Experimental Study Of Near-field Optical Coupling Between 2-Dimensional Plasmonic Nanostructures

In the last two decades,plasmonics has emerged as an important direction in the field of nano-photonics because of its unique capability of light manipulation in the subwavelength scale.Surface plasmon polaritons(SPPs)are collective oscillations of electrons bounded in the subwavelength zone at the interface between metal and dielectric.Depending on the boundary condition,SPPs can be propagating waves,standing wave,or Bloch waves with band structures,having many different applications including optical-antennas,subwavelength waveguides,nanoscale optical modulators,resonators,to name a few.These devices not only form a tool library for nanophotonics,but also provide powerful methods for the study of light-matter interactions at the nanoscale.In plasmonics,one of the key issues is how to tune the properties of SPPs.Recently,many works have been reported for this purpose.By changing the optical constants of metals or dielectrics,people can tune the amplitude,phase and wavelength at will.Based on this method,many active SPP devices were demonstrated,including phase modulators,optical switches,bio-sensors and polarization devices.To some extent,those SPP devices can considered as the optical cousins of microwave devices.In history of microwave,the rapid development of active devices is a sign of the large scale application of microwave devices in industry.We therefore expect that active SPP devices will become an indispensable jigsaw in integrated photonics in the near future.In addition to the tuning of optical constants,optical coupling is another important way for controlling the optical properties of SPP systems.According to the plasmon hybridization model proposed by Nordlander et al.,the resonance modes of a composite nanostructure can be treated as the hybridized result of the resonance modes of its components.Interestingly,the hybridization is very sensitive to the geometrical parameters of the system.It is therefore possible to engineer the optical properties of a composite plasmonic structure by manipulating the relative position of its components.In this thesis,we will focus on the optical coupling(or hybridization)between 2D plasmonic nanostructures,as well as the resulted effects,including resonance shifts and optical forces induced by the near-field coupling.In the introduction,we introduce the general principle of SPPs,including their forms,excitation methods,and coupling between SPP modes.Some of most representative applications in the field of plasmonics are also discussed,including modulator,metasurface,and sensing.In chapter 2,we discuss the position control system of plasmonic nanostructures.Here,the key component is the quartz tuning fork which serves as the force sensor.It measures the Van der Waals forces between plasmonic nanocomponents and functions as the gauge in the distance control system.The quartz tuning fork is a piezoelectric device supporting mechanical resonant motions with a high quality factor.To understand the behavior and further improve the performance of quartz tuning forks,we derived a new equivalent circuit model for them.The new model includes coupling and dissipation terms,and can be used to discuss how the load influence the quality factor.In chapter 3,we developed an ultrasensitive distance sensing device,the plasmonic flat,and performed quantitative measurements of the coupled plasmonic system using the high-precision distance control system.In the coupled SPP system,the coupling energy between the charges increases nonlinearly with distance and this gives the plasmonic system a high spectral sensitivity to the distance.Thanks to this near-field interference effect,we can obtain the distance information on the sub-nanometer scale by measuring the reflection spectrum of a coupled SPP system.To demonstrate this,we fabricated a 2D plasmonic nanodisk array and measured its near-field coupling effect with plasmonic thin film using the Newton's ring method.A very high spectral sensitivity was observed.This plasmonic optical flat breaks the limitations of distance measurement of conventional optical flats,and provides a fast,low-cost and highly sensitive surface inspection method.In chapter 4,we investigated near-field optical coupling induced forces between 2D plasmonic nanostructures in experiment.With the theory developed in chapter 2,we were able to achieve a high quality factor tuning fork sensor,and this allows us to measure the optical forces between 2D plasmonic nanostructures.The inherent noise ground of the entire measure system was determined lower than 0.1 pico-Newton.the ultralow noise performance of the tuning fork sensor also imply applications in ultra-precision electronics.