Experimental Study On Optically Levitated Nanoparticle

Optical tweezers is a technology that using light field radiation pressure to levitate tiny particles,such as atoms,molecules,micro-and nano-scale particles,cells and so on.It is an important direction in basic scientific research,and is widely used in physics and biology.Optically trapping nanoparticle is a typical application among them.Optically levitated nanoparticle is isolated form the thermal environment comparing with the mechanical oscillator connected by a cantilever.Thus the limitation of the thermalization and decoherence introduced by the cantilever are cancelled.The Q-factor of the system is predicted to approach 10¹² in high vacuum,which is extremely sensitive to some changes in the surrounding environment.It is expected to ultra-precision measurement.Moreover,if the nanoparticle is cooled down to the quantum ground state,it can be used to produce the quantum macroscopic superposition state and test the decoherence mechanism.In this paper,the experimental principle of optically levitated micro-and nano-particle is introduced firstly.According to the size of the trapped particle relative to the laser wavelength,the principle can be analysised by linear optics,Rayleigh approximation and general Lorentz-Mie scattering theory.Based on these theories,the effects of particle size and refractive index,the trapping laser wavelength and numerical aperture on the optically levitated particles are analyzed.A strongly focused laser beam generate a stable three-dimensional potential well at the focal region.Based on the Langevin's equation and the fluctuation-dissipation theorem,the power spectral density of center-of-mass motion of the particle and the relationship between the damping rate and pressure are given.A vacuum system is designed to optically trap nanoparticle in vacuum.The selection,storage,solution configuration and loading method of nanoparticle is important.The Co M motion signals of nanoparticle in three orthogonal directions can be measured based on the principle of balanced homodyne detection.As the core of the detection system,a self-made balanced homodyne detector with high gain,high bandwidth and high common mode rejection ratio is used to realize the measurement of particle motion signals with high-precision.An experimental device for optically trapping nanoparticle in vacuum is built.The optically trapping nanoparticle is realized experimentally using a high numerical aperture objective lens to focus a 532 nm laser beam.The loading and trapping process of nanoparticle in vacuum chamber is introduced.A balanced homodyne detection system is designed to measure the Co M motion signals of nanoparticle in three directions,and the motion trajectory in three-dimensional is described.The relationship between the eigen-motion frequencies and laser power is measured.The influence on the center-of-mass translational motion of a levitated nanoparticle in the case of the combination of an elliptical polarization and elliptical TEM00-mode Gaussian beam is measured.The field distribution of the strongly focused laser in the focus region depends on its polarization according to vector diffraction theory.Therefore,the motion of the nanoparticle depends on the relative orientation and ellipticity of the two ellipses parameters.For a linearly polarized light field,the eigen-frequencies and corresponding power spectra of the radial motions change periodically with the rotation of the linear polarized direction relative to the orientation of the elliptical TEM00-mode Gaussian beam.It is demonstrated that the effects could be enhanced or canceled by controlling the relative orientation and ellipticity of the two ellipses parameters.The hermitian and non-Hermitian normal-mode splitting in an optically levitated nanoparticle is realized.A nanoparticle is trapped by a strongly focused laser beam in free space,and could be regarded as a harmonic oscillator.An external oscillator is phase-locked to the nanoparticle motion as another harmonic oscillator,which is modulated on the power of trapping laser to feedback and interacts with the nanosphere.Therefore,the normal-mode splitting in an optically levitated nanoparticle is realized with strong coupling.Moreover,the normal mode splitting following the cooling or heating effect simultaneously is observed.The dipole scattering of a nanoparticle is observed using a high numerical aperture imaging system,which provides an environment free of particle-substrate interaction.We illuminate the optically levitated silica nanoparticle in vacuum with a 532 nm laser beam from the normal to the propagation direction of the strongly focused 1064 nm trapping laser beam,which results in a dark background and high signal-noise ratio for detecting the dipole scattering.The dipole orientations of the nanoparticle are studied by measuring scattering light distribution in the image and the Fourier space(k-space).The3D doughnut shaped dipole scattering is rotated by rotating the linear polarization of the incident 532 nm laser,whose orientations can be determined by the measured scattering light pattern.The polarization vortex(vector beam)is observed for the special case,when the dipole orientation of the nanoparticle is aligned along the optical axis of the objective lens.A record-breaking ultra-high rotation frequency about 6GHz in an optically levitated nanosphere system is observed.We optically trap a nanosphere in the gravity direction with a high numerical aperture objective lens,which shows significant advantages in compensating the influences of the scattering force and the photophoretic force on the trap,especially at intermediate pressures.This allows us to trap a nanoparticle from atmospheric to low pressure without using feedback cooling.We measure a highest rotation frequency about 4.3GHz of the trapped nanosphere without feedback cooling and a 6GHz rotation with feedback cooling,which is the fastest mechanical rotation ever reported to date.