Trapping atoms and molecules with the oscillating electric fields of a highly focused laser beam — a technique referred to as an optical tweezer — has been studied for several decades and was recognized by the 2018 Nobel Prize in physics. Standard optical tweezer techniques, however, do not yet provide enhanced manipulation of trapped molecules that require observations at extremely small time scales.

Kumar and Bechhoefer combined feedback with optical tweezers by incorporating a detection and beam-steering system. The input laser is split into two beams, one for trapping particles and the other for detection, thus achieving full control of the particle dynamics. With these detection and tracking methods, the system can create custom energy landscapes for objects with length scales down to 10 nanometers and time scales approaching one millisecond, typical scales at which proteins fold and unfold.

While numerous other passive processes can trap particles, including plasmonic-based tweezers, these rely on field enhancement by nearby surfaces, which limits their use to trapped particles in close proximity to a surface. The optical tweezer-based feedback-trap approach of Kumar and Bechhoefer opens the door for experiments that study the interaction of colloidal particles with large molecules like DNA and proteins.

This innovation makes it possible to alter the forces on a trapped particle without changing the laser intensity, thus preventing bleaching or damaging attached biological species, and furthermore leads to full control of the motion of molecules at short time scales, enabling new ways to interrogate fast thermodynamic processes such as protein folding.

Source: “Nanoscale virtual potentials using optical tweezers,” by Avinash Kumar and John Bechhoefer, Applied Physics Letters (2018). The article can be accessed at