Photonic Crystals: Suspended photonic-crystal mirrors could become lightsails for interstellar probes

Founded in 2015 by Russian entrepreneur and physicist Yuri Milner and his wife Julia, Breakthrough Starshot is a research and engineering program that aims to extend today's technology to demonstrate a proof of concept for laser-propelled miniature interstellar probes that could reach 20% of the speed of light. The goal of the $100 million program is to "lay the foundations for a flyby mission to Alpha Centauri within a generation."^{1, 2}

The idea is to create a phased array of ground-based lasers with a combined optical output of up to 100 GW, and to design and fabricate ultralight chip-based camera-carrying space probes with masses of a few grams and sails on the order of 16 m². If everything went right, the radiation force of the laser light would accelerate the spacecraft to a 0.2c velocity within about 20 min (an acceleration of about 100 km/s²). Thirty years or so later, images of the Alpha Centauri system, and ideally of its planets, would arrive at Earth. To boost chances of success, about 1000 of these probes would be launched.

As for the propulsion system, many challenges exist to the creation of not only the ground-based laser array and its phasing and transmission through the atmosphere, but also the probe's lightsail itself. In response to the latter, a research group at Delft University of Technology (Delft, Netherlands), led by Richard Norte, is developing and increasing the area of ultralight highly reflective photonic-crystal (PhC)-based membrane mirrors only nanometers thick.³

Nanoscale patterns over meter sizes needed

"One of the biggest technological problems for Starshot Breakthrough Initiative is conceiving of a way to make >99% reflectivity mirrors which span 4 × 4 meters and have a mass of only 1 gram," Norte says. "It's estimated that realizing such lightsails should allow us to send microchip nanoprobes with cameras, communication, and power systems to the nearest star within 20 years—as opposed to the nearly 10,000 years it would take with conventional propulsion systems. While many of the Starshot's goals rely on the usual miniaturization of nanotechnology for the microchip, the lightsails stand out as a unique problem. It's the only component which needs to expanded to meter scales while keeping its nanoscale thickness and patterning."

While highly reflective PhC membranes have been made before, their size has been limited to about 100 × 100 μm² by the types of microfabrication processes used. Clearly, this size is too small not only for spacecraft lightsails, but also for the numerous other utilities for which PhC membranes could be put to use.

In contrast, Norte and his group have now fabricated PhC membranes with sizes up to a centimeter square and reflectivities exceeding 99%. The size of these membranes could be straightforwardly scaled up further using well-known techniques such as nanoimprint lithography, lithography stepping, or interference lithography.

The experimental PhC membranes were fabricated from silicon nitride (SiN) in versions that were 56 and 210 nm thick, with the thinner version being only 0.038 of a wavelength thick at a 1550 nm wavelength and yet achieving a reflectivity of 90%—an experimental first for such a thin PhC membrane, say the researchers.

A typical structure consists of a rectangular array of holes (see figure), with about 6 × 10⁷ holes in a membrane 1 cm on a side. One interesting property of SiN is that at cold temperatures equivalent to those of outer space, the mechanical quality of SiN resonators increases—for example, when cooled to 10 K, the mechanical quality increases by an order of magnitude—interesting for experiments limited by the thermal displacement noise of conventional mirrors with distributed Bragg reflector (DBR) coatings.

In addition to use in space as a lightsail when scaled up further in size, the researchers see these PhC membranes proving useful for deformable mirrors and as interferometer mirrors in future cryogenic gravitational-wave experiments where the PhC membranes could reduce thermal noise.

REFERENCES

1. See https://goo.gl/JgHzf6.

2. See https://goo.gl/HajBTW.

3. J. P. Moura et al., arXiv:1707.08128v1 [physics.optics] (Jul. 25, 2017).