Research On Subwavelength Metamaterials For Polarization Manipulation

Polarization manipulation can be used for the generation,conversion and recognition of polarization state in applications of the biomedical,astronomy,defense,display and other fields.Infrared polarization imaging can not only identify the low contrast target that traditional intensity imaging can not or difficult to distinguish,but also can influence the contour of object.Thus,it has the advantage and broad application prospect in the aspects of remote sensing,fingerprint identification,medical diagnosis,navigation under harsh environment,camouflage identification and so on.Polarizers used for polarization identification and analysis are key components to polarimetric detection(imaging)systems.The optical constants of metamaterials can be controlled by artificial designing the structural units.Then,excellent polarized light selection/conversion performance can be obtained.In addition,metamaterials are integratable and with low-profile,so they are promising candidates to replace traditional crystal-based polarizing devices for polarization imaging systems.In this paper,the metamaterials based artificial subwavelength metal structures were studied for polarization manipulation in the infrared range,such as selective transmission of linear/circular polarization with high extinction ratio and polarization conversion.The main results obtained in this paper are as follows:1.The mechanism of the selective transmission for linear polarizations in subwavelength metallic gratings was studied.Results show that the transmission of TM incident lights in subwavelength metallic gratings mainly comes from the Fabry-Perot-like resonant modes.The resonant modes are caused by the coupling of surface plasmon polaritons and incident lights.And the response of the subwavelength metallic grating is related to the grating height,period,duty cycle,substrate and metal materilas.Subwavelength metallic gratings operating in 3-10?m was fabricated to realize wide-band linear polarization selective transmission.And a kind of novel dual-layer sub-wavelength metallic grating with dielectric structure between the metal and substrate is proposed,which not only can obtain high extinction ratio,but also can maintain high TM transmittance in a broad band.2.A concept of hybrid helix metamaterial with strong coupling of the localized and delocalized resonant modes has been proposed.The proposed metamaterial can achieve ultra-wide and giant circular dichroism with low insertion loss simultaneously,which solve the balancing problem of the three key parameters.A giant extinction ratio of 5.9×10~5 can be obtained with a six-pitch hybrid single-helixes array,which is almost 7 times larger than the best result reported by Behera et al.in 2015.A very high average extinction ratio of 8.5×10~3 is achieved in the operation band of 3.5-9.5?m.More importantly,the top tapered part well-matches the impedance and contributes to high average transmittance of 72%in the whole range.It is experimentally demonstrated in the microwave range to prove the validity of such a concept based on the general scalablility,which agrees well with theoretical ones.The proposed concept also can be broadly extended to other chiral metamaterial systems,and provides new approach in the design of novel high-performance circular polarizers.3.A kind of broadband circular polarizers based on helix-like chiral metamaterial was proposed,which can not only achieve high performance,but also has compatible fabrication process with the common nano-fabrication and semiconductor processes.Such design provides a greater possibility for planar metamaterial based circular micropolarizers to be applied to integrated polarized source and detection,especially for the full-Stokes polarization detectors.The structure consists of multi-layer gold arcs connected clockwise or counterclockwise and embedded in the transparent dielectrics.By tailoring the relative orientation and the distance of the adjacent arcs,the anisotropy of the arc is converted into the magneto-electric coupling of the three-dimensional inclusion,and contributes to a broad and strong bianisotropic optical response.The maximum extinction ratio can reach 19.7 in a four-layer helix-like metamaterial.The operation band is in the wavelength range from 4.69?m to 8.98?m with an average extinction ratio of 6.9,which is analogous to performance of the uniform helical metamaterial Gansel et al reported.And the transmittance for selective polarization is above 0.8 in the entire operation band.4.A three-layered planar metamaterial based on double cascading polarization conversion cavities is proposed to achieve giant and broadband asymmetric transmission of circularly polarized waves at near-infrared communication band.The metamaterial consists of two layers of identical 45°tilted chiral S-shaped structure array sandwiched with a subwavelength metallic grating,which forms two cascading cavities with polarization conversion effect.By delicately combining the broadband polarization conversion and cavity-enhanced effect,the asymmetric parameter can be as high as 0.87,which is 74%higher than that of the previous best reports(the maximum asymmetric parameter is about 0.5),and the asymmetric parameter is above 0.6 in the entire operation band of 1.2-2.0?m.In addition,since the two cavities are designed to work independently,a perfect robustness to misalignments is achieved,which greatly reduces the alignment precision in layer-by-layer photolithography processes.Such metamaterial is promising to be used as a polarization converter,polarization-dependent isolator and other optical passive devices for integrated optics and optical communication system.5.A hybrid metallic structure is proposed to achieve long-wavelength infrared quantum-well circularly polarized detector.By combining circular polarization selective conversion cavity and the plasmonic coupling microcavity,not only the coupling efficiency between the quantum well and the incident light is well improved,but also the circular polarizations are recognized.The preliminary design can get an extinction ratio of above 160 at 9.6?m.Such quantum well circular polarization detector will provide an important physical and technical basis for the circular polarization detection in infrared and even terahertz.