Halldor Gudfinnur Svavarsson

Leaky-mode resonance photonics: an applications platform

Resonant leaky modes can be induced on dielectric, semiconductor, and metallic periodic layers patterned in one or two dimensions. In this paper, we summarize their physical basis and present their applicability in photonic devices and systems. The fundamental amplitude and phase response of this device class is presented by computed examples for TE and TM polarizations for lightly and heavily spatially modulated gratings. A summary of potential applications is provided followed by discussion of representative examples. In particular, we present a resonant polarizer enabled by a single periodic silicon layer operating across 200-nm bandwidth at normal incidence. Guided-mode resonance (GMR) biosensor technology is presented in which the dual-polarization capability of the fundamental resonance effect is applied to determine two unknowns in a biodetection experiment. In principle, using polarization and modal diversity, simultaneously collected data sets can be used to determine several relevant parameters in each channel of the sensor system; these results exemplify this unique capability of GMR sensor technology. Applying the GMR phase, we show an example of a half-wave retarder design operating across a 50-nm bandwidth at λ~1550 nm. Experimental results using a metal/dielectric design show that surface-plasmon resonance and leaky-mode resonance can coexist in the same device; the experimental results fit well with theoretical simulations.

Fabrication of resonant patterns using thermal nano-imprint lithography for thin-film photovoltaic applications

A single-step, low-cost fabrication method to generate resonant nano-grating patterns on poly-methyl-methacrylate (PMMA; plexiglas) substrates using thermal nano-imprint lithography is reported. A guided-mode resonant structure is obtained by subsequent deposition of thin films of transparent conductive oxide and amorphous silicon on the imprinted area. Referenced to equivalent planar structures, around 25% and 45% integrated optical absorbance enhancement is observed over the 450-nm to 900-nm wavelength range in one- and two-dimensional patterned samples, respectively. The fabricated elements provided have 300-nm periods. Thermally imprinted thermoplastic substrates hold potential for low-cost fabrication of nano-patterned thin-film solar cells for efficient light management.

Experimental observation of leaky modes and plasmons in a hybrid resonance element

We provide experimental evidence of a hybrid photonic device supporting simultaneously surface-plasmon polaritons and resonant leaky modes. A fabricated metallo-dielectric structure exhibits a pronounced plasmonic resonance at 799 nm wavelength and a modal resonance at 669 nm in transverse magnetic polarization. In transverse electric polarization, a weak modal resonance appears at 725 nm wavelength. We identify the corresponding modes by computing the attendant internal field distributions. Numerically computed spectra are in good agreement with our measurements. Since traditional modal and plasmonic devices find many uses, their hybrid versions may enable the extension of their applicability.

Carbon dioxide from geothermal gas converted to biomass by cultivating coccoid cyanobacteria

ABSTRACT The Blue Lagoon is a geothermal aquifer with a diverse ecosystem located within the Reykjanes UNESCO Global Geopark on Iceland’s Reykjanes Peninsula. Blue Lagoon Ltd., which exploits the aquifer, isolated a strain of coccoid cyanobacteria Cyanobacterium aponinum (C. aponinum) from the geothermal fluid of the Blue Lagoon more than two decades ago. Since then Blue Lagoon Ltd. has cultivated it in a photobioreactor, for use as an active ingredient in its skin care products. Until recently, the cultivation of C. aponinum was achieved by feeding it on 99.99% (4N) bottled carbon dioxide (CO2). In this investigation, C. aponinum was cultivated using unmodified, non-condensable geothermal gas (geogas) emitted from a nearby geothermal powerplant as the feed-gas instead of the 4N-gas. The geogas contains roughly 90% vol CO2 and 2% vol hydrogen sulfide (H2S). A comparison of both CO2 sources was made. It was observed that the use of geogas did enhance the conversion efficiency. A 13 weeks’ average CO2 conversion efficiency of C. aponinum was 43% and 31% when fed on geogas and 4N-gas, respectively. Despite the high H2S concentration in the geogas, sulfur accumulation in the cultivated biomass was similar for both gas sources. Our results provide a model of a CO2 sequestration by photosynthetic conversion of otherwise unused geothermal emission gas into biomass.

Resonance-based nanophotonic device technology: Filters, reflectors, and absorbers

We review nanophotonic device technology that is based on fundamental photonic resonance effects. We present the physics behind resonance device operation, illustrate their design with rigorous methods, discuss fabrication processes, and present results of physical and spectral characterization. We indicate the application potential of this field, discuss some past device examples, and provide new and emerging aspects. In particular, we present new wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material thereby eliminating local reflections and phase changes. This critical interface therefore possesses zero refractive-index contrast; hence we call them “zero-contrast gratings.” For simple gratings with two-part periods, we show that zero-contrast grating reflectors outperform comparable high-contrast grating reflectors with nearly 700-nm bandwidth achieved at 99% reflectance. Resonance elements functioning as simultaneous spatial and spectral filters are introduced and substantiated with computed and experimental results that are in excellent agreement. Single-layer bandpass filters are presented and compared to their classic multilayer counterparts. An example bandpass filter with narrow transmission band fashioned with a single periodic layer compares in functionality with a classic Bragg stack with ~30 layers. We discuss deep Si grating structures that efficiently absorb fully-hemispherical unpolarized light in the entire visible spectral domain. This absorber provides a broad spectral continuum of densely populated resonant photonic states as well as a cooperating wide-angular antireflection effect, resulting in broadband, omnidirectional, and polarization-insensitive light absorption. We experimentally verify the absorber performance with precise fabrication and conical input beam spectral analysis. The promise and limitations of this class of devices is discussed.

Effect of hydrogenation on minority carrier lifetime in low-grade silicon

Silicon p–n junctions for solar cell applications were prepared by growing thin n-type films on p-type metallurgical-grade silicon (MG-Si) substrate in a liquid solution of arsenic-doped gallium/indium solution. MG-Si has much higher impurity concentrations than traditional solar-grade silicon, but is here being used as a low-cost alternative. The as-grown film showed negligible photovoltaic response. A post-growth exposure of the as-grown p–n junctions to a H2/Ar plasma resulted in an active photovoltaic device with VOC of up to several hundred mV. Furthermore, a significant increase in the minority carrier lifetime was observed. The short-circuit current density indicated low efficiency in accordance with the high defect concentration of the impure substrate. An x-ray diffraction study of MG-Si prior to and after film growth revealed textured crystal structure that diminished during film growth, indicating that the film is more homogeneous than the substrate.

Hopping conduction in lithium diffused and annealed GaAs

Temperature dependent conductivity and Hall effect measurements were carried out in the temperature range 30-300 K on Li-diffused and annealed GaAs. Li in-diffusion into GaAs reduces the free carrier concentration which leads to electrical conductivity as low as 10/sup -7/ S/cm. Annealing the lithium diffused samples at temperatures above 200/spl deg/C significantly increases the room temperature conductivity to 1-10 S/cm, depending on the annealing temperature. In samples annealed at 300-500/spl deg/C the temperature dependence of the conductivity indicates that hopping conduction sets in at temperatures below 100 K. We relate this to enhanced gallium vacancy (V/sub Ga/) and gallium antisite (Ga/sub As/) concentration in Li in-diffused and annealed samples.