Appl. Phys. Lett. 113, 183702 (2018)

Optical tweezers are a popular tool for manipulating and sorting individual nanoparticles. Now, Avinash Kumar and John Bechhoefer from Simon Fraser University, Canada have shown that when equipped with a suitable feedback scheme tweezers can be used to create a more complicated force field, such as single- or double-well harmonic potentials, for controlling particle dynamics. In the experiments, a polarized 532-nm laser was used for trapping and detection. The polarization of the detection beam was rotated by 90° with a half-wave plate so that a polarized beam splitter could separate the detection beam from backscattered light arising from the trapping laser. Quadrant photodiodes were used to detect the particle’s fluctuation and dynamics. Two feedback loops continuously regulated a pair of acousto-optic deflectors to compensate for any fluctuation in the total intensity. The shape of the double-well potential was reconstructed from the Boltzmann distribution of the position measurements. The well separation was 10.6 nm, which is far below the diffraction limit (≈220 nm). The ability to create and control energy landscapes at scales comparable to the size of proteins offers intriguing possibilities for biophysical applications.