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Micromanipulation of sub-millimeter objects is one of the more underappreciated technical challenges in modern scientific research. To fully understand their properties, objects such as dielectric spheres, viruses, bacteria, living cells, organelles, small metal particles, and even strands of DNA all require precise positioning under a microscope even though they can often only be tens of nanometers in size. Since these objects are too small to manipulate with a pair of tweezers, a non-contact method needs to be used to trap and then manipulate these objects. With high enough precision this can facilitate the study properties such as cell sorting, tracking of bacteria, measurement of small forces, and alteration of cell membranes. In this blog we will explore one methodology for trapping and manipulating these objects using a strong optical field to exert a gradient force on the object, allowing the object to be maneuvered in a controlled manner. This variant on optical trapping is known as “optical tweezers,” and it is widely used in biological sciences for the study of molecular motors and the physical properties of DNA.
The easiest way to understand how optical tweezers work is by taking a look at the momentum transfer associated with the scattering and bending of light caused by the object. Since every photon has momentum, whenever it changes its direction, its momentum will change as a result, and therefore conservation of momentum requires that the object must undergo an equal and opposite change in momentum. This conservation of momentum results in gradient force arising because as the light scatters off of the object, it tends to push it towards the region of maximum light intensity. A full analysis of how these forces arise is far beyond the scope of this blog, but to summarize if the object is much smaller than the wavelength of light it can be approximated as a dipole that feels a Lorentz force. When a single-mode continuous wave laser is utilized this force is proportional to the gradient of the electric field squared. Therefore, it will always point the object toward the laser focus, and even though it is typically in the piconewton range, it is more than strong enough to trap and manipulate these small objects. For a more detailed understanding of the fundamental physics behind this process, we recommend you reference “A Practical Guide to Optical Trapping” by Joshua Shaevitz. In this article, Dr. Shaevitz who is currently a Professor of Physics at Princeton University does an excellent job of not only introducing the fundamental principles of optical tweezers but also explore several practice considerations for integrating optical tweezers into a microscope.
The image above illustrates the basic principle of how the forces experienced by an optically transparent sphere are perfectly balanced when at the focus of the laser beam. For this reason, these spheres are often attached to more complex molecules to allow them to be more easily manipulated under a microscope. For example, if a biological specimen is biochemically attached to a micron-sized glass or polystyrene bead that is then trapped, or by attaching a single molecular such as kinesin, myosin, RNA polymerase to such a bead, people have been able to probe motor properties, elasticity, as well as the forces under which molecules breakdown or undergo a phase transition.
1064 nm laser light is the most commonly used in optical tweezers because of the availability of high-power single-mode laser sources. Traditionally these have been Nd:YAG diode-pumped solid-state laser sources, but with advances in diode technology there are now reliable high power single mode laser diodes which can also be used for these applications. Here at RPMC, we supply a wide range products such diodes from Sheaumann Laser, Inc based in Marlborough, MA. These lasers offer a perfect TEM00 output beam which is ideal for microscopy applications at wavelengths ranging from 785nm to 1064nm and output powers up to 500mW. These high brightness, high quality, and high-reliability diode lasers are available in a compact 9mm hermetically sealed laser package, with an AR coated window making it ideal for integration into OEM systems. In addition to the high-power output capability of these lasers, they also provide excellent slope efficiency, allowing them to be efficiently operated at lower dive currents making them more reliable and extending their lifetime.
RPMC Lasers is proud to be the North American distributor for Sheaumann. If you would like to get additional technical specifications on the high-powered single mode diodes lasers from Sheaumann, click here or talk to one of our laser experts today click here or call 1-636-272-7227.