Extreme-ultraviolet frequency comb generation by polarization-gated surface-enhanced optical fields

We show that two synchronized counter-propagating femtosecond pulses could be controlled by adjusting their relative delay to excite surface-enhanced optical fields of time-gated linear polarization in contrast with intrinsic elliptic polarization excited by the conventional one-pulse incidence scheme. Such surface-enhanced optical fields can be used to generate efficient high harmonics and thus extreme-ultraviolet frequency combs. The energy conversion efficiency as well as the frequency span of the generated extreme-ultraviolet frequency combs can be significantly increased with controllable emission probabilities along different directions relative to the surface.