California Institute of Technology researchers have worked out that super thin structures made of silicon and silica oxide convert waves of infrared light into a momentum that would accelerate a probe to speeds of around 60,000 kilometres (37,000 miles) per second [20% of light speed].
Optimized thin film structures could be promising candidates for a laser-driven light sail, providing the means of efficient propulsion as well as radiative cooling. For the optimal mass case (sail mass = payload mass). Researchers introduced the concept of reflectivity adjusted area density (RAAD) as one relevant figure of merit that seeks to minimize the constraints on the power and the size of the laser array. In addition, they demonstrated that structures which combine materials with high mid-infrared extinction coefficients (such as SiO2) and high near-infrared refractive index (such as Si) can be tailored to maintain a desired steady-state temperature for a range of near-infrared absorption coefficient values. The structures and materials analyzed in this study are intended to highlight the versatility of multilayer thin film structures to achieve a desired multiband optical response for a very low mass. In addition to materials analyzed here, other potential candidates could include IR-active materials such as silicon nitride, silicon carbide, or doped silicon, to name a few.
Light sails propelled by radiation pressure from high-power lasers have the potential to achieve relativistic spaceflight. In order to propel a spacecraft to relativistic speeds, an ultrathin, gram-sized light sail will need to be stably accelerated by lasers with ∼MW/cm2 intensities operating in the near-infrared spectral range. Such a laser-driven sail requires multiband electromagnetic functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-infrared and high emissivity in the mid-infrared for efficient radiative cooling. These engineering challenges present an opportunity for nanophotonic design. Here, we show that designed thin-film heterostructures could become multifunctional building-block elements of the light sail, due to their ability to achieve substantial reflectivity while maintaining low absorption in the near-infrared, significant emissivity in the mid-infrared, and a very low mass. For a light sail carrying a payload, they propose a relevant figure of merit—the reflectivity adjusted area density—that can capture the trade-off between sail mass and reflectivity, independent of other quantities such as the incident beam power, phased array size, or the payload mass. They present designs for effective thermal management via radiative cooling and compare propulsion efficiencies for several candidate materials, using a general approach that could apply to a broad range of high-power laser propulsion problems.
Thin-film structures with Si and SiO2 layers that minimize the reflectivity-adjusted area density W2 through constrained optimization, for different values of αSi. Here, P0/mp = 100 GW g−1 and the maximum allowed temperature is set at Ts = 1000 K. Structure parameters are given. Higher refractive index of silicon combined with radiative cooling properties of silica enables more efficient propulsion relative to silica-only structures. For large absorption (e.g., α = 10−2 per centimeter), thicker layers of silica are needed to ensure equilibrium sail temperature does not surpass Ts; as a result, there is a penalty in W. Red arrow denotes the incident laser light.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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