Degradation in aluminum resonant optical rod antennas
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Degradation in aluminum resonant optical rod antennas Patrick M. Schwab1,2, Carola Moosmann1, Katja Dopf1, Konstantin S. Ilin3, Michael Siegel3, Uli Lemmer1,2, and Hans-Juergen Eisler1 1 Light Technology Institute (LTI), Karlsruhe Institute of Technology, Karlsruhe, Germany 2 Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology, Karlsruhe, Germany 3 Institute for Micro- and Nanoelectronic Systems (IMS), Karlsruhe Institute of Technology, Karlsruhe, Germany ABSTRACT Resonant optical rod antennas are made from aluminum using electron-beam lithography and are optically characterized by linear dark-field microscopy and nonlinear multi-photon luminescence spectroscopy. It is demonstrated that by exciting close to the interband transition of aluminum at about 1.5 eV different radiative decay channels can be addressed. Over a period of weeks, a slight spectral red-shift and a decrease in the scattering intensity are observed due to the formation of a native oxide layer at the metal-air interface. To investigate the concurrent influence of shape transformation and dielectric environment on the spectral response function we carry out numerical calculations using finite difference time domain (FDTD) methods. It is found that the induced energy shift is mainly determined by the change of the dielectric constant in the nanovicinity resulting in an overall red-shift as seen in the experiment. These findings allow for a better understanding of designing and modeling plasmonic aluminum nanostructures for e.g. UV sensing where the shift in peak resonance and linewidth are key observables. INTRODUCTION Nanoplasmonic sensing [1] is one of the main research areas of plasmonics today. It is based on the detection of the particle plasmon resonance and its shift due to a change of the dielectric function of the nearby environment [2]. A large variety of structures has been synthesized and characterized whose plasmon resonances may be varied over the entire visible to mid-infrared frequency regime [3-6]. Due to its inert character and biocompatibility the noble metal gold (Au) is the most investigated nanoplasmonic material so far. For Au, as well as for silver (Ag) the optical response is inherently limited to wavelengths larger than 550 nm and 400 nm, respectively [7]. Besides the coinage metals, aluminum (Al) is another interesting candidate for nanoplasmonics. It is not only a relatively cheap and abundant material which offers the possibility of integration into existing CMOS technology [8] but it is also very interesting from a fundamental point of view. The Al dielectric function is significantly different than that of Au showing a strong but very narrow interband transition at about 1.5 eV [9]. In addition, the electron density of Al is three times larger than in Ag or Au resulting in an increased plasmon frequency ωp. Therefore, Al extends the plasmonic resonance bandwidth into the (deep)-UV regime that could not be reached by other plasmon metal candidates – as e.g. the real part of the dielectric func
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