Feedback Control Of The Optical Levitation In Vaccum

Optical levitation or optical tweezer in vacuum is an optomechanical system that has received great attentions in recent years and is becoming a hot research topic.Com-pared to other optomechanical systems,optical tweezer in vacuum has unique properties of ultra-high position detection sensitivity,no mechanical contact with the environment,optional effective mass range from fg to ng,additional rotation freedom and diverse control methods.In the precision measurement,the optical tweezers in vacuum can be used to construct an ultra-sensitive detector such as a zN-level weak force detector,an fg-level nanoparticle mass detector,and an ng-level accelerometer.In physics research,they have been used for macroscopic quantum phenomena,microscopic thermodynam-ics,ultra-high-speed rotors,and close-range force detection.Since the functions of the optical tweezers in vacuum rely on the control of the motion states of the levitated nanoparticles,photon momentum feedback control,pa-rameter feedback control,cavity cooling and other control technologies are the research priorities at this stage.The main research directions of the feedback control include un-derstanding the physical principle of vacuum levitation control scheme,improving the efficiency and performance of control schemes,and expanding the functions of feed-back control.Around these directions,the specific research contents and progresses of this thesis include:1.Construction of a vacuum optical levitation experimental system based on a single beam optical tweezer.The system can realize a vacuum levitation of silica beads with a diameter about 150 nm.The stable levitation pressure reaches 10-7 mbar,and the particle position detection sensitivity reaches the pm-level.The motion state of the trapped nanoparticles can be controlled by modulating the trapping laser intensity.2.Measurement of three-dimensional positions by a Dove prism.The role of the Dove prism in the position detections is studied in detail,which simplifies the position measuring devices.3.Implementation of digital parameters feedback control and cooling of the levi-tated particle in vacuum with square wave modulation.The performance and character-istics of the control scheme are theoretically analyzed.The algorithm and experimental device of square wave feedback control are realized by FPGA and verified on the vac-uum optical tweezer system.The experimental phenomenon of square wave parameter feedback cooling is analyzed in detail.Based on the digital parameters feedback control,a minimum cooling temperature of 10 mK is achieved.4.Calibration of optical tweezers with nonlinear frequency shift.Based on the nonlinear motion characteristics of the optically levitated particles and the tight focus-ing light field distribution characteristics,a scheme for the calibration of optical tweez-ers with nonlinear frequency shift is realized.A feedback amplitude locking scheme is proposed by the principle of parameter feedback control and implemented in the ex-periment.A measurement of the nonlinear frequency shift and the calibration of the optical tweezer in vacuum are realized in the experiment with the feedback amplitude locking.The systematic error of the calibration factor is approximately 1%.The error of amplitude lock can be controlled within 0.5%.A mass and density measurements of the optically levitated nanoparticles are achieved with the calibration results.