  Optically levitated multiple nanoparticles has emerged as a platform for
studying complex fundamental physics such as non-equilibrium phenomena, quantum
entanglement, and light-matter interaction, which could be applied for sensing
weak forces and torques with high sensitivity and accuracy. An optical trapping
landscape of increased complexity is needed to engineer the interaction between
levitated particles beyond the single harmonic trap. However, existing
platforms based on spatial light modulators for studying interactions between
levitated particles suffered from low efficiency, instability at focal points,
the complexity of optical systems, and the scalability for sensing
applications. Here, we experimentally demonstrated that a metasurface which
forms two diffraction-limited focal points with a high numerical aperture (0.9)
and high efficiency (31%) can generate tunable optical potential wells without
any intensity fluctuations. A bistable potential and double potential wells
were observed in the experiment by varying the focal points distance, and two
nanoparticles were levitated in double potential wells for hours, which could
be used for investigating the levitated particles nonlinear dynamics, thermal
dynamics, and optical binding. This would pave the way for scaling the number
of levitated optomechanical devices or realizing paralleled levitated sensors.
