Photonic Force Microscopy
Description
In a PFM, with the mechanical cantilever of an AFM replaced by the 3-D trapping potential of a laser focus, it is possible to trap and manipulate micrometer sized particles with the laser focused in a fluid chamber. This was first described in 1986 by Ashkin [Ashkin, A., Optical trapping and manipulation of viruses and bacteria. Science, 1987. 235: p. 1517-1520.1], the same year the first AFM [Binnig, G., C.F. Quate, and C. Gerber, Atomic Force Microscope. Phys. Rev. Lett., 1986. 56: p. 930-933.] was built.
The difference in the refractive index between fluid medium and particle, the particle’s diameter, as well as the laser intensity (together with its 3-D profile), determine the strength and the shape of the trapping potential. Depending on the application, this potential, which is harmonic and therefore can be described by its spring constant, can be adjusted by changing laser power. Usually this spring constant is two to three orders of magnitude less than that of the softest commercially available AFM cantilevers.
The particles used as tips can be as small as some tens of nanometers in the case of high refractive index materials such as metals, or even only a few nanometers when using resonant or modulated detection schemes. The significant difference between the optical trap of Ashkin and a PFM is the 3-D detection system [Pralle, A., M. Prummer, E. L. Florin, E. H. K. Stelzer, and J. K. H. Horber, 1999, Three-dimensional high-resolution particle tracking for optical tweezers by forward scattered light, Microscopy Research and Technique 44, 378-3864.] for the particle position with respect to the trapping potential; this allows forces and their directions to be measured with sub-picoNewton precision on a timescale of microseconds.
Thus, small particles can be used as scanning probe tips and moved along a surface to image or to make interaction force measurements (in the thermal force range) between the particle and a surface [Florin, E. L., A. Pralle, E. H. K. Stelzer, and J. K. H. Horber, 1998, Photonic force microscope calibration by thermal noise analysis, Applied Physics A-Materials Science & Processing 66, S75-S78]. Furthermore, using latex beads as tips, commercially available chemical surface modifications can be brought into play to make immunologically- or chemically-specific measurements.