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  • A Atlasov, Kirill, et al. (författare)
  • 1D photonic band formation and photon localization in finite-size photonic-crystal waveguides
  • 2010
  • Ingår i: OPTICS EXPRESS. - 1094-4087. ; 18:1, s. 117-122
  • Tidskriftsartikel (refereegranskat)abstract
    • A transition from discrete optical modes to 1D photonic bands is experimentally observed and numerically studied in planar photonic-crystal (PhC) L-N microcavities of length N. For increasing N the confined modes progressively acquire a well-defined momentum, eventually reconstructing the band dispersion of the corresponding waveguide. Furthermore, photon localization due to disorder is observed experimentally in the membrane PhCs using spatially resolved photoluminescence spectroscopy. Implications on single-photon sources and transfer lines based on quasi-1D PhC structures are discussed.
  • Abd Alrahman, Chadi, et al. (författare)
  • Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame
  • 2014
  • Ingår i: Optics Express. - 1094-4087. ; 22:11, s. 13889-13895
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate near-infrared cavity-enhanced optical frequency comb spectroscopy of water in a premixed methane/air flat flame. The detection system is based on an Er:fiber femtosecond laser, a high finesse optical cavity containing the flame, and a fast-scanning Fourier transform spectrometer (FTS). High absorption sensitivity is obtained by the combination of a high-bandwidth two-point comb-cavity lock and auto-balanced detection in the FTS. The system allows recording high-temperature water absorption spectra with a resolution of 1 GHz and a bandwidth of 50 nm in an acquisition time of 0.4 s, with absorption sensitivity of 4.2 x 10 (9) cm(-1) Hz(-1/2) per spectral element.
  • Abrahamsson, Christoffer, et al. (författare)
  • Time and wavelength resolved spectroscopy of turbid media using light continuum generated in a crystal fiber
  • 2004
  • Ingår i: Optics Express. - Optical Soc. America. - 1094-4087. ; 12:17, s. 4103-4112
  • Tidskriftsartikel (refereegranskat)abstract
    • We report a novel system for time-resolved diffuse remission spectral measurements, based on short light continuum pulses generated in an index-guided crystal fiber, and a spectrometer-equipped streak camera. The system enables spectral recordings of absorption and reduced scattering coefficients of turbid media in the wavelength range 500 - 1200 nm with a spectral resolution of 5 nm and a temporal resolution of 30 ps. The optical properties are calculated by fitting the solution of the diffusion equation to the time-dispersion curve at each wavelength. Example measurements are presented from an apple, a finger and a pharmaceutical tablet. (C) 2004 Optical Society of America.
  • Aghaeipour, Mahtab, et al. (författare)
  • Tunable absorption resonances in the ultraviolet for InP nanowire arrays
  • 2014
  • Ingår i: Optics Express. - Optical Soc Amer. - 1094-4087. ; 22:23, s. 29204-29212
  • Tidskriftsartikel (refereegranskat)abstract
    • The ability to tune the photon absorptance spectrum is an attracting way of tailoring the response of devices like photodetectors and solar cells. Here, we measure the reflectance spectra of InP substrates patterned with arrays of vertically standing InP nanowires. Using the reflectance spectra, we calculate and analyze the corresponding absorptance spectra of the nanowires. We show that we can tune absorption resonances for the nanowire arrays into the ultraviolet by decreasing the diameter of the nanowires. When we compare our measurements with electromagnetic modeling, we generally find good agreement. Interestingly, the remaining differences between modeled and measured spectra are attributed to a crystal-phase dependence in the refractive index of InP. Specifically, we find indication of significant differences in the refractive index between the modeled zinc-blende InP nanowires and the measured wurtzite InP nanowires in the ultraviolet. We believe that such crystal-phase dependent differences in the refractive index affect the possibility to excite optical resonances in the large wavelength range of 345 < lambda < 390 nm. To support this claim, we investigated how resonances in nanostructures can be shifted in wavelength by geometrical tuning. We find that dispersion in the refractive index can dominate over geometrical tuning and stop the possibility for such shifting. Our results open the door for using crystal-phase engineering to optimize the absorption in InP nanowire-based solar cells and photodetectors. (C) 2014 Optical Society of America
  • Ako, Thomas, et al. (författare)
  • Design of invisibility cloaks with an open tunnel
  • 2010
  • Ingår i: Optics Express. - 1094-4087. ; 18:26, s. 27060-27066
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we apply the methodology of transformation optics for design of a novel invisibility cloak which can possess an open tunnel. Such a cloak facilitates the insertion (retrieval) of matter into (from) the cloak's interior without significantly affecting the cloak's performance, overcoming the matter exchange bottleneck inherent to most previously proposed cloak designs. We achieve this by applying a transformation which expands a point at the origin in electromagnetic space to a finite area in physical space in a highly anisotropic manner. The invisibility performance of the proposed cloak is verified by using full-wave finite-element simulations. (C) 2010 Optical Society of America
  • Akram, Muhammad Nadeem, et al. (författare)
  • Influence of base-region thickness on the performance of Pnp transistor-VCSEL
  • 2014
  • Ingår i: Optics Express. - 1094-4087. ; 22:22, s. 27398-27414
  • Tidskriftsartikel (refereegranskat)abstract
    • We have recently reported a 980nm GaAs-based three terminal Pnp transistor-vertical-cavity surface-emitting laser (TVCSEL) operating at room temperature with optical power up to 1.8mW. However, the current gain beta = Delta I-c/Delta I-b was near zero just before lasing and became negative after the lasing threshold. The main cause of the negative current gain was found to be a gradual and position-dependent forward-biasing (saturation) of the base-collector junction with increasing bias even before lasing threshold. In this article, detailed multi-physics device simulations are performed to better understand the device physics, and find ways to avoid the premature saturation of the base-collector junction. We have optimized the thickness of the base region as well as its doping concentration and the location of the quantum wells to ensure that the T-VCSEL is in the active mode throughout its range of operation. That is, the emitter-base junction is forward biased and base-collector junction is reversed biased for sweeping the excess charges out of the base region.
  • Alerstam, Erik, et al. (författare)
  • Improved accuracy in time-resolved diffuse reflectance spectroscopy.
  • 2008
  • Ingår i: Optics express. - Optical Society of America. - 1094-4087. ; 16:14, s. 10440-10454
  • Tidskriftsartikel (refereegranskat)abstract
    • Significant improvements in the accuracy of time-resolved diffuse reflectance spectroscopy are reached by using a Monte Carlo scheme for evaluation of measured photon time-of-flight distributions. The use of time-resolved diffusion theory of photon migration, being the current standard scheme for data evaluation, is shown defective. In particular, the familiar problem sometimes referred to as absorption-to-scattering coupling or crosstalk, is identified as an error related to the breakdown of the diffusion approximation. These systematic errors are investigated numerically using Monte Carlo simulations, and their influence on data evaluation of experimental recordings are accurately predicted. The proposed Monte Carlo-based data evaluation avoids these errors, and can be used for routine data evaluation. The accuracy and reproducibility of both MC and diffusion modeling are investigated experimentally using the MEDPHOT set of solid tissue-simulating phantoms, and provides convincing arguments that Monte Carlo-based evaluation is crucial in important ranges of optical properties. In contrast to diffusion-based evaluation, the Monte Carlo scheme results in optical properties consistent with phantom design. Since the MEDPHOT phantoms are used for international comparisons and performance assessment, the performed characterization is carefully reported.
  • Amer, Eynas, et al. (författare)
  • UV laser interaction with a fluorescent dye solution studied using pulsed digital holography
  • 2013
  • Ingår i: Optics Express. - Optical Society of America. - 1094-4087. ; 21:21
  • Tidskriftsartikel (refereegranskat)abstract
    • A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. Simultaneously, a frequency doubled pulse (532 nm) from the same laser is used to probe the solvent perpendicularly resulting in a gain through stimulated laser induced fluorescence (LIF) emission. The resulting gain of the probe beam is recorded using digital holography by blending it with a reference beam on the detector. Two digital holograms without and with the pump beam were recorded. Intensity maps were calculated from the recorded digital holograms and used to calculate the gain of the probe beam due to the stimulated LIF. In addition numerical data of the local temperature rise was calculated from the corresponding phase maps using Radon inversion. It was concluded that about 15% of the pump beam energy is transferred to the dye solution as heat while the rest is consumed in the radiative process. The results show that pulsed digital holography is a promisingtechnique for quantitative study of fluorescent species
  • Andersson, Mats, et al. (författare)
  • Spectroscopic studies of wood-drying processes
  • 2006
  • Ingår i: Optics Express. - Optical Society of America. - 1094-4087. ; 14:8, s. 3641-3653
  • Tidskriftsartikel (refereegranskat)abstract
    • By the use of wavelength-modulation diode laser spectroscopy, water vapor and oxygen are detected in scattering media nonintrusively, at 980 nm and 760 nm, respectively. The technique demonstrated is based on the fact that free gases have extremely sharp absorption structures in comparison with the broad features of bulk material. Water vapor and oxygen measurements have been performed during the drying process of wood. The results suggest that the demonstrated technique can give information about the drying process of wood to complement that of commercially available moisture meters. In particular, the time when all the free water has evaporated from the wood can be readily identified by a strong falloff in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal. Furthermore, the same point is identified in the differential optical absorption signal for liquid water, with a sharp increase by an order of magnitude in the ratio of the signal intensities at 980 nm and 760 nm.
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