Measurements of the Trap Stiffness of Optical Tweezers

When using the optical tweezers, determing the exact value of the trap stiffiness is very important. In this work, displacements of micron size particle were measured as a function of time by using two different methods; the passive method which measures the trap stiffness of the trapped particle at a fixed position by optical tweezers and the active method which measures the trap stifffness by forcefully oscillating the trapped particle. When using the passive method, the trap stiffness which was determined from Boltzmann distribution and power spectrum was measured, respectively. When using the ative method, the trapstiffness which was analyzed through the data of the amplitude and phase information of the tradpped particle by using an lock-in amplifier was measured. The trap stiffness values obtained from two different methods were compared.