Vortex Beams for Photonic Metasails and Optomechanical Rotation

It has been proven experimentally that light can generate optical forces while interacting with nano-objects, resulting in various interesting applications such as optical trapping. Inspired by that, lightsailing has been introduced recently as a novel concept, exploiting the radiation pressure of light to reach the relativistic velocities with the hope of exploring the undiscovered universe. Metasurfaces, due to the taking advantage of the generalized Snell's law, can offer the possibility of controlling the wavefront of the reflected/transmitted light to engineer the amplitude and direction of the optical forces to obtain the desired goals. Lightsailing technology seems to be an ideal platform for probing the nearest inhabitable exoplanet within the human lifetime thanks to the exclusive functionalities that the photonic metasurfaces offer. Light tends to carry both the linear and angular momenta while propagating, where the exchange of the former one with the matter can result in linear acceleration, having been the subject of numerous studies so far. The linear momentum exchange between the light and the object exerts an optical force, which can be exploited for accelerating gram-scale space probes. Nevertheless, the transfer of angular momentum to an object will potentially impose a rotational motion, being desirable in a variety of applications. Here, we put our major focus on the study of the rotational behavior of our designed metasail while it exchanges the orbital angular momentum (OAM) with light. The rotational motion of the lightsail will be advantageous in various scenarios, including stability improvement. Moreover, enabling the self-stabilizing mechanism while minimizing the acceleration time in the propulsion stage is one of the key elements to realizing such nanocrafts. Conventional designs of metasails require a parachute configuration with a detached payload for enabling the passive self-stabilizing mechanism. Therefore, spinning the lightsail at high speeds can be introduced as a beneficial tool to provide a new degree of freedom in the stability criteria of the lightsails, which can be exploited to further widen the stability margins. In our study, we design an all-dielectric photonic metasail using multi-objective optimization, which consists of both the reflective and transmissive elements that can provide the required optomechanical performance for reaching the relativistic velocities in an acceptable period of time. Also, we carefully design each unitcell of the metasurface, considering the delicate trade-off between the linear acceleration and rotational motion of the sail. We illustrate the interaction between the vortex beam associated with different OAM states and photonic metasails for achieving desired propulsion performance. Generally, the rotational force would be expected if the incident light is carrying an orbital angular momentum or if the metasail imparts an orbital angular momentum to the incident beam. In the latter case, the propulsion beam upon reflection from a metasail with a broken azimuthal symmetry contains an orbital angular momentum (OAM) which seems to be a viable solution for generating large in-plane rotational torque. Our work addresses in detail the relation between the amount of OAM accompanied by the reflected beam and rotational speed of the sail as well as the linear acceleration. It is shown that a careful design can result in a metasail, having both the desirable linear acceleration and rotational motion, providing the self-stability feature to the sail, simultaneously. We believe that the studies in our work can be used to open a new perspective on the lightsailing field as our results show the impacts of both the linear and rotational motions on the functionality of the metasail.