Harry R. Clark

Versatile alignment layer method for new types of liquid crystal photonic devices

Liquid crystal photonic devices are becoming increasingly popular. These devices often present a challenge when it comes to creating a robust alignment layer in pre-assembled cells. In this paper, we describe a method of infusing a dye into a microcavity to produce an effective photo-definable alignment layer. However, previous research on such alignment layers has shown that they have limited stability, particularly against subsequent light exposure. As such, we further describe a method of utilizing a pre-polymer, infused into the microcavity along with the liquid crystal, to provide photostability. We demonstrate that the polymer layer, formed under ultraviolet irradiation of liquid crystal cells, has been effectively localized to a thin region near the substrate surface and provides a significant improvement in the photostability of the liquid crystal alignment. This versatile alignment layer method, capable of being utilized in devices from the described microcavities to displays, offers significant promise for new photonics applications.

Effects of humidity and surface on photoalignment of brilliant yellow

ABSTRACT Controlling and optimising the alignment of liquid crystals is a crucial process for display application. Here, we investigate the effects of humidity and surface types on photoalignment of an azo-dye brilliant yellow (BY). Specifically, the effect of humidity on the photoalignment of BY was studied at the stage of substrate storage before coating, during the spin-coating process, between film coating and exposure, and after exposure. Surprising results are the drastic effect of humidity during the spin-coating process, the humidity annealing to increase the order of the BY layer after exposure and the dry annealing to stabilise the layer. Our results are interpreted in terms of the effect of water on the aggregation of BY. The type of surface studied had minimal effects. Thin BY films (about 3 nm thickness) were sensitive to the hydrophilicity of the surface while thick BY films (about 30 nm thickness) were not affected by changing the surface. The results of this paper allow for the optimisation of the BY photoalignment for liquid crystal display application as well as a better understanding of the BY photoalignment mechanism. Graphical Abstract

Low dark current, back-illuminated charge coupled devices

Dark current for back-illuminated (BI) charge-coupled-device (CCD) imagers at Lincoln Laboratory has historically been higher than for front-illuminated (FI) detectors. This is presumably due to high concentrations of unpassivated dangling bonds at or near the thinned back surface caused by wafer thinning, inadequate passivation and low quality native oxide growth. The high dark current has meant that the CCDs must be substantially cooled to be comparable to FI devices. The dark current comprises three components: frontside surface-state, bulk, and back surface. We have developed a backside passivation process that significantly reduces the dark current of BI CCDs. The BI imagers are passivated using molecular beam epitaxy (MBE) to grow a thin heavily boron-doped layer, followed by an annealing step in hydrogen. The frontside surface state component can be suppressed using surface inversion, where clock dithering reduces the frontside dark current below the bulk. This work uses surface inversion, clock dithering and comparison between FI and BI imagers as tools to determine the dark current from each of the components. MBE passivated devices, when used with clock dithering, have dark current reduced by a factor of one hundred relative to ion-implant/laser annealed devices, with measured values as low as 10-14 pA/cm2 at 20°C.

A Scalable Fabrication Process for Liquid Crystal Based Uncooled Thermal Imagers

A novel sensor is being developed for a new uncooled imager technology that is scalable to large formats (tens of megapixels), which is greater than what is achieved by commercial microbolometer arrays. In this novel sensor, a liquid-crystal transducer is used to change a long-wavelength infrared scene into a visible image that can be detected using a conventional visible imager. This approach has the potential for making a more flexible thermal sensor that can be optimized for a variety of applications. In this paper, we describe the microfabrication processes required to create an array of sealed thermally isolated micro-cavities filled with liquid crystals to be used for an uncooled thermal imager. Experimental results from the fabricated arrays will also be discussed.

Development and application of spherically curved charge-coupled device imagers

Operation of a CCD imager on a curved focal surface offers advantages to flat focal planes, especially for lightweight, relatively simple optical systems. The first advantage is that the modulation transfer function can approach diffraction-limited performance for a spherical focal surface employed in large field-of-view or large-format imagers. The second advantage is that a curved focal surface maintains more uniform illumination as a function of radius from the field center. Examples of applications of curved imagers, described here, include a small compact imager and the large curved array used in the Space Surveillance Telescope. The operational characteristics and mechanical limits of an imager deformed to a 15 mm radius are also described.

Stable azodye photo-alignment layer for liquid crystal devices achieved by “turning off” dye photosensitivity

We have previously proposed a low cost, versatile process for stabilizing azodye photo-alignment layers for liquid crystal devices by utilizing a surface localized reactive mesogen (RM) layer. The RM is applied by dissolving the monomer in a liquid crystal material prior to filling the cell. In this paper, we show the significant effect of azodye layer thickness on the long term stability of these alignment layers when exposed to polarized light. We demonstrate, surprisingly, that thin azodye layers (∼3 nm) provide improved stability over thicker (∼40 nm) layers. Using this process, we show cells which have been stable to exposure with polarized light through one month. Additionally, we demonstrate the use of a photo-alignment layer to align the liquid crystals that afterwards can be rendered insensitive to polarized light. This was accomplished by using the process described above with the additional step of eliminating the photosensitivity of the azodye layer through photo-bleaching; the result is an RM...

Development of CCDs for REXIS on OSIRIS-REx

The Regolith x-ray Imaging Spectrometer (REXIS) is a coded-aperture soft x-ray imaging instrument on the OSIRIS-REx spacecraft to be launched in 2016. The spacecraft will fly to and orbit the near-Earth asteroid Bennu, while REXIS maps the elemental distribution on the asteroid using x-ray fluorescence. The detector consists of a 2×2 array of backilluminated 1k×1k frame transfer CCDs with a flight heritage to Suzaku and Chandra. The back surface has a thin p+-doped layer deposited by molecular-beam epitaxy (MBE) for maximum quantum efficiency and energy resolution at low x-ray energies. The CCDs also feature an integrated optical-blocking filter (OBF) to suppress visible and near-infrared light. The OBF is an aluminum film deposited directly on the CCD back surface and is mechanically more robust and less absorptive of x-rays than the conventional free-standing aluminum-coated polymer films. The CCDs have charge transfer inefficiencies of less than 10-6, and dark current of 1e-/pixel/second at the REXIS operating temperature of –60 °C. The resulting spectral resolution is 115 eV at 2 KeV. The extinction ratio of the filter is ~1012 at 625 nm.

Liquid crystal uncooled thermal imager development

An uncooled thermal imager is being developed based on a liquid crystal (LC) transducer. Without any electrical connections, the LC transducer pixels change the long-wavelength infrared (LWIR) scene directly into a visible image as opposed to an electric signal in microbolometers. The objectives are to develop an imager technology scalable to large formats (tens of megapixels) while maintaining or improving the noise equivalent temperature difference (NETD) compared to microbolometers. The present work is demonstrating that the LCs have the required performance (sensitivity, dynamic range, speed, etc.) to enable a more flexible uncooled imager. Utilizing 200-mm wafers, a process has been developed and arrays have been fabricated using aligned LCs confined in 20×20-μm cavities elevated on thermal legs. Detectors have been successfully fabricated on both silicon and fused silica wafers using less than 10 photolithographic mask steps. A breadboard camera system has been assembled to test the imagers. Various sensor configurations are described along with advantages and disadvantages of component arrangements.

Oxide-bonded molecular-beam epitaxial backside passivation process for large-format CCDs

We describe recent advances in backside passivation of large-format charge-coupled devices (CCDs) fabricated on 200- mm diameter wafers. These CCDs utilize direct oxide bonding and molecular-beam epitaxial (MBE) growth to enable high quantum efficiency in the ultraviolet (UV) and soft X-ray bands. In particular, the development of low-temperature MBE growth techniques and oxide bonding processes, which can withstand MBE processing, are described. Several highperformance large-format CCD designs were successfully back-illuminated using the presented process and excellent quantum efficiency (QE) and dark current are measured on these devices. Reflection-limited QE is measured from 200 nm to 800 nm, and dark current of less than 1e- /pixel/sec is measured at 40°C for a 9.5 μm pixel.

Process for a Reactive Monomer Alignment Layer for Liquid Crystals Formed on an Azodye Sublayer

In this work, the detailed studies of surface polymerization stabilizing liquid crystal formed on an azodye sublayer are presented. The surface localized stabilization is obtained by free-radical polymerization of a dilute solution of a bi-functional reactive monomer (RM) in a liquid crystal (LC) solvent. To optimize the process for surface localized stabilization, we investigate the effects of several process parameters including RM concentration in LC hosts, the types of materials (either RM or LC), the photo-initiator (PI) concentration, ultra-violet (UV) polymerization intensity, and the UV curing temperature. The quality of surface localized stabilization is characterized and/or evaluated by optical microscopy, electro-optical behavior (transmission/voltage curve), the life test, and photo-bleaching. Our results show that, by carefully selecting materials, formulating mixtures, and controlling the polymerizing variables, the RM polymerization can be realized either at the surface or through the bulk. Overall, the combination of surface localized stabilization and photo-alignment offers an elegant and dynamic solution for controlling the alignment for LC, which could play a profound role in almost all liquid crystal optical devices.