Optical traps are instruments that use a focused light beam to hold micron-scale objects with photon forces in a localized region in space. Optical trapping has become an important research technology in biology and physics, and more recently in commercial applications, useful for designing, manipulating, sorting and assembling objects at the nano-molecular scale. In addition, optical trapping can be used to evaluate picoNewton-scale force interactions between molecules (force-probe research) and to control nanostructures and nanoswitches.
A conventional optical trap is initiated by focusing a laser beam through an objective lens of high numerical aperture. The focused light produces a 3-dimensional, radial, intensity gradient, which increases as light converges upon the focus (focal point) and then diminishes as the light diverges from the focus. A dielectric object located closely down-beam of the focus will experience a combination of forces caused by transfer of momentum from photons, resulting from both scattering and refraction.
Dielectric objects used alone or as “handles” to manipulate other microscopic objects are typically in the range of about 0.2 to 5 microns, which is the same size range as many biological specimens that can be trapped directly, e.g., bacteria, yeast and organelles of larger cells.
It is known to construct optical traps using optical gradient forces from a single beam of light to manipulate the position of a small dielectric object immersed in a fluid medium whose refractive index is smaller than that of the particle. The optical trapping technique has been generalized to enable manipulation of reflecting, absorbing and low dielectric constant particles as well.
Typically, optical traps have been developed using standard microscopy substrates, primarily the traditional, standard, glass microscope slide. A microscopic object to be trapped will usually be immersed in an oil or aqueous fluid medium maintained between two glass slides separated by a spacer. In addition to stabilizing and limiting the movement of the object somewhat, the immersion fluid provides an index of refraction that can be selected to be less than the index of refraction for the object itself, with the ratio of these refractive indices being important to generating the optical trapping forces.
Traditionally, glass substrate slides have been used because they are commonly available for adaptation to microscopic sample stages and because they are substantially transparent to wavelengths of visible light (350 to 700 nanometers) commonly used with microscopy.
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