Path-integral study of a two-dimensional Lennard-Jones glass

The glass transition in a quantum Lennard-Jones mixture is investigated by constant-volume path-integral simulations. Particles are assumed to be distinguishable, and the strength of quantum effects is varied by changing h from zero (the classical case) to one (corresponding to a highly quantum-mechanical regime). Quantum delocalization and zero point energy drastically reduce the sensitivity of structural and thermodynamic properties to the glass transition. Nevertheless, the glass transition temperature T-g can be determined by analyzing the phase space mobility of path-integral centroids. At constant volume, the T-g of the simulated model increases monotonically with increasing h. Low temperature tunneling centers are identified, and the quantum versus thermal character of each center is analyzed. The relation between these centers and soft quasilocalized harmonic vibrations is investigated. Periodic minimizations of the potential energy with respect to the positions of the particles are performed to determine the inherent structure of classical and quantum glassy samples. The geometries corresponding to these energy minima are found to be qualitatively similar in all cases. Systematic comparisons for ordered and disordered structures, harmonic and anharmonic dynamics, classical and quantum systems show that disorder, anharmonicity, and quantum effects are closely interlinked.

Anisotropic optical properties of arrays of gold nanorods embedded in alumina

A series of thin films comprising gold nanorods embedded in an alumina matrix have been fabricated with lengths ranging from 75 to 330 nm. Their optical properties, expressed in terms of extinction - In(T), where T is optical transmittance, have been measured as a function of wavelength, rod length, angle of incidence, and incident polarization state. The results are compared to a Maxwell-Garnett based theory modified to take into account the strongly anisotropic nature of the medium. Transverse and longitudinal plasmon resonances are observed. The interaction between the nanorods leads to the splitting of the longitudinal resonance with the longer-wavelength resonance being forbidden for direct optical observations. The shorter-wavelength resonance related to the symmetric coupling between longitudinal plasma excitations in the nanorods depends on rod length, polarization state, and angle of incidence of the probing light. The impact of electron confinement on the optical properties of the gold rods is also seen and may be incorporated into the Maxwell-Garnett theory by restricting the mean free path of the conduction electrons to produce excellent agreement between observations and the complete theory. Annealing experiments that modify the physical structure of the gold confirm this conclusion.

Surface plasmon polariton propagation length: A direct comparison using photon scanning tunneling microscopy and attenuated total reflection

The propagation of surface plasmon polaritons (SPP's) is studied using a photon scanning tunneling microscope (PSTM) and conventional attenuated total reflection (ATR). The PSTM experiment uses localized (focused beam) launching or SPP's at a wavelength of 632.8 nm. Propagation of the SPP is observed as an exponentially decaying tail beyond the launch site acid the 1/e propagation length is measured directly for a series of Ag films of different thicknesses. The ATR measurements are used to characterize the thin film optical and thickness parameters, revealing, notably, the presence of a contaminating adlayer of Ag2S of typical dielectric function, 8.7 + i2.7, and thickness 1-2 nm. Values of the SPP propagation length, based on the ATR- derived film parameters used in the four-media implicit SPP dispersion relation, show very good agreement with those based on the PSTM images for the case of undercoupled or optimally coupled SPP modes. The observed propagation lengths are quantitatively analyzed taking explicit account of additional intrinsic damping due to the growth of the Ag2S layer and of reradiation of the SPP back into the prism outside the launch site. Finally, the PSTM images show excellent SPP beam confinement in the original propagation direction.