  Microscopic pyramidal pits in a reflective surface, a geometry similar to a
retroreflector, are frequently used to enhance signal strength. The enhancement
effect is generally attributed to surface plasmons, however, the sub-wavelength
to near-wavelength dimensions of the pyramidal 3D geometry suggest
contributions from diffraction and near-field effects. Our theoretical analysis
of the light intensity distribution in the similar (but simpler) 2D geometry
assuming a perfect conductor screen, that is, in the absence of any plasmon
effects, shows that interference patterns forming within the cavity cause a
significant resonant increase in local intensity. Such effect can be important
for many applications, especially for the widely used Raman spectroscopy.
Resonant enhancement without plasmons of the emitted Raman signal due to
enhanced local field amplitude is also possible, which implies that the
geometry practically implements a Raman laser. Comparison of diffraction
patterns obtained with near-field and far-field approaches reveals that the
near-field component is responsible for the observed dramatic intensity
enhancement, and thus the Raman enhancement as well.
