Broadband And High Performance Subwavelength Linear Polarizers In Infrared Region

Nanophotonics is a rapidly developed technology which has great potential and practical applications in many fields,such as integration and miniaturization of optoelectronic devices.In this thesis,polarization characterisctics of metal/dielectric subwavelength grating structures and multi-cavity subwavelength gratings in wide infrared band are investigated based on equivalent medium theory,time domain finite difference numerical method and FP-like cavity mode.Design and fabrication of linear polarizers based on the proposed structures in infrared region are performed and demonstrated.The main work is listed as follows:(1)The design and fabrication of a ZnSe-based linear polarizer operating in 8-12?m infrared region with multilayer nanogratings is demonstrated.The multilayer structure is formed by a low-and a high-refractive index thin layer that are evaporated successively on a ZnSe substrate,followed by a dielectric nanograting that is etched into the high-refractive index thin layer and then a double-layer metallic nanogratings that is sitting on the dielectric nanograting.Polarization characteristics of the proposed structure on structural parameters are investigated and an optimized multilayer grating structure is obtained Results show that a TM transmission of>90%and an extinction ratio(ER)of>55dB can be obtained in the whole 8-12?m wavelength band.Experimental fabrication of the multilayer nanograting structure is conducted using UV lithography and thin-film deposition in which no ion etching process is needed for the formation of metallic nanogratings.An extinction ratio(ER)of 35 dB and TM-transmission(TMT)of averagely higher than 80%are obtained experimentally in the whole 8-12?m waveband with a 250-nm-period multilayer nanogratings.The ZnSe-based multilayer structure shows the possibility of achieving large-area and high-performance polarization in 8-12?m infrared region in a fashion of relatively easy-fabrication with large area.(2)A novel broadband linear polarizer that is based on multi-cavity sub-wavelength grating(MC-SWG)is proposed.1)The polarization characteristics of the proposed MC-SWG structure is investigated in detail.It is found that TM transmission(especially in short wavelength end)and ER of the MC-SWG structure can be significantly enhanced by the introduced cavity-grating due to the resonance of the cavity effect.The resonant peak of the TM transmission can be tuned by changing the cavity length.2)The design of a ZnSe-based linear polarizer operating in 3-13?m ultra-broad infrared region with MC-SWG structures is conducted.The MC-SWG structure is formed by a dielectric nanograting that is etched into ZnSe substrate,followed by metal/dielectric/metal/dielectric/metal thin film structure that are evaporated successively on the top and bottom of the ZnSe dielectric nanograting,so that multiple optical cavities are formed on both top and bottom of the ZnSe dielectric nanograting.Polarization characteristics of the proposed polarizers on structural parameters are investigated and an optimized multi-cavity structure is obtained.An extinction ratio(ER)of higher than 95 dB and TM-transmission(TMT)of averagely higher than 85%in the whole 3-13?m broad waveband are obtained with a 250-nm-period multi-cavity nanogratings.The proposed MC-SWG provides a way of achieving ultra-wide bandwidth,high-performance and relatively easy-fabrication polarizers.