Magneto-controlled nonlinear optical materials

We exploit theoretically a magneto-controlled nonlinear optical material which contains ferromagnetic nanoparticles with a non-magnetic metallic nonlinear shell in a host fluid. Such an optical material can have anisotropic linear and nonlinear optical properties and a giant enhancement of nonlinearity, as well as an attractive figure of merit. Electronic address: [email protected] 1 Finding nonlinear optical materials with large nonlinear susceptibilities and fast responses is up to now a challenge [1–4]. Many applications of nonlinear optics that have been demonstrated under controlled laboratory conditions could become practical for technological uses if such materials were available. The most common way to obtain a nonlinear optical material is to search for materials in which the components possess an inherently large nonlinear response [1]. In contrast, in this Letter we shall exploit theoretically a nonlinear optical material whose nonlinear optical properties and nonlinearity enhancement can be tuned by applying an external magnetic field thus called magneto-controlled nonlinear optical materials. Devices that could benefit from these materials include optical switches, optical limiters, etc. Ferromagnetic nanoparticles, typically consisting of magnetite or cobalt, have a typical diameter of 10 nm, and carry a permanent magnetic moment (e. g., of the strength ∼ 2.4 × 10 μB for magnetite nanoparticles, where μB denotes the Bohr magneton) [5]. As the ferromagnetic nanoparticles are suspended in a host fluid like water, they can easily form particle chains under the application of external magnetic fields [5], thus yielding a magnetic-field-induced anisotropical structure. Recently, a non-magnetic golden shell was used to enhance the stability of the ferromagnetic nanoparticle against air and moisture [6]. In this work, we shall show that the effective nonlinear optical response of the suspension which contains ferromagnetic nanoparticles with metallic nonlinear shells can be enhanced significantly due to the effect of the magnetic-field-induced anisotropy. For the research on nonlinear optical responses, the introduction of a controllable element (e.g., external magnetic field) should be expected to open a fascinating field of new phenomena. The third-order nonlinear susceptibility χs of metallic (say typically, noble metals like gold and silver) shells is very large when compared to that of the magnetite or cobalt core and the host fluid like water. Let us start by considering ferromagnetic linear nanoparticles of linear dielectric constant ǫ 1 coated with a non-magnetic metallic nonlinear shell of ǫ ′ 1 and χs which are suspended in a linear host fluid of ǫ2. That is, in the shells, there is a nonlinear relation between the displacement Ds and the electric field Es, Ds = ǫ ′ 1Es+χs|Es| Es, where