Optical force

Matsuo, Y, Takasaki, H., Hotta, J. and Sasaki, K. (2001) Absorption analysis of a single microparticle by optical force measurement./. Appl. Phys., 89,5438-5441. 

The measured displacement value of the cell correlates to both the optical force exerted on the cell as well as the fluidic drag and surface interaction forces that oppose the cell s... 

In general, measuring beads requires less laser power than measuring cells because of their higher index of refraction (n 1.5 for polystyrene beads vs. n 1.37 for cells).15 The optical force imparted to a particle scales with the difference in index of refraction between the particle and the fluidic medium.16 For bead measurements, we typically operate at a laser power of 2.5 W, whereas for cell measurements the laser is operated at 10 W to obtain similar displacements. These relative power levels are in line with the comparative refractive index differences between the two different particle types and water. 

Development of scanning tunnel microscopy (STM) by Gerd Binnig and Heinrich Rohrer in 1981 pointed the way to breakthroughs in understanding basic chemical processes. Since then, STM and atomic force microscopy (AFM), as well as optical force microscopy proximal probes,1 have been used to manipulate individual atoms and molecules on surfaces. 

Rajagopalan, R. (2000), Atomic force and optical force microscopy Applications to interfacial microhydrodynamics, Coll. Surf. Physicochem. Eng. Aspects, 174(1-2), 253-267. 

Results will be split into various sections the first of which will be fundamentals of hot spots. This will include a summary of the most important developments in the theory of SERS hot spots for both the EM and CT enhancement mechanisms. The second section will cover developments in tip-enhanced Raman spectroscopy (TERS) which represents the idealized hot spot. Then some issues regarding hot spots and the single molecule will be tackled such as the magnitude of enhancement required for single-molecule detection, the effects of molecular orientation with respect to the hot spot as well as the possible influence of optical forces. Sections 4.4 and 4.5 will cover developments in the imaging and fabrication of SERS hot spots, respectively, which have important implications for theoretical modeling as well control of SERS hot spots. The chapter will conclude by summarizing some of the applications of SERS hot spots that have been recently reported. 

A particularly important variant of the optical force, interparticle forces, turns out to be crucial for SERS. This effect is similar to the attractive van der Waals force between small particles, which is due to interactions between spontaneously fluctuating dipoles, but the optical interaction is due to coupling between the actual particle dipoles induced by the trapping laser. Due to the interparticle optical forces, metal nanoparticles aggregate in an optical tweezers and produce hotspots, i.e., particle junctions with intense local fields for SERS. Raman probes can be excited either by the trapping laser or, preferably, by a separate low power beam that does not disturb the trapping. 

Jonas A, Zemanek P (2008) Light at work the use of optical forces for particle manipulation, sorting, and analysis. Electrophoresis 29(24) 4813 851... 

Tong LM, Miljkovic VD, Kail M (2010) Alignment, rotation, and spinning of single plasmonic nanoparticles and nanowires using polarization dependent optical forces. Nano Lett 10(l) 268-273... 

S.M. Block, Making light work with optical tweezers. Nature 360, 493-495 (1992) K. Svoboda, S.M. Block, Biological applications of optical forces. Annu. Rev. Biophys. Biomem. 23, 247-285 (1994)... 

W.J. Greenleaf, M.T. Woodside, E.A. Abbondanzieri, S.M. Block, Passive all-optical force clamp for high-resolution laser trapping. Phys. Rev. Lett. 95, 208102 (2005)... 

Demonstration of levitation of micron-sized latex particles by radiation pressure dates back to 1970 in the experiments reported by Ashkin [Ashkin 1970]. The average force accelerating (or slowing down) atoms in a laser field was derived by A.Kazantsev in 1972 [Kazantsev 1974]. Later in 1972-1974 he classified the optical forces as spontaneous, induced and mixed. In particular, it was he who first presented the dipole potentials for velocity broadened lines of resonance atoms in the logarithmic form... 

Microfluidic sorting of mammalian cells by optical force switching. Natrue Biotechnology 23 p. 83-87 (2005). 

Figure 7 Spatial dependence of optical force on an absorbing particle The radial and axial variation of the optical force is shown for both a TEMoo Gaussian beam and an LG03 Laguerre-Gaussian beam. Both beams have the same power (1 mW), spot size (2 urn) and wavenumber (free space wavelength 632.8 nm). The particle has a circular cross-section of radius 1 pm. Due to the cylindrical symmetry, there is no azimuthal variation of the force. The beam is propagating in the +z direction, with the beam waist at z = 0.

Block SM, Asbury CL, Shaevitz JW, Lang MJ. 2003. Probing the kinesin reaction cycle with a 2D optical force clamp. Proceedings of the National Academy of Sciences of USA 100 2351. 

Stanch T, Dreuw A (2014) Force-spectrum relations for molecular optical force probes. Angew Chem Int Ed Engl 53 2759-2761... 

The major impact of optical tweezers in biology arises from their use in the rapidly expanding field of single-molecule research. By chance, the optical forces produced by commonly available lasers (with output powers of a few hundred milliwatts) happen to be in the piconewton (pN) range, which is just right to experiment with... 

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