TIRT: Total Internal Reflection Tomography

Overview:

Total Internal Reflection Tomography (TIRT) builds on Total Internal Reflection Microscopy (TIRM), a technique that brings to conventional microscopy the added functionality of illumination by evanescent waves. Incorporation of evanescent waves in the illuminating field is an important development for several reasons. First, such waves are exponentially decaying along one direction. This property allows for the control of the depth of penetration of the illuminating field. Second, an evanescent field may be used to resonantly excite surface plasmon modes of the sample. Study of these modes is important in understanding the behavior of material interfaces. Third, evanescent waves may be employed to supersede the Abbe-Rayleigh limit of order 1/2.

TIRM has been in practical use for decades. It has primarily been used as a surface inspection technique, though the sensitivity of the field to distance along the decay axis has been utilized to advantage in applications such as the measurement of distance between two surfaces . Until recently the opportunities for transverse superresolution made possible by the high spatial frequency content of the probe field have been largely over-looked. However, there have been advances in the realization of super-resolved imaging utilizing TIRM reported recently in the literature. A direct imaging approach resulting from the marriage of standing-wave illumination techniques and TIRM has been described achieving transverse resolution of one seventh the wavelength. An additional approach has been brought forward in which structural information derived from a scattering object is extracted from the scattered field and a three dimensional reconstruction of the object is made. The reconstruction is accomplished by making use of an analytic solution to the inverse scattering problem with evanescent waves . We refer to to the method as TIRT.

TIRT offers several advantages over existing modalities for three-dimensional microscopy. First, the evanescent waves used for illumination encode on the scattered field the subwavelength structure of the scattering object. It is thus possible to obtain subwavelength resolved images of the sample as is done in other near-field techniques such as near-field scanning optical microscopy (NSOM), without the technical difficulties encountered with probe-sample interactions. Second, the results of the reconstruction are unambiguous. That is, non-uniqueness associated with simultaneous variations in the material optical properties and the topology of the sample is not an issue. Third, the reconstruction obtained offers a three dimensional view of the object. This is somewhat analogous to the transition from projection radiography to computed tomography.

FIG. 1. Illustration of the measurement scheme. Evanescent waves are generated at the prism face by total internal reflection. The total internal reflection is then partly frustrated by the presence of the scatterer, scattering evanescent modes to the homogeneous mode that propagate to the far zone.

Current efforts:

In concjunction with researchers at NASA Glenn Research Center, we are developing an experimental implementation of TIRT. We continue to investigate the theoretical aspects of the problem including analysis of the information content of the near-field.

References:

• P S Carney and J C Schotland, "Three-Dimensional Total Internal Reflection Microscopy," Opt. Lett. 26, 1072-1074 (2001). PDF


• P S Carney and J C Schotland, "Theory of total-internal-reflection tomography," J. Opt. Soc. Am. A 20, 542-547 (2003). PDF


• R A Frazin, D G Fischer, and P S Carney, "Information content of the near-field I: two-dimensional samples," Accepted by JOSA A (2004). PDF