A highly-focused laser beam can be used to trap microscopic particles. In this technique – known as optical tweezers – forces arise near the focal spot due to radiation pressure of the light beam (acting along the beam direction) and gradient forces which pull the particle towards the high-intensity focal spot. Since the 1980s, researchers have used optical tweezers to manipulate biological samples, starting from an individual tobacco mosaic virus and Escherichia coli bacterium. Further development, using optical trapping along with a sensitive cantilever, has made it possible to probe microscopic force fields ranging from femtonewtons (10-15 N) to piconewtons (10-12 N) and to characterize the mechanical properties of biomolecules and biological motors. Optical tweezers now play a major experimental role in many fields of physics, as well as in nanotechnology, spectroscopy, nano-thermodynamics and biology.
In a new tutorial article, a team of researchers including LML External Fellow Isaac Pérez Castillo offer a detailed primer on how to calibrate optical tweezers and use them for advanced applications. They focus on describing and comparing the various available calibration techniques, and discuss some of the most exciting cutting-edge applications of optical tweezers in a liquid medium, including use in the study of single-molecule and single-cell mechanics, microrheology, colloidal interactions, statistical physics and transport phenomena. They also consider optical tweezers in vacuum, which features different dynamics than a viscous medium, and poses its own experimental challenges. The aim of the tutorial is to provide a step-by-step guide ideal for non-specialists entering the field, as well as a comprehensive manual of advanced techniques useful for expert practitioners.
As the authors note, this technique has opened the door to a wide range of interesting physics in the classical regime, and is now entering into the quantum regime. The next challenges include achieving ground-state cooling for all three center-of-mass translational degrees of freedom, controlling the vibrational modes and precession of trapped particles, and preparing truly non-classical states. Optical tweezers in combination with cavity optomechanics will in the near future likely play an important role towards creating more exotic quantum mechanical states beyond the ground state. Once a strong cavity optomechanical interaction has been realized, a range of traditional quantum optics experiments can be realized with massive particles including quantum state transfer, quantum squeezing, entanglement and teleportation.
The paper is available here.