A. A. Cruz-Cabrera

Integration of fluorescence collection optics with a microfabricated surface electrode ion trap

We have successfully demonstrated an integrated optical system for collecting the fluorescence from a trapped ion. The system, consisting of an array of transmissive, dielectric micro-optics and an optical fiber array, has been intimately incorporated into the ion-trapping chip without negatively impacting trapping performance. Epoxies, vacuum feedthrough, and optical component materials were carefully chosen so that they did not degrade the vacuum environment, and we have demonstrated light detection as well as ion trapping and shuttling behavior comparable to trapping chips without integrated optics, with no modification to the control voltages of the trapping chip.

Metamaterial-inspired high-absorption surfaces for thermal infrared applications

We present design, fabrication, and characterization results of a highly absorptive surface in the thermal infrared that draws on concepts from the frequency selective surface and metamaterials communities. At normal incidence this optically thin surface has an absorption of over 99%. Furthermore, it has a broad angular range (over 90% absorption at 60 degrees from normal). The simple structure is composed of a reflective metal layer, a roughly quarter-wave layer of lossy dielectric, and a top metal layer that is patterned with an array of subwavelength apertures. The design of the aperture allows spectral and angular control of the absorption/emission band. We will present simulation and measured results. Change in waveband and polarization could easily be changed from pixel to pixel in a focal plane array.

Fabrication and measurement of wideband achromatic waveplates for the mid-infrared region using subwavelength features

Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-µm spectral region. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.

Polarimetric imaging cross talk effects from glue separation between FPA and micropolarizer arrays at the MWIR

We have numerically and experimentally determined the effect of crosstalk from adjacent gold wiregrid micropolarizer pixels in a midwave infrared (MWIR) focal plane array (FPA). Fabrication of a snapshot polarization-imaging device involves gluing a micropolarizer array substrate on top of an FPA. We evaluated several arrays of super-cells of four pixelated polarizers by modeling the near fields behind the devices. Each polarizer in the super-cell is oriented to allow solving three Stokes parameters by themselves or four Stokes parameters in conjunction with a birefringent waveplate. In addition, we fabricated sets of super-cells for determining optimum polarizer-FPA separation. Modeling and empirical data indicate cross talk between the adjacent pixels at several microns after crossing the polarizer plane. Cross talk between adjacent pixels increases uncertainty in the measured polarization states of a scene of interest. Data shows that the extinction ratio will decrease by 17% when moving the FPA from 0.5 &mgr;m to 1.0 &mgr;m away from the polarizer. These changes in extinction ratio are important given that typical glue separation is approximately 10 &mgr;m.

Fabrication and testing of finite aperture polarizers for determination of edge termination effects on polarimetric imaging applications at midwave infrared

Polarimetric imaging applications at the 2-to-5-µm or midwave infrared range use large pixel-count focal plane arrays (FPAs) with small pixel sizes. We report on the design, fabrication, and characterization of micropolarizers for the 2-to-5-µm regime. These micropolarizers will be used in conjunction with a FPA and will be in the near field of the imaging device. The pixel pitches for some commercial FPAs are small enough that the finite apertures of the polarizing devices may significantly affect their performance, because their aperture dimension varies between three and five waves. We are interested in understanding the effect on the extinction ratio due to variations in the edge terminations of a polarizer with a small aperture. Edge terminations are the spaces between the first and last wire with the perimeter of the aperture of the polarizer. To verify these effects, we fabricated micropolarizers with apertures of 5 to 20 µm and with termination edge spaces of one-quarter and three-quarters of the wiregrids period. The devices measured extinction ratios from 50:1 for the smallest aperture to 200:1 for the largest. Simulations and measurements show that the extinction ratio is larger for the smaller termination edge spacing.

Pixilated wideband achromatic waveplates fabricated for the mid IR using subwavelength features

Subwavelength diffractive features etched into a substrate lead to form birefringence that can be utilized to produce polarization sensitive elements such as waveplates. Using etched features allows for the development of pixilated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Typically, the main drawback from using diffractive devices is their high sensitivity to wavelength. Taking advantage of the dispersion of the form birefringence, diffractive waveplates with good achromatic characteristics can be designed. We will report on diffractive waveplates designed for minimal phase retardation error across the 2-5 micron spectral regime. The required fabrication processes of the sub-wavelength feature sizes will be discussed as well as the achromatic performance and transmission efficiency of final devices. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. In addition, the effect of the finite size of the apertures of the pixilated devices is of particular interest since they are designed to be used in conjunction with a detector array. The influence of small aperture sizes will also be investigated.

Edge termination effects on finite aperture polarizers for polarimetric imaging applications at mid wave IR

Polarimetric imaging applications at the 2 to 5 μm or Mid-Wave Infrared (MWIR) range use large pixel-count focal plane arrays (FPA) with small pixel size. This project is centered in designing, fabricating and testing micropolarizers that work in that wavelength regime and intended for that type of FPAs. The micro-polarizers will be used in conjunction with a FPA in snapshot mode and will be in the near field of the imaging device. The pixel pitches for some commercial FPAs are small enough that the finite apertures of the polarizing devices may significantly affect their performance given that their aperture size varies between 3 and 5 waves. We are interested in understanding the effect on extinction ratio due to variations in the edge terminations of a polarizer with a small aperture. Edge terminations are the spaces between the first or last wire with the perimeter of the aperture of the polarizer. While this parameter has negligible effects on a larger polarizer, it will be significant for apertures that are about 3 to 5 waves. We will present data that indicates significant variation in performance due to edge terminations.

Fabrication and testing of plasmonic optimized transmission and reflection coatings

We designed, fabricated, and tested surface-plasmon-based transmissive coatings in the MWIR (mid wave infrared) and LWIR (long wave infrared). This method offers certain advantages over current coatings technologies such as thin-film stacks and two-dimensional surface structuring (e.g. motheyes) while exploring an entirely different physical mechanism for achieving transmission. Thin-film stack technology relies on interference between layers of the stack, and often many layers are required for high efficiency performance. Two-dimensional surface structuring can optimize transmission over a broad spectral and angular domain1. Here the physical mechanism is an effective index matching between air and the substrate due to subwavelength surface features, such as tall pyramids. These pyramids must have a high-aspect ratio, resulting in a surface of many tall thin features, which may not be mechanically robust. In this work, we created a transmissive surface out of a metal skin perforated with an array of subwavelength apertures. The surface is the infrared analog of a frequency selective surface (FSS) common in the microwave regime. Such perforated metal surfaces are predicted to have nearly 100% transmission over selected wavelength and angular ranges. These ranges are determined by array geometry, period, and aperture size and shape, allowing the designer considerable freedom. Array geometry and aperture shape were investigated for tailoring spectral features.

Combustion diagnosis for analysis of solid propellant rocket abort hazards: Role of spectroscopy

Solid rocket propellant plume temperatures have been measured using spectroscopic methods as part of an ongoing effort to specify the thermal-chemical-physical environment in and around a burning fragment of an exploded solid rocket at atmospheric pressures. Such specification is needed for launch safety studies where hazardous payloads become involved with large fragments of burning propellant. The propellant burns in an off-design condition producing a hot gas flame loaded with burning metal droplets. Each component of the flame (soot, droplets and gas) has a characteristic temperature, and it is only through the use of spectroscopy that their temperature can be independently identified.

Demonstration of thermal emission control

In this paper, we describe our efforts to control the thermal emission from a surface utilizing structured surfaces with metal/dielectric interfaces. The goal was not to eliminate the emission, but to control the output direction and spectrum. We focus on methods that lead to high emissivity at grazing angles, with low emission near normal. We describe the fabrication and measurement of large passive devices (15×15 mm) and arrays of smaller chips for thermal emission control in the longwave infrared (8 to 12 micron) spectral region. All the devices consist of a metal base layer covered with dielectric/metal posts or lines, 0.5 microns tall. The posts (0.9×0.9 micron) and lines (0.3 micron wide) are subwavelength. One-dimensional and two-dimensional devices with a 3 micron pitch will be shown. The devices are measured with both a hemispherical directional reflectometer and a variable angle directional emissometer. Both simulated and experimental results show the thermal emission effectively limited to a small spectral region and grazing angles from the surface (≥ 80°) in stark contrast to the typical Lambertian radiation seen from unstructured material. Finally, the effect of this thermal emission control is illustrated using an infrared camera.