Dr. Halldór Guðfinnur Svavarsson


Position: Associate Professor
Website: http://www.ru.is/haskolinn/starfsfolk/halldorsv


Research projects

  • Photoconductive oxide films
  • Arrays of Silicon nanowires for photo- and thermovoltaics

My research areas of expertise include nanolithography, nanophotonics, nanoplasmonics, optical bio- and chemical sensors, nanofabrication, integrated nanoscale devices and thin-films. In particular I have been focusing on fabrication and characterization of periodic nanostructures of metals, dielectrics and semiconductors for thermoelectric, photovoltaic and photonic applications and with a special emphasize on silicon nanowires (SiNWs). These researches have been done in close collaboration with the Nanophotonics Device group at the Electrical Engineering department of the University of Texas at Arlington (http://leakymoderesonance.com/).


Publication

Google Scholar profile

  1. Muhammad Taha Sultan, Andrei Manolescu, Jon Tomas Gudmundsson, Kristinn Torfason, George [Alexandru Nemnes], Ionel Stavarache, Constantin Logofatu, Valentin Serban Teodorescu, Magdalena Lidia Ciurea and Halldor Gudfinnur Svavarsson.
    Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing.
    Applied Surface Science 469, 870 - 878 (2019).
    Abstract Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.
    URL, DOI BibTeX

    @article{SULTAN2019870,
    	title = "Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing",
    	journal = "Applied Surface Science",
    	volume = 469,
    	pages = "870 - 878",
    	year = 2019,
    	issn = "0169-4332",
    	doi = "https://doi.org/10.1016/j.apsusc.2018.11.061",
    	url = "http://www.sciencedirect.com/science/article/pii/S0169433218331295",
    	author = "Muhammad Taha Sultan and Andrei Manolescu and Jon Tomas Gudmundsson and Kristinn Torfason and George [Alexandru Nemnes] and Ionel Stavarache and Constantin Logofatu and Valentin Serban Teodorescu and Magdalena Lidia Ciurea and Halldor Gudfinnur Svavarsson",
    	keywords = "SiO, SiGe nanocrystals, Strain, Radio frequency magnetron sputtering, Thermal annealing, Photoconductivity",
    	abstract = "Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified."
    }
    
  2. M T Sultan, J T Gudmundsson, A Manolescu, V S Teodorescu, M L Ciurea and H G Svavarsson.
    Efficacy of annealing and fabrication parameters on photo-response of SiGe in TiO2 matrix.
    Nanotechnology 30, 365604 (June 2019).
    Abstract SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to ∼1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures.
    URL, DOI BibTeX

    @article{Sultan_2019,
    	doi = "10.1088/1361-6528/ab260e",
    	url = "https://doi.org/10.1088%2F1361-6528%2Fab260e",
    	year = 2019,
    	month = "jun",
    	publisher = "{IOP} Publishing",
    	volume = 30,
    	number = 36,
    	pages = 365604,
    	author = "M T Sultan and J T Gudmundsson and A Manolescu and V S Teodorescu and M L Ciurea and H G Svavarsson",
    	title = "Efficacy of annealing and fabrication parameters on photo-response of {SiGe} in {TiO}2 matrix",
    	journal = "Nanotechnology",
    	abstract = "SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to ∼1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures."
    }
    
  3. M T Sultan, J T Gudmundsson, A Manolescu, T Stoica, M L Ciurea and H G Svavarsson.
    Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation.
    Applied Surface Science 479, 403 - 409 (2019).
    Abstract We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent.
    URL, DOI BibTeX

    @article{SULTAN2019403,
    	title = "Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation",
    	journal = "Applied Surface Science",
    	volume = 479,
    	pages = "403 - 409",
    	year = 2019,
    	issn = "0169-4332",
    	doi = "https://doi.org/10.1016/j.apsusc.2019.02.096",
    	url = "http://www.sciencedirect.com/science/article/pii/S0169433219304398",
    	author = "M.T. Sultan and J.T. Gudmundsson and A. Manolescu and T. Stoica and M.L. Ciurea and H.G. Svavarsson",
    	keywords = "SiGe, SiO, Nanoparticles, Photocurrent, Hydrogen plasma, Passivation",
    	abstract = "We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent."
    }
    
  4. Muhammad Taha Sultan, Adrian Valentin Maraloiu, Ionel Stavarache, Tómas Jón Gudmundsson, Andrei Manolescu, Valentin Serban Teodorescu, Magdalena Lidia Ciurea and Halldór Gudfinnur Svavarsson.
    Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study.
    Beilstein Journal of Nanotechnology 10, 1873-1882 (2019).
    Abstract Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra.
    DOI BibTeX

    @article{Sultan2019,
    	author = "Muhammad Taha Sultan and Adrian Valentin Maraloiu and Ionel Stavarache and Jón Tómas Gudmundsson and Andrei Manolescu and Valentin Serban Teodorescu and Magdalena Lidia Ciurea and Halldór Gudfinnur Svavarsson",
    	title = "Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study",
    	journal = "Beilstein Journal of Nanotechnology",
    	year = 2019,
    	volume = 10,
    	pages = "1873-1882",
    	issn = "2190-4286",
    	doi = "10.3762/bjnano.10.182",
    	copyright = "Sultan et al.; licensee Beilstein-Institut.",
    	abstract = "Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra."
    }
    
  5. Daniel J Carney, Halldor G Svavarsson, Hafez Hemmati, Alexander Fannin, Jae W Yoon and Robert Magnusson.
    Refractometric Sensing with Periodic Nano-Indented Arrays: Effect of Structural Dimensions.
    Sensors 19 (2019).
    Abstract Fabrication and sensor application of a simple plasmonic structure is described in this paper. The sensor element consists of nano-patterned gold film brought about from two-dimensional periodic photoresist templates created by holographic laser interference lithography. Reflectance spectroscopy revealed that the sensor exhibits significant refractive index sensitivity. A linear relationship between shifts in plasmonic resonances and changes in the refractive index were demonstrated. The sensor has a bulk sensitivity (SB) of 880 nm/refractive index unit and work under normal incidence conditions. This sensitivity exceeded that of many common types of plasmonic sensors with more intricate structures. A modeled spectral response was used to study the effect of its geometrical dimensions on plasmonic behavior. A qualitative agreement between the experimental spectra and modeled ones was obtained.
    URL, DOI BibTeX

    @article{s19040897,
    	author = "Carney, Daniel J. and Svavarsson, Halldor G. and Hemmati, Hafez and Fannin, Alexander and Yoon, Jae W. and Magnusson, Robert",
    	title = "Refractometric Sensing with Periodic Nano-Indented Arrays: Effect of Structural Dimensions",
    	journal = "Sensors",
    	volume = 19,
    	year = 2019,
    	number = 4,
    	article-number = 897,
    	url = "https://www.mdpi.com/1424-8220/19/4/897",
    	issn = "1424-8220",
    	abstract = "Fabrication and sensor application of a simple plasmonic structure is described in this paper. The sensor element consists of nano-patterned gold film brought about from two-dimensional periodic photoresist templates created by holographic laser interference lithography. Reflectance spectroscopy revealed that the sensor exhibits significant refractive index sensitivity. A linear relationship between shifts in plasmonic resonances and changes in the refractive index were demonstrated. The sensor has a bulk sensitivity (SB) of 880 nm/refractive index unit and work under normal incidence conditions. This sensitivity exceeded that of many common types of plasmonic sensors with more intricate structures. A modeled spectral response was used to study the effect of its geometrical dimensions on plasmonic behavior. A qualitative agreement between the experimental spectra and modeled ones was obtained.",
    	doi = "10.3390/s19040897"
    }
    
  6. M T Sultan, J T Gudmundsson, A Manolescu, V S Teodorescu, M L Ciurea and H G Svavarsson.
    Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing.
    Applied Surface Science 511, 145552 (2020).
    Abstract Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.
    URL, DOI BibTeX

    @article{SULTAN2020145552,
    	title = "Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing",
    	journal = "Applied Surface Science",
    	volume = 511,
    	pages = 145552,
    	year = 2020,
    	issn = "0169-4332",
    	doi = "10.1016/j.apsusc.2020.145552",
    	url = "http://www.sciencedirect.com/science/article/pii/S0169433220303081",
    	author = "M.T. Sultan and J.T. Gudmundsson and A. Manolescu and V.S. Teodorescu and M.L. Ciurea and H.G. Svavarsson",
    	keywords = "TiO, SiGe, Nanoparticles, HiPIMS, GiXRD, Interface, Photo-spectra",
    	abstract = "Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering."
    }
    
  7. Ionel Stavarache, Constantin Logofatu, Muhammad Taha Sultan, Andrei Manolescu, Halldor Gudfinnur Svavarsson, Valentin Serban Teodorescu and Magdalena Lidia Ciurea.
    SiGe nanocrystals in SiO 2 with high photosensitivity from visible to short-wave infrared.
    Scientific Reports 10, 1–9 (2020).
    Abstract Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.
    DOI BibTeX

    @article{stavarache2020sige,
    	title = "SiGe nanocrystals in SiO 2 with high photosensitivity from visible to short-wave infrared",
    	author = "Stavarache, Ionel and Logofatu, Constantin and Sultan, Muhammad Taha and Manolescu, Andrei and Svavarsson, Halldor Gudfinnur and Teodorescu, Valentin Serban and Ciurea, Magdalena Lidia",
    	journal = "Scientific Reports",
    	volume = 10,
    	number = 1,
    	pages = "1--9",
    	year = 2020,
    	publisher = "Nature Publishing Group",
    	abstract = "Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.",
    	doi = "10.1038/s41598-020-60000-x"
    }
    
  8. Robert Magnusson, Kyu J Lee, Hafez Hemmati, Pawarat Bootpakdeetam, Jonathan Vasilyev, Fairooz A Simlan, Nasrin Razmjooei, Yeong Hwan Ko, Shanwen Zhang, Sun-Goo Lee and Halldor G Svavarsson.
    Properties of resonant photonic lattices: Bloch mode dynamics, band flips, and applications.
    In Connie J Chang-Hasnain, Andrei Faraon and Weimin Zhou (eds.). High Contrast Metastructures IX 11290. (2020), 1 – 10.
    Abstract We review guided-mode resonant photonic lattices by addressing their functionalities and potential device applications. The 1D canonical model is rich in properties and conceptually transparent, with all the main conclusions being applicable to 2D metasurfaces and periodic photonic slabs. We explain the operative physical mechanisms grounded in lateral leaky Bloch modes. We summarize the band dynamics of the leaky stopband. With several examples, we demonstrate that Mie scattering is not causative in resonant reflection. Illustrated applications include a wideband reflector at infrared bands as well as resonant reflectors with triangular profiles. We quantify the improved efficiency of a silicon reflector operating in the visible region relative to loss reduction as realizable with sample hydrogenation. A resonant polarizer with record performance is presented.
    URL, DOI BibTeX

    @inproceedings{10.1117/12.2547322,
    	author = "Robert Magnusson and Kyu J. Lee and Hafez Hemmati and Pawarat Bootpakdeetam and Jonathan Vasilyev and Fairooz A. Simlan and Nasrin Razmjooei and Yeong Hwan Ko and Shanwen Zhang and Sun-Goo Lee and Halldor G. Svavarsson",
    	title = "{Properties of resonant photonic lattices: Bloch mode dynamics, band flips, and applications}",
    	volume = 11290,
    	booktitle = "High Contrast Metastructures IX",
    	editor = "Connie J. Chang-Hasnain and Andrei Faraon and Weimin Zhou",
    	organization = "International Society for Optics and Photonics",
    	publisher = "SPIE",
    	pages = "1 -- 10",
    	abstract = "We review guided-mode resonant photonic lattices by addressing their functionalities and potential device applications. The 1D canonical model is rich in properties and conceptually transparent, with all the main conclusions being applicable to 2D metasurfaces and periodic photonic slabs. We explain the operative physical mechanisms grounded in lateral leaky Bloch modes. We summarize the band dynamics of the leaky stopband. With several examples, we demonstrate that Mie scattering is not causative in resonant reflection. Illustrated applications include a wideband reflector at infrared bands as well as resonant reflectors with triangular profiles. We quantify the improved efficiency of a silicon reflector operating in the visible region relative to loss reduction as realizable with sample hydrogenation. A resonant polarizer with record performance is presented.",
    	keywords = "guided-mode resonance, leaky-mode resonance, resonant waveguide gratings, periodic photonic films, metasurfaces, leaky-band dynamics, subwavelength nanophotonics, metamaterials",
    	year = 2020,
    	doi = "10.1117/12.2547322",
    	url = "https://doi.org/10.1117/12.2547322"
    }
    
  9. Halldor G Svavarsson, Johannes E Valberg, Hronn Arnardottir and Asa Brynjolfsdottir.
    Carbon dioxide from geothermal gas converted to biomass by cultivating coccoid cyanobacteria.
    Environmental Technology 0, 1-8 (2017).
    Abstract ABSTRACTThe 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.
    URL, DOI BibTeX

    @article{doi:10.1080/09593330.2017.1349840,
    	author = "Halldor G. Svavarsson and Johannes E. Valberg and Hronn Arnardottir and Asa Brynjolfsdottir",
    	title = "Carbon dioxide from geothermal gas converted to biomass by cultivating coccoid cyanobacteria",
    	journal = "Environmental Technology",
    	volume = 0,
    	number = 0,
    	pages = "1-8",
    	year = 2017,
    	publisher = "Taylor & Francis",
    	doi = "10.1080/09593330.2017.1349840",
    	note = "PMID: 28662603",
    	url = "https://doi.org/10.1080/09593330.2017.1349840",
    	eprint = "https://doi.org/10.1080/09593330.2017.1349840",
    	abstract = "ABSTRACTThe 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."
    }
    
  10. C Palade, A Slav, O Cojocaru, V S Teodorescu, S Lazanu, T Stoica, M T Sultan, H G Svavarsson and M L Ciurea.
    Enhanced Photocurrent in GeSi NCs / TiO2Multilayers.
    In 2018 International Semiconductor Conference (CAS) (). (2018), 73-76.
    BibTeX

    @inproceedings{8539740,
    	author = "C. {Palade} and A. {Slav} and O. {Cojocaru} and V. S. {Teodorescu} and S. {Lazanu} and T. {Stoica} and M. T. {Sultan} and H. G. {Svavarsson} and M. L. {Ciurea}",
    	booktitle = "2018 International Semiconductor Conference (CAS)",
    	title = "Enhanced Photocurrent in GeSi NCs / TiO2Multilayers",
    	year = 2018,
    	volume = "",
    	number = "",
    	pages = "73-76"
    }
    
  11. M T Sultan, J T Gudmundsson, A Manolescu, M L Ciurea, C Palade, A V Maraloiu and H G Svavarsson.
    Enhanced Photoconductivity of SIGE-Trilayer Stack by Retrenching Annealing Conditions.
    In 2018 International Semiconductor Conference (CAS) (). (2018), 61-64.
    BibTeX

    @inproceedings{8539775,
    	author = "M. T. {Sultan} and J. T. {Gudmundsson} and A. {Manolescu} and M. L. {Ciurea} and C. {Palade} and A. V. {Maraloiu} and H. G. {Svavarsson}",
    	booktitle = "2018 International Semiconductor Conference (CAS)",
    	title = "Enhanced Photoconductivity of SIGE-Trilayer Stack by Retrenching Annealing Conditions",
    	year = 2018,
    	volume = "",
    	number = "",
    	pages = "61-64"
    }
    
  12. M T Sultan, J T Gudmundsson, A Manolescu, M L Ciurea and H G Svavarsson.
    The Effect of H2/Ar Plasma Treatment Over Photoconductivity of Sige Nanoparticles Sandwiched Between Silicon Oxide Matrix.
    In 2018 International Semiconductor Conference (CAS) (). (2018), 257-260.
    BibTeX

    @inproceedings{8539761,
    	author = "M. T. {Sultan} and J. T. {Gudmundsson} and A. {Manolescu} and M. L. {Ciurea} and H. G. {Svavarsson}",
    	booktitle = "2018 International Semiconductor Conference (CAS)",
    	title = "The Effect of H2/Ar Plasma Treatment Over Photoconductivity of Sige Nanoparticles Sandwiched Between Silicon Oxide Matrix",
    	year = 2018,
    	volume = "",
    	number = "",
    	pages = "257-260"
    }
    
  13. A Slav, C Palade, I Stavarache, V S Teodorescu, M L Ciurea, R Müller, A Dinescu, M T Sultan, A Manolescu, J T Gudmundsson and H G Svavarsson.
    Influence of preparation conditions on structure and photosensing properties of GeSi/TiO2 multilayers.
    In 2017 International Semiconductor Conference (CAS) (). (October 2017), 63-66.
    Abstract The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO2 cap/(GeSi/TiO2)2 multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO2 cap/GeSi/TiO2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5-10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20-30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.
    DOI BibTeX

    @inproceedings{8101154,
    	author = "A. Slav and C. Palade and I. Stavarache and V. S. Teodorescu and M. L. Ciurea and R. Müller and A. Dinescu and M. T. Sultan and A. Manolescu and J. T. Gudmundsson and H. G. Svavarsson",
    	booktitle = "2017 International Semiconductor Conference (CAS)",
    	title = "Influence of preparation conditions on structure and photosensing properties of GeSi/TiO2 multilayers",
    	year = 2017,
    	volume = "",
    	number = "",
    	pages = "63-66",
    	abstract = "The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO2 cap/(GeSi/TiO2)2 multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO2 cap/GeSi/TiO2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5-10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20-30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.",
    	keywords = "Ge-Si alloys;X-ray diffraction;multilayers;nanofabrication;nanoparticles;nanostructured materials;photoconductivity;photodetectors;rapid thermal annealing;scanning electron microscopy;semiconductor materials;semiconductor superlattices;semiconductor thin films;sputter deposition;titanium compounds;transmission electron microscopy;GeSi-TiO2;SEM;TEM;XRD;high power impulse magnetron sputtering;multilayers;photocurrent measurement;photosensing properties;rapid thermal annealing;two-step annealing;visible-NIR spectroscopy;Annealing;Films;Nanocrystals;Nonhomogeneous media;Photoconductivity;Scanning electron microscopy;Silicon;GeSi nanocrystals;TiO2;photosensing",
    	doi = "10.1109/SMICND.2017.8101154",
    	issn = "",
    	month = "Oct"
    }
    
  14. R Magnusson, J W Yoon, M Niraula, K J Lee and H G Svavarsson.
    Resonance-based nanophotonic device technology: Filters, reflectors, and absorbers.
    In 2016 IEEE Aerospace Conference. (March 2016), 1-13.
    Abstract 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.
    DOI BibTeX

    @inproceedings{7500645,
    	author = "R. Magnusson and J. W. Yoon and M. Niraula and K. J. Lee and H. G. Svavarsson",
    	booktitle = "2016 IEEE Aerospace Conference",
    	title = "Resonance-based nanophotonic device technology: Filters, reflectors, and absorbers",
    	year = 2016,
    	pages = "1-13",
    	abstract = "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.",
    	keywords = "antireflection coatings;band-pass filters;diffraction gratings;elemental semiconductors;light absorption;light polarisation;nanophotonics;optical fabrication;optical filters;periodic structures;silicon;spatial filters;Si;broad spectral continuum;broadband light absorption;conical input beam spectral analysis;critical interface;deep Si grating structures;densely populated resonant photonic states;fully-hemispherical unpolarized light;grating ridges;identical material;narrow transmission band;omnidirectional light absorption;photonic resonance effects;physical characterization;polarization-insensitive light absorption;resonance device operation;resonance elements;resonance-based nanophotonic device technology;single periodic layer;single-layer bandpass filters;spatial filters;spectral characterization;spectral filters;visible spectral domain;wide-angular antireflection effect;wideband resonant reflectors;zero refractive index contrast;zero-contrast grating reflectors;Band-pass filters;Gratings;Nanoscale devices;Reflectivity;Silicon;Wideband",
    	doi = "10.1109/AERO.2016.7500645",
    	month = "March"
    }
    
  15. Halldor Gudfinnur Svavarsson, Birgir Hrafn Hallgrimsson, Manoj Niraula, Kyu Jin Lee and Robert Magnusson.
    Large arrays of ultra-high aspect ratio periodic silicon nanowires obtained via top–down route.
    Applied Physics A 122, 1–6 (2016).
    Abstract Metal-catalysed etching (MCE) is a simple and versatile method for fabrication of silicon nanowires, of high structural quality. When combined with laser interference lithography (LIL), large areas of periodic structures can be generated in only few steps. The aspect ratio of such periodic structure is however commonly not higher than several decades or very few hundred. Here, a combined MCE and LIL techniques were applied to fabricate dense (4 \texttimes 108 cm−3), periodic arrays of vertically aligned silicon nanowires with aspect ratio of up to 103. This is a considerable higher number than previously reported on for periodic silicon wire arrays prepared with top–down approaches. The wires were slightly tapered, with top and bottom diameters ranging from 370 to 195 nm and length of up to 200 $\mu$m. A potential use of the nanowires as light absorber is demonstrated by measuring reflection in integrating sphere. An average total absorption of \textasciitilde97 % was observed for 200-$\mu$m-long wires in the spectral range of 450–1000 nm. A comparison to simulated absorption spectra is given.
    URL, DOI BibTeX

    @article{Svavarsson2016,
    	author = "Svavarsson, Halldor Gudfinnur and Hallgrimsson, Birgir Hrafn and Niraula, Manoj and Lee, Kyu Jin and Magnusson, Robert",
    	title = "Large arrays of ultra-high aspect ratio periodic silicon nanowires obtained via top--down route",
    	journal = "Applied Physics A",
    	year = 2016,
    	volume = 122,
    	number = 2,
    	pages = "1--6",
    	abstract = "Metal-catalysed etching (MCE) is a simple and versatile method for fabrication of silicon nanowires, of high structural quality. When combined with laser interference lithography (LIL), large areas of periodic structures can be generated in only few steps. The aspect ratio of such periodic structure is however commonly not higher than several decades or very few hundred. Here, a combined MCE and LIL techniques were applied to fabricate dense (4 {\texttimes} 108 cm−3), periodic arrays of vertically aligned silicon nanowires with aspect ratio of up to 103. This is a considerable higher number than previously reported on for periodic silicon wire arrays prepared with top--down approaches. The wires were slightly tapered, with top and bottom diameters ranging from 370 to 195 nm and length of up to 200 $\mu$m. A potential use of the nanowires as light absorber is demonstrated by measuring reflection in integrating sphere. An average total absorption of {\textasciitilde}97 \% was observed for 200-$\mu$m-long wires in the spectral range of 450--1000 nm. A comparison to simulated absorption spectra is given.",
    	issn = "1432-0630",
    	doi = "10.1007/s00339-015-9589-y",
    	url = "http://dx.doi.org/10.1007/s00339-015-9589-y"
    }
    
  16. Halldor G Svavarsson, Sigurbjorn Einarsson and Asa Brynjolfsdottir.
    Adsorption applications of unmodified geothermal silica.
    Geothermics 50, 30 - 34 (2014).
    Abstract Abstract Silica, precipitated out of geothermal fluid discharged from a geothermal powerplant in Svartsengi on the Reykjanes peninsula in Iceland, was used as a chromatographic adsorbent to extract blue colored protein, C-phycocyanin, from coccoid blue-green algae. The only supplement used was salt obtained by evaporating the geothermal fluid. Analysis of the silica, using scanning electron microscopy, X-ray diffractometry and Brunauer–Emmett–Teller (BET) adsorption confirmed it has a high specific surface area and is amorphous. Upon adsorption and subsequent elution the purity of the extracted protein, measured as the ratio of the light absorbance of 620 and 280 nm, increased from 0.5 to above 2.0. Our results could facilitate utilization of a mostly unused byproduct of geothermal powerplants as chromatographic material.
    URL, DOI BibTeX

    @article{Svavarsson201430,
    	title = "Adsorption applications of unmodified geothermal silica",
    	journal = "Geothermics",
    	volume = 50,
    	number = 0,
    	pages = "30 - 34",
    	year = 2014,
    	note = "",
    	issn = "0375-6505",
    	doi = "10.1016/j.geothermics.2013.08.001",
    	url = "http://www.sciencedirect.com/science/article/pii/S0375650513000576",
    	author = "Halldor G. Svavarsson and Sigurbjorn Einarsson and Asa Brynjolfsdottir",
    	keywords = "Phycocyanin",
    	abstract = "Abstract Silica, precipitated out of geothermal fluid discharged from a geothermal powerplant in Svartsengi on the Reykjanes peninsula in Iceland, was used as a chromatographic adsorbent to extract blue colored protein, C-phycocyanin, from coccoid blue-green algae. The only supplement used was salt obtained by evaporating the geothermal fluid. Analysis of the silica, using scanning electron microscopy, X-ray diffractometry and Brunauer–Emmett–Teller (BET) adsorption confirmed it has a high specific surface area and is amorphous. Upon adsorption and subsequent elution the purity of the extracted protein, measured as the ratio of the light absorbance of 620 and 280 nm, increased from 0.5 to above 2.0. Our results could facilitate utilization of a mostly unused byproduct of geothermal powerplants as chromatographic material."
    }
    
  17. T Khaleque, H G Svavarsson and R Magnusson.
    Fabrication of nano-imprinted resonant structures for thin-film solar cell applications.
    In Photonics Conference (IPC), 2013 IEEE. (2013), 580-581.
    Abstract A simple fabrication approach of replicating nanoscale patterns with 300-nm periods on thermoplastic polymethyl methacrylate substrates using thermal nano-imprint lithography is reported. The resonant structures hold potential application in light trapping for thin-film solar cells.
    DOI BibTeX

    @inproceedings{6656696,
    	author = "Khaleque, T. and Svavarsson, H.G. and Magnusson, R.",
    	booktitle = "Photonics Conference (IPC), 2013 IEEE",
    	title = "Fabrication of nano-imprinted resonant structures for thin-film solar cell applications",
    	year = 2013,
    	month = "Sept",
    	pages = "580-581",
    	keywords = "nanofabrication;nanolithography;optical fabrication;radiation pressure;replica techniques;soft lithography;solar cells;light trapping;nanoimprinted resonant structure fabrication;thermal nanoimprint lithography;thermoplastic polymethyl methacrylate substrates;thin-film solar cells;Gratings;Heating;Optical device fabrication;Photovoltaic cells;Substrates;Temperature measurement",
    	doi = "10.1109/IPCon.2013.6656696",
    	abstract = "A simple fabrication approach of replicating nanoscale patterns with 300-nm periods on thermoplastic polymethyl methacrylate substrates using thermal nano-imprint lithography is reported. The resonant structures hold potential application in light trapping for thin-film solar cells."
    }
    
  18. Kristian Spilling, Ása Brynjólfsdóttir, Dagmar Enss, Heiko Rischer and Halldór Guðfinnur Svavarsson.
    The effect of high pH on structural lipids in diatoms.
    Journal of Applied Phycology 25, 1435-1439 (2013).
    Abstract We tested the hypothesis that increased pH reduces the amount of structural lipids. To do this, we used three different diatoms (Phaeodactylum tricornutum CCAP strain, P. tricornutum TV strain and Amphiprora sp). We tested the effect of rapid increase from pH?7.5 to 10 by adding NaOH. The total lipid content was reduced by 13, 36 and 47 % in the P. tricornutum CCAP strain, TV strain and Amphiprora sp., respectively, 1 h after increasing the pH. The P. tricornutum CCAP strain was used for further testing the effect of pH on the lipid content during active growth. This strain was cultivated at pH?7.5 and 10, and the pH was regulated by the CO2 inflow. The growth rate was similar (0.3 day?1) in both pH treatments, but the lipid content in the pH?10 treatment was on average 28 % lower than in the pH?7.5 treatment. Our data support the hypothesis that structural lipids are reduced when pH increases to high levels. The results suggest that regulating the pH during algae cultivation could be used to refine the lipid composition in the harvested algal biomass.
    URL PDF, DOI BibTeX

    @article{spilling2013effect,
    	year = 2013,
    	issn = "0921-8971",
    	journal = "Journal of Applied Phycology",
    	volume = 25,
    	number = 5,
    	doi = "10.1007/s10811-012-9971-5",
    	title = "The effect of high pH on structural lipids in diatoms",
    	url = "http://dx.doi.org/10.1007/s10811-012-9971-5",
    	publisher = "Springer Netherlands",
    	keywords = "Membrane lipids; Glycolipids; Phospholipids; Polyunsaturated fatty acids",
    	author = "Spilling, Kristian and Brynjólfsdóttir, Ása and Enss, Dagmar and Rischer, Heiko and Svavarsson, Halldór Guðfinnur",
    	pages = "1435-1439",
    	language = "English",
    	pdf = "http://www.researchgate.net/profile/Kristian_Spilling/publication/235332242_The_effect_of_high_pH_on_structural_lipids_in_diatoms/links/0fcfd510ec13ecf3c2000000.pdf",
    	abstract = "We tested the hypothesis that increased pH reduces the amount of structural lipids. To do this, we used three different diatoms (Phaeodactylum tricornutum CCAP strain, P. tricornutum TV strain and Amphiprora sp). We tested the effect of rapid increase from pH?7.5 to 10 by adding NaOH. The total lipid content was reduced by 13, 36 and 47 % in the P. tricornutum CCAP strain, TV strain and Amphiprora sp., respectively, 1 h after increasing the pH. The P. tricornutum CCAP strain was used for further testing the effect of pH on the lipid content during active growth. This strain was cultivated at pH?7.5 and 10, and the pH was regulated by the CO2 inflow. The growth rate was similar (0.3 day?1) in both pH treatments, but the lipid content in the pH?10 treatment was on average 28 % lower than in the pH?7.5 treatment. Our data support the hypothesis that structural lipids are reduced when pH increases to high levels. The results suggest that regulating the pH during algae cultivation could be used to refine the lipid composition in the harvested algal biomass."
    }
    
  19. Tanzina Khaleque, Halldor Gudfinnur Svavarsson and Robert Magnusson.
    Fabrication of resonant patterns using thermal nano-imprint lithography for thin-film photovoltaic applications.
    Opt. Express 21, A631–A641 (July 2013).
    Abstract 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.
    URL, DOI BibTeX

    @article{Khaleque:13,
    	author = "Tanzina Khaleque and Halldor Gudfinnur Svavarsson and Robert Magnusson",
    	journal = "Opt. Express",
    	keywords = "Waveguides; Deposition and fabrication; Guided waves; Solar energy; Nanolithography; Subwavelength structures",
    	number = "S4",
    	pages = "A631--A641",
    	publisher = "OSA",
    	title = "Fabrication of resonant patterns using thermal nano-imprint lithography for thin-film photovoltaic applications",
    	volume = 21,
    	month = "Jul",
    	year = 2013,
    	url = "http://www.opticsexpress.org/abstract.cfm?URI=oe-21-104-A631",
    	doi = "10.1364/OE.21.00A631",
    	abstract = "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."
    }
    
  20. H G Svavarsson, J W Yoon, M Shokooh-Saremi, S H Song and R Magnusson.
    Fabrication and Characterization of Large, Perfectly Periodic Arrays of Metallic Nanocups.
    Plasmonics 7, 653-657 (2012).
    Abstract Fabrication of plasmonic resonance devices composed of large arrays of highly ordered gold nanocups is presented. The nanostructures are generated from periodic photoresist templates created by interference lithography and subsequent reflow, deposition, and dislodging. The nanocups are hemispherical in shape and arranged in both rectangular and hexagonal arrays with periods of ~500 nm. Their ability to support surface plasmonic resonances is manifested experimentally by reflectance spectroscopy. Theoretical modeling to ascertain the plasmonic spectra of these nanostructures is performed. The computed spectra of the rectangular structure are in qualitative agreement with the measurements. A weaker correlation observed for the hexagonal structure is explained by its more intricate symmetry which complicates the spectral response.
    URL PDF, DOI BibTeX

    @article{Svavar2012,
    	year = 2012,
    	issn = "1557-1955",
    	journal = "Plasmonics",
    	volume = 7,
    	number = 4,
    	doi = "10.1007/s11468-012-9355-3",
    	title = "Fabrication and Characterization of Large, Perfectly Periodic Arrays of Metallic Nanocups",
    	url = "http://scitation.aip.org/content/aip/journal/apl/100/9/10.1063/1.3690951",
    	publisher = "Springer US",
    	keywords = "Nanocups; Periodic arrays; Nano-indented films; Plasmonics; Hexagonal arrays; Rectangular arrays",
    	author = "Svavarsson, H.G. and Yoon, J.W. and Shokooh-Saremi, M. and Song, S.H. and Magnusson, R.",
    	pages = "653-657",
    	abstract = "Fabrication of plasmonic resonance devices composed of large arrays of highly ordered gold nanocups is presented. The nanostructures are generated from periodic photoresist templates created by interference lithography and subsequent reflow, deposition, and dislodging. The nanocups are hemispherical in shape and arranged in both rectangular and hexagonal arrays with periods of ~500 nm. Their ability to support surface plasmonic resonances is manifested experimentally by reflectance spectroscopy. Theoretical modeling to ascertain the plasmonic spectra of these nanostructures is performed. The computed spectra of the rectangular structure are in qualitative agreement with the measurements. A weaker correlation observed for the hexagonal structure is explained by its more intricate symmetry which complicates the spectral response.",
    	language = "English",
    	pdf = "http://optics.hanyang.ac.kr/~shsong/Plasmonics%202012-03-Fabrication%20and%20Characterization%20of%20Large,%20Perfectly%20Periodic%20Arrays%20of%20Metallic%20Nanocups.pdf"
    }
    
  21. R Magnusson, H G Svavarsson, J Yoon, M Shokooh-Saremi and S H Song.
    Experimental observation of leaky modes and plasmons in a hybrid resonance element.
    Applied Physics Letters 100, 091106-091106-3 (2012).
    Abstract 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.
    PDF, DOI BibTeX

    @article{6163058,
    	author = "Magnusson, R. and Svavarsson, H. G. and Yoon, J. and Shokooh-Saremi, M. and Song, S. H.",
    	journal = "Applied Physics Letters",
    	title = "Experimental observation of leaky modes and plasmons in a hybrid resonance element",
    	year = 2012,
    	volume = 100,
    	number = 9,
    	pages = "091106-091106-3",
    	keywords = "light polarisation;polaritons;surface plasmon resonance;7320Mf",
    	doi = "10.1063/1.3690951",
    	issn = "0003-6951",
    	abstract = "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.",
    	pdf = "http://optics.hanyang.ac.kr/~shsong/APL%202012-02-Experimental%20observation%20of%20leaky%20modes%20and%20plasmons%20in%20a%20hybrid%20resonance%20element.pdf"
    }
    
  22. Halldor Gudfinnur Svavarsson, JaeWoong Yoon, SeokHo Song and Robert Magnusson.
    Fabrication of Large Plasmonic Arrays of Gold Nanocups Using Inverse Periodic Templates.
    Plasmonics 6, 741-744.
    Abstract A facile procedure to fabricate large arrays of highly ordered metal nanocups, 250 nm in diameter, is reported. The nanostructure is generated from periodic photoresist templates created by holographic laser interference lithography. A subsequent gold deposition and a peeling-off step respectively results in a large area of hemispherical nano-indentations or nanocups. A wide range of coating materials can be used, and the dimensions and periodicity of the structure are easily controlled. The structure’s ability to support localized surface plasmon polaritons was manifested by reflectance spectroscopy. A good correlation between experimental data and calculated data was observed.
    URL, DOI BibTeX

    @article{,
    	svavarsson2011 year = 2011,
    	issn = "1557-1955",
    	journal = "Plasmonics",
    	volume = 6,
    	number = 4,
    	doi = "10.1007/s11468-011-9258-8",
    	title = "Fabrication of Large Plasmonic Arrays of Gold Nanocups Using Inverse Periodic Templates",
    	url = "http://dx.doi.org/10.1007/s11468-011-9258-8",
    	publisher = "Springer US",
    	keywords = "Nanocups; Periodic arrays; Nano-indented films; Plasmonics",
    	author = "Svavarsson, Halldor Gudfinnur and Yoon, JaeWoong and Song, SeokHo and Magnusson, Robert",
    	pages = "741-744",
    	abstract = "A facile procedure to fabricate large arrays of highly ordered metal nanocups, 250 nm in diameter, is reported. The nanostructure is generated from periodic photoresist templates created by holographic laser interference lithography. A subsequent gold deposition and a peeling-off step respectively results in a large area of hemispherical nano-indentations or nanocups. A wide range of coating materials can be used, and the dimensions and periodicity of the structure are easily controlled. The structure’s ability to support localized surface plasmon polaritons was manifested by reflectance spectroscopy. A good correlation between experimental data and calculated data was observed.",
    	language = "English"
    }
    
  23. D M Danielsson, J T Gudmundsson and H G Svavarsson.
    Effect of hydrogenation on minority carrier lifetime in low-grade silicon.
    Physica Scripta 2010, 014005 (2010).
    Abstract 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 H 2 /Ar plasma resulted in an active photovoltaic device with V OC 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.
    URL BibTeX

    @article{1402-4896-2010-T141-014005,
    	author = "D M Danielsson and J T Gudmundsson and H G Svavarsson",
    	title = "Effect of hydrogenation on minority carrier lifetime in low-grade silicon",
    	journal = "Physica Scripta",
    	volume = 2010,
    	number = "T141",
    	pages = 014005,
    	url = "http://stacks.iop.org/1402-4896/2010/i=T141/a=014005",
    	year = 2010,
    	abstract = "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 H 2 /Ar plasma resulted in an active photovoltaic device with V OC 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."
    }