| 1. |
- Nowzari, Ali, et al.
(författare)
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A Comparative Study of Absorption in Vertically and Laterally Oriented InP Core–Shell Nanowire Photovoltaic Devices
- 2015
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Ingår i: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 15:3, s. 1809-1814
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Tidskriftsartikel (refereegranskat)abstract
- We have compared the absorption in InP core-shell nanowire p-i-n junctions in lateral and vertical orientation. Arrays of vertical core-shell nanowires with 400 nm pitch and 280 nm diameter, as well as corresponding lateral single core-shell nanowires, were configured as photovoltaic devices. The photovoltaic characteristics of the samples, measured under 1 sun illumination, showed a higher absorption in lateral single nanowires compared to that in individual vertical nanowires, arranged in arrays with 400 nm pitch. Electromagnetic modeling of the structures confirmed the experimental observations and showed that the absorption in a vertical nanowire in an array depends strongly on the array pitch. The modeling demonstrated that, depending on the array pitch, absorption in a vertical nanowire can be lower or higher than that in a lateral nanowire with equal absorption predicted at a pitch of 510 nm for our nanowire geometry. The technology described in this Letter facilitates quantitative comparison of absorption in laterally and vertically oriented core-shell nanowire p-i-n junctions and can aid in the design, optimization, and performance evaluation of nanowire-based core-shell photovoltaic devices. © 2014 American Chemical Society.
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| 2. |
- Borggren, Jesper, et al.
(författare)
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Diode laser-based thermometry using two-line atomic fluorescence of indium and gallium
- 2017
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Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 123:12
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Tidskriftsartikel (refereegranskat)abstract
- A robust and relatively compact calibration-free thermometric technique using diode lasers two-line atomic fluorescence (TLAF) for reactive flows at atmospheric pressures is investigated. TLAF temperature measurements were conducted using indium and, for the first time, gallium atoms as temperature markers. The temperature was measured in a multi-jet burner running methane/air flames providing variable temperatures ranging from 1600 to 2000 K. Indium and gallium were found to provide a similar accuracy of ~ 2.7% and precision of ~ 1% over the measured temperature range. The reliability of the TLAF thermometry was further tested by performing simultaneous rotational CARS measurements in the same experiments.
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| 3. |
- Brackmann, Christian, et al.
(författare)
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Experimental studies of nitromethane flames and evaluation of kinetic mechanisms
- 2018
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 190, s. 327-336
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Hosseinnia, Ali, et al.
(författare)
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Absorption through a coupled optical resonance in a horizontal InP nanowire array
- 2015
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Ingår i: Photonics Research. - 2327-9125. ; 3:4, s. 125-128
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Tidskriftsartikel (refereegranskat)abstract
- We study through electromagnetic modeling the absorption of light of a given wavelength in an array of horizontal InP nanowires of diameter less than 100 nm. Such absorption is performed most efficiently by using polarized light and by exciting a coupled optical resonance in a sparse array. In that case, we excite a resonance in the individual nanowires and couple the resonances in neighboring nanowires through a lattice resonance of the periodic array. At such a resonance, an array with nanowires of 80 nm in diameter can absorb more than eight times more strongly than a tight-packed array, despite containing a seven times smaller amount of the absorbing InP material. (C) 2015 Chinese Laser Press
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| 5. |
- Hosseinnia, Ali, et al.
(författare)
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Ethane thermometry using rotational coherent anti-Stokes Raman scattering (CARS)
- 2017
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4461-4468
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Tidskriftsartikel (refereegranskat)abstract
- The complete rotational CARS (coherent anti-Stokes Raman scattering) spectrum of ethane (C2H6) has for the first time been recorded instantaneously under high spectral resolution, and the potential for thermometry has been investigated. Experiments were performed in the temperature range 292-650K in pure ethane and in binary mixtures with nitrogen. A polarization technique was used to suppress the non-resonant contribution to the CARS signal. The ethane RCARS spectra show both S- and R-branch lines, which are more closely spaced than for the well-known nitrogen spectrum and located at much smaller Raman shifts. The peak signal strength was found to be around 240 times lower for ethane than for nitrogen (at 292K). Two main approaches for ethane thermometry are evaluated, which both show high potential. The first is a method in which a spectrum with unknown temperature is fitted using a library of experimental spectra recorded at various temperatures. The second is a method based on ratios of integrated signals in different spectral regions. A theoretical model for simulation of theoretical spectra is under development.
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| 6. |
- Hosseinnia, Ali, et al.
(författare)
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Femtosecond rotational CARS with quantum control of molecular rotation
- 2022
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Ingår i: Optical Sensors and Sensing Congress 2022 (AIS, LACSEA, Sensors, ES). - 9781557528209
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Konferensbidrag (refereegranskat)abstract
- Quantum control is implemented in a 2-beam fs/ns rotational CARS setup via a second control pulse with variable delay. The potential of the concept for improved diagnostics is demonstrated by species-selective enhancement/annihilation via field-free alignment.
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| 7. |
- Hosseinnia, Ali
(författare)
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On the Use of Rotational CARS on Polyatomics and in Time Domain
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work concerns the further developments of dual-broadband rotational coherent anti-Stokes Raman spectroscopy (RCARS) through experimental and theoretical investigations. Rotational CARS is an established technique for gas-phase thermometry and concentration measurement with major applications in combustion diagnostics. The technique has previously mainly been applied on species with linear molecular symmetry, for which the calculations of their spectra are relatively straightforward.The developments in the present thesis can be divided in two main categories. The first part of the thesis deals with the extension of nanosecond (ns) RCARS diagnostics towards two hydrocarbon fuels with complex molecular symmetries. The first is the symmetric-top molecule ethane (C2H6), and the second is the asymmetric top ethylene (C2H4).In the ethane case, RCARS spectra were recorded in pure ethane, as well as in different mixtures of ethane/nitrogen (N2), at atmospheric pressure and at temperatures between 300 K and 650 K. It was demonstrated that the rigid-rotor assumption was not accurate for successful calculations of the ethane spectra at the higher temperatures, and that the effects of internal rotation should be taken into account. A novel method based on Fourier analysis was developed for evaluation of RCARS spectra of ethane for thermometry and for concentration measurements in binary mixtures with nitrogen. The method was successfully demonstrated for thermometry and concentration measurements in an ethane diffusion flame.RCARS spectra of ethylene were recorded at atmospheric pressure and temperatures between 300 K and 800 K. A theoretical model was developed that could reproduce spectra in good agreement with the experimentally recorded data. Furthermore, linewidth parameters of ethylene were estimated for the first time through comparison of the experimental and theoretically calculated spectra.In the second part of the thesis, a novel technique was developed for time-domain studies of molecular coherences using femtosecond (fs)/ns RCARS. All the rotational Raman coherences of molecular species were prepared by two Fourier-limited fs pulses and probed by a single-mode ns pulse. The generated CARS signal was dispersed in a spectrometer and recorded by a streak camera. This combination resulted in simultaneous resolving of the coherences in spectral and temporal domain. Through this novel approach we demonstrated single-shot Raman linewidth measurements of N2 in mixtures with C2H4 and C2H6. Moreover, a theoretical model for time-resolved RCARS of N2 was developed that exploits the measured Raman linewidth data for single-shot thermometry using spectral fitting. This is an essential improvement in N2 RCARS thermometry, since it removes the limitations regarding the necessary pre-knowledge about the number density of the species in the mixtures and their impact on N2 linewidths.
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| 8. |
- Hosseinnia, Ali, et al.
(författare)
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Pure rotational coherent anti-Stokes Raman spectroscopy of ethylene, experiments and modelling
- 2019
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 234, s. 24-31
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Tidskriftsartikel (refereegranskat)abstract
- Rotational coherent anti-Stokes Raman spectroscopy (CARS) is an established diagnostic technique for temperature and concentration measurements in combustion-related experiments. CARS has mainly been applied on di- and tri-atomics with linear molecular symmetry, for which good agreement is achieved between experimental and theoretical spectra. Hydrocarbons have so far been less attractive for rotational CARS studies, partly because of their complex molecular symmetries making it challenging to obtain molecular parameters necessary for a successful modeling. Nevertheless, in this work we have studied the asymmetric top molecule ethylene (C2H4) using rotational CARS through experiments and theoretical modelling. Experimental spectra have been recorded at temperatures between 293 K and 804 K, and theoretical spectra have been calculated with a novel method for which the crucial temperature-dependent linewidth parameters were determined by comparison of experimental and theoretically calculated spectra. Line-mixing effects and temperature dependence of the isolated linewidths were studied using a semi-classical approach considering an exponential gap law and a scaling law. Subsequently, theoretically calculated spectra were used to evaluate the temperatures of the experimental spectra for validation of the method. The evaluated temperatures were in good agreement with thermocouple temperatures with differences up to ±7 K. Hence, this work not only investigates the potential of using rotational CARS on ethylene for diagnostic purposes, but also expands the applicability of the technique to a whole new class of molecules with asymmetric-top molecular symmetry.
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| 9. |
- Hosseinnia, Ali, et al.
(författare)
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Rotational CARS thermometry and concentration measurements in ethane-nitrogen mixtures using Fourier analysis
- 2018
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486. ; 49:7, s. 1096-1108
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Tidskriftsartikel (refereegranskat)abstract
- Rotational coherent anti-Stokes Raman scattering (CARS) is an established diagnostic technique for thermometry and species concentration measurements of di-atomics and tri-atomics in combustion processes; however, studies on hydrocarbon fuel molecules have been scarce. In this work, we have investigated the diagnostic potential for rotational CARS on ethane with emphasis on concentration measurements in mixtures with nitrogen. Rotational CARS spectra have periodic line structures, and because ethane and nitrogen spectra partially overlap, a novel Fourier analysis approach was developed and applied to facilitate evaluation of relative concentrations and temperatures. Instead of the standard procedure of spectral fitting using libraries of theoretically calculated spectra, the fitting was based on an experimental database of temperatures and relative ethane/nitrogen concentrations. The method was successfully demonstrated in an ethane diffusion flame where ethane, initially at room temperature, is heated downstream as well as mixed with increasing amounts of nitrogen. The evaluated temperatures and concentrations are in good agreement with computational fluid dynamic model simulations.
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| 10. |
- Hosseinnia, Ali, et al.
(författare)
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Simultaneous temporally and spectrally resolved Raman coherences with single-shot fs/ns rotational CARS
- 2020
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Ingår i: Optics Letters. - 0146-9592. ; 45:2, s. 308-311
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Tidskriftsartikel (refereegranskat)abstract
- A novel technique for studies of the dynamics of molecular coherences has been developed. The concept is based on prompt excitation using broadband femtosecond (fs) laser pulses, whereupon a narrowband nanosecond (ns) laser pulse probes the fast dynamics of the coherences. Detection of the spectrally dispersed coherent signal using a streak camera allows simultaneous spectrally and temporally resolved studies of all excited coherences in a single-shot acquisition. Here we demonstrate the high capacity and versatility of this hybrid fs/ns coherent anti-Stokes Raman scattering (CARS) technique through some illustrative examples, all single-shot measurements, namely time-resolved studies of rotational Raman coherences in nitrogen and air, dynamics of Stark effect on rotational lines, and beating phenomena originating from close-lying rotational lines.
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