Theodore Macioce
California Institute of Technology
My proposed work will focus on developing a gradient-index silicon lens with broad-band antireflection (AR) achieved with deep reactive ion etching (DRIE) for use in future millimeter and submillimeter astronomy missions. The primary science case for such lenses is the observation of primordial gravitational waves from inflation, as imprinted on the B-mode polarization pattern of the cosmic microwave background (CMB). Since the observed B-mode pattern is contaminated by foregrounds from dust and synchrotron emission, this measurement will require broad spectral coverage to separate out the foregrounds. As outlined in NASA’s 2017 Physics of the Cosmos (PCOS) Annual Technology report, there is a need for optics with antireflection over a broad spectral band; these will allow a wide frequency range to be imaged in a single focal plane, enabling compact designs for future space-based missions.
To realize such optics, the proposed work will design structures in silicon to lower its effective index of refraction using a novel DRIE technique that combines multi-layer etching and wafer bonding. This technique allows for varying the index both radially, to produce focusing, and axially, to implement broad-band antireflection. Our group has already produced and tested optical flats with two-layer coatings; four-layer coatings are currently being developed. The main emphasis of my work will be to incorporate a radial index gradient in a 15-cm diameter optic, but I will also need to expand to a 5-layer AR coating to provide sufficient bandwidth (75-300 GHz) for dust foreground subtraction to address the PCOS technology gap. This approach can scale to even broader bandwidths, making the proposed work a significant step towards extending to lower frequencies to enable synchrotron foreground removal.