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...

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A study of the UV photochemistry of organometallic molecules in the vapor and adsorbed phases is discussed. The gas-phase spectra is interpreted on the basis of molecular orbital theory and experimental data. The absorption spectra of the surface adlayers are determined by computer subtraction of the gas-phase spectra and an intepretation is provided. The influence of metalsubstrate microstructure on UV photochemical reactions of metal alkyls is briefly discussed.

Ultra-high-Q nanocavity resonance tuning by laser-assisted thermal oxidation of silicon is demonstrated by using a 532 nm continuous wave laser. The resonance is blue-shifted by >2nm. The quality factor remains >200,000. OCIS codes: (230.5298) Photonic crystals; (140.3945) Microcavities; (130.5296) Photonic crystal waveguides 1. Introduction: Photonic crystal nanocavities are increasingly employed in a range of photonic studies and applications because of their high Q/V m ratios [1]. For applications such as optical EIT [2] and cavity QED [3], accurate control of the resonant wavelength is critical. However, due to fabrication imperfections, resonances will deviate from their desired values. Many post-fabrication tuning techniques have been proposed and demonstrated. These methods can be separated into two categories: global and local. Global tuning methods create uniform changes to the entire photonic chip. Local tuning methods only modify a specific cavity region [4]. Laser-assisted thermal-oxidation tuning has been demonstrated on a GaAs L3 photonic crystal cavity with low-Q of 1800 [5].Deutschmann and Huber have previously studied the local oxidation of SOI substrates [6,7]. Here we demonstrate local oxidation tuning of a silicon photonic crystal double-heterostructure with an initial ultra-high-Q > 300,000. A 532 nm wavelength continuous wave laser beam is focused onto the cavity center with a total incident power of approximately 20 mW. The experiment was conducted at ambient room conditions with a relative humidity of 20%. The cavity mode is blue-shifted > 2 nm while maintaining an ultra-high-Q > 200,000. A larger tuning range should also be achievable. Faster tuning times are also possible using higher incident powers. This tuning method is well suited for complete automation, by using a computer-controlled stage and in-situ monitoring. This would enable the post-fabrication fine-tuning of massive numbers of nanocavities.

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