Tuning Of Optical Nanoantenna Via Femtosecond Laser And Electrically Metasurface-based Optical Switch

Metasurface,a special kind of artificial subwavelength structure is comprised of multiple periodically arranged nanoantennas.It can modulate electromagnetic waves and change the way electromagnetic waves propagate,achieving novel phenomena and a variety of applications.Nanoantenna is the basic unit of metasurface.Consequently,a series of studies on nanoantenna greatly contribute to the analysis of metasurface.Plasmonic nanoantennas are able to localize optical fields via excitation of strong plasmon resonances.On the other hand,low-loss high-index dielectric nanoantennas with inherent magnetic and electric Mie-type resonances.Moreover,the combination of these two paradigms in form of metal-dielectric(hybrid)nanoantennas have an artificial resonant magnetic response,whereas the plasmonic ones provide a strong electric field enhancement.Thus hybrid nanoantenna has unique optical properties and more significant research meaning.Recently,tremendous efforts have been dedicated to switchable metasurfaces,whose performances are tuned by external stimulus such as optical,magnetic,mechanical,electrical or thermal field.Therefore,they can be used to fabricate some dynamically adjustable optical devices,such as optical switches,optical modulators.For nanoantenna,passive control and active control are both effective ways to tune its optical properties.Passive control is achieved by changing the design of antenna,whereas active control can be obtained by changing the permittivity of antenna.In this thesis,we mainly focus on the tuning effect of optical nanoantenna(constituent part of metasurface)via femtosecond laser and electrically metasurface-based optical switch.For the beginning,femtosecond laser,one way of passive control,is induced to melt the plasmonic nanoantenna within the monopole and dimer hybrid nanoantenna,so the shape of the plasmonic nanoantenna is constantly changing.Furthermore,the near-field and far-field characteristics of the hybrid nanoantenna are obviously tuned,such as resonance frequency,electric field intensity,bandwidth,Purcell factor and power pattern.In addition,the tuning effect of the dimer hybrid nanoantenna is much better in comparison with that of the single nano-particle.Besides,we propose two kinds of electro-optical switches based on tunable metasurface.The first optical switch is comprised of tunable metasurface with discrete nanoantennas.It employs optical beam deflection and optical mode switching between different anomalous reflective angles(-10o and 42 o)at 0 o of normal incidence.The second one is an embedded triangular continuous metasurface optical switch.The switching takes place in the optical beam deflection between the normal reflectance(0 o)and 45 o anomalous reflection angle.In addition,the embedded continuous metasurface optical switch can obtain high extinction ratio under a small refractive index change,widen the working frequency and prior performance,and thus has highly practical value and significance.