## 1 Introduction  
Nonlinear optics in metal nanoparticles within transparent dielectric media has been studied extensively [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. In particular, third order optics using ultrashort (femtosecond) laser pulses in such metal nanoparticle systems have been widely studied and reported. The metallic nanoparticles of smaller diameters (\(<\)-10 nm) are also called quantum dots since many of their properties can be understood considering the quantum confinement of the electrons within nanometer dimensions. The third-order susceptibilities (\(\chi^{(3)}\)) and response times of various metallic nanoparticles in dielectric media have been measured for different diameters of nanoparticles. Saturation of absorption and two-photon absorption in specific nanometallic systems have also been studied [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. Very recently, we have reported quadratic electro-optic effect in metallic nanoparticles within dielectric media [6]. As our literature review has revealed no measurement of electroabsorption in metal nanoparticles has been published or known in the prior art. Using high intensity short pulses second order optical effect has also been shown for specific nanoparticles utilizing the asymmetry involved in shapes/interfaces [14]. Second order optics is not relevant to electroabsorption presented here. In our recent article, we have compared quadratic electro-optic effect in metal (gold) nanoparticles in glass with that of subnanometer-size metallic domains within doped nonconjugated conductive polymers [5]. Quadratic electro-optic effect and electroabsorption have applications in ultrafast electro-optic modulation/switching and other applications in lasers and optoelectronics. The focus of the present report is on measurement of electroabsorption in gold nanoparticles of various sizes to further elucidate the mechanisms of nonlinear optics in these important systems. The materials also include iodine doped nonconjugated conductive polymers, such as polyisoprene, which have the smallest known metallic particles in the subnanometer domain. While electroabsorption in iodine-doped nonconjugated conductive polymers was mentioned in previous reports [15; 16; 17; 18; 19; 20; 21], here we discuss more details and compare with gold nanoparticles in a dielectric medium. The quadratic electro-optic effect is given by: \(\Delta\)n = K\(\lambda\)E\({}^{2}\), where \(\Delta\)n is the change in refractive index (induced birefringence), K is defined as the real part of Kerr coefficient, \(\lambda\) is the wavelength, and E stands for the applied electric field. Since the magnitude of Kerr coefficient is typically small in metal nanoparticles within glass, a long--up to about 3 cm sample-length is used to obtain detectable modulation. For nonconjugated conductive polymers the electro-optic effect is the largest known, therefore only microns-thick samples are used for measurements. Quadratic electro-optic measurements were successfully carried out and a clear correlation between the real part of Kerr coefficient (K) and diameter of metal nanoparticles was established down to the subnanometer domain [6]. The magnitude of the real part of Kerr coefficient (K) was shown to have a \(\sim\)1/d\({}^{3}\) dependence, where d represents diameter of the metallic nanoparticle. In this report, we discuss electroabsorption which is given by: \(\Delta\alpha\) = 4\(\pi\)K\({}^{\prime}\)E\({}^{2}\), where K\({}^{\prime}\) is defined as the imaginary part of the Kerr coefficient and \(\Delta\alpha\) is the change in absorption coefficient. The magnitude of K\({}^{\prime}\) is related to saturation of absorption and two-photon absorption coefficient (\(\alpha_{2}\)), where two-photon absorption is given by \(\Delta\alpha\) = \(\alpha_{2}\)I, and I stands for optical intensity. The magnitude of \(\alpha_{2}\) is proportional to the imaginary part of the third order susceptibility (\(\chi^{(3)}\)).