Computational Lens for the Near Field

A method is presented to reconstruct the structure of a scattering object from data acquired with a photon scanning tunneling microscope . The data may be understood to form a Gabor type near-field hologram and are obtained at a distance from the sample where the field is defocused and normally uninterpretable. Object structure is obtained by the solution of the inverse scattering problem within the accuracy of a perturbative, two-dimensional model of the object.

Figure 1

The PSTM instrument. A He-Ne laser (L) emits a collimated beam that is divided at the beam splitter (BS) and propagated on one of two possible paths, (A) or (B), the other path being blocked. The beam is routed by mirrors (M) to the prism (P) at an angle such that the beam in the prism is totally internally reflected. The evanescent field outside the prism interacts with the sample (S) and the total field is detected by the tapered fiber probe (F), which is scanned in a plane at fixed height.

Figure 2

The data are displayed in panels (a) and (b). The colorbar indicates normalized photocount with the constant background subtracted. The result of the reconstruction algorithm is displayed in panel (c). An AFM image is displayed in panel (d) for comparison. The field of view in each image is $3.4\times 5\mu {\mathrm{m}}^{\mathrm{2}}$.