| Optical and electromagnetic devices with the characteristic of field focusing and enhancement have a wide application prospect in single molecule detection,optical tweezers technology,photoelectric conversion efficiency enhancement,nonlinear process,antenna radiation efficiency enhancement,electromagnetic control and so on.In the optical band,local field enhancement can be realized through metal nanostructures.However,in the optical band,the metal loss is relatively large,and field focusing and enhancement are restricted by quantum non-local characteristics.In the low frequency bands(microwave and terahertz),the loss of the metal can be neglected,which makes the low frequency components not only have more advantages in improving transmission efficiency and transmission distance,but also have higher local field enhancement when compared with corresponding devices in the optical band,thus having important application value in enhanced nonlinear effect,radiation efficiency enhancement and the control of electromagnetic waves,etc.Therefore,achieving significant field focusing and enhancement in the micro-and THz wave band has become an urgent problem to be solved.By designing metal metamaterial structures reasonably,spoof surface plasmons can be excited at low frequency,mimicking the effect of surface plasmons in the optical band,so as to break through the diffraction limit and confine electromagnetic waves into sub-wavelength scale.This thesis studied the physical phenomenon and applications of electromagnetic energy focusing based on spoof surface plasmons.The main contents and innovative achievements are summarized as follows:1.Structures based on spoof surface plasmons that can realize electromagnetic energy focusing are designed in the microwave and millimeter wave bands respectively,and the significant local field enhancement effect is verified.In the microwave band,an ultra-thin linear self-similar metal spiral chain is proposed,which can concentrate energy at the outside apex of the structure and realize electric field amplitude enhancement of more than 1400 times.The characteristics of resonant modes are observed through numerical simulation and experimental measurements,and the factors affecting the figure of local field enhancement are analyzed.In the millimeter wave band,a composite system composed of a spoof surface plasmon waveguide and a spoof localized surface plasmon resonator is designed for on-chip integration.The electromagnetic energy focusing phenomenon is observed at 300 GHz,and the influence of the gap between the waveguide and the resonator,and the length of the waveguide on field enhancement is analyzed.2.Spoof surface plasmon structures with fourfold and sixfold symmetry are designed.The structure is insensitive to the polarization state of incident electromagnetic waves,and can realize electromagnetic energy focusing under the excitation of any polarization state.The figure of local field enhancement is affected by the direction of excitation polarization to a small extend.On this basis,the spoof surface plasmon structure is coupled with a half-wave dipole.Experiments prove that the dipole antenna can radiate far below its original lowest operating frequency,and its original far-field radiation mode is preserved.This work not only verifies that designed spoof surface plasmon structure can enhance the radiation efficiency without changing the far-field radiation mode of the dipole antenna,but also provides a method to indirectly verify the local field enhancement characteristics of the designed spoof surface plasmon structure according to reciprocity principle. |
