<title>Progress toward photon force-based sensors: a system identification approach based on laser intensity modulation for measurement of the axial force constant of a single-beam gradient photon force t</title>

Abstract

New sensor technologies with the sensitivity and specificity capable of detecting biological and chemical agents at low concentration are of increasing importance for many environmental monitoring applications. We propose a potentially new class of microsensors that exploits the mechanical dynamics of a micrometer-sized particle held in a 3D optical force trap formed by a focused laser beam. Modulation of the laser trapping power axially perturbs the microparticle from its equilibrium position and permits measurement of the mechanical compliance transfer function (force input, displacement output) characterizing the particle micromechanical dynamics. In a mechanically homogeneous and isotropic environment, the particle motion is readily modeled as a forced harmonic oscillator; however, physico-chemical interactions between the particle and its surroundings impose external forces that modify the compliance transfer function. Our preliminary measurements indicate < 10 ppm changes in mass of a trapped microparticle can be detected with this method, suggesting possible applications as a chemical/biological sensor or for solubility measurements of microparticles.