## 3 Results and Discussion  
Three samples gold nanoparticles in glass with three different average particle-diameters were studied as discussed in the following. Optical absorption spectrum was recorded for light travelling through the thickness of the sample-plate. The absorption peak appears at shorter wavelengths and the relative intensity of the peak decreases for smaller nanoparticles [5,8,13]. These were all discussed in our recently published article [5]. For sample 1, the peak was at \(\sim\)534 nm wavelength which corresponds to a particle diameter of about 50 nm. For sample 2, the peak was at \(\sim\)527 nm wavelength which corresponds to a particle diameter of about 25 nm. For sample 3 (Figure 2), the peak was at \(\sim\)520 nm wavelength which corresponds to a particle diameter of about 15 nm. The absorption coefficient as given is small (\(\sim\)5.3 cm\({}^{-1}\) at 520 nm) consistent with a low concentration (\(\sim\)10\({}^{-9}\) M) of gold nanoparticles in glass [5,8]. Electroabsorption measurements were made in these plates at different applied ac fields as discussed above. The beam at 633 nm from a He:Ne laser was passed through the long dimension of the plate with interaction length of 3 cm. For samples 1 and 2 having particle diameters of 50 nm and 25 nm respectively, the modulation due to electroabsorption was too weak to measure. The results of the measurements for sample 3, having a particle diameter of 15 nm, are shown in Figure 3. The imaginary  
Figure 1: Experimental set-up for measurement of electroabsorption. Figure 2: Surface plasmon resonance spectrum of gold nanoparticles in glass (Sample 3, peak at \(\sim\)520 nm). The particle diameter \(\sim\)15 nm. part of Kerr coefficient (K\({}^{\prime}\)) as determined from the observed modulation signals is \(\sim\)4 \(\times\) 10\({}^{-15}\) m/V\({}^{2}\) at 633 nm. Results of measurements following identical experimental procedure but using about 1 \(\upmu\)m thick films of an iodine-doped nonconjugated conductive polymer (polyisoprene) are shown in Figure 4. Considering previous reports, subnanometer-size metallic domains are formed in nonconjugated conductive polymers upon doping with iodine [15; 16; 17; 18; 19; 20; 21]. A charge-transfer from an isolated carbon-carbon double bond to the dopant creates such highly confined metallic domains/quantum dots (surface plasmon resonance \(\sim\)400 nm and radical cation at \(\sim\)300 nm). The electrical conductivity, optical absorption, spectral line-width, EPR, magnetic susceptibility, vibration spectroscopic, and thermal characteristics of doped nonconjugated conductive polymer are all consistent with metallic particles of subnanometer sizes [15; 16; 17; 18; 19; 20; 21]. The optical absorption peak of nonconjugated conductive polymer is at \(\sim\)400 nm which is much smaller than that of the smallest metal nanoparticles reported so far (\(\sim\)2 nm). The absorption line-width is much wider compared to that of the smallest metal nanoparticles reported. These are as would be expected considering the particle size [8]. In addition, due to the subnanometer-sizes these metallic particles have shown the largest known Kerr coefficient and two-photon absorption coefficient for any material [13; 14; 15; 16; 17]. In this study, we have measured electroabsorption which is the imaginary part of Kerr coefficient (K). The imaginary part of Kerr coefficient (K) as measured is \(\sim\)3.2 \(\times\) 10\({}^{-11}\) m/V\({}^{2}\) at 633 nm for iodine-doped polyisoprene. All the magnitudes of the Kerr coefficient stated here are for about the same optical density at 633 nm. Figure 4: Electroabsorption modulation in a nonconjugated conductive polymer (iodine-doped polyisoprene) for different applied electric fields. Film thickness \(\sim\)1 \(\upmu\)m.  
Figure 3: Electroabsorption modulation in gold nanoparticles within glass for different applied electric fields. Particle diameter \(\sim\)15 nm. Interaction length \(\sim\)3 cm.