G. von Plessen

Spectroscopy of single metallic nanoparticles using total internal reflection microscopy

We have developed a simple, fast, and flexible technique to measure optical scattering spectra of individual metallic nanoparticles. The particles are placed in an evanescent field produced by total internal reflection of light from a halogen lamp in a glass prism. The light scattered by individual particles is collected using a conventional microscope and is spectrally analyzed by a nitrogen-cooled charge-coupled-device array coupled to a spectrometer. This technique is employed to measure the effect of particle diameter on the dephasing time of the particle plasmon resonance in gold nanoparticles. We also demonstrate the use of this technique for measurements in liquids, which is important for the potential application of particle plasmons in chemical or biological nanosensors.

Excitation of nanoscale vapor bubbles at the surface of gold nanoparticles in water

Intense nonequilibrium femtosecond laser excitation of gold nanoparticles in water leads to a transient heating of the nanoparticles, which decays via heat transfer to the water phase. It is shown that the water temperature rises to near the critical temperature and the water undergoes an explosive evaporation in the subnanosecond range. The formation of vapor bubbles shows a threshold dependence on laser fluence. The nascent nanoscale vapor bubbles change the heat dissipation drastically. The nanoscale structure is resolved directly with a combination of x-ray scattering methods sensitive to the particle lattice expansion and the change in the water structure factor.

Electrically controlled light scattering with single metal nanoparticles

A concept to electrically control the scattering of light is introduced. The idea is to embed noble metal nanoparticles in an electro-optical material such as a liquid crystal in order to induce a spectral shift of the particle plasmon resonance by applying an electric field. Light scattering experiments on single gold nanoparticles show that spherically shaped nanoparticles become optically spheroidal when covered by an anisotropic liquid crystal. The two particle plasmon resonances of the optically spheroidal gold nanoparticles can be spectrally shifted by up to 50 meV when electric fields of more than 10 kV/cm are applied.

Launching surface plasmons into nanoholes in metal films

We investigate optical transmission through individual nanometer-sized holes in opaque metal films using scanning near-field optical microscopy. We show unambiguously that excitation and lateral propagation of surface plasmons support the light transmission through these nanoholes. The direction of the surface plasmon propagation is given by the light polarization, thus controlled addressing of individual holes is possible. In addition, we find characteristic interference effects due to scattering of surface plasmons off holes.

Photochromic silver nanoparticles fabricated by sputter deposition

In this study a simple route to preparing photochromic silver nanoparticles in a TiO2 matrix is presented, which is based upon sputtering and subsequent annealing. The formation of silver nanoparticles with sizes of some tens of nanometers is confirmed by x-ray diffraction and transmission electron microscopy. The inhomogeneously broadened particle-plasmon resonance of the nanoparticle ensemble leads to a broad optical-absorption band, whose spectral profile can be tuned by varying the silver load and the annealing temperature. Multicolor photochromic behavior of this Ag–TiO2 system upon irradiation with laser light is demonstrated and discussed in terms of a particle-plasmon-assisted electron transfer from the silver nanoparticles to TiO2 and subsequent trapping by adsorbed molecular oxygen. The electron depletion in the nanoparticles reduces the light absorption at the wavelength of irradiation. A gradual recovery of the absorption band is observed after irradiation, which is explained with a slow therm...

Optimized Design of Plasmonic MSM Photodetector

We present an optimized design for a plasmonic metal-semiconductor-metal photodetector with interdigitated electrodes with subwavelength dimensions and a single GaInNAs quantum well (QW) as an absorbing layer. The excitation of surface plasmons at the metal-semiconductor interface leads to a strong field enhancement near the metallic electrodes. This results in an increased absorption in the QW, allowing both fast electrical response of the photodetector and high quantum efficiencies. With a grating periodicity of 820 nm and electrode finger width of 460 nm a 16-fold increase in the absorption of p-polarized light in the QW is achieved in comparison to the case without electrodes.

Capture and thermal re-emission of carriers in long-wavelength InGaAs/GaAs quantum dots

We investigate the ultrafast carrier dynamics in metalorganic chemical vapor deposition-grown InGaAs/GaAs quantum dots emitting at 1.3 μm. Time-resolved photoluminescence upconversion measurements show that the carriers photoexcited in the barriers relax to the quantum-dot ground state within a few picoseconds. At low temperatures and high carrier densities, the relaxation dynamics is dominated by carrier–carrier scattering. In contrast, at room temperature, the dominant relaxation process for electrons is scattering between quantum-dot levels via multiple longitudinal optical (LO)-phonon emission. The reverse process, i.e., multiple LO-phonon absorption, governs the thermal re-emission of electrons from the quantum-dot ground state.

Optical and structural changes of silver nanoparticles during photochromic transformation

Silver nanoparticles embedded in titanium oxide change their color upon irradiation with visible light. Here we investigate the origin of this photochromic effect. The color change is found to result chiefly from a reduction of the optical extinction peak of the photoexcited particle plasmons. From a comparison with x-ray diffraction experiments, we conclude that this reduction is caused by a photoinduced decrease of the mean size of the silver nanocrystals.

Localized plasmonic losses at metal back contacts of thin-film silicon solar cells

Investigations of optical losses induced by localized plasmons in protrusions on silver back contacts of thin-film silicon solar cells are presented. The interaction of electromagnetic waves with nanoprotrusions on flat silver layers is simulated with a three-dimensional numerical solver of Maxwells equations. Spatial absorption profiles and spatial electric field profiles as well as the absorption inside the protrusions are calculated. The results presented here show that the absorption of irradiated light at nanorough silver layers can be strongly enhanced by localized plasmonic resonances in Ag nanoprotrusions. Especially, localized plasmons in protrusions with a radius below 60 nm induce strong absorption, which can be several times the energy irradiated on the protrusions cross section. The localized plasmonic resonances in single protrusions on Ag layers are observed to shift to longer wavelengths with increasing refractive index of the surrounding material. At wavelengths above 500 nm localized plasmonic resonances will increase the absorption of nanorough μc-Si:H/Ag interfaces. The localized plasmon induced absorption at nanorough ZnO/Ag interfaces lies at shorter wavelengths due to the lower refractive index of ZnO. For wavelengths above 500 nm, a high reflectivity of the silver back contacts is essential for the light-trapping of thin-film silicon solar cells. Localized-plasmon induced losses at silver back contacts can explain the experimentally observed increase of the solar cell performance when applying a ZnO/Ag back contact in comparison to a μc-Si:H/Ag back contact.

Increasing Upconversion by Plasmon Resonance in Metal Nanoparticles—A Combined Simulation Analysis

Upconversion (UC) of subbandgap photons has the potential to increase solar cell efficiencies. In this paper, we first review our recent investigations of silicon solar cell devices with an attached upconverter based on β-NaYF4 :20%Er3+. Such devices showed peak external quantum efficiencies of 0.64% under monochromatic excitation at 1523 nm and an irradiance of 2305 Wm -2. Under broad spectrum illumination, an average UC efficiency of 1.07 ± 0.13% in the spectral range from 1460 to 1600 nm was achieved. The measured quantum efficiency corresponds to a relative efficiency increase of 0.014% for the used bifacial silicon solar cell with 16.70% overall efficiency. This increase is too small to make UC relevant in photovoltaics. Therefore, additional means of increasing the UC efficiency are necessary. In this paper, we investigate plasmon resonance in metal nanoparticles in the proximity of the UC material, with the aim of increasing UC efficiency. The local field enhancement by the plasmon resonance positively influences UC efficiency because of the nonlinear nature of UC. Additionally, the metal nanoparticles also influence the transition probabilities in the upconverter. To investigate the effects, we combine different simulation models. We use a rate equation model to describe the UC dynamics in β-NaYF4 :20%Er3+. The model considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multiphonon decay. The rate equation model is coupled with Mie theory calculations of the changed optical field in the proximity of a gold nanoparticle. The changes of the transition rates both for radiative and nonradiative processes are calculated with exact electrodynamic theory. Calculations are performed in high resolution for a 3-D simulation volume. The results suggest that metal nanoparticles can increase UC efficiency.