Plasmonic Nanoantennas

Abstract

The material-, size- and shape-tuned plasmonic nanostructures are fabricated by electron-beam lithography (EBL). Their linear and nonlinear optical responses are detected via dark-field scattering microscope and scanning confocal optical microscope, respectively. The elastic scattering offers the fingerprints for the localized surface plasmon resonances (LSPRs) of plasmonic nanoparticles (NPs), which give the assistance to the enhancement of nonlinear optical signals, such as second-harmonic generation (SHG) and two-photon photoluminescence (TPL). SHG from various types of nanoantennas is studied subsequently. The excitation polarization dependent far-field radiation of SHG shows a flipping effect, which is attributed to the resonant excitation condition and the SH phase interferenceduring the size of NP changes. The results clearly point out that the characterization of electromagnetic hot-spots using nonlinear optical processes is not straightforward, and that a competition between different contributions from parameters, such as polarization, geometry and resonance, to the enhancement often occurs. The radiations of metallic photoluminescence (MPL) in the weak and strong electromagnetic field are investigated systematically, involving experiments and theoretical modelling. In the weak excitation, it is found that the MPL consists of two emission channels: the particle plasmons (PPs) and the electron-hole (e-h) pair radiation channels. PPs are excited via Auger scattering of photo-excited d-band holes, the radiative decay of which develops the former components. The latter derives from the plasmon-enhanced radiative decay process of e-h pairs. A model of total emission quantum efficiency (TEQE) involving both contributions is established, to quantify the radiative emission capability per e-h pair, and explain the size effect regarding the MPL difference between the bulk and the NPs. The experiment results and the theoretical model supply a new approach to predict MPL. In the strong excitation, avalanche multiphoton photoluminescence (AMPL) is observed from the strong coupled Au-Al heterodimers once the incident beam exceeds critical laser intensity or incident polarization is closing to the longitudinal excitation. The emission intensity turns out approximately more than one order of magnitude larger and encounters dramatic spectral changes. The physic mechanism can be well explained via Keldysh’s rate equations in strong field. It is interpreted that AMPL derives from the recombination of avalanche ionized hot carriers seeded by multiphoton ionization (MI). The MI is greatly assisted by the dramatic local field of coupled Au-Al nanoantennas at the excitation stage. The threshold of optical breakdown can be evaluated via two-temperature model (TTM), taking the source term into account. At the emission step, a linear relationship between the power law exponent coefficient and the emitted photon energy is experimentally observed. The giant AMPL intensity can be evaluated as a function of the local field environment and the thermal factor of hot carriers. The spectral change from the LSPR modulated profile to the one that indicates the direct recombination from hot e-h pairs is explained by the diminishment of d-band hole scattering rate as the carriers’ temperature increases.