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Metasurfaces have been rapidly advancing our command over the many degrees of freedom of light within compact, lightweight devices. However, so far, they have mostly been limited to manipulating light in free space. Grating couplers provide the opportunity of bridging far-field optical radiation and in-plane guided waves, and thus have become fundamental building blocks in photonic integrated circuits. However, their operation and degree of light control is much more limited than metasurfaces. Metasurfaces integrated on top of guided wave photonic systems have been explored to control the scattering of light off-chip with enhanced functionalities – namely, point-by-point manipulation of amplitude, phase or polarization. However, these efforts have so far been limited to controlling one or two optical degrees of freedom at best, and to device configurations much more complex compared to conventional grating couplers. Here, we introduce leaky-wave metasurfaces, which are based on symmetry-broken photonic crystal slabs that support quasi-bound states in the continuum. This platform has a compact form factor equivalent to the one of conventional grating couplers, but it provides full command over amplitude, phase and polarization (four optical degrees of freedom) across large apertures. We present experimental demonstrations of various functionalities for operation at λ= 1.55 μm based on leaky-wave metasurfaces, including devices for phase and amplitude control at a fixed polarization state, and devices controlling all four optical degrees of freedom. Our results merge the fields of guided and free-space optics under the umbrella of metasurfaces, exploiting the hybrid nature of quasi-bound states in the continuum, for opportunities to advance in disruptive ways imaging, communications, augmented reality, quantum optics, LIDAR, and integrated photonic systems.
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Gradient metasurfaces for integrated optics, plasmonics, and optoelectronics
Gradient metasurfaces provide a well-established platform for the wavefront and polarization control of light propagating in free space. More recent studies have also focused on their integration with plasmonic and dielectric waveguides and optoelectronic devices such as light emitters and photodetectors. In this context, metasurfaces can be used to interface guided modes with free-space radiation, for example, to enforce the selective input coupling or detection of light with prescribed characteristics (in terms of direction of propagation, state of polarization, orbital angular momentum, etc.) or to manipulate the properties of the out-coupled or emitted light according to a desired system operation. These functionalities, which normally involve complex combinations of bulky optical elements, have important applications in multiple areas of high technological significance, from optical interconnects to microdisplays and multifunctional image sensors. Here, we provide a tutorial overview of this field of research, including a description of the basic building blocks (meta-atoms) used in integrated gradient metasurfaces, a review of the main capabilities reported to date, and an outlook on future directions of study and technological prospects.
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- Award ID(s):
- 2139451
- PAR ID:
- 10661653
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- APL Photonics
- Volume:
- 10
- Issue:
- 11
- ISSN:
- 2378-0967
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0284959
