Optical nonlinearity enhancement of graded metal-dielectric composite films

– We have derived the local electric field inside graded metal-dielectric composite films with weak nonlinearity analytically, which further yields the effective linear dielectric constant and third-order nonlinear susceptibility of the graded structures. As a result, the composition-dependent gradation can produce a broad resonant plasmon band in the optical region, resulting in a large enhancement of the optical nonlinearity and hence a large figure of merit. In contrast to bulk materials, the corresponding thin films often possess different optical properties [1,2]. It is also known that graded materials [3,4] have quite different physical properties from the homogeneous materials [5]. To one’s interest, one found that the graded thin films may have better dielectric properties than a single-layer film [6]. However, the traditional theories [7, 8] fail to deal with the composites of graded particles. For this purpose, we put forth a first-principles approach [9, 10] and a differential effective dipole approximation [11]. The situation becomes more complicated by the presence of nonlinearity which exists in real composites [12–22]. Moreover, there is a great need for nonlinear optical materials with large nonlinear susceptibility or optimal figure of merit (FOM). Therefore, much work has been done on how to gain a large nonlinearity enhancement or optimal FOM of bulk composites by the surface-plasmon resonance in metal-dielectric composites [19], as well as by taking into account the structural information [23, 24]. Recently, a large nonlinearity enhancement was found for a sub-wavelength multilayer of titanium dioxide and conjugated polymer [25]. However, the surface plasmon resonant nonlinearity enhancement often occurs concomitantly with a strong absorption, and unfortunately this behavior renders the FOM of the resonant enhancement peak too small to be useful. To circumvent this problem, we shall consider a kind of graded metal-dielectric composite film, in which a dielectric component is introduced as spherical particles embedded in the metallic component.