Episode 8: Two-beam interferometry experiment characterizes lightsail propulsion

In this podcast episode, MRS Bulletin’s Laura Leay interviews Harry Atwater from the California Institute of Technology about his study on lightsail propulsion in order to understand how the device can be developed to do fly-by space travel riding a beam of laser light. Atwater’s research group made a square prototype device where the researchers incorporated springs at each corner, etched out of a single sheet of silicon nitride, fastening it to the support frame. They tested its behavior in a two-beam interferometry experiment. Their comprehensive analysis provides a thorough understanding of key parameters that are essential for lightsail propulsion and paves the way for the next step of research: untethered flight. This work was published in a recent issue of Nature Photonics.  

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. In the near future, spacecraft could take just a few months to reach the outer edges of our solar system, rather than the years that it currently takes. They could do this riding a beam of laser light. It might sound like science fiction, but often ambitious scientific programs are there to push the boundaries of what is possible and invent new technology. The space craft would be comprised entirely of a membrane with sensors embedded in it. A new device has been created to measure the radiation pressure in this membrane, known as a lightsail. The device is made from the leading material for this application: silicon nitride. Professor Harry Atwater from the California Institute of Technology explains why this material is important.

HARRY ATWATER: Silicon nitride is a material that’s used in thin films in microelectronic circuits. The methods of fabrication of thin films are well developed. They’re used also in another research field which is quantum-optomechanics. This is an area where physicists are interested in cooling down mechanical structures to their quantum-mechanical ground states and this requires very low dissipation – very low so to speak – low noise materials where you can see individual mechanical quanta. And it’s that field that really pushed the limits of developing silicon nitride as a material. That was one of the reasons why we chose silicon nitride as a lightsail material but the characteristics of a lightsail as propelled by radiation pressure had not been previously studied.

LAURA LEAY: For space applications, the deployed lightsail would measure 10 m2 and be just 100 nm or less in thickness. For now, the experimental device that has been developed to measure forces with incredible sensitivity has a square profile measuring just 40 μm wide. The device incorporated springs at each corner of the lightsail, etched out of a single sheet of silicon nitride and tethering it to the support frame. To measure the displacement of the lightsail when exposed to a high intensity laser, a bespoke interferometer was created that could account for the inherent vibration of the atoms in the silicon nitride and measure displacement with picometer resolution. 

HARRY ATWATER: So what we actually do is a two-beam interferometry experiment where we use one beam that is on the rigid part of the silicon frame. That’s oscillating too in the lab, unavoidably, just because everything oscillates, everything vibrates. But we can measure the relative displacement of the membrane relative to its rigid frame. One beam goes on to the membrane, we can focus with a diffraction-limited beamspot, so we can actually profile the displacement across the membrane. We can watch the modes that the membrane will exhibit, so the fundamental mode or the higher order modes that have torsional oscillations.

LAURA LEAY: Observing these modes provided insight into how the lightsail would behave during flight. Riding a laser beam that originates on Earth means that the lightsail may not always be perfectly aligned with the beam. The incident angle and spot-size were varied which revealed a non-intuitive trend that was understood using mathematical modelling.

HARRY ATWATER: At the edges of the sail, what we found was as we illuminated it obliquely the beamspot size is elongated by a cosine expansion of the beamspot size. So what happened was, part of the beam was actually scattering from the edge of the membrane. So at the edge, you get something other than just a specular, simple reflection; you get more complex scattering forces.

LAURA LEAY: Creating a patterned membrane, one that resembles a diffraction grating, also allowed for in-plane motion to be measured and showed that the lightsail wobbles as it moves along the optical axis. This comprehensive analysis of the dynamics provides a thorough understanding of key parameters that are essential for lightsail propulsion and paves the way for the next step of research: untethered flight. The research also provides a platform for probing optomechanical properties of membranes on a macro-scale, which complements work on the microscale to develop optical tweezers. These advances are made possible by collaboration and Professor Atwater points to the efforts of other investigators on the team.

HARRY ATWATER: The work depends on the indomitable spirit of young people. Two people that played a major role in the work are Lior Michaeli – he’s currently a post-doc and will soon be a starting professor at Tel Aviv University in Israel – and Ramon Gao.

LAURA LEAY: This work was published in a recent issue of Nature Photonics. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.