Flat, Lightweight optics have the potential to significantly reduce the cost of space-based observing by allowing for reduced vehicle sizes and launch costs. We have designed, manufactured, and tested a metamaterial flat lens which operates at 480GHz. 480GHz was chosen as an intermediate step to designing a 557GHz lens, which is a frequency that has scientific importance as it is a ground-state water transition line, but nearly impossible to observe from the ground or from a balloon due to water in the atmosphere. The lens is constructed from polyimide (generic Kapton) and aluminum. The metamaterial design consists of ten layers of sub-wavelengthsized aluminum squares, sized via optimization to achieve the ideal phase transformation to create a 150mm focal distance at 480GHz. This optimization process also creates an effective anti-reflection quality. The lens has an aperture size of 124mm and an f-number of f/1.2. It weighs approximately three grams and is 110 microns thick. We have demonstrated the lens has near diffraction-limited beam performance with roughly 2.5dB of loss. The loss was measured using a radiometric y-factor method, using a room-temperature absorber as the hot load and an absorber submersed in liquid nitrogen as the cold load. The beam performance was measured using a near-field scan of the lens with a waveguide probe at the focus to illuminate the lens and a second probe to measure the phase and magnitude of the near-field collimated output. The loss was roughly 1.5 dB higher than expected in our design simulations.
In this manuscript, a rapid design process flow for multi-layered Terahertz (THz) optical components for Cosmic Microwave Background (CMB) telescopes using artificial dielectric metamaterials made up of periodic 2D grid of metal squares embedded in a dielectric material with a fixed interlayer spacing, termed ”capacitive grids”, is discussed. The modeling of such metamaterials using ideal RF transmission line sections to achieve an arbitrary value of refractive index and their fabrication using aluminum embedded in a polyimide dielectric is presented. Finally, the design and HFSS simulations results of a stepped-impedance low-pass quasi-optic metamaterial filter is discussed as an application of such a rapid design methodology in designing complex quasi-optical components with variable refractive indices in the millimeter wave and THz astronomy applications.
KEYWORDS: Time correlated photon counting, Photon counting, Field programmable gate arrays, Signal attenuation, Stars, Single photon, Photodetectors, Optical communications, High dynamic range imaging, Clocks
The continuous time-tagging of photon arrival times for high count rate sources is necessary for applications such as optical communications, quantum key encryption, and astronomical measurements. Detection of Hanbury- Brown Twiss photon correlations from thermal sources such as stars requires a combination of high dynamic range, long integration times and low systematics in the photon detection and time tagging system. The continuous nature of the measurements and the need for highly accurate timing resolution requires a customized time-to-digital converter (TDC). We used a custom built, two-channel, FPGA-based TDC to continuously time tag single photons with sub clock cycle timing resolution for correlation measurements. We used autocorrelation and cross-correlation measurement tools to constrain spurious systematic effects in the pulse count data as a function of system variables. These variables included, but were not limited to, incident photon count rate, incoming signal attenuation, and measurements of fixed signals. We present an overview of the results of these tests, the types of systematic effects that the results imply, and how those effects may be accounted for and corrected to levels below those required for photon correlation measurements.
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