Experimental data on the formation of the core−shell bimetallic spherical nanostructures in alkali silicate glasses is presented. The Ag−Cu and Cu−Ag core−shell bimetallic spherical nanostructures were formed in the nearsurface glass layers by the successive Na + → Ag + and Na + → Cu + or Na + → Cu + and Na + → Ag + ion exchange and subsequent thermal treatment above the glass transition temperature. The optical density measurements have shown that the Ag−Cu core−shell nanostructures provide three plasmon resonances and the Cu−Ag core−shell nanostructures provide at least four plasmon resonances. Experimental results obtained are confirmed by computer simulation in the dipole quasistatic approximation. ■ INTRODUCTIONOptical composite media (liquids, polymers, glasses) with metal nanoparticles are widely used in optics and photonics. As a rule, such media reveal the pronounced nonlinear optical properties and can be used for developing ultrafast all-optical switches. 1 In this case, glasses with the noble metal and copper nanoparticles attract the special attention of researchers because these metal nanoparticles have a large cubic nonlinear susceptibility. 2−6 The nonlinear susceptibility increases considerably near the plasmon resonances of the nanoparticles due to the local enhancement of the field amplitude of the electromagnetic wave. 7 However, the spectral localization of the plasmon resonances limits the spectral region favorable for the nonlinear application of the media with metal nanoparticles. This is why a search into the possibilities to shift spectrally the plasmon resonance is of practical unterest. To implement the spectral shift of the plasmon resonance, a change in either the size and shape of nanoparticles 8−10 or the refractive index of the surrounding host 8,11 can be used. Another way to produce the plasmon resonance spectral shift is the synthesis of nanoparticles or nanostructures containing two or more metals. 12−14 The realization of this method can be performed along two directions as follows: (i) the synthesis of nanoparticles consisting of an alloy of two metals, for example, Au−Ag or Au−In, 15−18 and (ii) the synthesis of nanostructures with the metal core and a shell of some other metal. Such nanostructures are studied experimentally and theoretically, in the most elaborate way, for the Ag−Au and Au−Ag core−shell cases. 19−24 Here, the first and second elements correspond to the core and shell, respectively. It was shown in refs 21 and 25−28 that composite media with the nanostructures mentioned are of interest not only for their nonlinear optical application but also for their application in sensors and solar cells. The nonspherical core−shell metal nanostructures were also widely investigated. It was shown in refs 29−31 that core− shell metal nanostructures having the form of prolate ellipsoids provide three plasmon resonances. Spherical core−shell Au−Ag and Ag−Au nanostructures were studied theoretically in refs 24, 32, and 33. Optical properties of nanostructures with two Ag and...

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