PERSPECTIVES
Sailing to the stars with photonics
dollars. However,“ Moore’ s law for lasers” predicts the same power laser would be up to three orders of magnitude cheaper within thirty years, becoming comparable to the budget of other large-scale science projects. Solar-cell farms can now fill areas in excess of one square kilometre – could phase-controlled, coherent, large-surface photonic crystal semiconductor lasers one day do the same [ 5 ]?
Once the sail reaches Alpha Centauri, it must send data back to Earth. The best bet is using an onboard laser, ideally repurposing the sail itself as a reflective mirror and using the immense aperture of the Earth-based accelerating laser as a detector. Powering the transmitter, pointing its beam towards Earth and isolating transmitter photons from those of the vastly brighter Proxima Centauri in proximity will be very challenging. No concrete engineering solutions have been proposed yet, but estimates of photon counts and possible modulation schemes have been analysed. Again, no laws of physics need to be violated, but this area will need much further study.
Instead, single-layer, thin, high-index dielectrics are often optimal in terms of acceleration. Photonic crystals and metasurfaces have emerged as better solutions to optimise acceleration: properly designed photonic crystals based on arrays of holes have lower mass than dielectric slabs( e. g. Figure 1c), all while having increased reflectivity. In addition, non-specular reflections from photonic crystals, and phase shift from metasurfaces, can provide transverse forces needed for stabilising the sail in the beam( see below and the insert). An important design aspect to consider when designing the photonic properties of sails is that, for a 0.2c mission, the laser wavelength shifts by up to 22 % due to the Doppler effect. Photonic
Figure 1. Experimental lightsail structures.( a) NASA’ s Advanced Composite Solar Sail System, a solar sail launched in 2024. The size of the sail is roughly that envisioned for a laser-driven mission. Reproduced from [ https:// www. nasa. gov / smallspacecraft / what-is-acs3 /]. Copyright 2024 NASA.( b) Scanning electron micrograph( SEM) of a hexagonally corrugated lattice trilayer( Al 2 O 3-MoS 2-Al 2 O 3, scale bar 50 micron), a highly reflective lightsail candidate. Image kindly provided by M. Campbell as part of the work [ M. Campbell et al., arXiv: 2508.05035( 2025)].( c) SEM of a pentagonal-lattice photonic crystal with numerically optimized hole shapes. The entire fabricated structure is 60 mm × 60 mm. Adapted from [ 7 ] under a CC BY 4.0 licence. Copyright 2025 Springer Nature.( d) Fabricated Si3N4 circular-hole photonic crystal on a Si wafer, possessing enhanced broadband reflection. Adapted from [ J. Chang et al., Nano Lett. 24, 6689−6695( 2024)] under a CC BY 4.0 licence. Copyright 2024 American Chemical Society.
SAIL DESIGN AND MATERIAL SELECTION If the sheer scale of the laser seems daunting, the constraints on the sail make it at least equally challenging: the sail must be extremely light, highly reflective, capable of withstanding tremendous accelerations in excess of 10,000g, and must remain taut and stable within the beam. All while surviving irradiation by the 100 GW laser beam.
To avoid overheating, the sail must be minimally absorptive at NIR wavelengths, where the laser is likely to operate, which precludes any metallic reflective coatings commonly used in solar sails, instead favouring dielectric reflectors [ 6 ]. Bragg reflectors come to mind as highly reflective dielectric-only structures, but the reflectivity benefit of additional layers is rapidly offset by the mass they add.
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