Abstract
A dielectric transmittance filter composed of subwavelength grating sandwiched between two few-layers distributed Bragg reflectors (DBRs) is proposed with the aim of being compatible with CMOS technology and to be tunable by lithographic means of the grating pattern without the need of thickness changes, in the broad spirit of metamaterials. The DBR mirrors form a Fabry-Perot (FP) cavity whose resonant frequency can be tuned by changing the effective refractive index of the cavity, here, by tailoring the in-plane filling factor of the grating. The structure has been studied and designed by performing numerical simulations using Fourier Modal Method (FMM). This filter proves to have high broad angular tolerance up to ±30˚. This feature is crucial for evaluating the spectral performance of narrow-band filters especially the so-called Ambient light sensors (ALS). By analyzing the transmittance spectral distributions in the band diagram, it is found that the angular tolerance is due to coupling between the FP and the guided mode inside the cavity in analogy to resonances occurring within multimode periodic waveguides in a different context.
Highlights
Driven by the increased use of smart devices such as mobile phones and compact cameras, rapid growth in digital color imaging devices is observed [1] [2]
A dielectric transmittance filter composed of subwavelength grating sandwiched between two few-layers distributed Bragg reflectors (DBRs) is proposed with the aim of being compatible with complementary metal oxide semiconductor (CMOS) technology and to be tunable by lithographic means of the grating pattern without the need of thickness changes, in the broad spirit of metamaterials
By analyzing the transmittance spectral distributions in the band diagram, it is found that the angular tolerance is due to coupling between the FP and the guided mode inside the cavity in analogy to resonances occurring within multimode periodic waveguides in a different context
Summary
Driven by the increased use of smart devices such as mobile phones and compact cameras, rapid growth in digital color imaging devices is observed [1] [2]. This last point is the main motivation of this paper, especially for (ALS) sensors [5] While these devices in the most common version just capture ambient light to correct smartphone displays, the present study rather addresses the more advanced versions that have multispectral capability (e.g. IR and UV added in the ST Microelectronics VD6281 product, see information on product datasheet) that generically aim at capturing the spectral content of a scene with better resolution than the usual RGB triple (and monochrome detectors). As the resolution is improved, it is observed that the decreased pixel area translates into larger degradation under ultraviolet illumination or high temperature environments and weaken efficiency To address this issue, nanostructured surfaces can be used such as plasmonic filters which are based on the use of thin films or patterned metallic (i.e., Au, Ag, or Al) nanostructures that exhibit large tunability over the visible spectral range [6] [7] [8]. For ALS of the forthcoming generation (with more resolution than basic ones, which operate not much beyond RGB selection), or for wider applications aiming for instance at
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