Dr. Ragip Pala Profile

Dr. Ragip Pala

Director of Metamaterial Research and Innovation at Meta Materials Inc.

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Proceedings Article | 14 March 2018 Presentation

>270-degree phase shift in aluminum gate tunable conducting oxide metasurfaces for continuous optical beam steering at 1550 nm (Conference Presentation)

Ghazaleh Kafaie Shirmanesh, Ruzan Sokhoyan, Ragip Pala, Harry Atwater

Proceedings Volume 10541, 105410B (2018) https://doi.org/10.1117/12.2286384

KEYWORDS: Aluminum, Oxides, Antennas, Phase shifts, Phased array optics, Beam steering, Phase shift keying, Optical components, Prisms, Lenses

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In the last several years, metasurfaces have demonstrated promise as both flat optical elements to replace conventional three-dimensional components (prisms or lenses) as well as to access functions that are unachievable in conventional optics. To date, the functional performance of metasurfaces have typically been encoded at the time of fabrication, which fixes the achievable phase and amplitude for each elements in an array. However if actively controlled metasurface elements can be designed to dynamically control the phase shift and amplitude change imposed by each metasurface element, we could realize phased arrays to enable complex spatio-temporal wavefront engineering. We report here design and experimental demonstration of a tunable conducting oxide metasurface that achieves such active control by incorporating materials with voltage-tunable optical permivitties, such as indium tin oxide (ITO), into a metasurface [1]. We design a metasurface that consists of an aluminum back plane, HfO2 gate dielectric followed by a 14 nm thick ITO active layer, and a periodic array of aluminum patch antennas. We choose the dimensions of the Al antennas so that the antenna magnetic dipole resonance occurs at 1550 nm. By applying a gate bias between the Al antenna and ITO active layer, charge accumulation or depletion occurs at the ITO/HfO2 interface. This results in modulation of the ITO complex permittivity, thus altering the metasurface reflection phase and amplitude. The designed metasurface is capable of >270° phase shift. Our design enables independent control of each metasurface element enabling electrical control of the metasurface phase profile, which is an essential requirement for demonstration of continious beam steering. [1] Y.-W. Huang et al., “Gate-Tunable Conducting Oxide Metasurfaces”, Nano Letters 16, 5319-5325 (2016).

Proceedings Article | 3 November 2016 Presentation

Angle- and polarization-insensitive, small area, subtractive color filters via a-Si nanopillar arrays (Conference Presentation)

Katherine Fountaine, Mikinori Ito, Ragip Pala, Harry Atwater

Proceedings Volume 9927, 992705 (2016) https://doi.org/10.1117/12.2238433

KEYWORDS: Optical filters, Amorphous silicon, Waveguides, Nanophotonics, Plasmonics, Thermography, UV optics, Nanolithography, Imaging devices, Ultraviolet radiation

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Spectrally-selective nanophotonic and plasmonic structures enjoy widespread interest for application as color filters in imaging devices, due to their potential advantages over traditional organic dyes and pigments. Organic dyes are straightforward to implement with predictable optical performance at large pixel size, but suffer from inherent optical cross-talk and stability (UV, thermal, humidity) issues and also exhibit increasingly unpredictable performance as pixel size approaches dye molecule size. Nanophotonic and plasmonic color filters are more robust, but often have polarization- and angle-dependent optical response and/or require large-range periodicity. Herein, we report on design and fabrication of polarization- and angle-insensitive CYM color filters based on a-Si nanopillar arrays as small as 1um2, supported by experiment, simulation, and analytic theory. Analytic waveguide and Mie theories explain the color filtering mechanism-- efficient coupling into and interband transition-mediated attenuation of waveguide-like modes—and also guided the FDTD simulation-based optimization of nanopillar array dimensions. The designed a-Si nanopillar arrays were fabricated using e-beam lithography and reactive ion etching; and were subsequently optically characterized, revealing the predicted polarization- and angle-insensitive (±40°) subtractive filter responses. Cyan, yellow, and magenta color filters have each been demonstrated. The effects of nanopillar array size and inter-array spacing were investigated both experimentally and theoretically to probe the issues of ever-shrinking pixel sizes and cross-talk, respectively. Results demonstrate that these nanopillar arrays maintain their performance down to 1um2 pixel sizes with no inter-array spacing. These concepts and results along with color-processed images taken with a fabricated color filter array will be presented and discussed.

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