On-Chip Metasurface for Optical Directional Rectification

On-chip two-dimensional (2D) metasurfaces create a new degree of freedom of light manipulation within a 2D surface, which is an emerging research area with fascinating possibilities for realization of very dense integration and miniaturization in photonic systems. As a longstanding challenge in optics steering, light rectification, which means to manipulate beams with multiple directional wavevectors to exit to the same outgoing direction, could hardly be made possible based on conventional optics unless with zero-index medium. However, on-chip metasurface could potentially come to rescue with deep-subwavelength features and its engineered phase signature depending on wavevectors. Here, we design and demonstrate a surface-wave rectification on-chip metasurface (SROM) for visible light, which realizes light rectification for oblique incidences within +/−20°. The SROM is formed by cascading two plasmonic on-chip nanowires of semi-elliptical and trapezoidal structures as its unit cell. Through the geometrical change induced by the semi-elliptical and trapezoidal structures and the plasmonic interaction between the two cascaded nanowires, the incident surface waves with multiple wavevector angles would exit and propagate normally along 0° around 500 nm wavelength, thus mimicking a zero-index-like phenomenon. The equal-frequency contour (EFC) of the designed SROM is also numerically retrieved to exhibit flat angular dispersion, which verifies the proof-of-concept light reification phenomenon. The on-chip miniature SROM is of merely ∼ 3 μm length, and the light rectification performance remains for the visible-frequency band of 500 - 560 nm. Such new on-chip architecture only involves the mature and conventional nanofabrication techniques to manufacture without any alignment for multilayer metasurfaces. We envision that the created beam-steering freedom of light rectification opens a viable and compatible route towards new invention of photonic integrated devices.