Automated Optical Tweezers
In the automation of the optical-tweezers system, the axial position of the trapped particle is needed for a number of calculations, including trap stiffness. To determine this position, the total voltage signal from the position sensing detector (PSD) must be calibrated and converted to distance. By first trapping a particle, the total signal is recorded as the particle is moved towards and through the focus of the trap laser. Next, the procedure is repeated for a particle stuck to the coverslip, while the total signal is also recorded. The plots are then superimposed onto one another, allowing for the determination of the convergence of the plots. This convergence corresponds to the coverslip contacting the particle, from which the axial equilibrium position is determined using the radius of the particle. The procedure for the determination of the axial position is currently being automated and incorporated into the main automation program.
One goal of the automated optical tweezers setup is the calculation of the real-time axial position of a particle in an optical trap. Due to a continuous phase shift of the electric field in the axial direction inherent in focused beams, known as the Gouy phase shift, it is possible to determine the axial position of a trapped particle with nanometer accuracy. In addition, through power spectrum analysis, a calculation of local temperature can be achieved which is necessary for determining trap stiffness.