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- 2019
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Journal article (peer-reviewed)
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| 3. |
- Bengtsson, Alexander, et al.
(author)
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Characterization and modeling of acousto-optic signal strengths in highly scattering media
- 2019
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In: Biomedical Optics Express. - 2156-7085. ; 10:11, s. 5565-5584
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Journal article (peer-reviewed)abstract
- Ultrasound optical tomography (UOT) is an imaging technique based on the acousto-optic effect that can perform optical imaging with ultrasound resolution inside turbid media, and is thus interesting for biomedical applications, e.g. for assessing tissue blood oxygenation. In this paper, we present near background free measurements of UOT signal strengths using slow light filter signal detection. We carefully analyze each part of our experimental setup and match measured signal strengths with calculations based on diffusion theory. This agreement between experiment and theory allows us to assert the deep tissue imaging potential of similar to 5 cm for UOT of real human tissues predicted by previous theoretical studies [Biomed. Opt. Express 8, 4523 (2017)] with greater confidence, and indicate that future theoretical analysis of optimized UOT systems can be expected to be reliable. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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| 4. |
- Bengtsson, Jenny, et al.
(author)
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Impact of non-solvents in the tetrabutylammonium acetate : dimethyl sulfoxide-cellulose system
- 2016
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In: The 7th Workshop on cellulose, regenerated cellulose and cellulose derivatives. ; , s. 19-22
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Conference paper (other academic/artistic)abstract
- This work examines the potential of tetrabutylammonium acetate: dimethyl sulfoxide (TBAAc:DMSO) as a solvent used in a process for producing man-made cellulose fibers. The tolerance towards nonsolvents is an important step to evaluate the recyclability of the solvent. TBAAc:DMSO was in this work further confirmed to be an efficient solvent for cellulose. Non-solvent tolerance depended on cellulose concentration, TBAAc:DMSO ratio and type of non-solvent. There was no significant change in mechanical properties for filaments regenerated from solutions containing 2 wt% non-solvent compared to those spun from virgin solvent. With 4 wt% ethanol present in solution very brittle filaments were produced, not suitable for use as textile fibers.
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- Bergmann, Michael Alexander, 1989, et al.
(author)
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Towards ultraviolet and blue microcavity lasers
- 2018
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In: Northen Optics and Photonics conference. - 9789163964886 ; 2018
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Conference paper (peer-reviewed)abstract
- The development of III-nitride-based (Al,Ga,In(N)) microcavity lasers is a challenging task. Significant progress in recent years has resulted in realizations of electrically pumped devices with optical output power in the mW-range and with threshold current densities below 20 kA/cm2. However, to become practical, the lifetime and power conversion efficiency of these devices must be improved. Among the challenges are achieving transverse optical mode confinement, highreflectivity mirrors and control over the resonator length. We will highlight our theoretical work on transverse optical mode confinement, emphasising the overwhelming risk of ending up with an optically anti-guided cavity, and its consequences such as very high optical losses that easily could double the threshold gain for lasing. We will show some anti-guided cavities with reasonable threshold gain and built-in modal discrimination. However, all anti-guided cavities are very sensitive to temperature effects and small structural changes in the cavity caused by fabrication imperfections. We have explored electrically conductive distributed Bragg reflectors (DBRs) in both AlN/GaN and ZnO/GaN. The AlN/GaN DBRs were grown with different strain-compensating interlayers, and the DBR without interlayers had the lowest vertical resistivity with a specific series resistance of 0.044 cmfor eight DBRpairs. In the ZnO/GaN DBR, the measured resistance was dominated by lateral and contact contributions, setting a lower measurable limit of ~10 for three DBR-pairs. Numerical simulations show the importance of having in-plane strained layers in the ZnO/GaN DBR, since that leads to cancellation of the spontaneous and piezoelectric polarization. This results in a dramatically reduced vertical resistance, potentially three orders of magnitude lower than what could be measured. cm An alternative to an epitaxially grown DBR is a dielectric DBR, which offers high reflectivity over a broader wavelength range, relaxing the requirements on resonator length control. To deposit a dielectric DBR on the bottom side of the cavity, the sample must first be bonded to a carrier wafer before the substrate can be removed. We used thermocompression gold-gold bonding to successfully bond the laser structure to a Si carrier wafer. The subsequent substrate removal is a challenging process due to the chemical inertness of the III-nitride-based materials. A doping-dependent electrochemical etch technique was used, which allows for the selective removal of a sacrificial (n-doped) layer between the cavity and the substrate. This resulted in nm-precise cavity lift-off with a low root-mean-square surface roughness down to 0.3 nm. Thus, the process is suitable for the fabrication of high-quality optical devices such as microcavity lasers. In addition, the technique offers a new alternative to create III-nitridebased optical resonators, mechanical resonators, thin film LEDs and transistors.
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| 7. |
- Brackmann, Christian, et al.
(author)
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Experimental studies of nitromethane flames and evaluation of kinetic mechanisms
- 2018
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In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 190, s. 327-336
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Journal article (peer-reviewed)abstract
- The present work reports new experimental data for premixed flames of nitromethane, CH3NO2, at atmospheric pressure, and an evaluation of two contemporary kinetic mechanisms based on these new flame studies as well as previously published experimental data on laminar burning velocity and ignition. Flames of nitromethane + air at lean (ϕ = 0.8) and rich (ϕ = 1.2) conditions were stabilized on a flat-flame burner, where profiles of CH2O, CO and NO were obtained using laser-induced fluorescence and temperature profiles using coherent anti-Stokes Raman spectroscopy. Laminar burning velocities for nitromethane + O2 + CO2 were measured using the heat flux method for ϕ = 0.8–1.3 at 348 K and ϕ = 0.8–1.6 at 358 K, and an oxidizer composition of 35% O2 and 65% CO2. In addition, the effect of the oxidizer composition was examined for a stoichiometric flame at 358 K by varying oxygen fraction from 30% to 40%. The mechanism by Mathieu et al. (Fuel 2016, 182, 597), previously not validated for flames, was able to reproduce experimental laminar burning velocities for nitromethane + air, but under predicted new results for CH3NO2 + O2 + CO2 mixtures. The mechanism by Brequigny et al. (Proc. Combust. Inst. 2014, 35, 703) under predicted experimental laminar burning velocities significantly at all investigated conditions. Previous studies have shown that none of the mechanisms can accurately predict ignition delay time over a wide range of conditions with respect to pressure, temperature, diluent and dilution ratio. The evaluation of the mechanisms reveals that the understanding of nitromethane combustion is at the present time not sufficient to produce a widely applicable mechanism.
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| 8. |
- Brackmann, Christian, et al.
(author)
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Structure of premixed ammonia plus air flames at atmospheric pressure: Laser diagnostics and kinetic modeling
- 2016
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In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 163, s. 370-381
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Journal article (peer-reviewed)abstract
- The structure of premixed ammonia air flames, burning at atmospheric pressure under strain-stabilized conditions on a porous-plug burner, has been investigated using laser-diagnostic methods. Profiles of OH, NH, and NO were acquired by laser-induced fluorescence (LIF) and quantitative concentrations of OH and NO were retrieved using a concept for calibration versus absorption utilizing the LIF-signal itself, whereas NH concentrations were evaluated employing a saturated fluorescence signal. In addition, temperatures and relative oxygen concentrations were measured by rotational Coherent Anti-stokes Raman Spectroscopy (CARS). The new experimental data for flames with equivalence ratios of 0.9, 1.0, and 1.2 were used to validate and rank the performance of four contemporary detailed kinetic models. Simulations were carried out using experimental temperature profiles as well as by solving the energy equation. Two models of the same origin, developed by Mendiara and Glarborg (2009) and by Klippenstein et al. (2011), in most cases showed good agreement in terms of radical concentrations, however, the model of Mendiara and Glarborg had better prediction of temperatures and flame front positions. The model by Shmakov et al. (2010) had comparable performance concerning radical species, but significant discrepancies appeared in the prediction of flame front positions. The model of Duynslaegher et al. (2012), in addition to the flame front positions, deviated from experiments or other models in terms of NH and NO concentrations. A sensitivity analysis for the Mendiara-Glarborg mechanism indicated that remaining uncertainties of the rate constants implemented in the recent H/N/O models are difficult to scrutinize unambiguously due to experimental uncertainties. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 9. |
- Cedergren, Alexander, et al.
(author)
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Understanding practical challenges to risk and vulnerability assessments : the case of Swedish municipalities
- 2019
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In: Journal of Risk Research. - : Informa UK Limited. - 1366-9877 .- 1466-4461. ; 22:6, s. 782-795
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Journal article (peer-reviewed)abstract
- Assessing risks and vulnerabilities in the multifaceted context of a municipality is challenging. There is a plethora of methods and tools to support this task. However, to achieve successful outcomes from Risk and Vulnerability Assessments (RVAs), merely applying theoretically and technically rigorous methods is far from enough; the question of how a method is implemented in a practical context is crucial for reaching intended outcomes. In this paper, we broaden the scope, from a strict consideration of technical and methodological matters, to a presentation of the more elusive challenges in accomplishing RVAs that are actually useful in a local context. We conduct and contrast two independent studies that were aimed at identifying general challenges in achieving a successful RVA process in Swedish municipalities. Based on the findings, we argue that there is a need for a considerably increased focus on municipal practices and on the implementation of methods in the practical context. Knowledge and understanding of such practical challenges are pivotal for efforts aimed at both developing methods for RVA and improving the entire RVA process.
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