| The optical field localization has been one of the driving forces behind the booming de-velopment of nanophotonics over the past two decades.“How small the light can be localized in space”is also one of the fundamental questions of physics.Recently,more localized opti-cal field have been observed in experiments,including Raman scattering and single-molecule photoluminescence.Futhermore,the concept of“picocavities”has been proposed based on the spatial distribution of the localized optical field and scientists attibuted its mechanism to the classical non-resonant lightning rod effect.However,several dilemmas arise when studying optical localization in such extreme cases.For multi-scale(nano and sub-nano scale)metal structures,how to depict the quantum and many-body effects of electrons?How can we rigorously characterize the localization of optical field?Only after addressing these issues can we delve into the fundamental physics inquiry of how small the optical field can be localized in space.The scientific questions encompass whether the classical non-resonant lightning rod effect is the primary cause for the“picocavities”when quantum and many-body effects are taken into account;whether there exist new mechanisms for more localized optical fields and how to surmount the ob-stacle that more localized optical fields typically entail more dissipation,so that they can be effectively excited by the far field.Surface plasmons naturally facilitate the integration of light,electricity,and nanotechnology.If more localized optical fields exist,they will in-evitably lead to more efficient coupling between light,matter,and low-frequency electric fields.Therefore,based on extremely localized light fields,direct electrical modulation of plasmons with ultrafast response is expected to be achieved.In order to address the above issues,the following theoretical study was carried out in this thesis:1.The quantum hydrodyanmic model and rigorous quasi-normal mode analysis are utilized to investigate the mode volume of the“picocavities”and elucidate the underlying mechanism.A extremely localized field and the corresponding new mechanism for extreme localization of optical fields have been found.The mode volume of extremely localized field is theoretically estimated to be 1 cubic nanometer.2.By coupling the extremely localized mode with highly radiative modes of the metal host structure,efficient far-field radiation is achieved.Numerical simulations indicate that systems with far-field excitation of this mode can have optical intensity enhancements ex-ceeding 10~7,and exhibit ultra-strong coupling between a single quantum emitter and the optical field of this mode.3.We propose the concept of a nano-electron reservoir and establish a theoretical model for the coupling between the optical field and low-frequency electric field within the frame-work of the quantum hydrodynamic model.Based on this,we present a theoretical scheme for direct electro-plasmonic and electro-optical modulation.Theoretical analysis of a de-signed modulation device based on this scheme shows that the relative change of the scat-tering cross section can reach up to 150%for far-field light incident at a wavelength of 950nm with a±2 V bias voltage.The response time for a single modulation operation can be as fast as 10-20 femtoseconds,and the energy consumption for a single signal switch is less than 100 aJ. |
