Brad Johnson

Simulations of luminescent solar concentrators: Effects of polarization and fluorophore alignment

We model the effects of dye molecule alignment on the collection efficiency of luminescent solar concentrators (LSCs). A Monte Carlo model for photon transport in LSC’s is derived and utilized, which incorporates the effects of fluorescent-dye-molecular alignment and the subsequent control over absorption, emission, and propagation properties. We focus on the effects of molecular alignment statistics on photon absorption and subsequent emission, including polarization and propagation direction imparted by dipole direction, to model device light-capture efficiency, defined as the ratio of the amount of light reaching particular slab edges to that incident on a face. We find that modest control of alignment, coupled with reasonable and attainable emission-absorption dipole angles, can produce very large collection efficiencies for a range of device parameters. We note that efficiencies for small values of dye molecule Stoke’s shift may be made as large as those for homogeneous (unaligned) systems with large...

Forbidden guided-wave plasmon polaritons in coupled thin films

We study the plasmon-polariton collective modes of a three-thin-film-on-a-substrate structure, where two of the thin films have free-charge carriers and are separated by a static dielectric. We find that this structure can support unusual guided-wave polaritons, which exist in a normally forbidden region of the surface polariton dispersion, and which are composed of coupled surface polariton modes of the individual films with a wavelike transverse electric-field amplitude in the spacer dielectric. The modes emerge from both bulk and surface collective mode bands and evolve into a ladder of solutions in the forbidden region. We also present the attenuated total reflection spectrum arising from the guided modes.

A simple model for Faraday waves

We show that the linear‐stability analysis of the birth of Faraday waves on the surface of a fluid is simplified considerably when the fluid container is driven by a triangle wave form rather than by a sine wave. The calculation is simple enough to use in an undergraduate course on fluid dynamics or nonlinear dynamics. It is also an attractive starting point for a nonlinear analysis.

Inertial forces and the Hall effect

We demonstrate that inertial forces give rise to a phenomenon similar to the Hall effect in metals and semiconductors. This phenomenon is useful for illustrating the structure of the Hall effect, since the Hamiltonian for the rotating reference frame problem has most of the same features as that for the problem of charges moving under a magnetic field. In addition, we investigate the quantum regime of a rotating system. We find that there exists an analog of the integer quantum Hall effect at extreme rotation rates, but of a qualitatively different nature. An analog of the fractional quantum Hall effect will not occur. The similarities and differences between the inertial-force and magnetic versions of the Hall effect are good pedagogical illustrations for the classical and quantum Hall regimes.

Organic-Vapor-Liquid-Solid Deposition with an Impinging Gas Jet

A method for rapid, mass-efficient deposition of highly crystalline organic films under near ambient conditions of pressure and temperature is reported based on delivery of an organic precursor via an impinging gas jet to a substrate coated by a thin liquid solvent layer. Films of the organic semiconductor tetracene were deposited by sublimation into a flow of argon carrier gas directed at an indium-tin-oxide/glass substrate coated by a thin layer of bis(2-ethylhexyl)sebecate, and growth was followed in situ with optical microscopy. A fluid dynamics model is applied to account for the gas phase transport and aggregation, and the results compared to experiment. The combination of gas jet delivery with an organic-vapor-liquid-solid growth mechanism leads to larger crystals and lower nucleation densities than on bare surfaces, with markedly different nucleation and growth kinetics. An explanation based on enhanced solution-phase diffusivity and a larger critical nucleus size in the liquid layer is proposed t...

Analysis of signal-to-noise ratio impact in heat assisted magnetic recording under insufficient head field

In this work, we present a systematic modeling analysis of signal-to-noise ratio (SNR) impact on recording under insufficient head field for heat assisted magnetic recording (HAMR). A typical head design in the absence of a medium soft underlayer is utilized in the modeling. The study shows that sufficient head field amplitude is critical for realization of expected HAMR recording performance. The strong correlation between recording time window and medium signal-to-noise ratio provides insightful understanding to medium and recording optimization when head field is not sufficiently high. The analysis is also performed with the inclusion of grain-to-grain Curie temperature (Tc) variation in the medium.

Detection of guided-wave plasmon polariton modes in a high-index dielectric MIM structure

Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These modes are supported by a simple physical structure that results from a minor symmetry modification of the traditional MIM structure, specifically the use of a central insulator layer with a higher refractive index than the supporting substrate. However, they display fundamental properties that are distinctly different from those of standard SPPs and from recently reported hybrid plasmonic modes. While GW-PPMs have been explored theoretically, they have not yet been realized experimentally. In this article, we present the first experimental demonstration of GW-PPMs. Specifically, we excite and detect GW-PPMs at visible frequencies and match model predictions to experimental results with remarkable accuracy using minimal parameter fitting. In addition to the experimental detection, we calculate and report on other interesting and relevant features of the detected modes, including the associated electric field profiles, confinement values, and propagation lengths, and discuss in terms of the applications-relevance of GW-PPMs.Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These mode...