Impact of Plasmon-Induced Atoms Migration in Harmonic Generation

Under illumination of a Ti:sapphire femtosecond oscillator, amplification of third harmonic generation by subwavelength plasmonic apertures is observed. However, the harmonic yield efficiency decays rapidly over time. In this work we investigate the physical phenomena behind the temporal attenuation of the harmonic signal. From high-resolution scanning electron micrographs and two-dimensional energy dispersive X-ray maps, we conclude that the attenuation of the third harmonic is attributed to trapping of a low-density carbon layer inside the plasmonic apertures. Furthermore, we show that the profile of the carbon deposit follows the enhanced electric near-field distribution, which indicates that the carbon atoms are transported to the field hotspot by the plasmonically enhanced optical tweezer effect. From the measurement of linear transmission spectra, we find that the dielectric constant inside the nanoholes is increased by the carbon deposit. However, numerical simulations demonstrate that the increase of dielectric constant does not reduce the electric near-field enhancement factor. Therefore, the decay of third harmonic radiation is primarily due to the strong reabsorption by the carbon deposit inside the gold-free aperture.