Mass Measurement Of Optical Levitated Nanopaticle In Medium Vacuum

Scientific research cannot be conducted without precision measurements.The improvement of measurement accuracy of physical quantities helps to verify theoretical conjectures and obtain new physical discoveries.Therefore,the development of scientific research is inseparable from the improvement of measurement accuracy.To achieve precision measurement of physical quantities,it is necessary to reduce the interference of the external environment,reduce the magnitude of the noise,and improve the signal-to-noise ratio.The optical trap in vacuum is an experimental setup that ultilizes the light’s momentum to levitate micro or nano particles in a vacuum environment.The levitated particles are not in direct contact with the external environment and are therefore shielded to the greatest extent from noise and disturbtion.This allows precise measurements of a wide range of physical quantities,including mass,and thus provides a platform for the characterisation of individual nanoparticles.However,some mass measurements in optical levitation rely on statistical properties about the motion of particles in thermal equilibrium.Therefore,these mass measurement methods cannot be used when the particles are heated.The study of nanoparticle mass variation with temperature is an important part of nanoparticle characterisation.It has been found that optically levitated particles undergo a sudden change in their properties in medium vacuum.This phenomenon cannot be explained by existing theories and deserves to be further investigated.However,the existing mass measurement method for optically levitated nanoparticle can not measure the particle’s mass accurately in medium vacuum.In order to further extend the application of optical levitation in the study of single nanoparticle properties,the following studies have been carried out,including:First,we achieve the accurate calibration of the position calibration coefficient of optically levitated nanoparticles in a medium vacuum.Due to laser heating,the temperature of the optically levitated particles changes when the pressure drops to a medium vacuum environment(1 mbar to 10-3 mbar),resulting in some existing position calibration methods not being able to be used in a medium vacuum environment.We propose a new position calibration method based on the Duffing nonlinearity of the optical trap.We first obtain the frequency shift of the measured trajectories by averaging the trajectories at the same initial position and fitting the averaged trajectories to the theoretical trajectories in the presence of the Duffing nonlinearity.By comparing the frequency shift with the theoretical result,combined with nonlinear coefficients obtained from the optical field analysis,the measured voltage signal can be calibrated to the displacement of the levitated particles.The error of the calibration coefficient is less than 7.5%when the pressure is at 1 mbar-10-3 mbar.Since the precision measurement is based on the detection of levitated particle’s trajectory,the precision measurement of position calibration coefficients can help improve the detection accuracy of physical quantities,thus facilitating the validation of theoretical models and the discovery of new physical phenomena.Secondly,we achieve the real-time tracking of optically levitated nanoparticle’s mass in a medium vacuum environment.The mass measurement is based on the analysis of the work done by the electric field forces on the levitated nanoparticles,assisted by known electric field forces.This mass measurement scheme does not need the prior knowledge of particle’s temperature.The variation of particle’s charge,temperature and oscillation frequency with respect to pressure is also measured in real time.We find that the nanoparticle has a sudden change in properties that cannot be explained by existing theories.Based on the experimental results,we propose a conjecture to explain the occurrence of the abrupt change.Thirdly,utilizing the nanoparticles that have been position-calibrated and massmeasured as probes for light field measurements,we measure the intensity distribution of the tightly focused light field.The measurement method is based on the intrinsic oscillation frequencies of the levitated nanoparticles at different positions.Finally,we investigate the characteristics of optically levitated particle when it is in the asymmetric optical field.