  In an optical configuration consisting of a flat plate of vacuum between
upper and lower spaces of uniform dielectric regions of n>1, we have calculated
two output light intensities for two input lights from the Maxwell's equations
as functions of the incision angle, a light intensity ratio, a phase difference
of the two input lights, and a thickness of the vacuum layer, where the two
input lights come from upper and lower dielectric regions with the same
incision angles, and one of the output light goes into upper dielectric and the
other goes into lower dielectric. We have found that, when evanescent lights
exist at the upper and lower boundary and interfere each other, there is one
set of incision angles and phase differences for any combination of an input
light ratio and a thickness of the vacuum layer where one of output lights
becomes zero. This finding will possibly lead to an innovative optical switch
with which an optical output light can be switched on and off with a control
light with an intensity much lower than that of the output light.
