Current Optics and Photonics

Optical Society of Korea (한국광학회)
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Directional Orbital Angular Momentum Generator with Enhanced Vertical Emission Efficiency

  • Tran, Thang Q. (Department of Electrical and Computer Engineering, Ajou University) ;
  • Kim, Sangin (Department of Electrical and Computer Engineering, Ajou University)
  • Received : 2019.03.28
  • Accepted : 2019.06.11
  • Published : 2019.08.25
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Abstract

We propose a ring resonator-based orbital angular momentum carrying vortex beam generator design with high vertical directional emission efficiency. By adopting a vertically asymmetric grating structure in the ring resonator, optimized for enhanced vertical emission, an emission efficiency in one direction reaches as high as 78%, exceeding the 50% theoretical limit of previously designed vertically symmetric grating-assisted ring resonator-based structures.

Keywords

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FIG. 1. (a) Side-grating-assisted ring resonator-based OAM generator, (b) our proposed top-grating assisted ring resonator- based OAM generator. The structural parameters of our OAM generator: R = 3.9 μm, tg = 59.76 nm, t = 310 nm, w = 480 nm, d = 100 nm, grating fill factor FF = 0.1, and number of grating n = 41. The structural parameters of the device in (a) are the same as given in [3]. In both devices, the waveguide is made of Si (nSi = 3.4) embedded in SiO2 (nSiO2 = 1.45).

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FIG. 2. Radiation directivity dependence on the waveguide thickness of the ring resonator. Other parameters are fixed: R = 3.9 μm, tg = 60 nm, w = 480 nm, grating fill factor FF = 0.5, and number of gratings n = 41. The directivity is defined as a ratio of the radiated power into the upper free-space region and the total radiation power. The calculation was performed without the input waveguide. The guiding mode of the ring resonator was excited by a modal source located inside the ring resonator.

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FIG. 3. Radiation efficiency and orbital angular momentum per photon (l) of (a) the top-grating and (b) the side-grating. l denotes the calculated angular momentum per photon at resonance. In the case of the side-grating, e represents the peak radiation efficiency at resonance for both upward and downward radiation.

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FIG. 5. Changes in vertical radiation efficiency of the device when certain parameters are out of optimization as a result of manufacturing defects: when (a) the ring width w, (b) the grating thickness tg or (c) the fill factor FF are, respectively, 5% larger than the optimal value. Their corresponding peak radiation efficiencies are 54%, 62% and 70%, respectively.

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FIG. 4. Amplitude and phase profiles of the generated OAM beam of our proposed structure compared to those of an array of azimuthally arranged dipoles with corresponding phase shift. The l number indicated was given by the formula l = p - q [3].

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