Abstract

Light scattering by fractal structures is a complicated and fascinating phenomenon because fractals do not possess translational invariance and, therefore, cannot transmit ordinary waves. Dipolar eigenmodes of fractal structures are typically localized in subwavelength-sized regions, and being the result of multiple scattering exhibit strong frequency and polarization dependencies of their spatial locations. This also means that the spatial locations of light-induced dipole modes are not dictated by the local topography. The resonant optics of fractals has progressed immensely over the last decade, and many issues related to optical excitations in fractals have been theoretically clarified [1]. However, experimental evidences of existence of localized dipolar eigenmodes in fractal structures are scarce. Usage of near-field techniques is probably inevitable for direct imaging these modes, and interesting observations made with a photon scanning tunneling microscope (PSTM) have been reported for fractal metal colloid clusters [2]. Optical modes of fractal aggregates were imaged in constant intensity mode (i.e., the detected optical signal was kept constant), and the observed spots were wavelength-sized or even larger. Therefore, optical enhancement and subwavelength dimensions of these bright spots could be deduced only indirectly, from elevations of a probe over the sample surface.

© 1998 IEEE