ISSN 0021-3640, JETP Letters, 2012, Vol. 96, No. 12, pp. 759–764. © Pleiades Publishing, Inc., 2012. Original Russian Text © N.S. Lapshina, R.E. Noskov, Yu.S. Kivshar, 2012, published in Pis’ma v Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2012, Vol. 96, No. 12, pp. 849– 854. 759 1. INTRODUCTION In the current usage, nanoantennas refer to devices for converting the angular and frequency characteris- tics of optical radiation at subwavelength scales. Stud- ies on nanoantennas represent a rapidly progressing area of modern optics in view of a large number of potential applications, which include thin-film solar cells, chemical and biological sensors, and ultrafast optical information processing devices [1–3]. A factor important for most of these applications is the possi- bility of active control over the radiation (scattering) pattern of a nanoantenna. However, the operating principle of most nanoantennas relies on the use of a single mode, the one enabling the best directivity and (or) Purcell factor [4, 5]. Nevertheless, methods have recently been proposed for steering the radiation (scattering) pattern via switching between different modal regimes upon the variation of the frequency in nanoantennas with broken geometrical or material symmetry [6, 7] or variation of the field intensity in nanoantennas with nonlinear components [8–14]. Here, we suggest a new concept of active dynamic control over the scattering pattern of the nanoantenna by using modulational instability. We demonstrate that the onset of this instability in a nanodimer consisting of two spherical silver nanoparticles may result in dynamic energy exchange between the eigenmodes of the system, which is accompanied by periodic rotation of the scattering pattern similar to that in classical phased-array antennas [15]. 2. THEORETICAL MODEL OF A NONLINEAR PLASMONIC NANOANTENNA We consider a nanoantenna consisting of two iden- tical spherical nanoparticles placed close to each other in a silica host with the relative permittivity ε h = 2.15. Let the particle radius a and the spacing d between the centers of the spheres be 10 and 30 nm, respectively. Under the condition a/d ≤ 1/3, the response of the particles can be described in the dipole approximation [16]. Assuming the nanoparticles made of silver with a Kerr-type nonlinear response, we take the dielectric constant in the form = + χ (3) . Here, the linear term is described by the Drude formula = ε ∞ – /[ω(ω – iν)] with ε ∞ = 4.96, ω p = 9.54 eV, and ν = 0.055 eV [17]; χ (3) is the cubic susceptibility; and are the local fields within the particles. Generally, the cubic optical susceptibility of metal- lic nanoparticles depends on the type of metal, particle size, frequency and duration of the incident laser radi- ation, and some other factors [18]. In particular, the analytical quantum model developed in [19, 20] and later confirmed by numerical simulations [21] indi- cates that, for 10-nm-radius silver particles driven at a frequency close to the frequency of the surface plas- mon resonance, the cubic susceptibility is purely real and equals χ (3)  3 × 10 –9 esu. This value is much larger than the cubic susceptibility of silica (~10 15 esu [22]), and the latter is disregarded in the following. In this paper, we analyze the dynamic mutation of the scattering pattern of such a nonlinear dimer nanoantenna when the response of this structure exhibits modulation instability. 3. BASIC EQUATIONS We study a nanoparticle dimer driven by a plane wave with the frequency close to the frequency of the surface plasmon resonance of an individual particle at ε Ag NL ε Ag L E 12 , in ( ) 2 ε Ag L ω p 2 E 12 , in ( ) Nonlinear Nanoantenna with Self-Tunable Scattering Pattern N. S. Lapshina a , R. E. Noskov a , and Yu. S. Kivshar a, b a National Research University of Information Technologies, Mechanics, and Optics, St. Petersburg, 197101 Russia e-mail: n.lapshina@phoi.ifmo.ru b Nonlinear Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, Australia Received October 9, 2012 A new concept of a nonlinear plasmonic nanoantenna with self-tunable scattering pattern (indicatrix) is sug- gested. It is shown that the onset of modulational instability in a nanoantenna consisting of two identical non- linear metallic nanoparticles results in dynamic energy exchange between the eigenmodes of the nanodimer, accompanied by periodic rotation and switching of the scattering pattern. Such a nanoantenna features a wide scanning sector, a relatively low working threshold, and a short response time. This makes it attractive for applications in nanophotonics and biology. DOI: 10.1134/S0021364012240071