Transfer functions in collection scanning near-field optical microscopy

Computational Lens for the Near Field

“…1) consists of a collection NSOM with an uncoated, sharp fiber tip used as a probe [20] and an arrangement for illumination of a sample by either of two counter-propagating evanescent waves, a slight modification of the setup described in detail in Ref. [21]. A beam from a 10 mW He-Ne laser (with 633 nm) is split into two beams that are coupled into a prism such that counterpropagating evanescent waves of equal magnitude are established at the prism face.…”

“…The probed optical field components are weakly coupled into the fiber and detected with a photomultiplier tube. The signal is proportional to the near-field intensity distribution only if the NSOM tip measures with the same efficiency all spatial frequency components of the total optical field [21]. The detection efficiency for uncoated fiber tips decreases with the increase of spatial frequency [22], resulting in an effective center of detection located inside the tip at a certain distance from the tip end [23].…”

Measuring amplitude and phase distribution of fields generated by gratings with sub-wavelength resolution

“…Since E x is perpendicular, but E z parallel to the tip, the problem will be to know what is really detected by the SNOM tip: only E x , or a combination of E x and E z ? This is a crucial question, because the answer explains the behavior of the tip probe and might lead to know its transfer function [8][9][10] for the electrical field vector.…”

“…Since they are orthogonal, they do not interfere. By introducing a fiber tip, the two components are coupled to the same propagating mode in the fiber [9,10] and can Fig. 5.…”

“…8. By introducing the coupling coefficients [9] c x and c z , for the E x and the E z , component, respectively, we can express the total collected electric field by…”