Optical nonlinearities of ZnO thin films, made by laser deposition, were investigated by the Z-scan method using a mode-locked femtosecond Ti:sapphire laser. The measured bound-electron nonlinear index of refraction ␥ and the two-photon absorption coefficient ␤ at near-IR wavelengths show an enormous enhancement compared with measurements on bulk ZnO at 532 nm. The results reveal that two-photon resonance to the band edge and exciton energy level is responsible for the nonlinear absorption and that the free carrier induced the optical nonlinearity. With the excitation wavelength operated between 810 to 840 nm, a negative ␤ value is measured due to the saturation of linear absorption of the defect states. Finally, we compared the values of ␤ from the closed aperture Z-scan data ͑by considering the multi-photon absorption induced thermal nonlinearity͒ with those obtained from the open aperture Z-scan data. The results show that nonlinear refraction in the near-IR region is dominated by the bound-electron and free-carrier effect, although the thermal optical nonlinearity cannot be completely ignored.

The authors numerically investigate the absorption enhancement of an amorphous Si solar cell, in which a periodical one-dimensional nanowall or two-dimensional nanopillar structure of the Ag back-reflector is fabricated such that a dome-shaped grating geometry is formed after Si deposition and indium-tin-oxide coating. In this investigation, the effects of surface plasmon (SP) interaction in such a metal nanostructure are of major concern. Absorption enhancement in most of the solar spectral range of significant amorphous Si absorption (320-800 nm) is observed in a grating solar cell. In the short-wavelength range of high amorphous Si absorption, the weakly wavelength-dependent absorption enhancement is mainly caused by the broadband anti-reflection effect, which is produced through the surface nano-grating structures. In the long-wavelength range of diminishing amorphous Si absorption, the highly wavelength-sensitive absorption enhancement is mainly caused by Fabry-Perot resonance and SP interaction. The SP interaction includes the contributions of surface plasmon polariton and localized surface plasmon.

Based on the fabrication of Ag nanoparticles (NPs) with controlled geometry and surface density on an InGaN/GaN quantum well (QW) epitaxial structure, which contains indium-rich nano-clusters for producing localized states and free-carrier (delocalized) states in the QWs, and the characterization of their localized surface plasmon (LSP) coupling behavior with the carriers in the QWs, the interplay behavior of LSP coupling with carrier delocalization in the QWs is demonstrated. By using the polystyrene nanosphere lithography technique with an appropriate nanosphere size and adjusting the post-fabrication thermal annealing condition, the induced LSP resonance wavelength of the fabricated Ag NPs on the QW sample can match the QW emission wavelength for generating the coherent coupling between the carriers in the QWs and the induced LSP. The coupling leads to the enhancement of radiative recombination rate in the QWs and results in increased photoluminescence (PL) intensity, red-shifted PL spectrum, reduced PL decay time, and enhanced internal quantum efficiency. It is found that the observed effects are mainly due to the LSP coupling with the delocalized carriers in the QWs.

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