Membrane-type Polarization-controlled Color Filters On HfO₂-on-silicon Platform

Color filters are key elements for truly restoring color,and have great value in display devices,image sensor etc.Researching and developing color filters with high color purity,high contrast,high performance and easy intergration,is the future research priority and direction.The output color of a CF can be tuned or changed when the polarization of the incident wave varies resulting from the polarization-sensitivity,which benefits integrated applications.With the development of micro-nano optics,sub-wavelength grating filters based on thin film structure expands the application fields of color filters in complex multicomponent systems.In this paper,we investigate two novel polarization-controlled CFs with a 200-nm-thick membrane structure,named CF-I and CF-II,which are implemented on an HfO₂-on-silicon platform using the（micro-）nano-machining technology.Main research contents and results are as follows:Firstly,two sets of grating structure parameters are designed and optimized by using Rsoft to realize polarization-controlled color filters.We visualize the guided-mode resonance（GMR）effect in grating CFs by the E-field distribution.Through the（micro-）nano-machining technologys of UV photolithography,DRIE and deep silicon etching etc,the CFs are surrounded by air and realized²00nm free-standing membrane structure by completely removing the silicon substrate beneath the HfO2 device area.The HfO₂ membrane structure is robust enough to bear the stress relaxation during the fabrication process with the aid of the four support beams.The morphology and structure of two finished Cfs are characterized by optical microscope,scanning electron microscope（SEM）and atomic force microscope（AFM）.From an overall view,the gratings are well fabricated without any crack.Secondly,the spectral responses are characterized by a self-built angle-resolved micro-reflection measurement system,which can verify the results of the simulation.For TM-and TE-polarized waves,the output colors of CF-I are blue and green centered at⁴85 nm and⁵53 nm,respectively,while the output colors of CF-II are yellow and red centered at⁵82 nm and⁶54nm wavelengths,respectively.Therefore,CF-I is a polarization-controlled blue-green CF,and CF-II is a polarization-controlled yellow-red CF.In addition,the silicon-based membrane-type devices here are ultra-compact and can be integrated with other silicon photonic devices to form multicomponent systems.Particularly,they have the potential of being transferred from the silicon substrate to other foreign platforms by mechanically releasing the support beams.Transferrable optoelectronics is a new emerging area for meeting the comprehensive requirements of flexible wearable devices.The CFs reported here may be applied in several fields such as image sensors,display elements,and even wearable devices.