Squeezing a Prism into a Surface: Emulating Bulk Optics with Achromatic Metasurfaces

• Published in: Advanced optical materials Vol. 8; no. 23
• Main Authors: Tsilipakos, Odysseas, Kafesaki, Maria, Economou, Eleftherios N., Soukoulis, Costas M., Koschny, Thomas
• Format: Journal Article
• Language: English
• Published: 01.12.2020
• Subjects: achromatic metasurfaces; broadband; flat optics; gradient metasurfaces; multiple resonances
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.202000942

Metasurfaces promise to replace bulky prisms and lenses with 2D surfaces, revolutionizing wavefront control with technologically significant advantages in size, weight, and planar fabrication. However, conventional implementations suffer from large chromatic aberrations and cannot sustain performance over practical bandwidths of real‐world signals because of the limited phase modulation margin available in a surface. How can an infinitely thin surface generate the arbitrarily large, broadband phase delay that bulk phase accumulation can provide? Here, equivalence between bulk optics and certain multiresonant metasurfaces is demonstrated, where phase delay arises from trains of multiple resonances in the effective sheet conductivities of the surface itself instead of accumulation of propagation phase. The fundamentally required electromagnetic surface conductivities of a purely achromatic metasurface are derived and general design rules for arbitrarily broadband beam steering and lensing are obtained. Both operation in transmission and reflection can be achieved in a unified way by proper alignment of the resonances, enabling 360° directionality. The arbitrary spectral bandwidth, dual reflection/transmission operation, and complete dispensability of propagation phase constitute a major advance in the state of the art of achromatic metasurfaces for wavefront manipulation. Equivalence between bulk optics and the proposed multiresonant metasurfaces is demonstrated by replacing the accumulation of propagation phase with resonant phase delay, making unprecedentedly thin realizations of optical components possible. The response is perfectly achromatic and the bandwidth can be extended arbitrarily. Operation in transmission and reflection mode are treated in a unified way by spectrally shifting the resonances.