Stoichiometric TiO2 nanoparticles (1-5 nm) were prepared by reactive-layer-assisted deposition (RLAD), in which Ti was initially deposited on a multilayer of H2O (or NO2) on a Au(111) substrate at approximately 90 K. The composition and atom-resolved structure of the nanoparticles were studied by XPS and STM. The approximately 5 nm TiO2 particles had either a rutile or anatase phase with various crystal facets. STS of the nanoparticles suggests size-dependent electronic structure. These well-defined nanoparticles can be used in molecular-level studies of the reactions and mechanisms of photocatalytic processes on TiO2 nanoparticle surfaces.

The enhancement of photochemical reactions on surface-supported metallic particles is reported. The result is obtained by uv photodissociation of organometallic molecules adsorbed on the supporting surface. Transmission electron-microscope micrographs of the resulting photodeposited film reveal enhanced deposition on and near the metal particles, and a clear interference pattern resulting from the mixing of the incident and reradiated light. The importance of the particle plasma resonance is described. In this Letter, we report a direct observation of a surface-enhanced photochemical reaction.The experiment is motivated in part by the recent studies of enhanced Raman scattering (SERS) on rough metal surfaces. ' Although the exact mechanism of SERS is still a matter of some controversy, it is clear that much of the enhancement results from a plasma resonance in the metal protrusions on the surface, which in turn greatly magnifies the local optical electric field. ' This explanation suggests that a similar enhancement should exist for photochemical phenomena occurring on similarly shaped metal surfaces.The possibility of such surface-enhanced photochemistry has been discussed theoretically by Nitzan and Brus. ' However, they emphasized that the rapid relaxation occurring on metal surfaces can be expected to quench long-lived photochemically active states. In this Letter, we use the photodecomposition of a metal-alkyl adlayer and its resultant metal deposit to show that enhanced di~ect photochemical reactions can occur in the vicinity of metal spheroids. According to the above discussion two conditions have to be fulfilled to observe such an enhanced reaction. First, the reaction chosen must involve a short-lived dissociative state; that is, the chemical reaction should occur in 1o ' -10~s. Second, the frequency of the incident light should be at a plasma resonance of the metal structure. The resonant frequency of a metal ellipsoid depends sharply on the dielectric constants of the metal and the shape of the metal feature. While silver has been almost exclusively used in SERS studies, the resonance frequencies typical of silver structures are too low to affect most direct one-photon photochemical reactions. However, optical and electron-beam energy-loss spectra4 show that several other metals such as Cd, In, and Al do exhibit sharp absorption peaks, as a result of a plasma resonance, in the mid-uv region4; see Table I. For small particles the resonant frequency,~",depends on the particle shape; see below. The resonant frequency~" for a sphere of a metal which can be described by a Drude-type free-electron model is &u, = &u~/ W3. Table I also lists u&~/v 3 for the same metals, as well as the experimental value of co".The photochemical reaction chosen was the photodissociation of Me,Cd (dimethyl cadmium) by a 257-nm laser beam. The photodissociation mechanism of Me, Cd has been studied carefully and the required instantaneous dissociation mechanism was confirmed.The absorption spectra of the Me, Cd vapor and physisor...

A waveguide optical isolator based on nonreciprocal interference is demonstrated. Ridge waveguides are fabricated in a Mach–Zehnder configuration on a single film of bismuth-, lutetium-, neodymium-iron garnet. With this design, no polarizers are required to achieve extinction in the backward propagation direction. This isolator exhibits a 19 dB extinction ratio at λ=1.54 μm. A flat wavelength dependence, to within 2 dB, has been observed in the range between 1.49 and 1.57 μm.