Yehuda Avniel

The failure of perfectly matched layers, and towards their redemption by adiabatic absorbers

Although perfectly matched layers (PMLs) have been widely used to truncate numerical simulations of electromagnetism and other wave equations, we point out important cases in which a PML fails to be reflectionless even in the limit of infinite resolution. In particular, the underlying coordinate-stretching idea behind PML breaks down in photonic crystals and in other structures where the material is not an analytic function in the direction perpendicular to the boundary, leading to substantial reflections. The alternative is an adiabatic absorber, in which reflections are made negligible by gradually increasing the material absorption at the boundaries, similar to a common strategy to combat discretization reflections in PMLs. We demonstrate the fundamental connection between such reflections and the smoothness of the absorption profile via coupled-mode theory, and show how to obtain higher-order and even exponential vanishing of the reflection with absorber thickness (although further work remains in optimizing the constant factor).

Robust design of slow-light tapers in periodic waveguides

This article concerns the design of tapers for coupling power between uniform and slow-light periodic waveguides. New optimization methods are described for designing robust tapers, which not only perform well under nominal conditions, but also over a given set of parameter variations. When the set of parameter variations models the inevitable variations typical in the manufacture or operation of the coupler, a robust design is one that will have a high yield, despite these parameter variations. The ideas of successive refinement, and robust optimization based on multi-scenario optimization with iterative sampling of uncertain parameters, using a fast method for approximately evaluating the reflection coefficient, are introduced. Robust design results are compared to a linear taper, and to optimized tapers that do not take parameter variation into account. Finally, robust performance of the resulting designs is verified using an accurate, but much more expensive, method for evaluating the reflection coefficient.

Global optimization of silicon photovoltaic cell front coatings

The front-coating (FC) of a solar cell controls its efficiency, determining admission of light into the absorbing material and potentially trapping light to enhance thin absorbers. Single-layer FC designs are well known, especially for thick absorbers where their only purpose is to reduce reflections. Multilayer FCs could improve performance, but require global optimization to design. For narrow bandwidths, one can always achieve nearly 100% absorption. For the entire solar bandwidth, however, a second FC layer improves performance by 6.1% for 256 microm wafer-based cells, or by 3.6% for 2 microm thin-film cells, while additional layers yield rapidly diminishing returns.

High-Throughput Signal Component Separator for Asymmetric Multi-Level Outphasing Power Amplifiers

This paper presents an energy-efficient high-throughput and high-precision signal component separator (SCS) chip design for the asymmetric-multilevel-outphasing (AMO) power amplifier. It uses a fixed-point piece-wise linear functional approximation developed to improve the hardware efficiency of the outphasing signal processing functions. The chip is fabricated in 45 nm SOI CMOS process and the SCS consumes an active area of 1.5 mm2. The new algorithm enables the SCS to run at a throughput of 3.4 GSamples/s producing the phases with 12-bit accuracy. Compared to traditional low-throughput AMO SCS implementations, at 0.8 GSamples/s this design improves the area efficiency by 25× and the energy-efficiency by 2×. This fastest high-precision SCS to date enables a new class of high-throughput mm-wave and base station transmitters that can operate at high area, energy and spectral efficiency.

Sufficient conditions for two-dimensional localization by arbitrarily weak defects in periodic potentials with band gaps

We prove, via an elementary variational method, 1d and 2d localization within the band gaps of a periodic Schrodinger operator for any mostly negative or mostly positive defect potential, V, whose depth is not too great compared to the size of the gap. In a similar way, we also prove sufficient conditions for 1d and 2d localization below the ground state of such an operator. Furthermore, we extend our results to 1d and 2d localization in d dimensions; for example, a linear or planar defect in a 3d crystal. For the case of D-fold degenerate band edges, we also give sufficient conditions for localization of up to D states.

Computation and visualization of photonic quasicrystal spectra via Bloch's theorem

Previous methods for determining photonic quasicrystal (PQC) spectra have relied on the use of large supercells to compute the eigenfrequencies and/or local density of states. In this paper, we present a method by which the energy spectrum and the eigenstates of a PQC can be obtained by solving Maxwells equations in higher dimensions for any PQC defined by the standard cut-and-project construction, to which a generalization of Blochs theorem applies. In addition, we demonstrate how one can compute band structures with defect states in the same higher-dimensional superspace. As a proof of concept, these general ideas are demonstrated for the simple case of one-dimensional quasicrystals, which can also be solved by simple transfer-matrix techniques.

Design strategies and rigorous conditions for single-polarization single-mode waveguides

We establish rigorous necessary analytical conditions for the existence of single-polarization single-mode (SPSM) bandwidths in index-guided microstructured waveguides (such as photonic-crystal fibers). These conditions allow us to categorize designs for SPSM waveguides into four strategies, at least one of which seems previously unexplored. Conversely, we obtain rigorous sufficient conditions for the existence of two cutoff-free index-guided modes in a wide variety of microstructured dielectric waveguides with arbitrary periodic claddings, based on the existence of a degenerate fundamental mode of the cladding (a degenerate light line). We show how such a degenerate light line, in turn, follows from the symmetry of the cladding.

Design trade-offs in signal component separators for outphasing power amplifiers

Implementation design space of piece-wise linear outphasing signal component separator is explored by utilizing the changes in micro-architecture, choice of storage elements and aggressive back-end leakage power optimization techniques. With combination of these techniques, ~2× energy and area savings are achieved, resulting in record energy-efficiency of 32pJ/sample for asymmetric multilevel outphasing and 22pJ/sample for linear amplification with nonlinear components, at throughput of 400MSample/s and areas of 0.2-0.4mm2 in 45nm SOI process.

Design Tools for Emerging Technologies

The rapidly expanding diversity of technology available at the nanoscale is disrupting the existing transistor-centric microelectronics design paradigm, resulting in nearly decade-long delays between prototype demonstration and technology deployment. The key to reducing these innovation-inhibiting delays is to develop algorithmic approaches that can start with first principles based descriptions of novel nanotechnology and rapidly and reliably synthesize manufacturable designs. Design tools are evolving this direction, with new extremely efficient yet customizable physical simulators, automatic parameterized low-order model extraction, and ever improving algorithms for robust optimization-new techniques that generate manufacturable designs by optimizing both system performance and robustness to manufacturing variations. In this paper we give a few examples of the coupling of such approaches, but mostly offer pointers to literature for researchers interested in contributed to this rapidly growing field of coupled simulation and robust optimization