Brownian Motion Captured with a Self-Mixing Laser

We measured the overall motion of Brownian particles suspended in water by a self-mixing thin-slice solid-state laser with extreme optical sensitivity. From the demodulated signal of laser intensity fluctuations through self-mixing modulations by the interference between the lasing field and Doppler-shifted scattered fields from Brownian particles, it was found that the changes over time in random walks of small particles suspended in water result in different overall dynamics when the field of vision for particles seen by the laser beam (scale of the observation) is changed. At a small focal volume of the laser beam, in which the relevant diffusion broadening is observed, the overall motion which can be represented by the motion of a ''virtual'' single particle whose velocity obeys a double-peaked non-Gaussian distribution function. The fast motion of a virtual particle, featuring fractional Brownian motion, which leads to a pure random walk with increasing the observation time, has been characterized in terms of mean-square displacements of a virtual particle.