Optical interaction of metal-induced-gap-state electrons and photon-assisted-tunneling electrons at the metal-insulator interfaces

We experimentally determined the delocalized electron density at metal-induced-gap-states (MIGS) in Au/Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ (i.e. metal-insulator or MI) interfaces by applying a sensitive second harmonic generation (SHG) technique. We also observed an enhancement limit in the third harmonic generation (THG) at Au/Al$_{\mathrm{2}}$O$_{\mathrm{3}}$/Au (i.e. metal-insulator-metal or MIM) interfaces due to photon-assisted-tunneling (PAT). The Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ layer was deposited on planar Au samples using atomic layer deposition (ALD) technique to form Al$_{\mathrm{2}}$O$_{\mathrm{3}}$/Au interface. Later, Au nanoparticles of diameter 20nm were immobilized on Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ layer to prepare Au/Al$_{\mathrm{2}}$O$_{\mathrm{3}}$/Au interface. Second and third harmonic signals were extracted in each step. Simulations were done using finite-element-method to compare with the experimental results. The experimental results match qualitatively to the prediction of quantum conductivity theory (QCT).