Kawasaki Akio, Fieguth Alexander, Priel Nadav, Blakemore Charles P, Martin Denzal, Gratta Giorgio
Department of Physics, Stanford University, Stanford, California 94305, USA.
Rev Sci Instrum. 2020 Aug 1;91(8):083201. doi: 10.1063/5.0011759.
A high sensitivity force sensor based on dielectric microspheres in vacuum, optically trapped by a single, upward-propagating laser beam, is described. Off-axis parabolic mirrors are used both to focus the 1064 nm trapping beam and to recollimate it to provide information on the horizontal position of the microsphere. The vertical degree of freedom is readout by forming an interferometer between the light retroreflected by the microsphere and a reference beam, hence eliminating the need for auxiliary beams. The focus of the trapping beam has a 1/E radius of 3.2 µm and small non-Gaussian tails, suitable for bringing devices close to the trapped microsphere without disturbing the optical field. Electrodes surrounding the trapping region provide excellent control of the electric field, which can be used to drive the translational degrees of freedom of a charged microsphere and the rotational degrees of freedom of a neutral microsphere, coupling to its electric dipole moment. With this control, the charge state can be determined with single electron precision, the mass of individual microspheres can be measured, and empirical calibrations of the force sensitivity can be made for each microsphere. A force noise of <1 × 10 N/Hz, which is comparable to previous reports, is measured on all three degrees of freedom for 4.7 µm diameter, 84 pg silica microspheres. Various devices have been brought within 1.6 µm of the surface of a trapped microsphere. Metrology in the trapping region is provided by two custom-designed microscopes providing views in the horizontal and one of the vertical planes. The apparatus opens the way to performing high sensitivity three-dimensional force measurements at a short distance.
描述了一种基于真空中介电微球的高灵敏度力传感器，该微球由一束向上传播的单激光束进行光学捕获。离轴抛物面镜用于聚焦1064 nm的捕获光束，并使其重新准直，以提供有关微球水平位置的信息。通过在微球后向反射的光与参考光束之间形成干涉仪来读出垂直自由度，从而无需辅助光束。捕获光束的焦点具有3.2 µm的1/E半径和小的非高斯尾部，适合在不干扰光场的情况下将设备靠近捕获的微球。围绕捕获区域的电极可对电场进行出色控制，该电场可用于驱动带电微球的平动自由度和中性微球的转动自由度，并与其电偶极矩耦合。通过这种控制，可以以单电子精度确定电荷状态，测量单个微球的质量，并对每个微球进行力灵敏度的经验校准。对于直径为4.7 µm、质量为84 pg的二氧化硅微球，在所有三个自由度上测得的力噪声<1×10 N/Hz，这与先前的报告相当。已将各种设备放置在距离捕获微球表面1.6 µm以内的位置。捕获区域的计量由两个定制设计的显微镜提供，分别用于观察水平平面和其中一个垂直平面。该装置为在短距离内进行高灵敏度三维力测量开辟了道路。
