Light dynamics in glass-vanadium dioxide nanocomposite waveguides with thermal nonlinearity

Abstract

We address the propagation of laser beams in SiO 2 -VO 2 nanocomposite waveguides with thermo-optical nonlinearity. We show that the large modifications of the absorption coefficient as well as notable changes of the refractive index of VO 2 nanoparticles embedded into the SiO 2 host media that accompany the semiconductor-to-metal phase transition may lead to optical limiting in the near-IR wave range. © 2009 Optical Society of America OCIS codes: 190.0190, 190.4400.The importance of thermal self-action of laser beams was realized almost 40 years ago [1,2]. In some materials, such as VO 2 , heating results in abrupt changes of absorption and refraction [3,4] owing to semiconductor-to-metal phase transition at T pt Ϸ 67°C. In VO 2 thin films such transitions can be driven optically at subpicosecond time scales [5,6]. The optical properties of VO 2 nanocomposites depend on sizes of the embedded particles [7][8][9], allowing various switching applications [7]. Such composites and films possess ultrafast nonlinearities [10] and enhanced absorption [11,12]. Phase transition is affected by the film morphology [13]. It can be used to achieve switchable reflectivity of nanocomposite layers [14]. However, for quasi-cw illumination the properties of VO 2 films [15] and nanoporous glass-VO 2 composites [16][17][18] are altered mostly by slow phase transition owing to light heating: a mechanism that is completely different from the ultrafast optically induced phase transition. Moreover, in thick nanocomposites with low concentration of doping nanoparticles ͑ ϳ 10 −2 % ͒ light propagation is affected not only by thermally induced changes of absorption but also by the corresponding refractive index variations [16,17]. Thus, exploration of laser beam dynamics in the waveguiding geometries is rather important. In this Letter we study light dynamics in SiO 2 -VO 2 nanocomposite waveguides for thermally induced semiconductor-to-metal phase transition. Optical limiting in this setting can be controlled by the initial system temperature and by the intensity of the input light beam.Light propagation in a planar waveguide formed by SiO 2 glass cladding and a glass core with embedded spherical VO 2 nanoparticles is described by the nonlinear Schrödinger equation for the dimensionless field amplitude q͑ , ͒: ͑1͒where is the propagation distance normalized to L dif = kx 0 2 , k =2n rh / , n rh is the real part of the refractive index of host material, and is the wavelength.The transverse coordinate is expressed in units of core width x 0 =10 m, while the function R͑͒ = exp͑− 8 ͒ describes the concentration profile of VO 2 nanoparticles. The parameter p r =2L dif ͑n rs − n rh ͒ / is the normalized difference of wavenumbers in the core (semiconducting phase is assumed) and the glass cladding; r = ͑n rm − n rs ͒ / ͑n rs − n rh ͒ is the normalized difference of refractive indices in metallic n rm and semiconducting n rs phases; and p i =2L dif ͑n is − n ih ͒ / characterizes the difference of imaginary parts of refractive indices, while i ...

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