Enhancement of plasmonic nonlinear conversion and polarization lifetime via fano resonances

Yıldız Karakul, Bilge Can
Boundary conditions and dispersion at dielectric-conductor interfaces, together, results in bands of energies or wavelengths of light within which light-matter interaction strength displays peaks as a result of fulfilled resonance conditions. At these resonance energies light propagation is strongly coupled to collective oscillation of free charges at the dielectric-conductor interface which qualifies to be given a quasiparticle name, "the surface plasmon polariton" (SPP). When dealing with zero-dimensional metal nanostructures (MNSs) in dielectric environment, these SPPs are necessarily bound to the structure and cannot propagate, hence become localized surface plasmons (LSPs). Around resonant LSP wavelengths, light is effectively confined to nanoscale sizes in the near field of the supporting MNSs, which offers a playground for effective light-management at the nanoscale, hence applicability of concept of plasmonics arises in nanoscience and nanotechnology. Despite its high amplitude, induced polarization field at the MNS suffers from rapid decay in time due to Ohmic losses such as electron-core interactions. In this thesis, it is shown that when conditions are favorable, Fano resonances may offer lifetime enhancement in plasmonic oscillators as a result of coupling of short lifetime bright and long-lived dark plasmon oscillation modes. Fano resonance is a destructive path interference effect, which emerges as an asymmetric dip in the spectral response of a driven harmonic oscillator, when a short lifetime oscillator is driven by a harmonic field at the resonance frequency of a long lifetime oscillator. At a certain frequency, the driven oscillator becomes under the influence of two driving forces which are out-of-phase, and hence their effects cancel each other. Time response of a plasmonic system can be extended by Fano resonance at a particular coupling strength, resonance and damping frequency. In addition, it is shown that Fano resonance mechanism is an effective way to enhance optical nonlinearities owing to plasmons in MNSs. Strong localized field leads to enhancement in higher harmonic fields compared to the linear responses. An analytical approach based on harmonic oscillator and a numerical approach based on 3D finite difference time domain Maxwell solution is presented to study the plasmonic coupling which resulted in prolonged lifetimes and enhanced or suppressed optical nonlinearity, in particular second harmonic generation. An experimental study on a silver coupled nanostructure system, results of which agree with the ones obtained in theory, is also presented as a verification of the developed model. Gaining the capability to obtain prolonged lifetime and enhanced nonlinear response of plasmonic MNS may play an important role for their successful integration to molecular switching, solar energy, photocatalysis and imaging applications.