Many of NASA’s direct imaging of exoplanet missions and projects require fabricated coronagraph masks to control scattering and diffraction of light. The designed, patterned mask intended for the coronagraphic testbeds are highly absorptive in the visible range on non-metallic regions. In this work, we employed the cryogenic etching process to fabricate black silicon (BSi) to achieve a high aspect ratio (HAR) structures with higher etch rate than conventional reactive ion etching (REI). Recent bidirectional reflectance distribution function (BRDF) measurements of uniformly etched BSi on silicon wafer show highly diffusive BSi with a specular reflective component in the orders of seven magnitudes lower than the total hemispherical reflectance when the polarized or non-polarized incident beam is used.
We are developing lumped-element kinetic inductance detectors (LEKIDs) designed to achieve background-limited sensitivity for far-infrared (FIR) spectroscopy on a stratospheric balloon. The Spectroscopic Terahertz Airborne Receiver for Far-InfraRed Exploration (STARFIRE) will study the evolution of dusty galaxies with observations of the [CII] 158 micron and other atomic fine-structure transitions at z = 0.5 - 1.5, both through direct observations of individual luminous infrared galaxies, and in blind surveys using the technique of line intensity mapping. The spectrometer requires large format arrays of dual-polarization-sensitive detectors with NEPs of 1e-17 W/sqrt(Hz). We pattern the LEKIDs in 20-nm aluminum film, and use an array of profiled feedhorns to couple optical radiation onto the meandered inductors. A backshort etched from the backside to a buried oxide layer insures high absorption efficiency without additional matching layers. Initial testing on small sub-arrays has demonstrated a high device yield and median NEP of 4e-18 W/sqrt(Hz). We describe the development and characterization of kilo-pixel arrays using a combination of dark noise measurements and optical response with our cryogenic blackbody.
The star formation mechanisms at work in the early universe remain one of the major unsolved problems of modern astrophysics. Many of the luminous galaxies present during the period of peak star formation (between redshifts 1 and 3) were heavily enshrouded in dust, which makes observing their properties difficult at optical wavelengths. However, many spectral lines exist at far-infrared wavelengths that serve as tracers of star formation during that period, in particular fine structure lines of nitrogen, carbon, and oxygen, as well as the carbon monoxide molecule. Using an observation technique known as intensity mapping, it would be possible to observe the total line intensity for a given redshift range even without detecting individual sources. Here, we describe a detector system suitable for a balloonborne spectroscopic intensity mapping experiment at far-infrared wavelengths. The experiment requires an “integralfield” type spectrograph, with modest spectral resolution (R~100) for each of a number of spatial pixels spanning several octaves in wavelength. The detector system uses lumped-element kinetic inductance detectors (LEKIDs), which have the potential to achieve the high sensitivity, low noise, and high multiplexing factor required for this experiment. We detail the design requirements and considerations, and the fabrication process for a prototype LEKID array of 1600 pixels. The pixel design is driven by the need for high responsivity, which requires a small physical volume for the LEKID inductor. In order to minimize two-level system noise, the resonators include large-area interdigitated capacitors. High quality factor resonances are required for a large frequency multiplexing factor. Detectors were fabricated using both trilayer TiN/Ti/TiN recipes and thin-film Al, and are operated at base temperatures near 250 mK.