P. T. Leung

Classical decay rates for molecules in the presence of a spherical surface: A complete treatment

Abstract A comprehensive treatment of the classical decay rates for a molecule in the vicinity of a spherical surface is presented through the application of the work of van der Pol and Bremmer and that of Fock. This theory takes full advantage of the Hertz vector formalism, which is mathematically simpler than the widely-adapted Lorenz-Mie approach which uses the complicated vector harmonic expansions. Results are obtained for both radiative and nonradiative transfer when the molecule is located outside or inside the surface. Numerical results are given for the cases of a surface, cavity and “aerosol”, and comparison with other works is made.

A theoretical model for the temperature-dependent sensitivity of the optical sensor based on surface plasmon resonance

Abstract 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.

OPTICAL PROPERTIES OF COMPOSITE MATERIALS AT HIGH TEMPERATURES

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.

The surface plasmon enhancement effect on adsorbed molecules at elevated temperatures

The surface plasmon enhancement effect on adsorbed molecules at elevated substrate temperatures is studied theoretically using surface enhanced Raman scattering (SERS) as an example. The surface structure is idealized to be a monodisperse spherical particle with its nonlocal dielectric response accounted for. The temperature effects are modeled using a temperature-dependent collision frequency in the Drude model. Numerical results show that only a small decrease in the SERS enhancement ratio occurs for temperatures up to the melting point of the substrate, even for scattering close to the surface plasmon resonance frequency of the metal. More definitive results are subjected to more realistic modeling as well as systematic experimental studies. The implication of this result to other surface photochemical processes is discussed.

Photodeflection probing of the explosion of a liquid film in contact with a solid heated by pulsed excimer laser irradiation

The explosion or rapid vaporization of a liquid film on an opaque surface by a pulsed laser is studied experimentally. Using a probe‐beam deflection sensing (PDS) scheme, together with a previously developed transmission monitor, the distortion of the PDS signal due to the generation of shock waves by the exploding liquid is investigated. Various liquids, including alcohols and pure water in contact with substrates such as polyimide, amorphous carbon, and silicon, are studied for a wide range of excimer laser fluences. It is concluded that the present PDS technique is highly sensitive to the explosion threshold.

High-resolution angular measurement using surface-plasmon-resonance via phase interrogation at optimal incident wavelengths

It is demonstrated that ultrahigh-resolution angular measurement can be achieved via surface-plasmon-resonance excitation in which the phase difference between p- and s-polarized reflected waves is monitored as a function of the incidence angle. Resolutions down to 1.9 x 10(-6) deg are obtained by performing the measurements at optimal incident wavelengths. This represents an order of magnitude improvement compared with previously reported values.

Temperature dependence of optical constants for amorphous silicon

The temperature dependence of the optical constants for amorphous silicon (a‐Si) is studied for two different sample thicknesses at two infrared wavelengths. It is observed that the extinction coefficient of a‐Si can increase significantly with temperature in the strong absorption regime. In addition, using the Mott–Davis formula, results are obtained for the variation of the optical gap energy for a‐Si with temperature, with similar feature observed for both amorphous and crystal silicon.

Nonlocal optical effects on the fluorescence and decay rates for admolecules at a metallic nanoparticle

A phenomenological model is implemented to study the decay rates of fluorescing molecules in the vicinity of a metallic nanoparticle, wherein the nonlocal optical response of the particle is accounted for via the hydrodynamic model for the description of the free electrons in the metal. These nonlocal effects are examined for each of the radiative rate and the nonradiative rate of the admolecule, respectively. In addition, the overall fluorescence rate which includes the enhancement ratio for the driving field intensity is also studied. It is found that for particles of very small sizes (<10 nm), the nonlocal effects, in general, lead to significantly greater fluorescence rates and smaller nonradiative decay rates for the admolecules, with the effects on radiative rates depending crucially on the orientation of the molecules. Furthermore, the effects are mostly noticeable for molecules close to the metal particle and in processes where higher multipolar interactions are significant such as those in nonradiative decay processes. Above all, these nonlocal effects can still be observable in the presence of large surface damping imposed on the metallic electrons due to the ultrasmall sizes of these nanoparticles. The relevance of these effects to some of the latest experiments is discussed.

Transmission studies of explosive vaporization of a transparent liquid film on an opaque solid surface induced by excimer‐laser‐pulsed irradiation

The dynamics of the explosion of a liquid film by an ultraviolet excimer pulsed laser is studied experimentally on top of an amorphous silicon (a‐Si) film deposited on fused quartz. In particular, the transient temperature as well as the nucleation of the superheated liquid at the interface for laser fluences below and above the spontaneous nucleation threshold are monitored using an optical transmission technique. Results for different pure and mixed liquids are presented for cases of different thicknesses of the a‐Si film. Details for the calibration of the transmission signal to obtain the interface temperature are provided through the application of the one‐dimensional diffusion model and the characteristic matrix theory using data obtained from previous steady‐state experiments. The calibrated results are in reasonable agreement with the known data for superheated liquids in the literature as well as theoretical calculations. In addition, the delay times for spontaneous nucleation of the liquids are ...

Optical temperature sensing based on the Goos-Hänchen effect

The possibility of constructing an optical sensor for temperature monitoring based on the Goos-Hänchen (GH) effect is explored using a theoretical model. This model considers the lateral shift of the incident beam upon reflection from a metal-dielectric interface, with the shift becoming a function of temperature due mainly to the temperature dependence of the optical properties of the metal. It is found that such a sensor can be most effective by using long wavelength p-polarized incident light at almost grazing incidence onto the metal, where significant variation of negative GH shifts can be observed as a function of the temperature.