IFS for Dark Matter Characterization

The high‐energy physics (HEP) community has identified integral field spectroscopy (IFS) as an area that could dramatically leverage investments in current and future sky surveys for the study of dark energy. Many lines of evidence now agree that most mass in the universe is in the form of dark matter, which interacts mainly via the force of gravity. Identification and detailed phenomenology of dark matter remain poorly understood. Although the last few years have seen significant progress in understanding these phenomena, the most important issues are still open and two requirements have emerged as important in obtaining better understanding: 1) obtaining spatially resolved spectroscopic data is essential; and 2) multi‐object survey capabilities need to be extended to the near infrared (NIR). The integral field unit (IFU) is the most critical part of the spectrograph, and optical components also typically comprise a major portion of an IFS‐equipped telescope’s capital budget, and its development time often paces the entire science project. Currently, three techniques are used in IFU optics: mirror‐slicers, fiber‐slicers and micro‐pupil‐arrays. Challenges common to each are: 1) obtaining high optical throughput (efficiency); 2) eliminating spectrum‐to‐spectrum crosstalk; 3) reducing Siedel and chromatic aberrations so diffraction‐limited spot sizes and uniform focal lengths are achieved across the spectral range; and 4) achieving high contrast. Due to deficiencies in addressing these challenges, existing IFS instruments cannot deliver the performance required for efficient dark energy surveys.Using additive manufacturing (AM) of 3D freeform gradient‐index optics, IFU optics will be developed that offer diffraction‐limited performance over the ~0.4 – 2.2‐μm spectral range. AM gradient‐index optics will be shown capable of correcting for geometric and chromatic aberrations, which—combined with integral pinholes and baffles—allow for the high signal‐to‐noise ratio (SNR), high contrast, and spectral resolution that must be achieved to meet the needs of HEP sky surveys at low cost. The ability to rapidly design, fabricate, and replicate IFU optics will be shown to benefit multiplexed‐aperture IFS instruments. A series of microlens arrays and stacked microlens assemblies will be fabricated and characterized. Microlens arrays (MLAs)—ranging in format from 32 x 32 to 200 x 200 elements, with square and hexagonal sampling, on pitches 120 μm to 3 mm—will be fabricated. The combined optical materials and fabrication technology will be shown to have reduced geometric and chromatic aberrations. The devices will be shown to have uniform foci over large areas, with near diffraction‐limited performance across the spectral range. After demonstrating feasibility, fully functional IFU optical assemblies, including micro‐pupil array and micro‐pupil‐fiber slicer assemblies will be fabricated, characterized, and demonstrated with a spectrograph.The printed 3D freeform lens technology allows complex optical assemblies to be implemented in thin planar optical films with a minimal number of components. The value of the innovation is best realized: in high‐performance optical systems, where the size, weight, and cost are necessarily dominated by the optics; and in miniature optical assemblies, where performance is constrained by size and weight restrictions. Applications include smaller more‐ efficient optics for high‐power industrial lasers, lower‐mass solar concentrators, 3D displays, head‐mounted displays, CMOS imager lens arrays, and camera lenses.