Mohamed Mohamed1
ETH Zürich1
Levitated nanoparticle systems have emerged as a promising approach for studying fundamental forces, investigating non-Gaussian quantum states, and probing material properties at the nanoscale. When operated under ultra-high vacuum (UHV), damping of nanoparticle mechanical motion is kept sufficiently low, reducing decoherence and sustaining a high-quality-factor mechanical oscillator which can be exploited for quantum-limited sensing.<br/><br/>One of the main challenges towards implementing nanoparticle levitation in vacuum is the required level of control over trapping potentials, feedback forces, and sources of noise. Control schemes should additionally satisfy vacuum-compatibility and stability requirements. A compact design implementation, especially in an integrated form factor, can further enable a wide range of experiments such as acceleration sensing in dynamic testing or quantum state manipulation in cryogenic systems.<br/><br/>Here, a hybrid chip-scale platform for trapping and control of dielectric nanoparticles in vacuum is presented. The platform features a primary optical trap generated through an optimized polarization-insensitive dielectric metalens which features focal length in the range of 50 – 200 µm and numerical aperture values of up to 0.98. Individual silicon meta-atoms are designed to provide complete 2π phase coverage in the near-infrared wavelength range, with average transmission values exceeding 90%. The near diffraction-limited performance of the metalens alleviates requirements on input power for stable optical trapping. The metalens is patterned in an epitaxial silicon-on-sapphire layer, which exhibits reduced optical losses in comparison to silicon layers prepared by alternative deposition methods. The sapphire substrate provides a broad transparency window for the incident trapping laser beam, while preventing heat build-up given its thermal properties.<br/><br/>The hybrid device design features metallic electrodes embedded within the metalens layer. A dual-ring electrode configuration serves to create a tunable-height Paul trap, where the trapping volume overlaps the focal spot of the optical metalens trap. This is to assist with the loading of nanoparticles into the device by virtue of the deep potential well of the Paul trap. Furthermore, the electrode layer comprises pads for nanoparticle feedback cooling by means of cold damping. The overall architecture can additionally accommodate customized static and dynamic electromagnetic potentials which can be readily applied to the trapped nanoparticle. The overall electrode design enables the above functionalities without significantly perturbing the generated optical potential of the metalens near the trapping zone.<br/><br/>The proposed design was fabricated using a refined approach relying on electron-beam patterning and etching, combined with metal lift-off. Initial testing confirmed UHV compatibility of the device down to a pressure of ≈ 10<sup>-7</sup> mbar, at the limit of the experimental vacuum chamber. Optical trapping was demonstrated with the device using femtogram SiO<sub>2</sub> nanoparticles, down to a pressure of 1 mbar. The collected forward scattering signal allowed for nanoparticle motion tracking using split detection.<br/><br/>The demonstrated platform provides a robust approach for chip-scale nanoparticle levitation and control, which opens the door to highly promising applications in both fundamental and applied science.