Electromagnetic interaction between a laser beam and semiconductor nanowires deposited on different substrates: Raman en
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Electromagnetic interaction between a laser beam and semiconductor nanowires deposited on different substrates: Raman enhancement in Si Nanowires J. Anaya1, J.Jimenez1, A.Rodriguez2, T.Rodríguez2 1 Optronlab Group, Departamento de Física de la Materia Condensada, Centro I+D, Universidad de Valladolid, Paseo de Belén 1, 47011 Valladolid, Spain 2 Departamento de Tecnología Electrónica, E.T.S.I. de Telecomunicación, Universidad Politécnica de Madrid, 28040 Madrid, Spain ABSTRACT Raman scattering of Si nanowires (NWs) presents antenna effects. The electromagnetic resonance depends on the electromagnetic coupling of the system laser/NW/substrate. The antenna effect of the Raman signal was measured in individual NWs deposited on different substrates, and also free standing NWs in air. The one phonon Raman band in NWs can reach high intensities depending on the system configuration; values of Raman intensity per unit volume more than a few hundred times with respect to bulk substrate can be obtained. INTRODUCTION Semiconductor NWs are attracting a great deal of attention as the building blocks of the future nanodevices. Most of the useful properties of NWs arise from the dimension of its diameter, as compared to characteristic lengths, e.g. exciton Bohr radius, phonon mean free path, wavelength of the incident electromagnetic waves… One of their most interesting properties concerns its ability to enhance the optical absorption; because of the dielectric mismatch between the NW and its surrounding medium, it gives absorption resonances for certain NW diameters when interacting with an incident electromagnetic wave, e.g. illumination with an external light source [1,2]. Recently, antenna effects have been reported in the photocurrent response of Ge NWs [3]. This means that the electromagnetic field inside the NW can be enhanced for predetermined wavelengths by tuning the NWs diameter. This behaviour should permit the development of devices, such as efficient light sources, solar cells, and sensitive photodetectors among other. The NWs diameter dependent absorption resonances occur when the light wavelength is commensurate with the diameter size of the NW [3,4]. The study of this interaction is very important for selecting the NWs dimension for the desired application. The problem can be treated by numerical methods solving the Maxwell equations inside the NW. The measurement of the absorption of NWs is not an easy task; however, Raman spectroscopy can be an alternative method to experimentally study the optical resonances in semiconductor NWs, as well as the response of the NWs to the light polarization [4,5]. We present herein an analysis of the interaction of a laser beam with Si NWs using finite element methods for solving the Maxwell equations, considering different substrates supporting the NWs. Experimental microRaman measurements on individual Si NWs deposited on different substrates are used for illustrating the resonance effects. METHODS AND SAMPLES The Si NWs were grown by the vapor-liquid-solid (VLS) method, using
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