![]() ![]() The sensitivities ∂ N ∕ ∂ n C depend on the polarization, mode number and effective waveguide thickness: below a certain minimum cutoff thickness no propagation can take place and as the thickness increases the sensitivity rapidly reaches a maximum and then slowly diminishes the sensitivities decrease for higher-order modes within conventional waveguides. Only discrete values of N are allowed, corresponding to the mode numbers m = 0, 1, 2, …, each value of m being represented by two orthogonal polarizations, transverse magnetic (TM) and TE. This condition can be written as a set of mode equations linking the effective refractive index N of a particular mode with n S, n F, d F and n C. The condition for the guided waves to propagate is for the different contributions to Φ to sum to zero or an integral multiple of 2 π. Both these materials are much thicker than 1 ∕ s, and hence can be considered semi-infinite. The optical waveguide is constructed by sandwiching a slab of the high refractive index material F between lower refractive index materials, the mechanically strong support S and the medium C containing the cells. Taken together, these necessarily constrain the shapes that can be holographically achieved in the near field, and clearly require the development of specialized algorithms. The disadvantages of evanescent fields are that one must obviously work very close to the surface, create patterns using only the range of incident angles beyond the critical angle for total internal reflection, and allow for the strong variation in penetration depth as a function of incident angle (which, on the other hand, allows 3D shaping of the evanescent field ). Several schemes have been proposed for holographic control of evanescent fields, and a tailored algorithm for doing this sort of light shaping has recently appeared. Interestingly, in an unpatterned, but resonantly enhanced, evanescent field, arrays of trapped particles have recently been observed to self-organize, due to the onset of nonlinear optical phenomena (optical solitons). Also, it has been shown that patterned evanescent fields can create large numbers of traps spanning macroscopically large areas. One is not subject to the free-space diffraction limit and can therefore create significantly subwavelength structures in the optical fields. Roberto Di Leonardo, in Structured Light and Its Applications, 2008 6.4.6.3 Evanescent-wave optical trap arraysĮvanescent fields hold promise for future generation of trap arrays, primarily for two reasons.
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