Fig. 4.

Reconstruction of stable and unstable equilibrium points. a Multiwell optical potential generated by two focused Gaussian beams slightly displaced along the x-direction. FORMA identifies three stable ($x_1^{*}$, $x_3^{*}$, $x_5^{*}$; full circles) and two unstable ($x_2^{*}$, $x_4^{*}$; empty circles) equilibrium points, and measures their stiffness (orange solid and dashed lines). The corresponding x-potential obtained from the potential method is shown by the green solid line (the green shaded area represents one standard deviation obtained repeating the experiment 20 times). b 2D plot of the force field measured with FORMA (arrows) and of the potential measured with the potential analysis (background colour). The stable and unstable equilibrium points are indicated by the full and empty circles, respectively. We have acquired trajectories of the motion of a spherical particle with radius R = 0.48 μm in an aqueous medium of viscosity η = 0.0011 Pa s⁻¹ at a sampling frequency f_s = 4504.5 s⁻¹, and used 20 windows of 4.5 × 10⁵ samples for the analysis