Gero von Plessen

Influence of interband electronic transitions on the optical absorption in metallic nanoparticles

Electronic interband transitions influence the surface plasmon resonance in metallic nanoparticles significantly. We derive expressions for the resonance frequency, the bandwidth and the maximum of the light absorption cross section of noble metal nanoparticles, taking into account the interband transitions in the dielectric function. We propose a simple method for determining the width of the plasmon resonance based on an analysis of the dielectric permittivity of the constituent metal.

Substrate effect on the optical response of silver nanoparticles

We study the influence of a substrate on the surface plasmon resonance absorption in silver nanoparticles depending on the distance between the nanoparticles and the substrate. The experimentally observed red shift of the resonant absorption is explained in the frame of the image-induced charges at the interface between the ambient medium and the substrate. The influence of a metallic substrate is negligible when the clusters are at a distance d>2R from the substrate. A dielectric substrate has much less influence on the optical response of silver clusters than a metallic one.

Plasmon enhanced upconversion luminescence near gold nanoparticles–simulation and analysis of the interactions

We investigate plasmon resonances in gold nanoparticles to enhance the quantum yield of upconverting materials. For this purpose, we use a rate equation model that describes the upconversion of trivalent erbium based upconverters. Changes of the optical field acting on the upconverter and the changes to the transition probabilities of the upconverter in the proximity of a gold nanoparticle are calculated using Mie theory and exact electrodynamic theory respectively. With this data, the influence on the luminescence of the upconverter is determined using the rate equation model. The results show that upconversion luminescence can be increased in the proximity of a spherical gold nanoparticle due to the change in the optical field and the modification of the transition rates.

Reversible Photothermal Melting of DNA in DNA–Gold‐Nanoparticle Networks

Temperature is the most commonly applied control parameter for chemical reactions. In contexts where a very local control of reactions is important, such as in molecular self-assembly processes or lab-on-a-chip applications, it would be desirable to have a fast and precise means of manipulating the temperature on micrometer or even nanometer scales while the surrounding temperature remains unaffected. One possible tool to achieve this goal is photothermal temperature manipulation via laser irradiation of metal nanoparticles. The nanoparticles convert the absorbed light energy into heat, which is subsequently transferred to the local environment of each nanoparticle. Metal nanoparticles are very suitable absorbers since they possess extremely high absorption cross-sections associated with surface plasmons, that is, collective oscillations of conduction electrons in the metal. By choosing the particle composition, size, and geometry, the plasmonic absorption maximum can be tuned to spectral positions where no other species in the particle environment is strongly absorbing, thus enabling spectral selectivity of the heating effect. Several studies have investigated destructive processes caused by nanoparticle-assisted photothermal heating. A focus of interest lies in the investigation of nanoparticleassisted hyperthermia to be applied in cancer therapy. Csaki et al. applied metal nanoparticles to generate lesions in predefined areas of chromosomes. Nanoparticle-supported photothermia can be employed in externally triggered drug release by a selective, nanoparticle-assisted disruption of the drug-carrying capsules. Very few examples have shown reversible, that is, non-destructive, photothermally induced processes based on metal nanoparticles. Das et al. presented a photothermal volume phase transition of gold nanorod-loaded hydrogels while Jones et al. used spherical nanoparticles to generate microlenses from hydrogel phase transitions. Controlled

Optical Effects of Metallic Nanoparticles

Metallic nanoparticles show a rich optical behaviour because of their strong light absorption and scattering, wide spectral tunability, and interesting optical near-field effects. Research into optical effects of metallic nanoparticles, a field with a long tradition, has been developing rapidly in recent years as a result of progress in nanoparticle fabrication, spectroscopic techniques, and computational methods. This article provides a survey of optical effects of metallic nanoparticles, covering both fundamental phenomena and emerging applications.

Enhanced upconversion quantum yield near spherical gold nanoparticles – a comprehensive simulation based analysis

Photon upconversion is promising for many applications. However, the potential of lanthanide doped upconverter materials is typically limited by low absorption coefficients and low upconversion quantum yields (UCQY) under practical irradiance of the excitation. Modifying the photonic environment can strongly enhance the spontaneous emission and therefore also the upconversion luminescence. Additionally, the non-linear nature of the upconversion processes can be exploited by an increased local optical field introduced by photonic or plasmonic structures. In combination, both processes may lead to a strong enhancement of the UCQY at simultaneously lower incident irradiances. Here, we use a comprehensive 3D computation-based approach to investigate how absorption, upconversion luminescence, and UCQY of an upconverter are altered in the vicinity of spherical gold nanoparticles (GNPs). We use Mie theory and electrodynamic theory to compute the properties of GNPs. The parameters obtained in these calculations were used as input parameters in a rate equation model of the upconverter β-NaYF4: 20% Er3+. We consider different diameters of the GNP and determine the behavior of the system as a function of the incident irradiance. Whether the UCQY is increased or actually decreased depends heavily on the position of the upconverter in respect to the GNP. Whereas the upconversion luminescence enhancement reaches a maximum around a distance of 35 nm to the surface of the GNP, we observe strong quenching of the UCQY for distances <40 nm and a UCQY maximum around 125 to 150 nm, in the case of a 300 nm diameter GNP. Hence, the upconverter material needs to be placed at different positions, depending on whether absorption, upconversion luminescence, or UCQY should be maximized. At the optimum position, we determine a maximum UCQY enhancement of 117% for a 300 nm diameter GNP at a low incident irradiance of 0.01 W/cm2. As the irradiance increases, the maximum UCQY enhancement decreases to 20% at 1 W/cm2. However, this UCQY enhancement translates into a significant improvement of the UCQY from 12.0% to 14.4% absolute.

Stepwise thermal and photothermal dissociation of a hierarchical superaggregate of DNA-functionalized gold nanoparticles.

Interest in oligonucleotide-functionalized gold nanoparticles has gradually risen due to the molecular-recognition capabilities of oligonucleotides and the outstanding optical properties of gold nanoparticles (AuNPs). [ 1 − 4 ] Applications of these systems in biodiagnostics, antisense gene regulation, intracellular messenger RNA detection, and delivery systems are in use today or seem within reach in the near future. [ 5 ]

Excitonic quantum beats in semiconductor quantum-well structures

Abstract We study quantum beats from excitonic states in semiconductor quantum-well structures. The quantum beats manifest themselves as modulations in transient four-wave mixing (FWM) signals. Using time-integrated, time-resolved, and spectrally resolved detection of the FWM signal, beat phenomena of various origins are studied, including interferences between the bound and unbound (continuum) states of a quantum-well Wannier exciton, and interference between excitonic transitions associated with spatial regions of different quantum-well thickness. An attempt is made to specify in how far excitonic quantum beats can be described as interferences between the states of a multi-level system like in atomic and molecular spectroscopy. We investigate to which extent quantum beats are modified or even generated by Coulomb, disorder, and propagation induced effects characteristic of semiconductors and semiconductor heterostructures.

Calculation of up-conversion photoluminescence in Er3+ ions near noble-metal nanoparticles

In conventional silicon solar cells, photons with energies lower than the silicon band gap (1.12 eV) are not absorbed in the silicon layer. However, the near-infrared portion of the solar spectrum may still be able to contribute to photocurrent generation if use can be made of up-conversion processes that transform two or more infrared photons into a photon of sufficient energy to be absorbed in silicon. One possible material in which up-conversion processes occur are rare-earth ions such as Er3+. It has recently been shown that up-conversion in such ions could be enhanced by optical near-field coupling to metal nanoparticles in a highly controlled geometry. However, potential photovoltaic applications of the upconversion enhancement will certainly be characterized by different geometric arrangements, with random distances between ions and nanoparticles. Whether or not an overall enhancement of the up-conversion efficiency may be expected under such realistic conditions is an open question. In this work, we address an important aspect of this question, namely the particle-induced enhancement of the optical excitation rate in the rare-earth ions. Our model calculations show that the excitation rate in Er3+ ions can be enhanced using spherical gold nanoparticles. The model includes random distances between ions and nanoparticles, as well as random polarizations of the exciting light. The enhancement of the rate of excitation of the fundamental transition results in increases of the up-conversion rate by up to 20% for an excitation wavelength of 1523 nm, provided that photoluminescence-quenching effects due to nonradiative relaxation in the metal can be neglected.

Controlled Gold Nanorod Reorientation and Hexagonal Order in Micromolded Gold Nanorod@pNIPAM Microgel Chain Arrays

A one-step soft lithography based pathway to manufacture aligned gold nanorod@poly-(N-isopropylacrylamide) (GNR@pNIPAM) hybrid chains with hexagonal arrangement of the nanorods and with an anisotropic optical response is presented. After demonstration of an efficient synthesis protocol, yielding uniform composite microgels in high concentration, a micromolding procedure using wrinkled polydimethylsiloxane (PDMS) templates to fabricate aligned hybrid chains is introduced. It is found that the self-assembled GNR@pNIPAM microgels inside the PDMS wrinkle grooves can be transferred onto solid substrates, on which they exhibit a hexagonal order, as confirmed by small-angle X-ray scattering. Further, it is shown that the application of minimized PDMS wrinkle dimensions aligns GNRs inside the pNIPAM microgels, and that the optical response of such molded assemblies is anisotropic.