  We study theoretically the interplay between the surface confined wave modes
and the linear and nonlinear gain of the dielectric layer in the Otto
configuration. The surface confined wave modes such as surface plasmons or
waveguide modes are excited in the dielectric-metal bilayer by obliquely
incident $p$ waves. In the purely linear case, we find that the interplay
between linear gain and surface confined wave modes can generate a large
reflectance peak with its value much greater than 1. As the linear gain
parameter increases, the peak appears at smaller incident angles, and the
associated modes also change from surface plasmons to waveguide modes. When the
nonlinear gain is turned on, the reflectance shows very strong multistability
near the incident angles associated with surface confined wave modes. As the
nonlinear gain parameter is varied, the reflectance curve undergoes complicated
topological changes and sometimes displays separated closed curves. When the
nonlinear gain parameter takes an optimally small value, a giant amplification
of the reflectance by three orders of magnitude occurs near the incident angle
associated with a waveguide mode. We also find that there exists a range of the
incident angle where the wave is dissipated rather than amplified even in the
presence of gain. We suggest that this can provide the basis for a possible new
technology for thermal control in the subwavelength scale.
