All entries for ami yaacobi

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Frequency-chirped Sub-wavelength Nanoantennas

• Authors: A.Yaacobi, M R. Watts
• Sponsorship: DARPA Microsystems Technology Office SWEEPER Project DE-AC04-94AL85000

Vertical coupling can be used in silicon on insulator-based optical phased arrays (OPA) to emit light traveling in a waveguide to a free space mode. Vertical input/output coupling has been extensively investigated for coupling to or from optical fibers e.g.^[1][2][3]; thus coupler size generally equals the diameter of a fiber mode (~10 µm or more). This large side (>>the wavelength) inherently cause the coupler to be narrow banded (< 60 nm band width). Moreover, in an OPA, a small footprint  is desired in an emitter  (ideally <1 wavelength), as it is critical to fitting many emitters in a small area. Short emitters have been demonstrated using nanoantenna designs translated from the radio frequency (RF) to optical bands^[4][5][6]. However, RF antennas can be fed from thin transmission lines, enabling excitation of the antenna to occur from a single location. In a photonic nanoantenna, the open dielectric waveguide precludes separation of the guided wave from the antenna structure, so the guided wave interacts with the entire antenna.  This difference requires a new approach to enable compact, efficient emission from such antennas.

Our work^[7] proposes a new nanoantenna structure that uses resonance-chirped components to create a short, broadband vertical coupler (Figure 1). The coupler couples light from a Si photonic waveguide to free space and consists of Al nanorods above a Si₃N₄ waveguide, and a metal ground plane to reflect up downward radiation. We simulated this antenna in the telecom C-band using finite-difference time-domain techniques.  We predict 55% peak efficiency and ~ 500 nm of bandwidth (Figure 2) in a structure only λ/3 long. Using different metal and dielectric materials would allow this coupler to work from the optical band to RF. To our knowledge this structure is the first proposed and numerically verified compact (<1 wavelength) emitter design of its kind.



Figure 1. (a) Side view of single resonator (here a dipole) driven by plane wavefront, (b) Set of chirped resonators design for right-angle wave coupling driven with same source as (a), (c) Side and top views of set of resonators designed for vertical coupling from waveguide to free space. Total structure length is <1>



Figure 2. (a) Simulated vertical emission for different structures, (b) Final structure with time integrated field box showing radiated field, (c) Free space far field pattern of same structure at λ=1550 nm.




1. G. Roelkens, D. Vermeulen, S. Selvaraja, R. Halir, W. Bogaerts and D. V. Thourhout, “Grating-Based Optical Fiber Interfaces for Silicon-on-Insulator Photonic Integrated Circuits,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 17, no. 3, pp. 571-580, (2011). [↩]
2. M. Fan, M. A. Popović, and F. X. Kärtner, “High Directivity, Vertical Fiber-to-Chip Coupler with Anisotropically Radiating Grating Teeth,” in OSA/CLEO CTuDD3 (2007). [↩]
3. G. Roelkens, D. V. Thourhout and R. Baets, “High efficiency grating coupler between silicon-on-insulator waveguides and perfectly vertical optical fibers,” Optics Letters vol. 32, pp. 1495-1497, (2007). [↩]
4. A. Yaacobi, E. Timurdogan, and M. Watts, “Vertical emitting aperture nanoantennas,” Optics Letters  vol. 37, pp. 1454-1456 (2012). [↩]
5. T. Kosako, Y. Kadoya, H. F. Hofmann, “Directional control of light by a nano-optical Yagi-Uda antenna,” Nature Photonics, vol. 4, no. 5, pp. 312-315 (2010). [↩]
6. J. Li and N. Engheta, “Core-shell nanowire optical antennas fed by slab waveguides” IEEE Transactions on Antennas and Propagation, Vol. 55, No. 11, Nov. 2007 [↩]
7. A. Yaacobi and M. Watts, “Frequency-chirped sub-wavelength nanoantennas,” Optics Letters, vol. 37, pp. 4979-4981 (2012). [↩]

Vertical Emitting Aperture Nanoantennas

• Authors: A. Yaacobi, E. Timurdogan, M. R. Watts
• Sponsorship: DARPA Microsystems Technology Office SWEEPER Project DE-AC04-94AL85000

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 (~10µm). Vertical coupling can also be used in optical phased arrays^[1]),^[2]) 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.

To overcome this limitation, our work^[3] 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 Si₃N₄ 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.



Figure 1. Antenna designs: (a) side view, (b) top view of laterally repeating apertures, and (c) longitudinally repeating apertures (period Λ=0.9µm<λ/n). Bottom layer of Al represents ground plane positioned at d=3λ/4n (680nm) from emitter metal.



Figure 2. (a) Simulated vertical emission for structures with ground-plane: (blue dash-doted) single aperture, (dashed green) 3 apertures arranged laterally to waveguide direction, (solid cyan) 3×2 array design to emit exactly to zenith, (magenta solid-w-dots) 3×2 array design for 10º backward emission, (red dashed-line-w-dots) 3×6 array design for 5º backward emission, and (black line-w-Xs) ohmic losses of 3×6 array, (b) simulated 3×6 structure with time Fourier integrated field box showing emitted fields. (c) Far field pattern of same structure at λ=1550nm.




1. K. V. Acoleyen, H. Rogier and R. Baets, “Two-dimensional optical phased array antenna on silicon-on-Insulator,” Optics Express, vol. 18, pp. 13655-13660 (2010 [↩]
2. J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, L. A. Coldren, and J. E. Bowers, “Two-dimensional free-space beam steering with an optical phased array on silicon-on-insulator,” Optics Express, vol. 19, pp. 21595-21604 (2011 [↩]
3. A. Yaacobi, E. Timurdogan, and M. Watts, “Vertical emitting aperture nanoantennas,” Optics Letters,  vol. 37, pp. 1454-1456 (2012). [↩]