932 research outputs found
Dielectric Metasurfaces for Complete Control of Phase and Polarization with Subwavelength Spatial Resolution and High Transmission
Metasurfaces are planar structures that locally modify the polarization,
phase, and amplitude of light in reflection or transmission, thus enabling
lithographically patterned flat optical components with functionalities
controlled by design. Transmissive metasurfaces are especially important, as
most optical systems used in practice operate in transmission. Several types of
transmissive metasurfaces have been realized, but with either low transmission
efficiencies or limited control over polarization and phase. Here we show a
metasurface platform based on high-contrast dielectric elliptical nano-posts
which provides complete control of polarization and phase with sub-wavelength
spatial resolution and experimentally measured efficiency ranging from 72% to
97%, depending on the exact design. Such complete control enables the
realization of most free-space transmissive optical elements such as lenses,
phase-plates, wave-plates, polarizers, beam-splitters, as well as polarization
switchable phase holograms and arbitrary vector beam generators using the same
metamaterial platform.Comment: Nature Nanotechnology (2015
Wide bandwidth and high resolution planar filter array based on DBR-metasurface-DBR structures
We propose and experimentally demonstrate a planar array of optical bandpass
filters composed of low loss dielectric metasurface layers sandwiched between
two distributed Bragg reflectors (DBRs). The two DBRs form a Fabry-P\'erot
resonator whose center wavelength is controlled by the design of the
transmissive metasurface layer which functions as a phase shifting element. We
demonstrate an array of bandpass filters with spatially varying center
wavelengths covering a wide range of operation wavelengths of 250 nm around
{\lambda} = 1550 nm ({\Delta}{\lambda}/{\lambda} = 16%). The center wavelengths
of each filter are independently controlled only by changing the in-plane
geometry of the sandwiched metasurfaces, and the experimentally measured
quality factors are larger than 700. The demonstrated filter array can be
directly integrated on top of photodetector arrays to realize on-chip
high-resolution spectrometers with free-space coupling
Multiwavelength polarization insensitive lenses based on dielectric metasurfaces with meta-molecules
Metasurfaces are nano-structured devices composed of arrays of subwavelength
scatterers (or meta-atoms) that manipulate the wavefront, polarization, or
intensity of light. Like other diffractive optical devices, metasurfaces suffer
from significant chromatic aberrations that limit their bandwidth. Here, we
present a method for designing multiwavelength metasurfaces using unit cells
with multiple meta-atoms, or meta-molecules. Transmissive lenses with
efficiencies as high as 72% and numerical apertures as high as 0.46
simultaneously operating at 915 nm and 1550 nm are demonstrated. With proper
scaling, these devices can be used in applications where operation at distinct
known wavelengths is required, like various fluorescence microscopy techniques
Planar retroreflector
We propose a planar retroreflector composed of two cascaded high contrast periodic structures with slowly varying features. One of the high contrast structures focuses the light while the other reflects it as a concave mirror
Guided resonance reflective phase shifters
We present reflective phase shifters based on high contrast gratings resting on a low-index spacer backed by a metallic mirror. The guided resonance of the grating combined with the reflection from the metallic mirror leads to an all-pass filter with 2Ï€ phase shift variation and unity reflectivity across the resonance. We present simulations, fabrication and measurement of passive devices fabricated in silicon over gold using a polymer as the spacer layer. Active control at high modulation speeds can be achieved by shifting the guided resonance wavelength using carrier injection or thermo-optic effect in silicon
On-chip broadband spectral filtering using planar double high-contrast grating reflectors
We propose a broadband free-space on-chip spectrometer based on an array of integrated narrowband filters consisting of Fabry-Perot resonators formed by two high-contrast grating (HCG) based reflectors separated by a low-index thin layer with a fixed cavity thickness. Using numerical simulations, broadband tunability of resonance wavelengths was achieved only by changing the in-plane grating parameters such as period or duty cycle of HCGs while the substrate geometry was kept fixed. Experimentally, the HCG reflectors were fabricated on silicon on insulator (SOI) substrates and high reflectivity was measured, fabrication process for the proposed double HCG-based narrowband filter array was developed. The filtering function that can be spanned over a wide range of wavelengths was measured
Angle-multiplexed metasurfaces: encoding independent wavefronts in a single metasurface under different illumination angles
The angular response of thin diffractive optical elements is highly
correlated. For example, the angles of incidence and diffraction of a grating
are locked through the grating momentum determined by the grating period. Other
diffractive devices, including conventional metasurfaces, have a similar
angular behavior due to the fixed locations of the Fresnel zone boundaries and
the weak angular sensitivity of the meta-atoms. To alter this fundamental
property, we introduce angle-multiplexed metasurfaces, composed of reflective
high-contrast dielectric U-shaped meta-atoms, whose response under illumination
from different angles can be controlled independently. This enables flat
optical devices that impose different and independent optical transformations
when illuminated from different directions, a capability not previously
available in diffractive optics
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