Sailing to the stars with photonics PERSPECTIVES solutions must thus be relatively broad band. Greater than 50 % reflectivity averaged over the full Doppler band has been demonstrated in the lab on fabricated Al 2 O 3-MoS 2-Al 2 O 3 photonic crystals that are less than 200-nm-thick( Figure 1b), while other groups have developed membranes as large as 60 mm × 60 mm [ 7 ]( Figure 1c), a substantial step towards metre-scale lightsails.
Even minimal absorption in the sail leads to heating, which must be re-radiated into the vacuum of space, requiring high emissivity at mid-infrared( MIR) wavelengths. Materials like silicon that have high refractive index, and thus strong reflection, can be marred by excessive absorption without providing sufficient emissivity in the MIR. To prevent the temperature reaching material limits, the simplest solution is to add a highly MIR-emissive layer such as silica on the space-facing side. An even more promising strategy for simultaneous propulsion and thermal regulation comes from nanopatterning. Photonic crystals can be patterned at the NIR length scale, achieving high reflectivity, but further structured at the larger MIR length scale to resonantly enhance the sail emissivity. Alternatively, more advanced active photonic cooling mechanisms have been proposed, such as rare-earth-ion doping in the sail as used in solid state laser cooling. The most likely sail material candidates are thus dielectrics with high tensile strength to withstand the acceleration, high refractive index, low NIR absorption and crucially, the ability to be manufactured as large scale patterned freestanding films.
An area that requires more investigation is the sail’ s spatial temperature distribution and evolution. Dust particles are anticipated to be a significant source of heating, both from impact on the sail and, if they attach to the membrane, as a localised absorption site. Strategies to mitigate dust impacts( e. g. through detaching locally heated segments or shielding) are needed. In comprehensive thermodynamic simulations of sail-temperature evolution, a silicon nitride sail was shown to be effective at staying below its melting point over the full extent of the simulation, however, the simulation time was limited to just a few seconds due to computational costs( in comparison to the several-minutes-long acceleration phase).
STABILITY DESIGN One of the biggest challenges with the lightsail mission is the sail’ s dynamical stability. The sail should intercept as much beam power as possible and stay on course to its target, so it must be confined within the laser beam for the entire acceleration phase lasting several minutes. Stability is nontrivial given continuous perturbations from, for example, atmospheric beam distortions or laser noise. There is no mass budget available for traditional stabilisers like thrusters, and the round-trip time of photons back to the laser is too long for closed-loop stabilisation. Stabilisation must thus be achieved locally on the sail, and be passive, ideally stemming from the radiation-pressure profile itself [ 2 ].
In essence, two mechanisms are required: restoring forces / torques and damping forces / torques. The restoring forces and torques trap the sail within the laser beam area, whereas damping forces and torques diminish the amplitudes of oscillations excited by perturbations. Passively generating restoring and damping can be done using the incoming laser momentum. However, this presents an inherent trade-off: laser momentum that is otherwise reflected specularly, thus providing maximum propulsion, must be partly scattered perpendicularly to the beam propagation to create transverse forces and torques( see the insert).
Restoring forces act like a spring, growing in magnitude with the distance between the sail centre and the laser-beam axis, guiding the sail back to the beam centre. In practice, this would be achieved by the sail scattering light in the same direction as the displacement from the laser-beam axis, thus repositioning towards the beam centre by conservation of momentum [ 4 ]. Restoring torques are similar, relying on changes in scattering with changes in angle relative to the ideal orientation. The simplest designs that exhibit such restoring properties are
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