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The need for larger arrays of millimeter and submillimeter wavelength detectors for Cosmic Microwave Background (CMB) experiments is driving a demand for focal planes which can field large numbers of detectors with both high sensitivity and wide bandwidth. Current CMB experiments have $\sim 10^{4}$ detectors, with next generation focal planes requiring $\sim 10^{5}$ or more. One challenge of expanding the array size is coupling the detectors to instrument optics with a method that is broadband, low loss, and scalable.
Current state of the art methods of coupling incident radiation include phased array antenna-coupled detectors, corrugated feedhorn arrays, and hemispherical lenslet array-coupled planar antennas. Phased array antennas are fabricated using planar lithography techniques and therefore easily scalable, but are typically narrow band ($\sim 30\%$). Silicon platelet feedhorns are scalable and low loss, but typically achieve only an octave of bandwidth. Lenslets have been produced using silicon hemispheres stacked on silicon plates to approximate an elliptical lens. Low loss and broadband behavior is accomplished by individually molding anti-reflection layers made of materials with appropriate refractive indices and individually glued to arrays; however this approach does not easily scale to larger arrays.
We are developing planar lenslet arrays using metamaterials fabricated with standard microlithograpy techniques on silicon wafers. Instead of using difficult to manufacture curved optical surfaces, the lenslets consist of stacks of silicon wafers which are each patterned with an array of sub-wavelength features to produce optical features which form a well defined beam at measurement wavelengths. These arrays are being developed using two approaches: GRadient INdex (GRIN) lenslets which are fabricated by etching holes on a sub-wavelength grid to produce a spatially varying effective index of refraction, and metal-mesh lenslets which are produced by depositing spatially varying metallic features which act as a series of Transmission Line (TL) lumped element features to control phase delay across the wafer.
GRIN lenslets are fabricated by etching sub-wavelength holes on a periodic, sub-wavelength grid using standard microlithography techniques. The wafers can be stacked, allowing the spatial index to be altered along all dimensions, which allows for arbitrary anti-reflective coatings to be integrated in the lenslet design. Simulations in finite element modeling (FEM) software have been used to both evaluate the effective index of an individual element and simulate full lenslet structures.
Dielectrically embedded mesh-lenses are based on existing mesh-filter technology. Differently from the mesh-filters, the grids are inhomogeneous and their geometry is designed in such a way to impart variable phase shifts across the surface. The local phase shifts reproduce those that would be introduced by a classical dielectric lens. In this work we are developing mesh-lenslets on silicon substrates. The metal grids are supported by silicon nitride (SiN) membranes and kept at specific distances in an air-gap configuration. Finite element analysis is used to quantify and optimize the performance of these devices.
We report on progress in both lenslet design approaches. In each case we have developed a set of design equations which guide the design of the full lenslet structure. These structures are simulated using finite element modeling simulations. We report on measurements and efficacy of the design and simulation process and agreement with laboratory measurements of prototype lenslet arrays.
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