Chulwoo Oh

Achromatic diffraction from polarization gratings with high efficiency

We demonstrate a broadband, thin-film, polarizing beam splitter based on an anisotropic diffraction grating composed of reactive mesogens (polymerizable liquid crystals). This achromatic polarization grating (PG) manifests high diffraction efficiency (approximately 100%) and high extinction ratio (> or = 1000:1) in both theory and experiment. We show an operational bandwidth Deltalambda/lambda0 approximately 56% (roughly spanning visible wavelength range) that represents more than a fourfold increase of bandwidth over conventional PGs (and significantly larger than any other grating). The diffraction angle and operational region (visible, near-infrared, midwave infrared, and ultraviolet wavelengths) may be easily tailored during fabrication. The essence of the achromatic design is a stack of two chiral PGs with an opposite twist sense and employs the principle of retardation compensation. We fully characterize its optical properties and derive the theoretical diffraction behavior.

Time-domain analysis of periodic anisotropic media at oblique incidence: an efficient FDTD implementation.

We describe an efficient implementation of the finite-difference time-domain (FDTD) method as applied to lightwave propagation through periodic media with arbitrary anisotropy (birefringence). A permittivity tensor with non-diagonal elements is successfully integrated here with periodic boundary conditions, bounded computation space, and the split-field update technique. This enables modeling of periodic structures using only one period even with obliquely incident light in combination with both monochromatic (sinusoidal) and wideband (time-domain pulse) sources. Comparisons with results from other techniques in four validation cases are presented and excellent agreement is obtained. Our implementation is freely available on the Web.

Simplified Spectropolarimetry Using Reactive Mesogen Polarization Gratings

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.

Wide-angle, nonmechanical beam steering with high throughput utilizing 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.

Polarization-independent modulation for projection displays using small-period LC polarization gratings

— Progress in the use of liquid-crystal polarization grating (LCPG) to modulate unpolarized (and polarized) light with a grating period as small as 6.3 μm is reported. Similar to LCPGs formed at larger periods (11 μm) reported previously, polarization-independent switching, predominantly three diffraction orders, maximum contrast ratios of ∼100:1 for unpolarized broadband light, very low scattering, and diffraction efficiencies >98% continue to be observed. The smaller period led to an expected lower threshold voltage, even though the thickness was greater. Because the smaller grating period enables a brighter result from a Schlieren projection scheme for a microdisplay using the LCPG light valve, the inherent tradeoffs involved with both material and design parameters are discussed, and prospects for a polarization-independent projection display are commented upon.

High-Throughput Continuous Beam Steering Using Rotating Polarization Gratings

A new beam steering concept comprising independently rotating, inline polarization gratings (PGs) is experimentally demonstrated. The approach, which we term Risley gratings, achieves high steering throughput within a large field-of-regard (FOR) in a fashion similar to Risley prisms, composed of wedged prisms. However, because PGs are patterned in thin liquid crystal layers, they enable a system with far less thickness, weight, and beam walk-off. Furthermore, large apertures are feasible and wavelengths from visible to infrared can be chosen. Any direction within a solid angle defined by twice the diffraction angle of each PG can be addressed mechanically. Here we demonstrate a Risley grating system with a 62° FOR and 89%-92% transmittance at 1550-nm wavelength, using two PGs with 6-¿m grating period.

Polarization Gratings: A Novel Polarimetric Component for Astronomical Instruments

Polarization gratings (PGs) have been recently been developed for ultraefficient liquid crystal displays, nonmechanical optical beam steering, and telecommunication devices at optical and near-infrared wavelengths (0.4–2.0 μm). A PG simultaneously acts as both a spectroscopic and polarimetric disperser for circularly polarized light. With the use of a quarter-wave retarder (or analog) to convert linearly to circularly polarized light, these devices can be used as linear polarimetric analyzers. PGs offer high throughput and high levels of birefringence and can currently be constructed inexpensively to diameters of 150 mm, and development projects are in progress to double that size. In this article we report on the characterization of a PG sample at mid-infrared wavelengths (2–40 μm), including the birefringence, throughput, spectral response, and cold cycling survivability. We discuss these devices in the context of astronomical polarimetry, especially as the polarimetric components for a conceptual study of a SOFIA-based polarimeter.

P-167: FDTD and Elastic Continuum Analysis of the Liquid Crystal Polarization Grating

liquid crystal polarization grating (LCPG) has advantages of polarization independence of the zero diffraction order, nearly 100% diffraction efficiency in the first orders, and higher resolution capability over previously reported binary LC gratings. Here we analyze the LCPG, an electrically controlled, polarization-independent light modulator using the finite- difference time-domain (FDTD) method and the elastic continuum theory. The optical performance is studied and critical electrical parameters for a LCPG cell are presented. 1. Introduction of liquid crystal (LC) light modulators operating on unpolarized light have been proposed in an effort to overcome the substantial losses in conventional LC displays that require polarized light. Most prominently, Bos and coworkers suggested several potentially efficient and polarization-independent diffraction gratings using liquid crystals (1-4). These approaches, however, face crucial limitations for real applications due to their binary nature: fabrication difficulty and the easy appearance of defects. They therefore tend to have inherently large periods that result in many diffraction orders with small diffraction angles. A newly demonstrated liquid crystal polarization grating (LCPG) (5) is a switchable diffractive optical element with a continuously varying, periodic, anisotropic index profile. As shown in Fig. 1(a), this can be embodied as a nematic director that follows (6): n(x) = ˆ cos(x / ) + ˆ y sin(x /) + ˆ (0) (1)

34.4L: Late‐News Paper: Polarization Independent Projection Systems Using Thin Film Polymer Polarization Gratings and Standard Liquid Crystal Microdisplays

We report on our progress in using polymer Polarization Gratings (PGs) to enhance the brightness of standard Liquid Crystal (LC) microdisplays. By replacing conventional polarizers with PGs, high transmittance (>85%) and contrast ratios (>300:1) can be achieved with common LC modes and unpolarized input light. Using this approach, we demonstrate a polymer-PG projection display prototype with double the brightness compared to a polarizer-based system using the same illumination (input acceptance angles≥±15°).

Reflective liquid crystal polarization gratings with high efficiency and small pitch

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.