A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Such miniaturization is expected to lead to compact, nanoscale optical devices with applications in cameras, lighting, displays and wearable optics. However, retaining functionality while reducing device size has proven particularly challenging. For example, so far there has been no demonstration of broadband achromatic metalenses covering the entire visible spectrum. Here, we show that by judicious design of nanofins on a surface, it is possible to simultaneously control the phase, group delay and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth. We demonstrate diffraction-limited achromatic focusing and achromatic imaging from 470 to 670 nm. Our metalens comprises only a single layer of nanostructures whose thickness is on the order of the wavelength, and does not involve spatial multiplexing or cascading. While this initial design (numerical aperture of 0.2) has an efficiency of about 20% at 500 nm, we discuss ways in which our approach may be further optimized to meet the demand of future applications. Controlling the geometry of each dielectric element of a nanostructured surface enables frequency-dependent group delay and group delay dispersion engineering, and the fabrication of an achromatic metalens for imaging in the visible in transmission.

/pdf/a-broadband-achromatic-metalens-for-focusing-and-imaging-in-1jmcuho99u.pdf

A broadband achromatic metalens for focusing and imaging in the visible.

We present a method allowing for the imposition of two independent and arbitrary phase profiles on any pair of orthogonal states of polarization-linear, circular, or elliptical-relying only on simple, linearly birefringent wave plate elements arranged into metasurfaces. This stands in contrast to previous designs which could only address orthogonal linear, and to a limited extent, circular polarizations. Using this approach, we demonstrate chiral holograms characterized by fully independent far fields for each circular polarization and elliptical polarization beam splitters, both in the visible. This approach significantly expands the scope of metasurface polarization optics.

/pdf/metasurface-polarization-optics-independent-phase-control-of-3djyufb8ya.pdf

Metasurface Polarization Optics: Independent Phase Control of Arbitrary Orthogonal States of Polarization.

A shape-programmable liquid crystal elastomer comprises polymerized, nematic monomers. The monomers are organized into a plurality of voxels with each voxel having a director orientation.

http://science.sciencemag.org/content/sci/347/6225/982.full.pdf

Voxelated liquid crystal elastomers

Methods of manufacturing a liquid crystal device including depositing a layer of liquid crystal material on a substrate and imprinting a pattern on the layer of liquid crystal material using an imprint template are disclosed. The liquid crystal material can be jet deposited. The imprint template can include surface relief features, Pancharatnam-Berry Phase Effect (PBPE) structures or diffractive structures. The liquid crystal device manufactured by the methods described herein can be used to manipulate light, such as for beam steering, wavefront shaping, separating wavelengths and/or polarizations, and combining different wavelengths and/or polarizations.

Virtual and augmented reality systems and methods

Recent developments have enabled the practical realization of optical elements in which the polarization of light may vary spatially. We present an extension of Fourier optics-matrix Fourier optics-for understanding these devices and apply it to the design and realization of metasurface gratings implementing arbitrary, parallel polarization analysis. We show how these gratings enable a compact, full-Stokes polarization camera without standard polarization optics. Our single-shot polarization camera requires no moving parts, specially patterned pixels, or conventional polarization optics and may enable the widespread adoption of polarization imaging in machine vision, remote sensing, and other areas.

Matrix Fourier optics enables a compact full-Stokes polarization camera

Throughout optics and photonics, phase is normally controlled via an optical path difference. Although much less common, an alternative means for phase control exists: a geometric phase (GP) shift occurring when a light wave is transformed through one parameter space, e.g., polarization, in such a way as to create a change in a second parameter, e.g., phase. In thin films and surfaces where only the GP varies spatially—which may be called GP holograms (GPHs)—the phase profile of nearly any (physical or virtual) object can in principle be embodied as an inhomogeneous anisotropy manifesting exceptional diffraction and polarization behavior. Pure GP elements have had poor efficiency and utility up to now, except in isolated cases, due to the lack of fabrication techniques producing elements with an arbitrary spatially varying GP shift at visible and near-infrared wavelengths. Here, we describe two methods to create high-fidelity GPHs, one interferometric and another direct-write, capable of recording the wavefront of nearly any physical or virtual object. We employ photoaligned liquid crystals to record the patterns as an inhomogeneous optical axis profile in thin films with a few μm thickness. We report on eight representative examples, including a GP lens with F/2.3 (at 633 nm) and 99% diffraction efficiency across visible wavelengths, and several GP vortex phase plates with excellent modal purity and remarkably small central defect size (e.g., 0.7 and 7 μm for topological charges of 1 and 8, respectively). We also report on a GP Fourier hologram, a fan-out grid with dozens of far-field spots, and an elaborate phase profile, which showed excellent fidelity and very low leakage wave transmittance and haze. Together, these techniques are the first practical bases for arbitrary GPHs with essentially no loss, high phase gradients (∼rad/μm), novel polarization functionality, and broadband behavior.

Fabrication of ideal geometric-phase holograms with arbitrary wavefronts

We introduce and demonstrate a compact, nonmechanical beam steering device based on liquid Crystal (LC)
Polarization Gratings (PGs). Directional control of collimated light is essential for free-space optical communications,
remote sensing, and related technologies. However, current beam steering methods often require moving
parts, or are limited to small angle operation, offer low optical throughput, and are constrained by size and
weight. We employ multiple layers of LCPGs to achieve wide-angle (> ±40°), coarse beam steering of 1550
nm light in a remarkably thin package. LCPGs can be made in switchable or polymer materials, and possess
a continuous periodic birefringence profile, that renders several compelling properties (experimentally realized):
~ 100% experimental diffraction efficiency into a single order, high polarization sensitivity, and very low scattering.
Light may be controlled within and between the zero- and first-diffraction orders by the handedness of
the incident light and potentially by voltage applied to the PG itself. We implement a coarse steering device
with several LCPGs matched with active halfwave LC variable retarders. Here, we present the preliminary
experimental results and discuss the unique capability of this wide-angle steering.

Wide-angle nonmechanical beam steering using thin liquid crystal polarization gratings

We introduce and demonstrate a ternary nonmechanical beam steering device based on polarization gratings (PGs). Our beam steering device employs multiple stages consisting of combinations of PGs and wave plates, which allows for a unique three-way (ternary) steering design. Ultrahigh efficiency (∼100%) and polarization sensitive diffraction of individual PGs allow wide steering angles (among three diffracted orders) with extremely high throughput. We report our successful demonstration of the three-stage beam steerer having a 44° field of regard with 1.7° resolution at 1550 nm wavelength. A substantially high throughput of 78%–83% is observed that is mainly limited by electrode absorption and Fresnel losses.

Wide-angle, nonmechanical beam steering with high throughput utilizing polarization gratings

Improved fabrication techniques are creating a new generation of gratings, lenses, and other elements that are physically thin and optically thick.

Controlling Light with Geometric-Phase Holograms

We present a novel optical element that efficiently generates orbital angular momentum
(OAM) of light and transforms light between OAM modes based on a polarization grating with a
fork-shaped singularity This forked polarization grating (FPG) is composed of liquid crystalline
materials, and can be made either static or switchable with high diffraction efficiency (ie, 100%
theoretically) into a single order By spatially varying the Pancharatnam–Berry phase, FPGs
shape the wavefront and thus control the OAM mode We demonstrate theoretically and empirically that
a charge lg FPG creates helical modes with OAM charge ±lg when a Gaussian
beam is input, and more generally, transforms the incident helical mode with OAM charge
lin into output modes with OAM charge lin±lg We also show for
the first time that this conversion into a single mode can be very efficient (ie, ∼95%
experimentally) at visible wavelengths, and the relative power between the two possible output modes
is polarization-controllable from 0% to ∼100% We developed a fabrication method that
substantially improves FPG quality and efficiency over prior work We also successfully fabricated
switchable FPGs, which can be electrically switched between an OAM generating/transforming state and
a transmissive state Our experimental results showed >92% conversion efficiency for both
configurations at 633 nm These holographically fabricated elements are compact (ie, thin
glass plates), lightweight, and easily optimized for nearly any wavelength from ultraviolet to
infrared, for a wide range of OAM charge, and for large or small clear apertures They are ideal
elements for enhanced control of OAM, eg, in optical trapping and high-capacity
information

Jihwan Kim

Papers

Fabrication of ideal geometric-phase holograms with arbitrary wavefronts

Wide-angle nonmechanical beam steering using thin liquid crystal polarization gratings

Wide-angle, nonmechanical beam steering with high throughput utilizing polarization gratings

Controlling Light with Geometric-Phase Holograms

Orbital angular momentum generation and mode transformation with high efficiency using forked polarization gratings.