Semiconductor nanoclusters resonant nonlinearity

One subject that attracted much attention is the nonlinear optical properties of these semiconductor nanoclusters [17], The primary objective is to find materials with exceptional nonlinear optical response for possible applications such as optical switching and frequency conversion elements. When semiconductors such as GaAs are confined in two dimensions as ultrathin films (commonly referred to as multiple quantum well structures), their optical nonlinearities are enhanced and novel prototype devices can be built [18], The enhancement is attributed mostly to the presence of a sharp exciton absorption band at room temperature due to the quantum confinement effect. Naturally, this raises the expectation on three-dimensionally confined semiconductor nanoclusters. The nonlinearity of interest here is the resonant nonlinearity, which means that light is absorbed by the sample and the magnitude of the nonlinearity is determined by the excited state... 

Different parameters are required to characterize the resonant and the nonresonant optical nonlinearity. This has often been a source of confusion in the literature, even today. For nonresonant processes, the magnitude of the nonlinearity is measured by either x(3) or n2. However, for resonant processes, x(3)> a2> or ni alone cannot measure the magnitude of the nonlinearity. For example, a different x(3) value can result from the same material when lasers with different pulse widths are used for the measurement. A complete characterization of the nonlinearity requires a set of parameters, including % 3), the ground state absorption coefficient, the laser pulse width, and the excited state relaxation time. In a simple two- or three-level system, once all these factors are properly taken into account, the best parameter for measuring the resonant nonlinearity is simply the ground state absorption cross section of the material. In the following section I focus on the resonant nonlinearity only as this is closely related to the photophysical properties. The discussion of nonresonant nonlinearity of semiconductor nanoclusters can be found elsewhere [17, 84-86],... 

With the basic mechanism understood, the resonant nonlinearity of semiconductor clusters can now be quantitatively analyzed. Since one trapped electron-hole pair can bleach the exciton absorption of the whole cluster, the bleaching efficiency per absorbed photon of a nanocluster is the same as that of a molecule, as described by Eq. (20). For a given rp and r, the resonant third-order optical nonlinearity of a nanocluster is simply determined by the (a - ax) term. 

One major difference between a semiconductor nanocluster and molecule is the size dependence of a. As discussed in Section II.B and illustrated in Figure 5, there is a rapid rise in the absorption cross section of the first excited state as the cluster size is reduced below the exciton size. The resonant third-order nonlinearity of a nanocluster is therefore predicted to increase with decreasing cluster size. In reality it is limited by the cluster size dispersion and the presence of surface defects [15]. To maximize the resonant nonlinearity, one needs (1) a sharp exciton absorption band (which means smaller and monodisperse clusters), and (2) semiconductor clusters with larger absorption coefficients, such as GaAs or PbS. 

Another approach has been proposed to enhance the optical nonlinearity of semiconductor nanoclusters based on surface plasmon resonance [99,100], In the proposed method, the semiconductor nanocluster is coated with metals such as silver. The local electric field inside the cluster can be enhanced because of the surface plasmon resonance of the metal particles. The local field enhancement effect on nonresonant xl3) of CdS clusters has already been demonstrated using the third harmonic generation technique [17, 84, 85]. In this case enhancement in the local field originates from the difference in dielectric properties between the clusters and the host. The proposed enhancement of x 3) of metal-coated semiconductor nanoclusters owing to surface plasmon resonance has not been demonstrated experimentally. 

---