A concept to electrically control the scattering of light is introduced. The idea is to embed noble metal nanoparticles in an electro-optical material such as a liquid crystal in order to induce a spectral shift of the particle plasmon resonance by applying an electric field. Light scattering experiments on single gold nanoparticles show that spherically shaped nanoparticles become optically spheroidal when covered by an anisotropic liquid crystal. The two particle plasmon resonances of the optically spheroidal gold nanoparticles can be spectrally shifted by up to 50 meV when electric fields of more than 10 kV/cm are applied.

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Electrically controlled switching of light is an important ingredient in modern display technology and optical communication systems. Optoelectronic devices usually modulate the properties of transmitted, reflected or diffracted light beams. Optoelectronic light scattering devices are rare and mainly used for smart switchable glazing. Until now such optoelectronic scattering devices do not take advantage of noble metal nanoparticles. Metal nanoparticles are known to act as strong light scatterers, when the frequency of the light and the particle plasmon resonance are in the same range. The plasmon resonance frequency can easily be tuned over the whole visible range by varying material, size and shape of the nanoparticle.We report on electrically controlled light scattering where electric fields are used to spectrally shift the plasmon resonance of spherical noble metal nanoparticles. The idea is based on the fact that the resonance energy not only depends on intrinsic particle properties, but also very strongly on the refractive index of the surrounding medium. Embedding spherical nanoparticles in an anisotropic dielectric matrix therefore results in optically anisotropic particles. The orientation of this anisotropy can be varied with an electric field. -a 0 ) c -L c d m P 0 . 0 -1.8 2.0 2.2 2.4 Photon Energy [aq F d P % Iy C w 5 d c 0 cn 0 20 40 60 80 100 Electric Field [kV/cm] Fig.1: a) Experimental light scattering spectra of an individual gold nanoparticle embedded in a liquid crystal cell for two different polarization orientations. b) Dependence of the plasmon energy of a nanoparticle on the electric field in a liquid crystal cell. Inset: Principle of the experiment (with electric field applied).A darkfield microscope setup is used to record the scattering spectra of individual gold nanoparticles. Changing the surrounding medium from air to liquid crystal results in a red shift of the nanoparticle plasmon as compared to air due to an increased refractive index. Due to the optical anisotropy of the embedding matrix the spectral position of the nanoparticle plasmon peak now depends on polarization (Fig la). The peak positions of the scattering spectra differ by 50 meV for two orthogonal polarizations. The spherically shaped particle now behaves optically like an ellipsoid.Applying external electric fields to the liquid crystal induces a reorientation of the molecules and therefore a reorientation of the anisotropy. This results in a field-induced shift of the plasmon peak (Fig Ib), which maybe useful for the realization of novel optoelectronic switching devices.