Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.

Surface-enhanced spectroscopy

Surface raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

Band structure of indium antimonide

Anomalous Optical Absorption Limit in InSb

Tunneling Phenomena in Solids

The evidence, from the photoelastic properties of alkali halides and MgO, and from the deviations from the Cauchy relation among the elastic constants in these crystals, for the existence of an appreciable deformation of the charge distribution about the atoms during lattice vibration, is discussed. The deformation of the charge distribution is suggested as a possible alternative explanation to that of anharmonic forces for the auxiliary bands in the infrared absorption and reflection spectra of the alkali halides and MgO. It is suggested, on the other hand, that the infrared absorption spectra of pure homopolar crystals, such as diamond, silicon, and germanium in which a linear moment is absent, must involve the deformation of the charge distribution during vibration, since anharmonic forces alone cannot account for any infrared absorption in the absence of a linear moment.The deformation of the charge distribution in a crystal during vibration is shown to lead to a second order electric moment (quadratic in the displacements) as well as to a modification of the first order moment. A qualitative understanding of the contribution of the second order moment to infrared absorption is obtained with the help of a one-dimensional calculation.An analysis is made of the infrared absorption in diamond, silicon, and germanium. Part of the absorption is judged to be intrinsic and to be reasonably explained by a second order electric moment arising from charge deformation. The remainder seems to vary with the specimen and is explained as an impurity-induced first order moment. The second order electric moment can also be used to explain the side bands in the absorption and reflection spectra of the alkali halides. However, it fails to explain the observed broadening of the main absorption line, indicating that anharmonic forces are probably the principal mechanism in the latter phenomenon.

Infrared Lattice Absorption in Ionic and Homopolar Crystals

“Giant” Raman scattering by adsorbed molecules on metal surfaces

A theory of surface magnetoplasmons in semiconductors is developed with the inclusion of retardation for the geometry in which the magnetic field is parallel to the surface and the direction of propagation is perpendicular to the magnetic field. If the background dielectric constant ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}$ lies in a suitable range for a given value of the magnetic field, gaps appear in the dispersion relation for the surface magnetoplasmons. The possible experimental observation of these gaps is discussed.

Elias Burstein

Papers

Anomalous Optical Absorption Limit in InSb

Tunneling Phenomena in Solids

Infrared Lattice Absorption in Ionic and Homopolar Crystals

“Giant” Raman scattering by adsorbed molecules on metal surfaces

Theory of Surface Magnetoplasmons in Semiconductors