Nonlinear refractive index measurement of a trapped particle with Shack–Hartmann wavefront sensor

A technique for nonlinear refractive index measurement of a single trapped nanoparticle is investigated using Shack–Hartmann sensor. In this technique, trapping laser is used as a probe beam and an interacting green laser is illuminated as a pumping beam. Optical path difference and effective height of particle is measured with dual wavelength unwrapping technique. Using the absorption relation, we obtain the profile of nonlinear refractive index. Also, the results prove the quadric relation of the nonlinear refractive index to the nonlinear absorption. This result may have an enormous effect in investigating and characterization of nonlinear refractive index of Nano and Micro-sized particles. Also, measuring the thermal profile of the trapping site after illuminating the high power trapping laser leads to measuring the gradient intensity that could have an important role in nonconservative forces measurements. Another advantage is in having the effective thickness of particle which is important while geometry of the particle is crucial for determining trap stiffness. This measurement may lead to adaptive trap shape control and potential well measurement.