Detectors & Imaging

Optical Antennas: Bundt-pan-shaped optenna demonstrates 80 dB infrared concentration

Oct. 1, 2019

An optical antenna fabricated in the shape of a Bundt pan can concentrate energy over targeted infrared wavelength bands by up to 8 orders of magnitude.

Gail Overton

The optenna array (a) consists of a series of conical Bundt-pan-shaped structures (b) with dimensions on the order of few microns (c) that are optimized for impedance matching with IR radiation in a certain wavelength region. FDTD simulations (d) show how the IR energy is funneled into the annular ring and concentrated at the IR-absorbing layer.

Attempts to miniaturize radiation-collecting devices such as photodetectors and solar cells often result in poor collection efficiency; however, a variety of plasmonic nanoscale optical antennas made with gold nanoparticles and 2D graphene layers and with horn shapes and circular and spiral arrays, for example, have been fabricated that can concentrate infrared (IR) energy over various wavelength bands by up to seven orders of magnitude.

Improving on existing designs by demonstrating 80 dB (eight orders of magnitude) polarization-insensitive IR concentration in each of the near-IR (NIR), short-wavelength IR (SWIR), and mid-IR (MWIR) bands is a tube- or Bundt-pan-shaped optical antenna developed by professor Ehab Awad at King Saud University (Riyadh, Saudi Arabia).¹The conical shape of the device creates a large aperture that allows it to gather radiation from an 80° wide field of view.

The recipe

On a bulk silicon substrate, a 700-nm-thick silicon nitride (Si₃N₄) IR absorbing layer is topped with conical, horn, and waveguide structures that constitute one Bundt-shaped cell (see figure). A series of these cells is built into an array with physical parameters optimized through 3D finite-difference time-domain (FDTD) simulations to maximize radiation capture over a particular IR wavelength range through impedance matching between free space and the array.

Each Bundt-shaped structure has a gold conical horn region that focuses IR energy by exciting surface plasmon polaritons (SPPs) on the conical surfaces and funnels the TE₁₁ mode energy down to a vertical waveguide structure with 50 nm width in a ring shape at the base of the Bundt structure. The array has a central cone-to-cone separation L and each Bundt structure has a depth and diameter optimized to concentrate a NIR, SWIR, or MWIR wavelength band (for example, the MWIR-optimized array has center-to-center L = 3.25 µm, Bundt-pan outer diameter A = 3.2 µm, conical-region Bundt-pan depth E = 5 µm, and waveguide depth G = 1 µm). The 50 nm waveguide width and 700 nm absorbing layer thickness are fixed for all designs.

Simulations show that the eight-orders-of-magnitude concentrated IR energy is funneled via the waveguides onto an annular ring structure that terminates at the absorbing IR material layer and is rapidly attenuated in the absorber.

“The Bundt optenna is promising for nanoscale IR detection devices such as solar cells, telecommunications photodetectors, shortwave cameras, and midwave microbolometers,” says Awad. “During the optenna design and optimization process of trying to reach peak performance, it was surprising to find out that the best optenna shape looks like a Bundt cake pan.”

REFERENCE

1. E. Awad, Sci. Rep., 9, 12197 (Aug. 21, 2019); www.nature.com/articles/s41598-019-48648-6.

About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.