13 March 2007 Effects of surface asymmetry on femtosecond second-harmonic generation from metal nanoparticle arrays
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We describe experiments aimed at distinguishing possible mechanisms of second-harmonic generation (SHG) in lithographically prepared arrays of metal nanoparticles. It is well-known that even-order harmonics cannot be generated by electric dipole-dipole interactions in centrosymmetric systems. The experiment employs two basic sample geometries. In our first geometry, as in our previous work, the NPs are left exposed to air, producing an asymmetric local dielectric environment with ITO on one side and air on the other. In the second geometry, we propose coating the arrays with the same material as they are created on, thus producing a centrosymmetric environment in which any SHG observed can not be due to asymmetry in the medium, but to nonlocal or retardation mechanisms in the particles. The arrays are fabricated using focused ion-beam lithography and vapor deposition of the metal, followed by standard lift-off protocols. This procedure yields typical NP dimensions between 60 nm and 200 nm in diameter, and between 15 nm and 30 nm in height, as characterized by scanning electron and atomic-force microscopy. By tuning the NP resonances to the excitation wavelength the SHG signal can be substantially enhanced. Surface melting effects are minimized by the use of ultra-short (50-fs) pulses which give high intensity while allowing us to work at relatively low fluence.
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D. Ferrara, D. Ferrara, M. D. McMahon, M. D. McMahon, R. Lopez, R. Lopez, R. F. Haglund, R. F. Haglund, } "Effects of surface asymmetry on femtosecond second-harmonic generation from metal nanoparticle arrays", Proc. SPIE 6458, Photon Processing in Microelectronics and Photonics VI, 64581K (13 March 2007); doi: 10.1117/12.708293; https://doi.org/10.1117/12.708293

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