Giant second-harmonic generation in few-atomic layer metals

Quantum confinement restructures the electronic phase space of ultrathin metals, but the resulting density-of-states (DOS) change is not generically large. We show that, within a three-dimensional confinement theory based on the suppression of long-wavelength electronic states, the Fermi-level DOS enhancement on the weak-confinement branch is bounded by $(4\mathrm{/}3{)}^{1\mathrm{/}3}-1\simeq 10.1\mathrm{\%}$. Instead, confinement-induced redistribution of electronic phase space can be strongly amplified by resonant denominators in the second-order susceptibility. A minimal double-resonance closure predicts giant amplification when the film thickness approaches $L_c=(2\pi \mathrm{/}n{)}^{1\mathrm{/}3}$, where $n$ is the carrier density, and a dominant optical pathway lies within a dephasing linewidth of resonance. Applied to recent Ag(111) measurements, the theory gives an effective phase-space carrier density parameter $n_{\mathrm{e}\mathrm{f}\mathrm{f}}\simeq 3.6\times {10}^{20}{\mathrm{c}\mathrm{m}}^{-3}$, a tenfold enhancement of $\mathrm{\mid }{\chi }^{(2)}\mathrm{\mid }$, and hence the observed $\sim 100$-fold increase of thickness-normalized second-harmonic-generation efficiency, together with the observed non-oscillatory thickness dependence.