Ardavan Oskooi

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).

Partially disordered photonic-crystal thin films for enhanced and robust photovoltaics

We present a general framework for the design of thin-film photovoltaics based on a partially disordered photonic crystal that has both enhanced absorption for light trapping and reduced sensitivity to the angle and polarization of incident radiation. The absorption characteristics of different lattice structures are investigated as an initial periodic structure is gradually perturbed. We find that an optimal amount of disorder controllably introduced into a multi-lattice photonic crystal causes the characteristic narrow-band, resonant peaks to be broadened resulting in a device with enhanced and robust performance ideal for typical operating conditions of photovoltaic applications.

Accurate finite-difference time-domain simulation of anisotropic media by subpixel smoothing

Finite-difference time-domain methods suffer from reduced accuracy when discretizing discontinuous materials. We previously showed that accuracy can be significantly improved by using subpixel smoothing of the isotropic dielectric function, but only if the smoothing scheme is properly designed. Using recent developments in perturbation theory that were applied to spectral methods, we extend this idea to anisotropic media and demonstrate that the generalized smoothing consistently reduces the errors and even attains second-order convergence with resolution.

Robust optimization of adiabatic tapers for coupling to slow-light photonic-crystal waveguides

We investigate the design of taper structures for coupling to slow-light modes of various photonic-crystal waveguides while taking into account parameter uncertainties inherent in practical fabrication. Our short-length (11 periods) robust tapers designed for ? = 1.55?m and a slow-light group velocity of c/34 have a total loss of < 20 dB even in the presence of nanometer-scale surface roughness, which outperform the corresponding non-robust designs by an order of magnitude. We discover a posteriori that the robust designs have smooth profiles that can be parameterized by a few-term (intrinsically smooth) sine series which helps the optimization to further boost the performance slightly. We ground these numerical results in an analytical foundation by deriving the scaling relationships between taper length, taper smoothness, and group velocity with the help of an exact equivalence with Fourier analysis.

Design of single-mode narrow-bandwidth thermal emitters for enhanced infrared light sources

We design efficient thermal emitters based on intersubband transitions (ISB-Ts) in quantum wells and two-dimensional photonic crystal (PC) slabs that have single-mode, very narrowband emission with high emissivity. Our design strategy involves positioning a single isolated mode of the PC within the absorption range of the ISB-T, where the mode’s radiation rate is precisely matched with the absorption rate of the ISB-T. The optimized design for this class of thermal emitters has a single-peak emission with a quality factor of ∼600, an emissivity of ∼0.9, and a radiation divergence cone of ∼20°, surpassing, by a large margin, the performance of previous designs. We also demonstrate, for practical application purposes, that the required input power for our best-performing emitter to reach a given temperature threshold is less than a factor of 200 compared to that of a blackbody.

Short note: Distinguishing correct from incorrect PML proposals and a corrected unsplit PML for anisotropic, dispersive media

We show that some previous proposals for perfectly matched layer (PML) absorbers in anisotropic media or for waveguides at oblique incidence are not, in fact true PMLs; in previous work we similarly showed a failure of several PML proposals for periodic media (photonic crystals). We therefore argue that a more careful validation scheme is required for PML proposals, in contrast to past authors who have typically checked only that reflections are small for a fixed resolution, and suggest a simple validation scheme that can be readily applied to any PML proposal regardless of derivation or implementation. We demonstrate this test for a corrected, unsplit-field PML valid for anisotropic, dispersive media, implemented in both planewave-expansion and finite-difference time-domain (FDTD) methods.

Experimental Demonstration of Quasi-resonant Absorption in Silicon Thin Films for Enhanced Solar Light Trapping

We experimentally demonstrate that the addition of partial lattice disorder to a thin-film microcrystalline silicon photonic crystal results in the controlled spectral broadening of its absorption peaks to form quasi resonances: increasing light trapping over a wide bandwidth while also reducing sensitivity to the angle of incident radiation. Accurate finite-difference time-domain simulations are used to design the active-layer photonic crystal so as to maximize the number of its absorption resonances over the broadband interval where microcrystalline silicon is weakly absorbing before lattice disorder augmented with fabrication-induced imperfections is applied to further boost performance. Such a design strategy may find practical use for increasing the efficiency of thin-film silicon photovoltaics.

Tailoring repulsive optical forces in nanophotonic waveguides

We describe a mechanism and propose design strategies to selectively tailor repulsive-gradient-optical forces between parallel, nanophotonic waveguides via morphology augmented by slow-light band-edge modes. We show that at small separation lengths, the repulsive force can be made nearly 2 orders of magnitude larger than that of standard dielectric waveguides with a square cross section. The increased coupling interactions should enable a wider dynamic range of optomechanical functionality for potential applications in sensing, switching, and nanoelectromechanical systems.

Enhancement of photocurrent in ultrathin active-layer photodetecting devices with photonic crystals

We demonstrate an enhancement of the photoelectric-conversion efficiency of an ultrathin (50 nm) silicon active-layer photodetecting device using a two-dimensional photonic crystal positioned nearby to boost the optical absorption. We show both experimentally and with simulations that the incident-light absorption within the active layer is enhanced by optical-resonance effects at the photonic band edge. We also find that a photonic crystal with deeper holes can lead to an even larger absorption enhancement due to better quality (Q)-factor matching between the photonic band-edge modes and the intrinsic material absorption. The experimentally observed photocurrent of the fabricated photonic-crystal sample is increased by a factor of ∼20 at the photonic band-edge wavelength relative to that of a control sample without the photonic crystal which is attributed to the improved Q matching.

Texturing the cathode of white organic light-emitting diodes with a lattice of nanoscale scatterers for enhanced light out-coupling

The external quantum efficiency of white organic light-emitting diodes is often limited by light out-coupling losses due to surface plasmons. We demonstrate how texturing of the metal-cathode surface using a two-dimensionally periodic lattice of nanoscale scatterers with limited disorder can be used to reduce plasmonic losses while simultaneously enhancing both the light out-coupling and the spontaneous-emission rate of the excitons. We use electrodynamic simulations and statistical modeling to explore the relationship between the topology of the surface texture and its corresponding scattering efficiency. From this, we outline attributes of textures that can most enhance device performance.