. van Hulst, Nature 465, 905-908 (2010).

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