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High-index dielectric metasurfaces performing mathematical operations
Andrea Cordaro, Hoyeong Kwon, Dimitrios Sounas, A. Femius Koenderink, Andrea Alù, Albert Polman
TL;DR
Digital image processing has size, speed, and power limitations, while Fourier-optics alternatives require bulky components. This work develops dielectric nanobeam metasurfaces for optical processing and reports angle-dependent transmission behavior, with intrinsic silicon absorption limiting transmission.
Problem
Digital image processing has size, speed, and power limitations, while Fourier-optics processing requires bulky optical components.
Method
The work designs dielectric metasurfaces composed of nanobeams whose angular transmission response, or transfer function, is tuned through dispersion.
Results
2.2% residual at normal incidence is attributed to minor fabrication imperfections, while the measured trend agrees with numerical calculations shown in Figure 2.
Takeaways & Limitations
Optical processing using dielectric metasurfaces provides a route toward overcoming conventional image-processing limitations while supporting on-chip integration.
Takeaways & Limitations
Transmission saturates below unity because of intrinsic absorption in silicon, and the one-dimensional geometry differentiates only along the x-axis.
Abstract
from arXiv · showhide
Image processing and edge detection are at the core of several newly emerging technologies, such as augmented reality, autonomous driving and more generally object recognition. Image processing is typically performed digitally using integrated electronic circuits and algorithms, implying fundamental size and speed limitations, as well as significant power needs. On the other hand, it can also be performed in a low-power analog fashion using Fourier optics, requiring however bulky optical components. Here, we introduce dielectric metasurfaces that perform optical image edge detection in the analog domain using a subwavelength geometry that can be readily integrated with detectors. The metasurface is composed of a suitably engineered array of nanobeams designed to perform either 1st- or 2nd-order spatial differentiation. We experimentally demonstrate the 2nd-derivative operation on an input image, showing the potential of all-optical edge detection using a silicon metasurface geometry working at a numerical aperture as large as 0.35.
Methods
The metasurface samples were fabricated using electron-beam lithography, deposition or etching, and resist-processing steps on glass or c-Si/Al2O3 substrates.
- Silicon metasurfaces: c-Si on Al2O3 substrates were cleaned, etched to the metasurface thickness, resist-coated, patterned, developed, and etched again.The silicon pattern transfer used two-step reactive ion etching with Cl2, HBr, and O2.
- Silicon metasurfaces: The developed pattern was transferred into c-Si by a second two-step reactive ion etching process using Cl2, HBr, and O2.
- Final cleaning: The completed samples were cleaned in anisole at 65 °C and then subjected to acid piranha cleaning.
- Glass image samples: Glass slides were cleaned, coated with MMA and PMMA resist, baked, patterned by electron-beam lithography, and developed.The resist stack used 150 nm MMA and 95 nm PMMA, with baking at 150 °C and 180 °C for 2 minutes.
- Glass image samples: A 40 nm Cr layer was evaporated and the residual resist was removed by lift-off in anisole at 65 °C.
Competing interests
The authors declare no competing interests.
- The authors declare no competing interests.
- The passage lists ORCID identifiers for the named authors.