Mapping the optical intensity distribution in photonic crystals using a near-field scanning optical microscope

A near-field optical microscope (NSOM) is used in collection mode to map the optical intensity distribution inside channel glass photonic crystal samples. The intensity distribution at wavelengths spanning the fundamental transmission minimum (i.e., the fundamental photonic band gap) was studied. The dependence of the NSOM signal on tip-sample separation allows us to confirm that the NSOM is probing the evanescent tail of the light confined to the crystal. At a given wavelength, NSOM images of low-index-contrast unetched samples, consisting of glass cylinders in a matrix of a different glass material, show bands of high and low optical intensity. The spacings of these bands are comparable to the cylinder separation, but display a nontrivial wavelength dependence. We show that these results can be explained by interference between incoming and reflected light inside the photonic crystal. Based on this, we can then determine dispersion relations of the photonic crystal modes directly from the NSOM images. Our experimental results are consistent with the calculated dispersion relations of the lowest order photonic crystal modes with no adjustable parameters. In addition we show an example where the NSOM images reflect the variation in coupling due to sample features rather than the guided mode structures. Based on these results, we discuss possible improvements in photonic crystal design for NSOM studies. (C) 2001 American Institute of Physics.