NOPET: Near-field Optical Power-Extinction Tomography

Overview:

There has been considerable recent interest in the development of methods which extend the spatial resolution of optical microscopy beyond the classical diffraction limit. Approaches such as Near-field Scanning Optical Microscopy (NSOM) and Photon Scanning Tunneling Microscopy (PSTM), have been used to obtain

There has also been significant recent activity in the development of 3-D near-field imaging. One such method is a destructive method called nanotomography, in which a sample is successively eroded and then imaged layer by layer with a scanning probe microscope. A nondestructive approach based upon the solution to the linearized near-field inverse scattering problem for 3-D inhomogeneous media was recently present by Carney and Schotland. For this method, the input data for the image reconstruction algorithm requires both the amplitude and phase of the scattered field. Measuring the optical phase is notoriously difficult.

We have seen in Optical Power-Extinction Tomography (OPET) that it is possible to circumvent the phase problem by making use of two-beam power extinction measurements. The basic principle at work in OPET with homogeneous waves also applies when evanescent waves illuminate the sample. Evanescent waves offer several advantages. First because evanescent waves decay exponentially with distance away from the plane in which they are generated, they work very well to localize objects at or just below the surface of the sample. Second, evanescent waves are super-oscillatory, i.e. they vary in space more rapidly than homogeneous waves, and so they provide a means to probe the high-frequency components of the object normally not accessible with homogeneous waves. In this manner NOPET potentially provides super-resolved images.

Current efforts on this project include an extension of the optical theorem to electromagnetic waves in a half-space geometry and the development of the OPET experiment.

FIG. 1. Illustrating the experiment. Beams (a) and (b) generate evanescent waves which are incident on the scatter. The extinguished power is then measured at the output via difference measurements with and without the scatterer present.

References:

• P S Carney, V Markel and J C Schotland, "Near-Field Tomography without Phase Retrieval," Phys. Rev. Lett. 86, 5874-5877 (2001). PDF


• P S Carney "The optical cross-section theorem with incident fields containing evanescent components," J. Mod. Opt. 46, 891-899 (1999). PDF


• P S Carney, E Wolf, and G S Agarwal, "Diffraction tomography using power extinction measurements," J. Opt. Soc. Am. A 16, 2643-2648 (1999). PDF


• P S Carney, E Wolf, and G S Agarwal, "Statistical generalizations of the optical cross-section theorem with application to inverse scattering," J. Opt. Soc. Am. A 14, 3366-3371 (1997). PDF