We have studied the effects of planar inversion symmetry and particle-coupling of gold nanoparticle (NP) arrays
by angle dependent second-harmonic generation (SHG). Time- and angle- resolved measurements were made
using a mode-locked Ti:sapphire 800 nm laser onto gold NP arrays with plasmon resonance tuned to match the
laser wavelength in order to produce maximum SHG signal. Finite-difference time domain simulations are used
to model the near-field distributions for the various geometries and compared to experiment. The arrays were
fabricated by focused ion-beam lithography and metal vapor deposition followed by standard lift-off protocols,
producing NPs approximately 20nm high with various in-plane dimensions and interparticle gaps. Above a
threshold fluence of ~ 7.3 × 10<sup>-5</sup> mJ/cm<sup>2</sup> we find that the SHG scales with the third power of intensity, rather
than the second, and atomic-force microscopy shows that the NPs have undergone a reshaping process leading
to more nearly spherical shapes.
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.
Closely spaced pairs or "dimers" of elongated gold nanoparticles may be expected to exhibit electric field hotspots. We investigate the possible influence of hotspots on second harmonic generation. Preliminary results show that arrays of nanoparticle dimers exhibit reduced second-harmonic generation compared with arrays of single nanoparticles having similar extinction spectra, contradicting a simple model of second-harmonic generation (varying as the fourth power of the local fundamental field) if hotspots can be shown to exist in such gaps.
We present experimental results from second-harmonic generation studies of lithographically-prepared arrays of centrosymmetric gold nanorods, extending a previous treatment. The arrays serve as diffraction gratings, allowing control over the emission directions. The intrinsic radiation patterns from the nanoparticles are superimposed on the diffraction pattern, creating a unique angular distribution of second-harmonic light. The surface plasmon resonance mode of the particles is tuned to match the wavelength of the ultrafast Ti:sapphire excitation laser, dramatically enhancing the second-harmonic intensity but also increasing photodesorption effects. The details of the diffracted peak intensities depend sensitively on the geometry of the system and require a complex normalization of the data.