We introduce a new beam steering concept of the "Risley grating" that consists of independently rotating inline
polarization gratings (PGs). The Risley grating concept replaces the bulky prismatic elements of the Risley prisms
with thin plates containing polarization gratings, and employs their highly polarization-sensitive diffraction. As
rotating two PGs, the output beam tracks within a field-of-regard (FOR), which is determined by the grating
period and their relative orientations. Since PGs are typically patterned in thin liquid crystal layers (a few μm
thick), the system can be implemented with far less thickness and weight. In addition, these thin gratings can
be placed with virtually zero proximity and the beam walk-off becomes negligible. We demonstrate the Risley
grating that performs continuous steering with 62° FOR and 89-92% transmittance at 1550 nm wavelength. The
governing equations for the steering angles of the Risley grating in the direction cosine space are also presented.
We report our experimental success in realizing high efficiency liquid crystal polarization gratings (LCPGs) on
reflective substrates, with periods as small as 2.2μm, enabling the largest switchable LCPG diffraction angles
reported yet for red light. Moreover, these gratings retain nearly ideal electro-optical properties, including
> 95% hologram efficiency, high polarization contrast, sub-millisecond total switching times, and relatively low
voltage operation (thresholds ~1.5V). We discuss two different fabrication approaches, each with its own set of
advantages, which have resulted in gratings with the above compelling properties. We anticipate broad utility
of these diffractive elements in a variety of applications.
We have experimentally demonstrated broadband light modulation by achromatic liquid crystal (LC) polarization
gratings (PGs), which manifest polarization-independent modulation with high efficiencies (≥95%). Recently, we
introduced achromatic PGs with a unique double-layer, reversed-twist structure as efficient, broadband polarizing
beamsplitters. We now report on our successful implementation of electrically switchable achromatic LCPGs on a
reflective substrate. To pattern a spiraling, periodically varying LC profile, we utilize polarization holography and
photoalignment techniques. Use of reflective substrates enables the same retardation compensation of double-layer
achromatic PGs. In addition, perhaps most importantly, the single cell structure allows the electro-optical
switching/modulation by applying an electric field across the cell. The achromatic LCPG sample shows steeper
voltage responses and less spectral shifts while operating in grayscale with respect to previously reported LCPGs.
Relatively faster switching times (~6 msec for 3 μm-thickness) were measured compared to a conventional
LCPG with the same thickness (~10 msec). Interesting electro-optical behaviors were also observed including
zero-voltage threshold and a hysteresis in the voltage response.
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.
We introduce and experimentally demonstrate an achromatic polarization grating (PG), which manifests high
diffraction efficiency (> 99.5%) over a broad range of spectrum. Unlike conventional phase gratings, this family
of PGs has unique diffraction properties including three non-zero diffraction orders (m = 0,±1) with up to
100% efficiency and strongly polarization sensitive first-order diffraction. It has long been recognized that
these diffractive optical elements are useful for beamsplitting, polarimetry, displays, and more. A conventional
(Circular-type) PG implemented with a spiraling, in-plane, linear birefringence has a modest spectral range
(Δλ/λ0 congruent to 6.8%) over which it possesses > 99.5% efficiency. We have identified a two-layer twisted PG structure
that achieves achromatic diffraction that achieves a five-fold improvement of the high efficiency bandwidth
(Δλ/λ0 congruent to 34.3%). We have successfully implemented this structure with reactive mesogens (polymerizable
liquid crystals) with a small amount of left- and right-hand chiral agents, and here report on its operation
over nearly the entire range of visible light. We also investigated the behavior of the achromatic PG with the
finite-difference time-domain method using an Open Source software package WOLFSIM, developed at NC State
University, in order to evaluate the angular selectivity and the paraxial limit.
We report a numerical analysis of the liquid crystal polarization grating (LCPG) as an electro-optically controlled, polarization independent light modulator. The 2D finite-difference time-domain (FDTD) modeling for periodic anisotropic structures has been developed as a numerical tool to study optical properties of anisotropic gratings. Both normal and oblique incidence cases are successfully implemented for wide-band analysis. Nematic director profiles of the LCPG are obtained from elastic free-energy calculations using a commercial software tool, called LC3D. A study of the essential diffraction characteristics of the LCPG is presented, which manifests pixel-level light modulation with a nearly 100% efficiency on unpolarized light. The effect of an off-axis input and the grating regime on the LCPG diffraction is investigated. Finally, we present a study of the electro-optical response of the LCPG when an electric field applied for both static and dynamic cases. The FDTD results show that a highly efficient, polarization-independent light modulation with capability of an electrical switching/tuning is possible by the LCPG.
The measurement of complete polarimetric parameters for a broad spectrum of wavelengths is challenging because of the multi-dimensional nature of the data and the need to chromatically separate the light under test. As a result, current methods for spectropolarimetry and imaging polarimetry are limited because they tend to be complex and/or relatively slow. Here we experimentally demonstrate an approach to measure all four Stokes parameters using three polarization gratings and four simultaneous intensity measurements, with potential to dramatically impact the varied fields of air/space-borne remote sensing, target detection, biomedical imaging/diagnosis, and telecommunications. We have developed reactive mesogen polarization gratings using simple spin-casting and holography techniques, and used them to implement a potentially revolutionary detector capable of simultaneous measurement of full polarization information at many wavelengths with no moving or tunable elements. This polarimeter design not only enables measurements over a likely bandwidth of up to 70% of the center wavelength, it is also capable of measurements at relatively high speed (MHz or more) limited only by the choice of photo-detectors and processing power of the system. The polarization gratings themselves manifest nearly ideal behavior, including diffraction efficiencies of greater than 99%, strong polarization sensitivity of the first diffraction orders, very low incoherent scattering, and suitability for visible and infrared light. Due to its simple and compact design, simultaneous measurement process, and potential for preserving image registration, this spectropolarimeter should prove an attractive alternative to current polarization detection and imaging systems.