High harmonic generation from surface states of solids

We demonstrate that high-harmonic generation (HHG) in solids dominantly originates from strongly localized surface states through non-perturbative processes. Measurements reveal that HHG from bulk states is suppressed by at least 1-2 orders of magnitude due to the lack of phase matching, when generated perturbatively, or by at least 3-4 orders of magnitude, when generated non-perturbatively. We derive a theory that fully supports this observation and quantitatively describes the generation of harmonics from the surface states as well as from interfaces between solids; it also predicts a much weaker generation of harmonics from the bulk states. Our results pave the way for the development of very high repetition rate high harmonic sources for vacuum ultraviolet spectroscopy and high precision frequency comb metrology using surface states from solids.