Colloquia

Over the course of the last two decades, neutron reflectometry has become established as an important structural probe of thin films and multilayered composites, most notably of hydrogenous and magnetic materials. As an introduction, the basic principles and typical applications of neutron reflectometry are briefly reviewed. Examples of neutron reflectometry studies of thin film systems of interest in condensed matter physics, chemical physics, and biophysics are presented. In particular, the scattering length density (SLD) depth profile along the surface normal, averaged over in plane, can be deduced from specular neutron reflectivity measurements (wavevector transfer Q normal to the surface). The SLD profile, in turn, is directly related to the corresponding material composition distribution. Under favorable conditions, specular neutron reflectometry can resolve variations in the compositional depth profile on a length scale of the order of a nanometer for a thin film having a single unit repeat, whereas for a periodic multilayered system, the spatial resolution can approach an Angstrom. For specular neutron reflection, the complex reflection amplitude or phase associated with an "unknown" segment of a composite film structure can be determined exactly, using reference segments, and a subsequent direct inversion can be performed, thereby ensuring, in principle, a unique result [1]. Thus, the phasesensitive neutron reflection / inversion process results in a realspace picture without fitting or any adjustable parameters. We will discuss how, because of the onetoone correspondence between the complex reflection amplitude and the SLD, phasesensitive NR can be viewed, in effect, as being equivalent to a realspace imaging process one in which the inversion computation plays an analogous role to that of the brain, for instance, in interpreting the optical image of an object focused on the retina of the eye [2]. In performing phasesensitive reflectivity measurements in practice, what ultimately limits the accuracy and spatial resolution of the depth profile are the maximum range of Q attainable and the statistical uncertainty in the measured reflected intensities. These effects can be analyzed quantitatively [3] and we will consider the spatial resolution currently possible as well as what can be reasonably expected in the future with more advanced neutron sources and instrumentation (e.g., employing polychromatic beams at continuous sources). Finally, we will critically examine a possible alternative approach to performing neutron reflectivity measurements, which involves the quantum phenomenon of "Interaction Free Measurement" (IFM) of the type first proposed by Dicke [4] and realized in rudimentary fashion by Kwiat et al. with visible light [5]. The scheme utilized by Kwiat et al. purportedly optimizes the efficiency for performing an IFM of the reflectivity (or transmission) by application of the quantum Zeno effect (which requires polarized photons or neutrons) within an interferometer.