KOÇ UNIVERSITY PHYSICS SEMINAR
Speaker : Alpan Bek, METU
Title : Nonlinear and Linear Optical Phenomena by Molecular-Plasmonic Hybrid Nanostructures
Date : November 14, 2017 Tuesday
Time : 2:30 P.M.
Cookie & Tea : SCI 103 2:15 P.M.
Place : SCI 103
web : https://physics-seminars.ku.
Finding a possibility to control the activation of a single molecule is interesting since it paves the way to realize logic devices as small as molecules. This can be achieved by carefully designed devices which can interface a single molecule and interact with it at the same size scale. Such devices can be of electrical, magnetic or optical origin  and may induce changes in the properties of a single or few molecules which can act as switching registers for data processing or data storage at the nanometer scale. Electrical devices at the single molecule size level include break-junctions, scanning tunneling junctions and molecular landers. Optical devices that can interface a single molecule among an ensemble of many molecules are yet lacking. The main difficulty lies in the diffraction limit of practically ~150-180 nm focal size for a wavelength of 600 nm light. Compared to a few nm sizes of individual molecules, unambiguous activation of single molecules by means of absorption of light can only be possible for a device prepared from a sparse ensemble of molecules. Thanks to advancement in near-field optical instrumentation technology this main difficulty can be circumvented as the near-field light intensities typically have a very strong nonlinear dependence on distance (from ~r3 to ~r6). Strongly confined, highly intense fields of light called “hot spots,” can be achieved by engineering the nanoscale environment of a single molecule. By utilizing plasmonic resonances of nanoscale metal structures, such spots can be arranged to occupy from a few down to a single molecule. Nevertheless, in these schemes, a broad far-field background component of the activation wavelength is inevitably superposed on to the near-field of the nanoparticles. This imposes limitations on single molecule activation experiments to be performed with high fidelity.
In this study we propose a method which is feasible for deterministic activation of few molecules. We demonstrate effective background-free continuous wave nonlinear optical excitation of enhanced yellow fluorescent protein molecules that are sandwiched between asymmetrically constructed plasmonic gold nanoparticle clusters . We observe that infrared photons are converted to visible photons through efficient plasmonic second harmonic generation. Our theoretical model and simulations demonstrate that nonlinear conversion of continuous wave light becomes possible by Fano resonance in the nonlinear response. We show that nonlinearity enhancement of plasmonic nanostructures via coupled quantum mechanical oscillators such as molecules can be several orders larger as compared to their classical counterparts [3-4]. Our numerical simulations demonstrate that observation of second harmonic generation with continuous wave laser becomes possible owing to the cooperative action of conversion enhancement through Fano resonance, hybridization in the plasmon absorption spectrum and the size asymmetry of nanoparticle dimers.
We further investigate the Fano resonances in intrinsic nonlinear optical response of single or coupled metal nanoparticles. We show that Fano resonance conditions can be tuned to enhance nonlinear optical conversion. When a bare metal nanoparticle is coupled with a plasmonic nonlinear converter, second harmonic generation can be enhanced by several orders of magnitude. This phenomenon emerges due to path interference effects. It is shown that second harmonic generation enhancement in hybrid structures can be obtained even in the absence of coupled quantum emitters . This is an important simplification for facilitating the use of purely metal nanoparticles with appropriate functionality.
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- Salakhutdinov I., Kendziora D., Abak M. K., Turkpence D., Piantanida L., Fruk L., Tasgin M. E., Lazzarino M. & Bek A., Photonics and Nanostructures – Fundamentals and Applications 21, 32–43 (2016).
- Türkpence D., Akguc G. B., Bek A. & Tasgin M. E., Journal of Optics, 16, 105009 (2014).
- Tasgin M. E., Preprint at http://arxiv.org/abs/1404.3901 (2014).
- Yildiz B. C., Tasgin M. E., Abak M. K., Coskun S., Unalan H. E. & Bek A., Journal of Optics 17, 125005 (2015).
Bio: Alpan Bek has completed his BSc and MSc studies at Bilkent University Department of Phyics. He has taken part in experimental research on optical properties of low dimensional systems and integrated optics under supervision of Prof. Atilla Aydınlı. He has obtained his PhD degree from Ecole Polytechnique Federale de Lausanne (EPFL) while he conducted his PhD work at the Max-Planck Institute for Solid State Research in Stuttgart, Germany under supervision of Prof. Klaus Kern. He has constructed an optical microscope that can resolve structures down to 5 nm with visible light. He has had a post-doctoral stay at Ludwig Maximillian’s University of Munich (LMU) in the group of Prof. Jochen Feldmann and conducted research together with Prof. Thomas Klar on plasmonic systems. Afterwards he has moved to Cluster in Biomedicine (CBM) in Trieste, Italy to work together with Dr. Marco Lazzarino on tip enhanced Raman spectroscopy (TERS). In 2011 he has returned back to Turkey as a faculty member in Physics Department of Middle East Technical University. He is currently conducting research in the fields of plasmonic solar cells and nonlinear plasmonics as the leader of the Nano-Optics Research Group and as a member of The Center for Solar Energy Research and Applications (GÜNAM).