Electromagnetic Polarization-converting And Active Metamaterials

Metamaterials have provided promising platforms for manipulating electromagnetic(EM)waves and developing novel EM functional devices.As an essential property of EM waves and broadly applied in various fields including imaging,display,communication and information storage etc.,polarization has been a significant manipulating object.Besides,active metamaterials enable dynamic modulation of EM waves and a wider range of functions and practical application,which are now receiving increasing attention.In this dissertation,through metamaterial design and use of phase transition material VO2,devices offering polarization conversion and active control of EM waves have been studied,the main contents are listed as follow:1.An ultra-thin linear polarization converter(LPC)and an efficient broadband LPC that work in the transmission mode are experimentally achieved via a design framework of perfect linear polarization conversion,which requires device to possess characteristics of being equivalent to two degenerate and complementary oriented perfect conducting surfaces under symmetrical bi-directional excitations,with one being electric and the other being magnetic.Furthermore,properties of the two LPCs are well explained with intuitive EM scattering analysis and equivalent circuit models.2.An efficient broadband and wide field-of-view half-wave plate have been designed employing Multi-reflection interference.Multi-reflection interferential polarization conversion analysis and transfer matrix computation for the dielectric/wire grid/dilectric/metal mirror structure arrive at conditions to achieve broadband half-wave plate,which are confirmed with EM simulation.Experimental sample exhibits efficient polarization conversion and outgoing polarization extinction ratio greater than 20 dB within 6.4?12.5 GHz.Also,the device has good oblique incident performance and can still be a freely tunable broadband polarization rotator under the condition that the incident angle is as large as 45 degrees.At the end,polarization conversion under oblique incidence has been investigated with eigenmode and transfer matrix methods.3.Broadband and high modulation-depth THz modulator using electric bias controlled VO2-integrated metamaterial has been studied.Measurement results show the modulation depth of the device is larger than 87%in a broad frequency range from 0.3 to 1.0 THz.The physical mechanism underlying the device's electrical tunability is found to be attributed to the ohmic heating by heat balance model together with a comparison between temperature and power dissipation dependences of terahertz transmission in the respective thermal and electrical control modulations.Furthermore,dynamic modulation experiment with continuous THz waves are conducted,and modulating performances is discussed.4.Spectral property manipulation of a configurable metamaterial with embedded VO2 by thermal,electrical and optical excitations has been studied.The metamaterial consists of a periodic array of two metal structures connected with VO2 patches.Through insulator-metal phase transition of VO2,conductive coupling between the metal structures can be actively controlled and makes resonant mode transition process to happen,realizing reconfigurability of spectral property and THz wave manipulating function of the device.Dynamic control experiments via three types of excitations including varying temperature,changing electrical current and power-adjusting femtosecond laser at different temperatures have been carried out,and we have obtained the manipulating conditions and explained the transimission spectra variations.Besides,dynamic process of manipulation through optical control has been measured and analysed.The research has demonstrated the significant value of the phase transition and large conductivity range of VO2 in THz reconfigurable device design.