Pentagonal Photonic Crystal Mirrors: Scalable Lightsails with Enhanced Acceleration via Neural Topology Optimization

Abstract

The Starshot Breakthrough Initiative aims to send one-gram microchip probes to Alpha Centauri within 20 years, using gram-scale lightsails propelled by laser-based radiation pressure, reaching velocities nearing a fifth of light speed. This mission requires lightsail materials that challenge the fundamentals of nanotechnology, requiring innovations in optics, material science and structural engineering. Unlike the microchip payload, which must be minimized in every dimension, such lightsails need meter-scale dimensions with nanoscale thickness and billions of nanoscale holes to enhance reflectivity and reduce mass. Our study employs neural topology optimization, revealing a novel pentagonal lattice-based photonic crystal (PhC) reflector. The optimized designs shorten acceleration times, therefore lowering launch costs significantly. Crucially, these designs also enable lightsail material fabrication with orders-of-magnitude reduction in costs. We have fabricated a 60 x 60 mm$^2$, 200nm thick, single-layer reflector perforated with over a billion nanoscale features; the highest aspect-ratio nanophotonic element to date. We achieve this with nearly 9,000 times cost reduction per m$^2$. Starshot lightsails will have several stringent requirements but will ultimately be driven by costs to build at scale. Here we highlight challenges and possible solutions in developing lightsail materials - showcasing the potential of scaling nanophotonics for cost-effective next-generation space exploration.

Figures (8)

• Figure 1: High power earth-based laser propelling a fleet of lightweight sails to 20% of the speed of light, to reach Alpha Centauri in 20 years Starshot. The lightsail needs to be reflective over a broad bandwidth due to the Doppler red-shift of the laser resulting from the change in velocity of the sail. The minimum feature size of a photonic crystal based lightsail is related to the fabrication cost. A commonly used performance metric for a lightsail is the acceleration distance. The launch cost is mainly determined by the energy consumption of the laser EUEprice.
• Figure 2: a, Working principles of different photonic crystal architectures. Multilayered PhC consists of stacked layers with varying refractive indices. The bilayer PhC consists of a repeating PhC pattern on top of a solid membrane. Single layer PhC is a membrane with a repetitive PhC hole pattern. For both the bilayer and single-layer PhC, the incident light creates an optical mode within the material that deconstructively interferes with the transmitted light and constructively with the reflected light. The best optimized single layer PhC design without area constraint for square lattice (b) and hexagonal lattice (c), where black is material and white is vacuum. The square and hexagonal lattice thicknesses are 0.2 $\mu m$ and 0.3 $\mu m$ respectively. d, The pentagonal lattice design for an Area fraction $A_f$ of 55% with a thickness of 0.18 $\mu m$.
• Figure 3: a, Acceleration distance ($D$) for different lattice structures with varying minimum features size (MFS). The red line indicates the D for a 200 nm thick un-patterned PhC membrane. b, The reflectivity spectrum of the selected photonic crystal designs from a for the full Doppler shift region. The rest of the energy is transmitted due to the ppm absorption of SiN Steinlechner2017c, The velocity of the hexagonal and pentagonal PhC lightsail during acceleration compared with the speed of light.
• Figure 4: a, Reflectivity spectrum of two hexagonal PhCs. The red arrow indicates the shift of the reflectivity peak to the laser wavelength (i.e. to the left) when optimizing for T.b, the final design of hexagonal (blue) and pentagonal (green) PhC optimized for T. c, The velocity of the PhC lightsail during acceleration compared with the speed of light. Regions I (orange) and II (yellow) indicate how the beginning of the reflectivity spectrum translates to the initial acceleration of the sail.
• Figure 5: a, Photograph of a 100 mm wafer with a $60 \times 60$ mm^2, 200 nm thick suspended SiN PhC membrane, covered with a pentagonal pattern having a period of 3.0 µm. b, Microscope image of two arrows etched into the substrate pointing towards a $350 \times 350$ µm suspended PhC membrane. The bottom of the orange-framed inset shows the edge of the $60 \times 60$ mm^2 suspended membrane in the same magnification. The $350 \times 350$ µm membrane puts the large membrane in perspective by showcasing the largest single-layer suspended PhC membranes at Starshot's announcement (2016) Chen2017c, 50x magnification of the edge of the membrane. One can see the repeating pattern covering the $60 \times 60$ mm^2 phononic crystal (PhC). The SiN is still attached to the silicon frame in the purple regions. The light pink indicates where the silicon has been removed under the PhC, leaving a suspended SiN PhC membrane. The yellow-framed inset shows a further zoom of the pentagonal lattice taken with a scanning electron microscope.