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We propose an explanation for the recently measured slow fluctuations and ``blinking'' in the surface enhanced Raman scattering (SERS) spectrum of single molecules adsorbed on a silver colloidal particle. We suggest that these fluctuations may be related to the dynamic relaxation of the surface roughness on the nanometer scale and show that there are two classes of roughness with qualitatively different dynamics. The predictions agree with measurements of surface roughness relaxation. Using a theoretical model for the kinetics of surface roughness relaxation in the presence of charges and optical electrical fields, we predict that the high-frequency electromagnetic field increases both the effective surface tension and the surface diffusion constant and thus accelerates the surface smoothing kinetics and time scale of the Raman fluctuations in manner that is linear with the laser power intensity, while the addition of salt retards the surface relaxation kinetics and increases the time scale of the fluctuations. These predictions are in qualitative agreement with the Raman experiments.

The primary objective of this program has been to understand and subsequently reduce the sources of scattering in planar Ti-diffused LiNbO3 waveguides. On the basis of previous work these were perceived to be surface roughness and surface index inhomogeneities associated with Li-/ti-O compound formation during diffusion. A secondary objective has been to characterize state-of-the-art waveguides to determine their potential for use in an integrated optical spectrum analyzer providing at least 40 dB dynamic range at the fourth off-axis pixel. A key ingredient to meeting these objectives was found to be the accurate determination of waveguide scattering parameters from in-plane scattering measurements. These were frustrated by superimposed background scattering associated with input and output coupling. Both prism and end-fire techniques were used. A statistical analysis employed to effectively subtract the background contribution provided this best estimate: Existing waveguides provide the 40 dB dynamic range at the fourth pixel that is marginally acceptable for the spectrum analyzer application; however, a +or=5 dB uncertainty in dynamic range is within one standard deviation of our average value. In the case of 35 dB dynamic range at the fourth pixel, consistent with this uncertainty, an analysis of dynamic range versus pixel number is used to show that 40 dB dynamic range is still achieved at the tenth off-axis pixel.

The primary objective of this program has been to understand and subsequently reduce the sources of scattering in planar Ti-diffused LiNbO3 waveguides. On the basis of previous work these were perceived to be surface roughness and surface index inhomogeneities associated with Li-/ti-O compound formation during diffusion. A secondary objective has been to characterize state-of-the-art waveguides to determine their potential for use in an integrated optical spectrum analyzer providing at least 40 dB dynamic range at the fourth off-axis pixel. A key ingredient to meeting these objectives was found to be the accurate determination of waveguide scattering parameters from in-plane scattering measurements. These were frustrated by superimposed background scattering associated with input and output coupling. Both prism and end-fire techniques were used. A statistical analysis employed to effectively subtract the background contribution provided this best estimate: Existing waveguides provide the 40 dB dynamic range at the fourth pixel that is marginally acceptable for the spectrum analyzer application; however, a +or=5 dB uncertainty in dynamic range is within one standard deviation of our average value. In the case of 35 dB dynamic range at the fourth pixel, consistent with this uncertainty, an analysis of dynamic range versus pixel number is used to show that 40 dB dynamic range is still achieved at the tenth off-axis pixel.

We investigate the effects of electron and acoustic-phonon confinement on the low-field electron mobility of thin square silicon nanowires (SiNWs) that are surrounded by SiO$_2$ and gated. We employ a self-consistent Poisson-Schr\"{o}dinger-Monte Carlo solver that accounts for scattering due to acoustic phonons (confined and bulk), intervalley phonons, and the Si/SiO$_2$ surface roughness. The wires considered have cross sections between 3 $\times$ 3 nm$^2$ and 8 $\times$ 8 nm$^2$. For larger wires, as expected, the dependence of the mobility on the transverse field from the gate is pronounced. At low transverse fields, where phonon scattering dominates, scattering from confined acoustic phonons results in about a 10% decrease of the mobility with respect to the bulk phonon approximation. As the wire cross-section decreases, the electron mobility drops because the detrimental increase in both electron--acoustic phonon and electron--surface roughness scattering rates overshadows the beneficial volume inversion and subband modulation. For wires thinner than 5 $\times$ 5 nm$^2$, surface roughness scattering dominates regardless of the transverse field applied and leads to a monotonic decrease of the electron mobility with decreasing SiNWs cross section.

We investigate the effects of electron and acoustic-phonon confinement on the low-field electron mobility of thin square silicon nanowires (SiNWs) that are surrounded by SiO$_2$ and gated. We employ a self-consistent Poisson-Schr\"{o}dinger-Monte Carlo solver that accounts for scattering due to acoustic phonons (confined and bulk), intervalley phonons, and the Si/SiO$_2$ surface roughness. The wires considered have cross sections between 3 $\times$ 3 nm$^2$ and 8 $\times$ 8 nm$^2$. For larger wires, as expected, the dependence of the mobility on the transverse field from the gate is pronounced. At low transverse fields, where phonon scattering dominates, scattering from confined acoustic phonons results in about a 10% decrease of the mobility with respect to the bulk phonon approximation. As the wire cross-section decreases, the electron mobility drops because the detrimental increase in both electron--acoustic phonon and electron--surface roughness scattering rates overshadows the beneficial volume inversion and subband modulation. For wires thinner than 5 $\times$ 5 nm$^2$, surface roughness scattering dominates regardless of the transverse field applied and leads to a monotonic decrease of the electron mobility with decreasing SiNWs cross section.

The band structures of single-layer MoS$_2$ and WS$_2$ present a coupling between spin and valley degrees of freedom that suppresses spin-flip scattering and spin dephasing. Here we show that out-of-plane deformations, such as corrugations or ripples, enhance spin-flip scattering in armchair MoS$_2$ and WS$_2$ nanoribbons. Spin transport in the presence of surface roughness is systematically investigated, employing the non-equilibrium Green's function method along with the tight-binding approximation. Both transmission and conductance have been calculated as a function of surface roughness. Our results indicate that the spin-flip rate, usually neglected in flat pristine samples, increases significantly with the surface roughness amplitude. These results are important for the design and fabrication of transition metal dichalcogenides based spintronic devices.

A modeling approach, based on an analytical solution of the semiclassical multi-subband Boltzmann transport equation, is presented to study resistivity scaling in metallic thin films and nanowires due to grain boundary and surface roughness scattering. While taking into account the detailed statistical properties of grains, roughness and barrier material as well as the metallic band structure and quantum mechanical aspects of scattering and confinement, the model does not rely on phenomenological fitting parameters.

Bistatic configurations are being considered in several modern surveillance radar concepts. This introduces new clutter and target scattering cross section representations as a result of the more complex geometry. In this report three bistatic clutter topics are addressed. First, a bistatic two-scale of roughness terrain scattering model is validated by comparison with data. Second, the contributions from each roughness level are evaluated as a function of the scattering angle regimes. Third, comparisons of monostatic and bistatic clutter cross section maps for a loam surface are made for a bistatic radar configuration. Results showed generally good agreement between model and data except at low grazing angles where problems arise for experimental cross section measurements and single scattering assumptions in the model. Examination of the individual component contribution shows distinct behavior patterns and regions of dominance. Finally, the clutter mapping shows that for the configuration studied the bistatic cross sections were always much larger than the corresponding monostatic values. (kr)

This paper discusses the effect of periodic roughness and surface defects on the electromagnetic scattering of terahertz waves from cylindrical objects. The cylinders, possessing periodic roughness imparted during their fabrication process, had average roughness values ranging from approximately 0.1 mum - 0.50 mum. Metallic cylinders were fabricated from lathe-turned aluminum rods and dielectric cylinders were fabricated using a rapid prototype technique (stereolithography). The scattering behavior of the rough cylinders was measured in 160 GHz and 350 GHz compact radar ranges. In addition, the effect of seams and grooves on the scattering behavior of cylinders will also be presented.