Search

The modified emission characteristics of a source dipole near a metal-coated topological insulator sphere are studied with the application of axion electrodynamics in the long wavelength limit. Our focus is on the effect due to the topological magneto-electric (TME) response of the sphere and its influence on the multipolar plasmonic resonance of the metallic nanoshell. It is found that the TME response mainly affects the bonding modes of the metal shell plasmons with the lower order multipole resonances experiencing appreciable red-shifts. In addition, the TME effects can lead to either greater or smaller decay rates and lower emission frequencies, depending crucially on the frequency of the emitting dipole; but otherwise rather insensitive to the orientation of the dipole moments. These findings thus provide an alternative approach to probing the TME effects via molecular fluorescence experiments, as well as new possibility of controlling the emission properties of molecules using these novel nanoparticles.

The recent study of the Lorenz-Mie scattering from a graphene-coated sphere is extended to the case when the sphere is of negative refractive index, focusing on the two intriguing phenomena observed previously for a “bare” negative index sphere of sub-wavelength sizes, namely, the non-Rayleigh behavior and the low-frequency resonances. It is observed that while the graphene coating can regulate the non-Rayleigh scattering back towards Rayleigh, manifested with its own low frequency plasmonic resonances in the IR region; the low-frequency peak observed previously can be significantly suppressed with the result rather insensitive to the doping level of the graphene layer. These graphene-induced effects are clarified via a study of the Bohren-Hunt theory in the limit of small size parameters for the negative refracting sphere.

The optical response of charged polymeric and metallic colloids is investigated using effective medium theories for composite systems of nanoparticles. Based on the Bohren-Hunt theory for generalized Mie scattering from charged particles, an effective quasi-static dielectric function previously obtained is applied to the present study to characterize the response from the various colloidal particles. It is found that such effects are more prominent for polymeric and nonmetallic colloidal solutions in general. In addition, the effects of clustering among the colloidal particles are also studied via a fractal model available from the literature. Detailed numerical studies of the dependence of these effects on the amount of extraneous charge, as well as on the geometry and volume fraction of the colloidal particles are presented.

The effects of extraneous surface charges on the anomalous extinction from metallic nanoparticles are studied via an application of the extended Mie theory by Bohren and Hunt. Due to the sensitivity of the higher multipolar resonance on the surface charges, it is found that quenching of the anomalous resonance can be observed with presence of only a modest amount of charges on these particles. The observed effects thus provide a rather sensitive mechanism for the monitoring of the neutrality of these nanoparticles using far field scattering approaches.

The recent demonstration of the plasmonic-enhanced Forster resonance energy transfer (FRET) between two molecules in the vicinity of planar graphene monolayers is further investigated using graphene-coated nanoparticles (GNP). Due to the flexibility of these nanostructures in terms of their geometric (size) and dielectric (e.g., core material) properties, greater tunability of the FRET enhancement can be achieved employing the localized surface plasmons. It is found that while the typical characteristic graphene plasmonic enhancements are manifested from using these GNPs, even higher enhancements can be possible via doping and manipulating the core materials. In addition, the broadband characteristics are further expanded by the closely spaced multipolar plasmon resonances of the GNPs.

Axion electrodynamics is applied to study the response of a plasmonic nanoshell with a core made of topological insulator (TI) materials. The electric polarizability of such a system is calculated in the long wavelength limit via the introduction of two scalar potentials satisfying the various appropriate boundary conditions. Our focus is on the topological magneto-electric effect (TME) as manifested in the coupled plasmonic resonances of the nanoshell. It is found that for a TI with broken time-reversal symmetry, such TME will lead to observable red-shifts in the coupled plasmonic modes, with more significant manifestation of such shifts for the bonding modes of a metallic nanoshell. It is speculated that such universal red-shift could be a manifestation of the fundamental dual symmetry as generalized for axion electrodynamics. Keywords: Surface plasmons, Axion electrodynamics, Topological insulator, Nanoshells

The terahertz (THz) emission from quantum dots in close proximity to graphene-coated nanoparticles is studied via phenomenological modeling with particular interest in the possibility of enhancement for such emission via the excitation of the graphene plasmons. It is shown that depending on various factors such as the damping factor and the Fermi level of the graphene, as well as the size and core material of the coated particle, such plasmonic-enhanced THz emission is indeed possible. This thus opens up a new pathway to provide intense THz sources for future applications.

Motivated by the recent demonstration of asymmetric transmission of light through stratified systems of various anisotropic metamaterials, we have established generalized reciprocal relations for both transmission and reflection for these systems referring to the two cases of incidence from opposite directions. These relations reduce to those derived previously for simple isotropic systems which guarantee reciprocal symmetry in both transmission and reflection for non-dissipative medium, and provide constraint for reflection from both sides in the presence of absorption. Illustrations of our generalized relations are provided for different material systems.

The fluorescence characteristics of a dipole molecule in the vicinity of a spherical multilayered metallic nanoshell (a plasmonic nanomatryoska) of ultra-small dimensions is studied via electrodynamic modeling, where we have computed the fluorescence decay rates, the shifts in emission frequency, and the overall fluorescence yields for molecular dipoles of both tangential and radial orientations. Our focus is on structures of ultra small dimensions in order to elucidate the possibly novel nonlocal optical effects in such a phenomenon. The results show that at very close distances between the molecule and the nanoshell, the nonlocal effects in general lead to smaller structure-induced effects with broadened and blue-shifted plasmonic resonances. These effects include overall smaller induced decay rates, smaller red-shifts in emission frequency, and somewhat larger fluorescence yields at low emission frequencies. Physical interpretation of our simulation results is provided.

Optical sensing of temperature variations is explored by studying the Goos–Hanchen (GH) lateral shift of a reflected light beam from various device based on the surface plasmon (SP) excitation at metal-dielectric interfaces. Both the Kretchman and the Sarid geometry will be considered, where the temperature variations of the GH shifts associated with excitation of both the regular and the long-range SP will be studied. It is found that while the SP-induced shifts and their temperature sensitivities are much greater than those from a bare metallic surface, these sensitivities are comparable between the shifts induced by the different kinds of SP, although the long-range SP can in general induce much greater values in the GH shifts, as reported recently in the literature.

This paper presents a new idea to produce matter in the high energy density physics (HEDP) regime in the laboratory using an intense ion beam. A gas bubble created inside a solid metal may collapse by driving it with an intense ion beam. The melted metal will compress the gas bubble and supply extra energy to it. Simulations show that the spherical implosion ratio can be about 5 and at the stagnation point, the maximum density, temperature and pressure inside the gas bubble can go up to nearly 2 times solid density, 10 eV and a few megabar (Mbar) respectively. The proposed experiment is the first to permit access into the Mbar regime with existing or near-term ion facilities, and opens up possibilities for new physics gained through careful comparisons of simulations with measurements of quantities like stagnation radius, peak temperature and peak pressure at the metal wall.

The optical response of graded-index spherical metallic nanoparticles is studied in the modified long wavelength approximation with electrodynamic effects accounted for to the lowest order of the inverse of the wavelength. An effective-medium approach is adopted which leads to the conclusion that the first-order dynamical effects will enter mainly via the polarizability and not the effective dielectric function of the system. Numerical studies using various graded Drude functions show that these effects are not only significant for particles of large sizes but can also be appreciable for smaller particles with varying index profile.

The Goos–Hanchen (GH) shift of a p-polarized light beam reflected from an interface of a composite material of particulate metals in a dielectric host is studied theoretically using effective medium approaches, with focus on the effects due to the clustering of the metal particles. With application of a fractal-clustering model, it is shown that the composite can have optically metallic behavior even for relatively low volume fraction of metal when clustering takes place, with appreciable negative GH shifts to take place for light of long wavelengths close to grazing incident angles. Furthermore, we confirm that large reflectance is always accompanied with this metal behavior, thus rendering these shifts easily observable.

The problem of radiationless Forster energy transfer between a donor and an acceptor molecule is studied in the vicinity of a metallic nanorice. Using a recently formulated effective medium theory, the modified dipole–dipole interaction between the molecules in the vicinity of a spheroidal metallic nanoshell can be easily formulated, from which huge enhancement of the energy transfer rate is obtained due to the resonant excitation of the bonding and the antibonding plasmonic modes of the nanoshell. Effects due to the different locations and orientations of the molecules are also studied. The results show that the plasmonic resonances depend mainly on the nanorice geometry and much less on the configuration of the molecules, whereas the enhancement is more sensitive to the relative orientations and locations of the molecules.

The optical properties of composites with metallic nanoparticles are studied, taking into account the effects due to the nonlocal dielectric response of the metal and the coalescing of the particles to form clusters. An approach based on various effective medium theories is followed, and the modeling results are compared with those from the cases with local response and particles randomly distributed through the host medium. Possible observations of our modeling results are illustrated via a calculation of the transmission of light through a thin film made of these materials. It is found that the nonlocal effects are particularly significant when the particles coalesce, leading to blue-shifted resonances and slightly lower values in the dielectric functions. The dependence of these effects on the volume fraction and fractal dimension of the metal clusters is studied in detail.

The effects of temperature variation on the electric polarizability of mesoscopic metallic nanoparticles is studied via a theoretical model in which the temperature dependence of the electron–phonon and electron–electron scattering rates are taken into account. Furthermore, the nonlocal response of the metal, which is significant for particles of mesoscopic dimensions, is also included. It is found that the significance of the nonlocal effects can be very different with different metals, for the same smallness in size of the nanoparticles. In addition, while the imaginary part of the polarizability in general increases with temperature, the real part can both decrease or increase as temperature rises—in contrast to the case with the static polarizability of atomic clusters, which most of the time increases with temperature.

The effects due to a finite photon mass on atomic spectroscopy are explored, for the purpose of providing yet another approach for setting the upper limit of the photon mass. In particular, the hydrogen 21 cm radiation is recalculated using Proca electrodynamics for the description of the hyperfine interaction, as well as the Yukawa wave function for the ground state atom. The result shows that the leading order correction will be quadratic in the photon mass. With the current limitation of experimental techniques, it will be challenging for this spectroscopic approach to provide a more stringent upper limit for the photon mass.

The optical properties of a metallic nanoshell composite are studied theoretically using different effective medium theories with particular focus on the effects of variation in temperature and particle clustering on these properties. One unique result from our modeling is the persistent manifestation of the single-particle resonances of the individual nanoshells which cannot be found in a composite of solid particles. In addition, we observe red-shifts in the composite plasmon modes, as well as damping in the dielectric function spectrum as the temperature increases. Furthermore, a much greater red-shift will occur when these nanoshells coalesce to form fractal clusters in the composite, with the value of this shift increasing with decrease in the fractal dimension or increase in the cluster size. We further calculate the transmission at normal incidence through such composite thin films as a demonstration of how these effects can be observed experimentally.

In experiments of Single-bubble Sonoluminescence (SBSL), the bubble is heated to temperatures of a few eV in the collapse phase of the oscillation. Our hydrodynamic simulations show that the density inside the bubble can go up to the order of 1 g/cm3, and the electron density due to ionization is 1021/cm3. So the plasma coupling constant is found to be around 1 and the gas inside the bubble is in the Warm Dense Matter (WDM) regime. We simulate the light emission of SL with an optical model for thermal radiation which takes the finite opacity of the bubble into consideration. The numerical results obtained are compared with the experimental data and found to be very sensitive to the equation of state (EOS) used. As theories for the equation of state, as well as the opacity data, in the WDM regime are still very uncertain, we propose that SL may be a good low-cost experimental check for the EOS and the opacity data for matter in the WDM regime.

Hydrodynamic simulations have been carried out using the multi-physics radiation hydrodynamics code HYDRA and the simplified one-dimensional hydrodynamics code DISH. We simulate possible targets for a near-term experiment at LBNL (the Neutralized Drift Compression Experiment, NDCX) and possible later experiments on a proposed facility (NDCX-II) for studies of warm dense matter and inertial fusion energy-related beam-target coupling. Simulations of various target materials (including solids and foams) are presented. Experimental configurations include single-pulse planar metallic solid and foam foils. Concepts for double-pulsed and ramped-energy pulses on cryogenic targets and foams have been simulated for exploring direct drive beam-target coupling, and concepts and simulations for collapsing cylindrical and spherical bubbles to enhance temperature and pressure for warm dense matter studies.

The decay rates and emission frequencies of a fluorescing molecule in the vicinity of an anisotropic metamaterial are studied within a phenomenological model. A special kind of indefinite medium recently shown to have a broadband all-angle negative refraction is studied for its interaction with the emitting dipole in the long wavelength limit at close molecule-substrate distances. In addition, materials of alternating metal-dielectric stratified layers are also studied. For the first kind which consists of a composite of metallic nanowires and a dielectric host, the results show that large molecular decay rates (hence exceedingly short lifetimes) occur when negative refraction takes place within the medium; and large blue-shifts can occur for the emission frequencies of the admolecues. For the stratified layered system, abrupt changes in both the emission lifetimes and frequencies can also take place upon negative refraction, although to a much less extent. These abrupt changes can thus provide signatures for probing the transition to such unusual optical property for the medium via surface-fluorescence experiments.

The dispersion relation for the plasmon at the interface of a metal and a nonlinear Kerr medium is studied by applying a semi-analytic theory established in the literature. Explicit analytical results are obtained and are compared to those from a certain approximate treatment appeared in the literature. It is found that for large electric field strengths, both the dispersion relation and the surface plasmon frequency from the approximate treatment deviate significantly from those obtained in the exact approach, especially for the case with a negative Kerr susceptibility.

The temperature dependence of the optical/spectroscopic properties of metallic nanoparticles (in particular nanoshells) including absorption, scattering, and enhanced-Raman scattering for admolecules is studied via theoretical modeling. For core-shell particles, we focus on both the high frequency (symmetric) and low frequency (antisymmetric) plasmon resonance modes of the nanoshells. It is found, that while these properties in general have optimal values at a certain temperature specific to the shell configuration except for relatively thin shells, the SERS enhancement generally has more appreciable variations and decreases when the nanoshells get heated up from room temperature. Implications for various applications with these nanoparticles are discussed.

In formulating the Maxwell equations for electrodynamics in the presence of a macroscopic continuous medium, it is common among standard textbooks to simply borrow expressions for the polarization charges and magnetization currents derived in electrostatics and magnetostatics, without much explicit justifications for using them in electrodynamics. Here we emphasize that these quantities must be introduced without resorting to results in statics, and must be derived directly from their definitions which remain valid in the general situation of electrodynamics.

The singularity of the exact electromagnetic fields is derived to include the 'source terms' for harmonically oscillating electric (and magnetic) dipoles, so that the fields will be consistent with the full Maxwell equations with a source. It is shown explicitly, as somewhat expected, that the same δ-function terms for the case of static dipole fields will have to be included in the dynamic case for this purpose, while the associated magnetic (electric) field will remain unaffected by these singular source terms.

Adopting the Lindblom-Detweiler formalism for polar oscillations of neutron stars, we study the $w$-mode oscillation and find that the Lagrangian change in pressure, measured by the physical quantity $X$, is negligibly small. Based on this observation, we develop the generalised inverse-Cowling approximation (GICA) with the approximation $X=X'=0$, where $X'$ is the derivative of $X$ with respect to the circumferential radius, for $w$-mode oscillations of neutron stars. Under GICA, $w$-mode oscillations are described by a second-order differential system, which can yield accurate frequencies and damping rates of quasi-normal modes., 11 pages, 4 figures, accepted for publication in MNRAS

Utilizing the continuous frequency mode quantization scheme, we study from first principle the efficiency of a feedback scheme that can generate maximally entangled states of two atoms in an optical cavity through their interactions with a single input photon. The spectral function of the photon emitted from the cavity, which will be used as the input of the next round in the feedback process, is obtained analytically. We find that the spectral function of the photon is modified in each round and deviates from the original one. The efficiency of the feedback scheme consequently deteriorates gradually after several rounds of operation., Comment: 11 pages, 5 figures, accepted for publication in Journal of Physics B

The Gersten–Nitzan model is applied to study the surface enhanced Raman scattering from a molecule adsorbed on a spherical metallic nanoshell. The nonlocal dielectric response from the nanoshell has been accounted for by using a phenomenological model. For a fixed outer radius of the shell, the enhancement ratio is calculated over a large range of scattering frequency and inner radius of the shell, including the case of a solid metallic particle. The effect due to different core materials is also studied. It is concluded that the nonlocal effects can be significant for very close molecule–shell distances, and for shells with very small sizes and/or thickness, leading to smaller enhancement in general. This can affect the plasmon resonances as well as the optimal shell thickness for the shell acting as a Raman enhancer.

The effects due to elevated temperatures on the surface plasmon (SP) at a metal–semiconductor interface are studied both experimentally and theoretically. In particular, a junction made of silver and amorphous silicon is fabricated and the interfacial plasmon is excited optically via the Kretschmann geometry. Both the reflectance and phase monitoring of the response of the junction have been studied as a function of temperature from 300 K to 380 K. Theoretical simulations have been carried out to understand the observed data, using a previously established model for the temperature-dependent optical constants of the metal, together with empirically fitted data for those of the semiconductor. Reasonable qualitative comparison between experimental data and simulation is obtained. It is found that the strength of the SP at the junction will decrease as temperature increases, and the methodology of the present experiment may provide a way to quantify such a decrease in the operation efficiency of the junction. In addition, it is shown that, by monitoring the resonance angle, such a junction may act as a temperature sensor with sensitivity possibly higher than the previous ones which employed a bare metal film in the Kretschmann geometry.

We perform theoretical studies on the plasmonic enhancement for the Forster resonance energy transfer (FRET) between a donor and an acceptor molecule in the vicinity of a metallic particle or cavity, with focus on the possible role of the addition of a clad layer of gain material can play in such a process. The results show that while the plasmonic resonances can be shifted with higher order plasmonic enhancements emerged in the presence of such a layer of gain material, optimal enhancement of the FRET rate can be achieved when gain just balances with the loss in the metal. This then leads to the existence of an optimal thickness for the gain material layer, for both particle and cavity enhancement. In addition, it is observed that the FRET efficiency can always be increased with the coating of the gain material even at the dipole plasmonic resonance when nonradiative transfer from the donor to the metal is high, provided that the gain level is not beyond a certain critical value.

A phenomenological model for the optical response of composite materials with metallic nanoparticles is presented. This model applies the conventional effective medium theories (EMT) but takes into account the spatial dispersion effects in the dielectric response of the metallic nanoparticles. This leads to an EMT that depends on the size of the particles. Numerical results from a model computation shows that this effect due to the nonlocal optical response of the nanoparticles can increase the resonant absorption frequency of the composite significantly for particles of very small sizes; and can lead to resonant absorption even in the Bruggeman symmetric EMT—a feature which is believed to be absent in the conventional treatment where local response for the metal particles has been assumed.

The application of surface plasmon resonance to the monitoring of the temperature of a metal film and its environment is well-established. A new feature in our present experimental work is to carry out this monitoring via the measurement of the phase difference between a s and a p-polarized wave at different wavelengths, 632.8 nm and 1.15 μm, based on a technique established previously in the literature. By monitoring the change of this phase as a function of the film temperature, it is found that this approach leads to very sensitive measurements of temperature, in comparison with previous approaches in which reflectance was measured instead. Sensitivity is 0.027 K at incident wavelength of 632.8 nm and 0.1 K at incident wavelength of 1.15 μm. A simple model based on the temperature dependence of the optical constants of the metal is applied to simulate our measurements, and it is found that the general qualitative behavior and trend of the experimental results can be reasonably accounted for using such a model.

The possible nonlocal electrodynamic effects on the optical excitation of the surface plasmon resonance (SPR) at a metal–dielectric interface are studied theoretically adapting the Kretschmann attenuated-total-reflection (ATR) geometry. Calculations of the SPR reflectivity are performed using both the Lindhard–Mermin and the hydrodynamic models for the dielectric response of the metal film. The results are compared with those obtained from the classical Drude model, as well as those from previous study for a bimetallic system. It is concluded that, unlike the bimetallic system, these nonlocal effects are relatively insignificant in the Kretschmann device, except perhaps at cryogenic temperatures when appreciable changes in dip-reflectivity values can take place due to the significance of single-particle excitation.

The recent work reporting the observation of the increase in sensitivity for surface plasmon resonance (SPR) spectroscopy at smaller incident angles, is further studied with variation in the temperature of the SPR sensor. Using a simple theoretical model established previously, it is found that in spite of the increase in damping for the resonance at higher temperatures, the recent finding of the increase of sensitivity with decrease in incident angles remains valid.

The spectroscopic characteristics for a single-molecule in the vicinity of a metallic structure are studied as a function of the substrate temperature via a phenomenological model. Emphasis is put on the role of the surface plasmon excitation of the structure and its variation with the change of temperature. From the emission characteristics such as the decay rates and frequency-shifts of the molecule, it is concluded that such effects should be accounted for in certain recent experiments in single-molecule imaging using metallic tips. In addition, from the photoabsorption line-shape of the molecule, it is speculated that certain surface-enhanced photochemical reactions under realistic catalytic conditions may indeed be feasible, provided that other complications to the surface processes can be put under control during the variation of the substrate temperature.

It is well established that the optical sensor based on the surface plasmon resonance at a metal–dielectric interface can provide very high sensitivity for many applications. It is also known that, however, this sensitivity is affected by the variation of temperature of the sensing environment, leading possibly to lower sensitivity at elevated temperatures. Here we provide a detailed theoretical model which can predict such variation of sensitivity with temperature. From the numerical results, we observe that the so-called “angular interrogation” approach seems to have the preferred stability against temperature variations. We also postulate that sensitivity gain can likely be achieved by operating the optical sensor at much lower temperatures.

The Green dyadic formulation for calculating classical decay rates of admolecules at multiple planar interfaces first published by Chance, Prock and Silbey is reexamined. It is pointed out that, for the case of fluorescing molecules sandwiched between a system of super- and substrate interfaces, the original formalism requires significant modifications in order to lead to results consistent with those obtained from the Sommerfeld radiation theory.

It has been previously established that for p-polarized light incident onto a semi-infinite absorbing medium, large negative Goos–Hanchen (GH) shifts can be expected in the case of weak absorption at incidence close to the Brewster angle. The effect has been demonstrated for certain semiconducting media at optical frequencies. Here we point out that similar phenomenon can take place for strongly reflecting and attenuating medium such as metal at IR frequencies, with large incident angles close to grazing incidence. Moreover, unlike the previously-studied case with semiconductors, the Brewster angle in the present case with metals plays an insignificant role in the possible hindrance of the observation of such large negative shifts.

The numerical determination of quasinormal mode frequencies is notoriously difficult for long-ranged potentials. Of particular interest are potentials with exponential tails, for which direct numerical integration fails. The reason for such a failure is analysed, and an effective numerical algorithm is developed.

A general scheme is presented for obtaining systematic relativistic corrections to quantum-mechanical sum rules for various calculations in atomic physics. The single-particle picture and the Foldy-Wouthuysen transformation [Phys. Rev. 78, 29 (1950)] are adopted to derive such corrections. Results are obtained for the ``generalized Bethe sums,'' which include all higher moments of the energy transfer, and explicit results are given to the lowest order of relativistic corrections.

The simple classical image theory is applied to describe the shift in emission frequencies for single molecules at a metallic aperture. The validity of this approach is further clarified using well known results for the case of molecules at flat surfaces. For molecules oriented parallel to the plane of the aperture, a numerical example shows that aside from the general small red shift, the two orthogonal “in-plane” directions behave quite differently in their emission frequencies as they are scanned from the center to the edge of the tip, suggesting a possible way of distinguishing the two orientations through observation of their emission frequencies.

The optical properties of composite materials are studied theoretically as a function of temperature via a phenomenological model for temperatures up to the melting points of the materials. Both the Maxwell-Garnett and Bruggeman models are considered and the temperature variation of the optical constants of the metallic particles is obtained with an account of the dependence of both the electron-phonon and electron-electron scattering on temperature. The results show that the extinction coefficient of the composite generally increases with temperature and that the Maxwell-Garnett and Bruggeman models can give very different results at certain optical frequency. Transmittance through a thin composite film is calculated providing a means for a simple experimental study of the various modeling results.

Ag nanostructures with surface-enhanced Raman scattering (SERS) activities have been fabricated by applying laser-direct writing (LDW) technique on silver oxide (AgOx) thin films. By controlling the laser powers, multi-level Raman imaging of organic molecules adsorbed on the nanostructures has been observed. This phenomenon is further investigated by atomic-force microscopy and electromagnetic calculation. The SERS-active nanostructure is also fabricated on transparent and flexible substrate to demonstrate our promising strategy for the development of novel and low-cost sensing chip.

A brief review is presented on fluorescence spectroscopy of molecules in the vicinity of a microstructure, which could be a surface structure, clustered ions/atoms, a colloidal particle in a solution, or a tip in certain molecular probes. This structure can be viewed as microscopic, mesoscopic or macroscopic, depending on the specific environment in which the molecule is located. An overview is given for selected experimental and theoretical aspects of this phenomenon, emphasizing some of the previous modeling work of the authors. Discussion of possible future attempts to explore the phenomenon in more detail is presented at the end.