S. R. Keiding

Aarhus University, Aarhus, Central Jutland, Denmark

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Publications (122)269.39 Total impact

  • Optics Express 05/2015; 23(10):13141. DOI:10.1364/OE.23.013141 · 3.53 Impact Factor
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    ABSTRACT: The vibrational energy relaxation following UV excitation of nitrate ions in aqueous solution is investigated by infrared transient absorption spectroscopy. The experiments show that after their return to the electronic ground state the nitrate ions relax to the vibration ground state with a time constant of 2 ps. The experimental data are compared to simulations of two relaxation scenarios in which the nitrate molecules either relax by intra-molecular vibrational redistribution or by inter-molecular energy transfer. The comparison suggests that the energy relaxation is dominated by the latter.
    Chemical Physics 10/2014; 442. DOI:10.1016/j.chemphys.2014.03.014 · 2.03 Impact Factor
  • J. Ramsay, S. Dupont, S. R. Keiding
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    ABSTRACT: Infrared supercontinuum sources suffer from amplitude and polarization fluctuations. These fluctuations are seeded by stochastic noise, and thus limits the applicability of such sources. Here it is shown that implementation of polarization insensitive pulse-to-pulse normalization enhances the signal-to-noise ratio up to 18 times compared to conventional 45° beam splitting. This serves as a promising approach to achieve highly sensitive supercontinuum spectroscopy.
    Laser Physics Letters 09/2014; 11(9-9):095702. DOI:10.1088/1612-2011/11/9/095702 · 2.96 Impact Factor
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    ABSTRACT: We investigate the formation of aqueous nitrogen dioxide, NO2 formed through femtosecond photolysis of nitrate, [Formula: see text] and nitromethane CH3NO2(aq). Common to the experiments is the observation of a strong induced absorption at 1610 ± 10 cm(-1), assigned to the asymmetric stretch vibration in the ground state of NO2. This assignment is substantiated through isotope experiments substituting (14)N by (15)N, experiments at different pH values, and by theoretical calculations and simulations of NO2-D2O clusters.
    The Journal of Chemical Physics 08/2014; 141(6):064310. DOI:10.1063/1.4892342 · 3.12 Impact Factor
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    ABSTRACT: We report the use of a dispersed supercontinuum generated in an all-normal-dispersion fibre to record low-noise spectra from atmospheric molecules at least an order of magnitude faster than has been previously reported. Supercontinuum generation in standard, anomalous dispersion photonic-crystal fibres is inherently connected with large pulse-to-pulse fluctuations resulting in detrimental consequences for high resolution spectroscopy if temporal averaging is not permitted. Replacing the standard photonic-crystal fibre (PCF) with a specially designed all-normal dispersion PCF we find that a substantially superior noise performance is achieved and present its use for high repetition rate absorption spectroscopy where spectra covering hundreds of nm in spectral bandwidth can be captured of gases at hundreds of kHz repetition rates.
    Laser Physics Letters 05/2014; 11(7):075601. DOI:10.1088/1612-2011/11/7/075601 · 2.96 Impact Factor
  • Jacob Ramsay, Sune Dupont, Søren R. Keiding
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    ABSTRACT: We present an cross-correlation frequency-resolved optical gating (XFROG) measurement of a megahertz IR supercontinuum generated in a step-index ZBLAN fiber. The resulting spectrogram gives the dispersion characteristics of the fiber and reveals that it has three zero-dispersion wavelengths. A comparison of the measured spectrogram with numerical simulations shows that this dispersion profile allows a notable dispersive-wave generation toward long wavelengths. Furthermore, the sum-frequency generation process in the XFROG measurement gives the possibility of measuring the IR light with fast Si-based detectors, such as CCD arrays.
    Journal of the Optical Society of America B 10/2013; 30(10):2570-2575. DOI:10.1364/JOSAB.30.002570 · 1.81 Impact Factor
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    ABSTRACT: Using femtosecond upconversion we investigate the time and wavelength structure of infrared supercontinuum generation. It is shown that radiation is scattered into higher order spatial modes (HOMs) when generating a supercontinuum using fibers that are not single-moded, such as a step-index ZBLAN fiber. As a consequence of intermodal scattering and the difference in group velocity for the modes, the supercontinuum splits up spatially and temporally. Experimental results indicate that a significant part of the radiation propagates in HOMs. Conventional simulations of super-continuum generation do not include scattering into HOMs, and including this provides an extra degree of freedom for tailoring supercontinuum sources.
    Optics Express 05/2013; 21(9):10764-71. DOI:10.1364/OE.21.010764 · 3.53 Impact Factor
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    ABSTRACT: Photoprotection of the RNA nucleotides adenosine 5′-monophosphate and cytidine 5′-monophosphate, and the nucleobase cytosine was studied using UV pump, IR probe femtosecond transient absorption spectroscopy. The excitation energy is contained in the aromatic ring system, protecting the RNA backbone. All three molecules dissipate the excitation energy by internal conversion and subsequent vibrational relaxation to the electronic ground state in less than 10 ps. In addition, a second deactivation channel is found in cytidine 5′-monophosphate, illustrated by a signal at 1563 cm−1 with a lifetime of 33 ps assigned to an nπ∗ state in agreement with observations in the UV region.
    Chemical Physics Letters 04/2013; 567:50–54. DOI:10.1016/j.cplett.2013.03.001 · 1.99 Impact Factor
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    ABSTRACT: Aqueous nitrate, NO3¯(aq), is studied by 2D-IR, UV-IR and UV-UV time resolved spectroscopy in combination with molecular dynamics (MD) simulations with the purpose of determining the hydration dynamics around the anion. In water the D3h symmetry of NO3¯ is broken and the degeneracy of the asymmetric stretch modes is lifted. This provides a very sensitive probe of the ion-water interactions. The 2D-IR measurements reveal excitation exchange between the two non-degenerate asymmetric stretch vibrations on a 300 fs timescale concomitant with fast anisotropy decay of the diagonal peak signals. The MD simulations show that this is caused by jumps of the transition dipole orientations related to fluctuations of the hydrogen bonds connecting the nitrate ion to the nearest water molecules. Reorientation of the ion, which is associated with the hydrogen bond breaking, is monitored by time resolved UV-IR- and UV-UV spectroscopy, revealing a 2 ps time constant. These time scales are very similar to those reported for isotope-labeled water, suggesting that NO3¯(aq) has a labile hydration shell.
    The Journal of Physical Chemistry B 03/2013; 117(12). DOI:10.1021/jp310090u · 3.38 Impact Factor
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    ABSTRACT: The hydration dynamics of aqueous nitrate, NO3-(aq), is studied by 2D-IR spectroscopy, UV-IR- and UV-UV transient absorption spectroscopy. The experimental results are compared to Car-Parinello molecular dynamics (MD) simulations. The 2D-IR measurements and MD simulations of the non-degenerate asymmetric stretch vibrations of nitrate reveal an intermodal energy exchange occurring on a 0.2 ps time scale related to hydrogen bond fluctuations. The transient absorption measurements find that the nitrate ions rotate in 2 ps. The MD simulations indicate that the ion rotation is associated with the formation of new hydrogen bonds. The 2 ps rotation time thus indicates that the hydration shell of aqueous nitrate is rather labile.
    The European Physical Journal Conferences 03/2013; 41:06002-. DOI:10.1051/epjconf/20134106002
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    ABSTRACT: An experimental strategy for post-eliminating thermal noise on position measurements of optically trapped particles is presented. Using a nanosecond pulsed laser, synchronized to the detection system, to exert a periodic driving force on an optically trapped 10 μm polystyrene bead, the laser pulse-bead interaction is repeated hundreds of times. Traces with the bead position following the prompt displacement from equilibrium, induced by each laser pulse, are averaged and reveal the underlying deterministic motion of the bead, which is not visible in a single trace due to thermal noise. The motion of the bead is analyzed from the direct time-dependent position measurements and from the power spectrum. The results show that the bead is on average displaced 208 nm from the trap center and exposed to a force amplitude of 71 nanoNewton, more than five orders of magnitude larger than the trapping forces. Our experimental method may have implications for microrheology.
    Optics Express 01/2013; 21(2):1986-96. DOI:10.1364/OE.21.001986 · 3.53 Impact Factor
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    ABSTRACT: We present an analysis of nonlinear coupling between fibers. We introduce the nonlinear coupling coefficient and investigate solitons coupling from one fiber into another. We will also present simulated supercontinuum from concatenated fiber systems.
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    ABSTRACT: Based on the experience gained developing our market leading visible spectrum supercontinuum sources NKT Photonics has built the first mid-infrared supercontinuum source based on modelocked picosecond fiber lasers. The source is pumped by a ~ 2 um laser based on a combination of erbium and thulium and use ZBLAN fibers to generate a 1.75-4.4 μm spectrum. We will present results obtained by applying the source for mid-infrared microscopy where absorption spectra can be used to identify the chemical nature of different parts of a sample. Subsequently, we discuss the possible application of a mid-IR supercontinuum source in other areas including infrared countermeasures.
    Proceedings of SPIE - The International Society for Optical Engineering 05/2012; DOI:10.1117/12.920094 · 0.20 Impact Factor
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    ABSTRACT: We present a detailed comparison between modeling and experiments on supercontinuum (SC) generation in a commercial ZBLAN step-index fiber. Special emphasis is put on identifying accurate material parameters by incorporating measurements of the ZBLAN Raman gain, fiber dispersion, and loss. This identification of accurate parameters is of great importance to substantiate numerical simulations of SC generation in soft-glass fibers. Good agreement between measurement and simulation is obtained when pumping both in the normal and anomalous dispersion regimes.
    Journal of the Optical Society of America B 03/2012; 29(4):635-645. DOI:10.1364/JOSAB.29.000635 · 1.81 Impact Factor
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    ABSTRACT: Combining the molecular specificity of the infrared spectral region with high resolution microscopy has been pursued by researchers for decades. Here we demonstrate infrared supercontinuum radiated from an optical fiber as a promising new light source for infrared microspectroscopy. The supercontinuum light source has a high brightness and spans the infrared region from 1400 nm to 4000 nm. This combination allows contact free high resolution hyper spectral infrared microscopy. The microscope is demonstrated by imaging an oil/water sample with 20 μm resolution.
    Optics Express 02/2012; 20(5):4887-92. DOI:10.1364/OE.20.004887 · 3.53 Impact Factor
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    ABSTRACT: Motion analysis of optically trapped objects is demonstrated using a simple 2D Fourier transform technique. The displacements of trapped objects are determined directly from the phase shift between the Fourier transform of subsequent images. Using end- and side-view imaging, the stiffness of the trap is determined in three dimensions. The Fourier transform method is simple to implement and applicable in cases where the trapped object changes shape or where the lighting conditions change. This is illustrated by tracking a fluorescent particle and a myoblast cell, with subsequent determination of diffusion coefficients and the trapping forces.
    Optics Express 01/2012; 20(3):1953-62. DOI:10.1364/OE.20.001953 · 3.53 Impact Factor
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    ABSTRACT: An all fiber based supercontinuum light source is demonstrated for infrared microscopy. The high brightness and spatial coherence of the source facilitate fast high resolution measurements.
    Lasers and Electro-Optics (CLEO), 2012 Conference on; 01/2012
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    ABSTRACT: We have measured the absolute Raman gain spectrum in short fluoride soft glass fibers with a pump wavelength of 1650 nm . We found a peak gain of g R = 4.0 ± 2 × 10 − 14 m W − 1 .
    Journal of the Optical Society of America B 09/2011; 28(10):2310-2313. DOI:10.1364/JOSAB.28.002310 · 1.81 Impact Factor
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    ABSTRACT: In this Letter, we propose a generic nonlinear coupling coefficient, η(NL)²=η|γ/β₂|(fiber2)/|γ/β₂|(fiber1), which gives a quantitative measure for the efficiency of nonlinear matching of optical fibers by describing how a fundamental soliton couples from one fiber into another. Specifically, we use η(NL) to demonstrate a significant soliton self-frequency shift of a fundamental soliton, and we show that nonlinear matching can take precedence over linear mode matching. The nonlinear coupling coefficient depends on both the dispersion (β₂) and nonlinearity (γ), as well as on the power coupling efficiency η. Being generic, η(NL) enables engineering of general waveguide systems, e.g., for optimized Raman redshift or supercontinuum generation.
    Optics Letters 07/2011; 36(13):2596-8. DOI:10.1364/OL.36.002596 · 3.18 Impact Factor
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    ABSTRACT: The relaxation dynamics of the DNA nucleotide deoxyguanosine 5'-monophosphate (dGMP) following 266 nm photoexcitation has been studied by transient IR spectroscopy with femtosecond time resolution. The induced dynamics of the amide I (carbonyl) stretch, the asymmetric guanine ring stretch and the phosphate asymmetric stretch are monitored in the region 1000-1800 cm(-1). Excitation and subsequent rapid internal conversion to a "hot" ground state is reflected by depletion of the vibrational ground states of the amide I stretch and guanine ring stretch. However, the vibrational ground state of the phosphate is left unperturbed, indicating the absence of vibrational coupling between the guanine ring system and the phosphate group. The vibrational ground state of the amide I is repopulated in 2.5 ps (±0.2 ps) while it takes 3.7 ps (±0.5 ps) to repopulate the guanine ring vibration. This article discusses two possible relaxation pathways of dGMP, as well as the implications of the weak phosphate dynamics.
    Physical Chemistry Chemical Physics 06/2011; 13(30):13821-6. DOI:10.1039/c1cp20918c · 4.20 Impact Factor

Publication Stats

3k Citations
269.39 Total Impact Points

Institutions

  • 1995–2014
    • Aarhus University
      • Department of Chemistry
      Aarhus, Central Jutland, Denmark
  • 1998
    • University of Freiburg
      • Freiburg Materials Research Center
      Freiburg, Baden-Württemberg, Germany
  • 1992
    • Odense University Hospital
      Odense, South Denmark, Denmark