Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 2 / Section A(iii) / 19 / iv

Absorption converts part of the incident optical power into heat. Increasing laser power can therefore raise the local temperature, potentially driving a thermotropic nematic liquid crystal through its nematic-isotropic transition. The associated loss or change of orientational order, refractive index and anchoring can alter the trapping force or destabilize the trap.

Even below that transition, thermal gradients can induce convection, thermophoretic drift and changes in viscosity or Brownian fluctuations; sufficiently strong heating can damage the particle or host. Thus increasing laser power need not simply strengthen an otherwise unchanged optical trap.