The near-field and far-field spectral response of plasmonic systems are often assumed to be identical, due to the lack of methods that can directly compare and correlate both responses under similar environmental conditions. We develop a widely tunable optical technique to probe the near-field resonances within individual plasmonic nanostructures that can be directly compared to the corresponding far-field response. In tightly coupled nanoparticle-on-mirror constructs with nanometer-sized gaps we find >40 meV blue-shifts of the near-field compared to the dark-field scattering peak, which agrees with full electromagnetic simulations. Using a transformation optics approach, we show such shifts arise from the different spectral interference between different gap modes in the near- and far-field. The control and tuning of near-field and far-field responses demonstrated here is of paramount importance in the design of optical nanostructures for field-enhanced spectroscopy, as well as to control near-field activity monitored through the far-field of nano-optical devices.
We theoretically compare the scattering and near field of nanoparticles from different types of materials, each characterized by specific optical properties that determine the interaction with light: metals with their free charge carriers giving rise to plasmon resonances, dielectrics showing zero absorption in wide wavelength ranges, and semiconductors combining the two beforehand mentioned properties plus a band gap. Our simulations are based on Mie theory and on full 3D calculations of Maxwell’s equations with the finite element method. Scattering and absorption cross sections, their division into the different order electric and magnetic modes, electromagnetic near field distributions around the nanoparticles at various wavelengths as well as angular distributions of the scattered light were investigated. The combined information from these calculations will give guidelines for choosing adequate nanoparticles when aiming at certain scattering properties. With a special focus on the integration into thin film solar cells, we will evaluate our results.
a b s t r a c tIntegration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se 2 (CIGSe) solar cells is investigated. X-ray photoelectron spectroscopy results show that Ag nanoparticles underneath a Sn:In 2 O 3 back contact could not be thermally passivated even at a low substrate temperature of 440 • C during CIGSe deposition. It is shown that a 50 nm thick Al 2 O 3 film prepared by atomic layer deposition is able to block the diffusion of Ag, clearing the thermal obstacle in utilizing Ag nanoparticles as a back reflector in ultra-thin CIGSe solar cells. Via 3-D finite element optical simulation, it is proved that the Ag nanoparticles show the potential to contribute the effective absorption in CIGSe solar cells.
We use scanning near-field optical microscopy (SNOM) to characterize different plasmonic-nanoparticle situations with high spatial and spectral resolution in this comparative study. The near-field enhancement is measured with an aperture probe (Al coated glass fiber) and two CCD spectrometers for simultaneous detection of reflection and transmission. The images of transmission and reflection show a correlation to the topography. We present a new way to access the relative absorption and discuss the results with consideration of artifact influences. Near-field enhancements are deeper understood by imaging isolated particles. This near field will be compared to measurements of random-particle distributions. Therefore, we will show normalized reflection and transmission images of random structures that lay the foundation for an absolute interpretation of near-field images. The normalization considers both the far-field UV/VIS results and a reference image of the substrate. The near-field reflection of nanoparticle arrays shows an enhancement of 25 %. In view of specific applications, particle distributions implemented in two ways: as far-field scatters and as near field enhancing objects.
The local efficiency of lamellar shaped Cu(In,Ga)Se solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm).
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