Source: next Big Future
Proposals using large light sails like the Starshot Breakthrough, require mirrors with lateral sizes of 4 × 4 m2, thicknesses of 0.05 lambda, reflectivities of 90 %, ppm level optical absorption and a total mass of only 1 gram. Photonics Crystal are designed with a lattice of holes which remove about 30 % of the mass of the membrane. Additionally, they are made of LPCVD SiN which has an imaginary refractive index of about 1.0E-6 at 1064 nm and has been shown to withstand high laser powers of around 250 gigaWatt per square meter – nearly 2 orders of magnitude more than what is required for the initiative. The mirrors created here mark the first proof-of-concept which could realistically be implemented at larger scales for the Starshot Breakthrough Initiative. The high intrinsic tensile stress of LPCVD SiN allows to expand these mirrors to large diameters
while remaining relatively flat. This is a crucial feature for the fabrication of continuous, suspended films with enough stability to avoid segmented mirror geometries supported by small frames, dramatically decreasing the overall weight of a light sail.
Researchers experimentally demonstrated freestanding SiN photonic crystal mirrors with thicknesses of 56 and 210 nm and diameters of up to 10 mm. Not only do we increase the area of suspended PhC mirrors by nearly 4 orders of magnitude compared to previous works. They also show that these large aspect-ratios allow us to achieve high reflectivity from membranes 3 times thinner than previously measured. Initiatives like the Starshot Breakthrough (targeting travel at 20% of lightspeed) require meter-sized light sails with a thickness of only tens of nanometers.
The fact that these mirrors are suspended allows them to be used in a variety of applications that profit from mechanical tuning of mirrors. Deformable mirrors could be realized with these PhC structures for example through electrostatic-tuning with arrays of electrodes close to the mirror, or even as displacement noise tunable mirrors, using techniques such as optomechanical feedback control. Further experiments are planned to study the transversal mode composition of the reflected beam. These developments open up a new paradigm in nanotechnology – one that steers away from the focus on simply miniaturizing components, but instead tries to bring the performance of nano-engineered materials to large scales.
Thus the needed material is at hand. Initially sprite-sats to other star systems may suffice, but eventually we’ll want to send bigger vehicles. A much higher power level means higher acceleration and so a Sail-Beam can be contemplated for pushing a bigger vehicle via a stream of sails. A Beamer for pushing the micro-sails at one frequency, and a Blaster to ionise the sails at another frequency, should be straightforward to engineer. Albeit at a GW power-scale.
Photonic crystal (PhC) membranes are suspended dielectric sheets patterned with sub-wavelength, low-index two-dimensional periodic structures. These patterns give rise to resonances that can couple out-of-plane radiation to in-plane guided modes, and can be engineered to transform a flat membrane into a mirror, a lens, or even a curved mirror. Here we study a PhC consisting of a periodic lattice of holes in a membrane, whose hole radius and lattice constant can be tuned to reflect light at a wavelength of choice. When fabricated from materials with low optical absorption such as low-pressure chemically vapor-deposited silicon nitride (LPCVD SiN), one can realize mirrors with subwavelength thicknesses and reflectivities over 99 %, only limited by scattering losses. LPCVD SiN thin films also enable the combination of PhC mirrors with low thermal noise mechanical oscillators, due to their high intrinsic stress, thin geometry, and weak coupling to undesired thermal modes. Limitations in microfabrication processes have so far restricted suspended PhC mirrors to areas not much bigger than 100 microns × 100 microns. This size sets an upper bound to the waist of incident Gaussian beams, since wider waists do not completely interact with the PhC, resulting in decreased reflectivity.
Demand for lightweight, highly reflective and mechanically compliant mirrors for optics experiments has recently seen a significant surge. Due to their bulky geometry, standard mirror solutions have a high mass, which severely limits their use in applications such as light sails, evanescent field sensors, or deformable mirrors. Advances in nanofabrication have shown that photonic crystal (PhC) membranes are an ideal alternative to conventional mirrors, as they provide high reflectivity with only a single layer of dielectric material. In particular, devices made of silicon nitride constitute the state-of-the-art in PhC mirrors with low optical absorption and mechanical loss. However, fabrication technology has constrained their effective area to a few square-micrometers. Here we experimentally demonstrate the first example of suspended PhC mirrors spanning areas up to 10×10 mm. We overcome the limitations imposed by the finite size of the PhC, which allows us to measure reflectivities greater than 99% on 210 nm thin devices at 1550 nm wavelength and beyond 90% on 56 nm thick mirrors — an unrivaled performance compared to PhC mirrors with micro scale diameters. We also consider their use as mirrors in gravitational wave detectors where they could potentially reduce mirror coating noise at cryogenic temperatures. These structures bridge the gap between nano scale technologies and macroscopic optical elements.