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Abstract
A theoretical study of differences in broadband high-index-contrast grating (HCG) reflectors for TM and TE polarizations is presented, covering various grating parameters and properties of HCGs. It is shown that the HCG reflectors for TM polarization (TM HCG reflectors) have much thicker grating thicknesses and smaller grating periods than the TE HCG reflectors. This difference is found to originate from the different boundary conditions met for the electric field of each polarization. Due to this difference, the TM HCG reflectors have much shorter evanescent extension of HCG modes into low-refractive-index media surrounding the HCG. This enables to achieve a very short effective cavity length for VC-SELs, which is essential for ultrahigh speed VCSELs and MEMS-tunable VCSELs. The obtained understandings on polarization dependences will be able to serve as important design guidelines for various HCG-based devices.
Highlights
The grating structure referred to as high contrast grating (HCG) is near subwavelength gratings formed in a thin high-refractive-index layer surrounded by lower refractive index materials [1]
It is shown that the high-indexcontrast grating (HCG) reflectors for TM polarization (TM HCG reflectors) have much thicker grating thicknesses and smaller grating periods than the TE HCG reflectors
The TM HCG reflectors have much shorter evanescent extension of HCG modes into low-refractive-index media surrounding the HCG
Summary
The grating structure referred to as high contrast grating (HCG) is near subwavelength gratings formed in a thin high-refractive-index layer surrounded by lower refractive index materials [1]. Since the first demonstration of VCSELs employing a HCG reflector [2, 3], many novel HCG-based VCSEL structures have been reported, featured by strong single-transverse-mode operation [4,5,6], efficient MEMS tuning of emission wavelength [7,8], integration onto a silicon-on-insulator (SOI) wafer [9,10,11], and control of transverse mode confinement [12, 13] These 1D HCG reflectors can be classified into TM HCG and TE HCG reflectors, depending on the polarization of an incident wave to which a HCG is highly reflective. The results in this paper should be valid for other wavelengths and other sets of materials with a high refractive-index contrast
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