Subwavelength grating couplers implemented on free-standing HfO2 membrane are proposed for out-of-plane coupling of visible light. The device is realized by double-side fabrication, which combines front patterning of grating grooves with a back-releasing of the Si substrate. The free-standing HfO2 membrane with diameter ~200 μm and thickness 200 nm is fabricated by including an auxiliary structure into the membrane to release residual stress.
The fabricated grating parameters are characterized by scanning electron microscope and atomic force microscope. TE and TM light coupling into and out of the planar membrane waveguide is thoroughly investigated by a numerical finite element method simulation and experiment. Two pairs of gratings with different periods 350 and 330 nm, and filling factors 0.4 and 0.6, but the same etching depth 70 nm, were prepared. TE and TM light entered and exited the planar membrane through the grating coupling. The removal of the substrate suppresses radiation leakage.
The coupling can be tuned by controlling the grating period, filling factor, and light incident angles. Free-standing HfO2 planar photonics in the visible spectrum may be used to realize specialized or highly sensitive sensors, beam splitters, or display components, and for particular wavelength extraction.
Liu, Q., Wang, W., Sa, T., He, S., Li, X., Zhu, G. and Wang, Y., 2015. Subwavelength gratings on a free-standing HfO2 membrane for out-of-plane coupling of visible light. Applied Physics B, 121(3), pp.353-361.
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