  The resolution of far-field optical microscopes, which rely on propagating
optical modes, is widely believed to be limited because of diffraction to a
value on the order of a half-wavelength $\lambda /2$ of the light used.
Although immersion microscopes have slightly improved resolution on the order
of $\lambda /2n$, the increased resolution is limited by the small range of
refractive indices n of available transparent materials. Here we demonstrate a
new far-field optical microscope design, which is capable of reaching
nanometer-scale resolution. This microscope uses the fact that the effective
refractive index $n_{eff}$ of a planar dielectric lens or mirror placed on a
metal surface may reach extremely large values, up to $10^3$, as seen by
propagating surface optical modes (plasmons). In our design a magnified planar
image produced originally by surface plasmons in the metal plane is viewed by a
regular microscope. Thus, the theoretical diffraction limit on resolution is
pushed down to nanometer-scale $\lambda /2n_{eff}$ values. Used in reverse,
such a microscope may become an optical lithography tool with nanometer-scale
spatial resolution.
