In terms of the effective medium theory, we develop a novel technique for designing nanostructured metamaterials
(photonic crystals) with predetermined dielectric and optical properties over a frequency band. The technique is based on
materials' architecture and reduces to the design of one-dimensional or two-dimensional nanostructured metal-dielectric
composites with a specified graded geometry. As particular examples, we show how to tailor the materials with epsilonnear-
zero (ENZ), ultralow-refractive-index (ULRI), and refractive index-near-unity (RINU) over a frequency band using
a fitting procedure. The quality of fitting is discussed by the example of RINU metamaterials.
In terms of the effective medium theory, we propose a general solution to such a fundamental problem as designing nanostructured materials with the very low permittivity (epsilon-near-zero metamaterials) over a frequency range. We show, in particular, that this can be accomplished for a columnar metal-dielectric composite consisting of parallel cylinders with varied thickness that resembles a forest of identical stems. The applicability of our method and some limitations are also discussed. Whilst the corresponding production procedure does not appear to be very simple, it nevertheless is realizable.
Laser ablation is one of the most effective ways of making single-wall carbon nanotubes. Although the process is poorly understood, the importance of nanoparticle formation to initiate tube growth is evident. While some groups have concluded that nanoparticles can form in vacuum, we have argued that this is unlikely because the expansion of the plume is so rapid that the 'freezing limit' is reached too rapidly for nucleation and growth to the observed size. A background gas changes the dynamics completely. Calculations show that in a few microseconds the ablated plume is dramatically slowed by the 'snowplowing' of the background gas into a peak whose density is much greater than its initial density. The ablated material is trapped within this peak. The question then arises as to how this peak dissipates by diffusion. A simple calculation shows that it is at this point that a drastic change in the timescale of the process occurs so that there is ample time (milliseconds) for nanoparticles to nucleate and grow.
It is now well established that thin films of a wide variety of materials can be deposited by ablation of a target material by a laser. Here, expansion of the laser-ablated plume in vacuum and in a background gas is comparatively studied using continuum hydrodynamics models, molecular gas dynamics models, and a recently developed multiple scattering model which combines continuum hydrodynamics and inter-species collisions. Continuum hydrodynamics models and molecular gas dynamics models predict, for the most part, that background plasma would reach the deposition substrate (or an ion probe placed at the same distance away from the target) first. On the other hand, the multiple scattering model shows that a component of the plume can indeed reach the substrate at vacuum speed, followed by a second plume component which is more or less slowed down by the presence of the background gas depending on its ambient pressure. Quantitative fits to the experimental data have been obtained with this multiple scattering model for expansion of Silicon in Helium and in Argon. The successful application of the multiple scattering model serves to explain the phenomenon of 'plume splitting' which is frequently observed in laser ablation processes for thin film deposition.
We describe the Ion-Ripple Laser as an advanced scheme for generating coherent radiation. A relativistic electron beam obligately propagating through an ion ripple excites electromagnetic radiation which is coupled to slow electrostatic waves with peak growth rate at the resonance frequency ω≈ 2λ<sup>2</sup><sub>O</sub>k<sub>w</sub>c via backward Raman scattering. This new scheme may provide novel tunable sources of coherent high-power radiation. By proper choice of device parameters, sources of microwaves, optical, and perhaps even x rays can be made. By employing fluid theory the dispersion relation for wave coupling is derived and used to calculate the radiation frequency and linear growth rate. The nonlinear saturation mechanism is due to trapping of the beam electrons by the ponderomotive potential. For an energetic electron beam, the peak growth rate is ω<sub>i</sub> = ω<sup>5/2</sup><sub>pe</sub>ε<sub>i</sub>sinθ/λ<sub>O</sub><sup>3/4</sup>(2k<sub>ω</sub>c)<sup>3/2</sup> and the efficiency is η= ω<sub>pe</sub>/(2k<sub>ω</sub>cλ<sub>O</sub><sup>3/2</sup>). A 1 2/2 D-PIC simulation code was developed to verify the ideas, scaling laws, and nonlinear mechanism. From the observed power spectrum, backward Raman scattering is show to be responsible for the radiation. The growth rates and efficiencies given by the simulation match the ones of theory for different wiggler wave lengths and beam λ. Both of them show a slow decrease with momentum spread. Momentum spread also broadens the radiation spectrum.