Multiple Femtosecond Optical Harmonics from Nanoparticle Sandwiches

For over sixty years, the generation of optical harmonics in non-centrosymmetric crystals by a high-intensity laser has been a signature manifestation of nonlinear optics. In bulk materials, generating each successive harmonic is typically orders of magnitude less probable. Recently, however, we have generated the third through the fifth harmonics of a femtosecond, near-infrared laser beam with roughly similar probabilities in ultrathin layers of gold, aluminum and copper sulfide nanoparticles. The harmonic generation mechanism — non-radiative plasmon-plasmon coupling on a sub-picosecond time scale — has unique features of a nonlinear process that takes place entirely in the optical near field.