Since the development of the first scanning near-field optical microscope g
reat effort has been put into the development of nearfield optical probe ti
ps that have both a good lateral resolution, in the range of several tens o
f nanometers, and a high power throughput. Several groups have proposed the
use of scanning near-field optical tips with a coaxial structure. From ele
ctromagnetic theory it is well known that such structures can attain power
transmissions of close to one and they do not have a cut-off frequency. In
this paper we discuss some questions concerning the stimulation of optical
coaxial modes and their propagation behavior.