04 December 2012 Congratulations to New ICFO PhD Graduate

Dr. Sukanya Randhawa


Thesis Committee

Dr. Sukanya Randhawa has successfully defended her thesis on active plasmonics. Dr. Sukanya Randhawa joined the Plasmon nano-optics group at ICFO led by ICREA Prof. Romain Quidant after completing her Masters in Electrical and Computer Engineering at Georgia Institute of Technology, USA. Her work at ICFO has centred on the design, fabrication and characterisation of passive and active plasmonic components.

Dr. Randhawa has defended her thesis entitled ‘Active control of surface plasmons in hybrid nanostructures’. Her thesis advisor was ICFO Group Leader Prof. Romain Quidant and her co-advisor was Dr. Jan Renger.

Abstract:

Plasmonics nanostructures are becoming remarkably important as tools towards manipulating photons at the nanoscale. They are poised to revolutionize a wide range of applications ranging from integrated optical circuits, photovoltaics, and biosensing. They enable miniaturization of optical components beyond the ``diffraction limit\'\' as they convert optical radiation into highly confined electromagnetic near-fields in the vicinity of subwavelength metallic structures due to excitation of surface plasmons (SPs). These strong electromagnetic fields generated at the plasmonic ``hot spots\'\' raise exciting prospects in terms of driving nonlinear effects in active media. The area of active plasmonics aims at the modulation of SPs supported at the interface of a metal and a nonlinear material by an external control signal. The nonlinear material changes its refractive index under an applied control signal, thereby resulting in an overall altered plasmonic response. Such hybrid nanostructures also allow for the creation of new kinds of hybrid states. This not only provides tools for designing active plasmonic devices, but is also a means of re-examining existing conventional rules of light-matter interactions. Therefore, the need for studying such hybrid plasmonic nanostructures both theoretically and experimentally cannot be understated. The present work seeks to advance and study the control of SPs excited in hybrid systems combining active materials and nanometallics, by an external optical signal or an applied voltage. Different types of plasmonic geometries have been explored via modelling tools such as frequency domain methods, and further investigated experimentally using both near-field and far field techniques such as scanning near field optical microscopy and leakage radiation microscopy respectively. First, passive SP elements were studied, such as the dielectric plasmonic mirrors that demonstrate the ability of gratings made of dielectric ridges placed on top of flat metal layers to open gaps in the dispersion relation of surface plasmon polaritons (SPPs). The results show very good reflecting properties of these mirrors for a propagating SPP whose wavelength is inside the gap. Another passive configuration employed was a plasmonic resonator consisting of dielectric-loaded surface plasmon polariton waveguide ring resonator (WRR). Also, a more robust variant has been proposed by replacing the ring in the WRR with a disk (WDR). The performance in terms of wavelength selectivity and efficiency of the WDRs was evaluated and was shown to be in good agreement with numerical results. Control of SPP signal was demonstrated in the WRR configuration both electro-optically and all-optically. In the case of electro-optical control, the dielectric host matrix was doped with an electro-optical material and combined with an appropriate set of planar electrodes. A 16% relative change of transmission upon application of a controlled electric field was measured. For all-optical control, nonlinearity based on trans-cis isomerization in a polymer material is utilized. More than a 3-fold change between high and low transmission states of the device at milliwatt control powers (100 W/cm^2 intensity) was observed. Beyond the active control of propagating surface plasmons, further advancement can be achieved by means of nanoscale plasmonic structures supporting localized surface plasmons (LSP). Interactions of molecular excitations in a pi-conjugated polymer with plasmonic polarizations are investigated in hybrid plasmonic cavities. Insights into the fundamentals of enhanced light-matter interactions in hybrid subwavelength structures with extreme light concentration are drawn, using ultrafast pump-probe spectroscopy. This thesis also gives an overview of the challenges and opportunities that hybrid plasmonic functionalities provide in the field of plasmon nano optics.

THESIS COMMITTEE:
President: Prof. Alain Dereux, Director, Laboratoire Interdisciplinaire Carnot de Bourgogne, FRANCE
Secretary: Prof. Valerio Pruneri, ICREA Professor, Optoelectronics Group, ICFO-Institute of Photonic Sciences, SPAIN.
Vocal: Prof. Anatoly Zayats, Kings’s College London, UK

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