PERSPECTIVES
Sailing to the stars with photonics
geometrically shaped mirrors, such as an inverted cone flying apex-down in a parabolic-intensity beam. Yet, photonic crystals and metasurfaces have emerged as superior candidates with restoring mechanisms because they can provide stability with a planar geometry( having better beam interception efficiency for a given mass) in a Gaussian beam, and can come with the enhanced scattering dispersion needed for torques and damping forces.
Restoring forces alone under continuous excitation by perturbations lead to oscillations with growing amplitude, with the sail eventually leaving the beam and shooting off course. To keep these in check, damping forces and torques are required, which oppose velocities away from the beam axis and oppose destabilising angular velocities. Damping in the vacuum of space is tricky. One way to achieve it is by altering the magnitude of restoring forces and torques over time through parametric or adiabatic damping. This can be done through active feedback by periodically switching the laser off, or more naturally, through the sail’ s dispersive scattering response and slow change of wavelength( Doppler shift) over the course of acceleration. More promising suggestions involve explicitly velocity-dependent forces, such as adding an internal damped degree of freedom into the sail( like earthquake dampeners in skyscrapers) or using relativistic optical effects( e. g. the relativistic Doppler effect, used in fields of study like cavity optomechanics). For instance, damping the rotational motion of a lightsail is possible using the Doppler redshift and blueshift of light on the portions of the sail rotating away from and towards the laser, respectively. The advantage of photonic structures like gratings is apparent for these methods, as they can dramatically enhance any change in scattering due to minute changes in wavelength – but taking advantage of this dispersion over the full Doppler shifted band is challenging.
The best way to stabilize sails is still an active topic of theoretical investigation, and we don’ t yet have the technology to make finely structured thin membranes on the scales required for interstellar missions. However, increasingly large, high-quality membrane photonic crystals have been fabricated in the lab( Figure 1c, d). Researchers have measured non-specular scattering from fabricated gratings and found consistency with the requirements of self-stabilizing lightsail motion. Moreover, direct radiation pressure measurements of restoring forces using a torsional pendulum apparatus or tethered membranes have been successful.
CONCLUSION Lightsails are the most promising spacecrafts for reaching near-relativistic speeds, opening highways into new star systems, and providing answers to the fundamental question of our place in the universe. While not entirely realistic with today’ s technology, based on current pace of progress in laser and fabrication technology, many researchers believe an interstellar mission could be launched within a few decades.
Many challenges, such as largescale lasers and nanofabrication, will find progress driven by other
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industries and scientific endeavours including telecoms, semiconductor manufacturing, defence, and fusion research. We expect that once these technologies mature enough for the price tag of the required lasers to be within grasp of large-scale science projects such as LIGO or the LHC, the prospect of an interstellar mission will encourage more targeted research into practical implementations of lightsail missions, which in turn will spur technological advances across scientific fields. The comparison with LIGO is one that often appears among lightsail researchers. To us, lightsails are where LIGO was fifty years ago, when the detection of gravitational waves, requiring the resolution of shifts less than the width of a nucleus over 4 km interferometric arms, seemed to be farfetched science fiction. Many technical challenges were posed without obvious solutions. But with the resolve of visionary scientists, gravitational wave astronomy is now a reality, and perhaps in another fifty years, incentivised by progress in photonic technologies, interstellar probes will be too.
ACKNOWLEDGEMENTS This research was supported by an Australian Government Research Training Program( RTP) scholarship.
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