{"id":1696,"date":"2013-07-25T18:32:09","date_gmt":"2013-07-25T18:32:09","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/?p=1696"},"modified":"2013-08-14T16:31:45","modified_gmt":"2013-08-14T16:31:45","slug":"vertical-emitting-aperture-nanoantennas","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2013\/vertical-emitting-aperture-nanoantennas\/","title":{"rendered":"Vertical Emitting Aperture Nanoantennas"},"content":{"rendered":"

Vertical input\/output coupling in the telecom C-band (1535 nm << 1565 nm) has been extensively investigated for coupling to or from an optical fiber. Hence, most vertical couplers to date have size similar to fiber modal diameters (~10\u00b5m). Vertical coupling can also be used in optical phased arrays[1<\/a>]<\/sup>),[2<\/a>]<\/sup>) to steer a beam or generate holograms. However in a phased array, a compact structure (a few wavelengths long or less) is critical in order to fit many emitters in a small area and enable wide rotation angles with a single lobe. Most vertical couplers rely on gratings in semiconductors or insulators alone to perturb the electromagnetic field and emit efficiently. Due to the index contrasts available in dielectric structures, this method allows only weak perturbation and inherently limits the extraction rate.<\/p>\n

To overcome this limitation, our work[3<\/a>]<\/sup> considers the use of metals to perturb the electromagnetic field and achieve efficient vertical coupling in a compact structure. We show that the inherent large permittivities and conductivities of metals enable large coupling coefficients and efficient output in an aperture nanoantenna, a structure only a few wavelengths long. We theoretically investigated and numerically simulated variations on the structure. In the case shown in Figure 1, we assume an aluminum nanoantenna structure is fabricated on top of a Si3<\/sub>N4<\/sub> waveguide and an aluminum ground plane is placed below the waveguide. The ground plane breaks up\/down symmetry, allowing us to couple > 50% of the light upward. Finite-difference time-domain (FDTD) simulations shown in Figure 2 reveal up to 65% coupling efficiency and a coupling bandwidth of 300nm. The use of metallic nanoantennas allows for efficient coupling in a short structure which in turn, enables the large bandwidth. The specific structure designed here is a nanophotonic aperture antenna, but the results here are general; designed properly, nanoantennas enable rapid emission from dielectric waveguides.<\/p>\n\n\t\t