ZnO has attracted growing research attention as a strong candidate material for various optoelectronic device applications. It is important to understand and control the interactions between surface plasmons (SPs) and charge carriers in metal-ZnO hybrid nanostructures to improve the optical characteristics. In this work, we fabricated ZnO/Ag nanogratings using patterned polymer and Si templates. Excitation of the surface plasmon polaritons (SPPs) well explained the optical reflectance and photoluminescence spectra of the ZnO/Ag nanogratings [1,2]. Nanoscopic mapping of surface photovoltage (SPV), i.e., changes in the surface potential under illumination, obtained by Kelvin probe force microscopy (KPFM) enabled us to investigate the local behaviors of the photo-generated carriers. The magnitude and relaxation time of the measured SPV depended on the wavelength and polarization of the incident light . This showed that the SP excitation in the nanogratings directly affected the creation and recombination processes of the charge carriers. All of these results suggested that SPV measurements using KPFM should be very useful for studying the SP effects in metal/semiconductor hybrid nanostructures.
 Gwon et al., Opt. Express 19, 5895 (2011).
 Gwon et al., ACS Appl. Mater. Interfaces. 6, 8602 (2014).
 Gwon et al., Sci. Rep. 5, 16727; doi: 10.1038/srep16727 (2015).
We investigated optical properties of planar Si wafers and Si microwire (MW) arrays with and without ZnO thin films using the finite-difference time-domain (FDTD) method. Reflectance of the MW array (diameter: 4 μm and period: 12 μm) was smaller than that of the planar wafer in the wavelength range from 400 to 1100 nm, which could be originated from antireflection effects due to low optical density and guided-mode-assisted field enhancement. The reflectance of ZnO (thickness: 50 and 80 nm)-coated MW array was drastically reduced compared with the bare array but somewhat larger than that of the coated planar wafer. This could be attributed to less-confined guided modes in the wires, which was supported by the field distribution simulation results. Our results provide some insights into possible roles of transparent conducting layers on MW arrays for photovoltaic applications.
We have investigated optical characteristics of silicon nanowire (Si NW) on Al disk arrays using the finite-difference
time-domain (FDTD) simulations. Without the Al disk, the Si NW arrays alone exhibit strong absorption peaks,
originated from guided mode resonance. The arrays of SiNW with Al disk possess additional broad peaks, at slightly
larger wavelengths than those of the resonant guided mode peaks. The FDTD simulations show formation of
concentrated electromagnetic field at the Si NW/Al interface, indicating excitation of localized surface plasmons. These
results suggest that bottom-contact electrodes can work as means to enhance the optical absorption of the Si NWs as well
as to collect carriers in Si NW-based optoelectronic devices.
We investigated the optical properties of ZnO/Ag grating structures, with the periods of 1000 and 1400 nm, fabricated by
sputtering and nanoimprint lithography. The grating structures exhibited multiple peak features in visible-range
photoluminescence (PL) spectra. Whereas a ZnO/Ag planar thin film showed two broad PL peaks in UV and visible
region. Moreover, the PL intensity of the periodic structures was ~100 times larger than that of the planar counterpart.
Several reflectance dips in the visible range were seen only in the grating structures, which could be caused by photoninduced
surface plasmon polariton (SPP) excitation via the grating coupling. The PL peaks well matched with the
reflectance dips. This represented that the PL enhancement should be originated from the SPP excitation. The finitedifference
time-domain simulations also supported the plasmonic effects in the periodic structures.