Radially polarized light excites ring dipoles --- Thesis appreciation
（ See references at the end of the paper for the original text ）
1957 year , Former Soviet scientists Zel'dovich In order to solve the parity breaking problem of weak interaction in atomic physics and nuclear physics , The concept of ring dipole is proposed for the first time . Ring dipole is a kind of electromagnetic wave excitation source which is different from electric dipole and magnetic dipole , It has some basic properties of materials, such as absorption 、 Optical response such as dispersion has an important influence . Ring dipole resonance can significantly reduce the material of
Radiation loss , So as to form an optical resonator with high quality factor —— Ring dipole resonator . The excitation of this resonant cavity mode can greatly promote the interaction between light and matter , And then we can realize such things as low threshold semiconductor lasers 、 High sensitivity sensors 、 Optical switches and modulators are important optical devices .
However , In nature , Its radiation energy is much weaker than that of electric dipole or magnetic dipole because it is difficult to observe in experiment . And the rational design of metamaterial unit structure , The ring dipole can be generated .
Radially polarized light
Polarization is one of the most important vector characteristics of light field , Nowadays, the application principles of many optical instruments and systems are based on the vector nature of light field and its interaction with matter . The interaction between polarized light and matter in optical detection and metrology 、 Show 、 Optical communication 、 Data storage 、 material science 、 Astronomy and biomedicine have been widely used . Previous studies focused on spatially uniformly polarized light （ For example, linearly polarized light , Elliptically polarized light , Circularly polarized light, etc ）, This kind of light field does not depend on the spatial position of its cross section . However , With the continuous development of technology and the continuous expansion of demand , People began to think about how to get the light field with the change of polarization state with spatial position . Studies have shown that , If the spatial distribution of the polarization state of light wave is controlled purposefully , Some inhomogeneous distributions of polarization states are introduced , A series of novel optical effects and phenomena can be induced . This kind of beam with nonuniform polarization is called vector beam . With respect to scalar light fields , At the same time , There are different polarization states at different spatial positions in the same wave front of vector light field . Vector light field in surface plasma 、 Super resolution imaging 、 Optical micromachining and optical micromanipulation have great potential applications .
The cylindrical vector beam is a typical example of vector beam , Its polarization state is axially symmetric with respect to the center of the beam . Theoretically , The cylindrical vector beam is a characteristic solution of vector Helmholtz equation in cylindrical coordinates , The uncertainty of the state of polarization at the center of the beam （ There are polarization singularities ） This leads to zero light intensity here .
Why use radially polarized light ？ Previously reported metamaterials based on ring dipoles are complex , This is due to the symmetry of spatial inversion of linearly polarized light , So the ring dipole can only be generated by geometric design of metamaterial structure .
This paper uses radially polarized light as incident light , This simplifies the complexity of metamaterial structure design , It's worth a taste .
Metamaterial structure and result analysis
The surface of the structure is hexamer , The material is gold ; The dielectric layer material is SiO2; The bottom plate is gold , Pictured (a Shown ). The incident light used --- The electric field distribution of radially polarized light is shown in the figure (b) Shown . The results show that , The structure has a wavelength of 785nm It has a formant at , chart （c）, The distribution of the magnetic field is as follows (d) Shown , Produce an obvious magnetic field closed-loop response .
The results show that ,z The directional annular dipole is dominant in the multipole spreading scattering .
: Bao, Y. et al. Plasmonic Toroidal Dipolar Response under Radially Polarized Excitation. Sci. Rep. 5, 11793; doi: 10.1038/srep11793 (2015).