Linear and Nonlinear Optical Properties of Metal Nanoparticle Composites

Faculty Participants: Haglund, Feldman, Magruder, Perakis and Weller

Michael Faraday, in his Bakerian Lectures of 1857, proposed that microscopic particles of silver and gold are responsible for the beautiful coloration of the famed ruby-gold decorative glasses. Fifty years later, Gustav Mie explained the phenomenon quantitatively as arising from a collective oscillation of the metal electrons confined in these microscopic particles. This collective oscillation is widely known as the surface plasmon resonance, and it is one of the two fundamental optical excitations in solids. (The other is the electron-hole pair called the exciton.) More recently, noble-metal nanocrystals have attracted attention because of their large optical nonlinearities. The current DOE-supported project is the most recent stage of research, begun by Professors Haglund and Magruder more than a decade ago, to study the nonlinear optical properties of noble metal nanoparticles embedded in dielectric hosts such as silica glass. One of their important early results was the first demonstration of the quantum size effect on a metal nanocrystal.

The newest experiments have been based on the idea of making ordered arrays of metal nanocrystals by directed self-assembly processes. The most successful of these techniques combines focused ion-beam lithography with pulsed laser deposition of the desired metal species to create optical heterostructures with unusual linear and nonlinear optical properties (Figure 1). For example, the peak of the plasmon resonance shifts with interparticle spacing (Figure 2); such shifts can be used to quantify the interactions between neighboring nanoparticles. Nanoparticle arrays are also extremely sensitive to environmental effects, and are being investigated as potential substrates for surface-enhanced Raman scattering. Calculational tools based on the coupled-dipole approximation are being developed to assist in modeling interactions between nanoparticles as well as the effects of adsorbates on nanoparticle surfaces (Figure 3).

Figure 1. Array of silver-nanoparticle triangles fabricated by focused ion-beam lithography followed by pulsed laser deposition of silver. The substrate is indium tin oxide.

Figure 2. Surface plasmon resonance spectrum of silver nanoparticle arrays with varying lattice constants, measured in a dark-field confocal microscope. The nanoparticles are 60 nm in diameter.

Figure 3. Calculation of expected plasmon resonance spectrum of silver nanoparticles exposed to sulfur impurity in laboratory air at a level of 0.2 ppb, showing changes expected as outer layer of the nanoparticle changes to silver sulfide. The results are consistent with laboratory data and imply that a shell approximately 1 nm thick of Ag₂S has formed outside the Ag core.

References

“Size Dependence of the Third-Order Nonlinear Susceptibility of Cu Nanoclusters Observed by Four-Wave Mixing,” Li Yang, K. Becker, R. H. Magruder, III, F. M. Smith, R. F. Haglund, Jr., Lina Yang, R. Dorsinville and R. R. Alfano, J. Opt. Soc. Am. B 11, 457-461 (1994).

“Pulsed Laser Deposition of Cu:Al₂O₃ Nanocrystal Thin Films with High Third-Order Optical Susceptibility,” J. M. Ballesteros, R. Serna, J. Solís, C. N. Afonso, A. K. Petford-Long, D. H. Osborne and R. F. Haglund, Jr., Appl. Phys. Lett. 71, 2445-2447 (1997).

“Fabrication of metallic quantum-dot arrays for nanoscale nonlinear optics,” A. B. Hmelo, M. D. McMahon, R. Lopez, R. H. Magruder III, R. A. Weller, R. F. Haglund, Jr. and L. C. Feldman, in Ceramic Nanomaterials and Nanotechnology II, eds. M. R. DeGuire, M. Hu, Y. Gogotsi and S. Lu, Ceramic Transactions 148, 68 (2003).