EN
We demonstrate the possibility of generation of
coherent radiation with tunable frequencies higher than
the frequency of the driving field vd in a nonlinear medium
utilizing the difference combination resonance that occurs
when vd matches the difference of the frequencies of the
two generated fields ω1 and ω2. We find that such a resonance
can appear in materials which have opposite signs
of refractive index at ω1 and ω2. It can also occur in positive
refractive index materials with strong anomalous dispersion
if at one of the generated frequencies the group
and phase velocities are opposite to each other. We show
that the light amplification mechanism is equivalent to a
combination resonance in a system of two coupled parametric
oscillators with the opposite sign of masses. Such
a mechanism holds promise for a new kind of light source
that emits coherent radiation of tunable wavelengths by
an optical parametric amplification process with the frequency
higher than vd.