Active control of the optical properties of nanoscale coatings using 'smart' nanoparticles

Coatings that can self-modulate their optical properties as a function of an external stimulus are of significant technological interest. In this regard, the possibilities for thermoor electrochromic materials such as V02 and W03 are already comparatively well-known. Here, however, we explore a new kind of 'smart' coating, based on the active control of a plasmon resonance in nanoparticles. One possible system is based on the modulation of the plasmon resonance of a precious metal nanorod or nanosphere by an active dielectric shell. The active dielectric undergoes an insulator-to-metal transition on increase of temperature which modulates the plasmon resonance of the underlying precious metal nanoparticle, thereby changing the wavelength at which its optical extinction is maximum. In the case of nanorods, the absorption maximum of the longitudinal plasmon is particularly sensitive to the aspect ratio of the nanoparticle and the dielectric properties of the environment, and may be readily tuned across the visible and near-infrared portions of the spectrum. In addition, nanoparticles of certain thermochromic dielectrics, such as V02, are expected to have a plasmon resonance of their own which can be switched on or off by control of the temperature. We consider some of the possibilities, using both the discrete dipole approximation and the exact analytical solution due to Mie to calculate the optical properties.