@article{LiebigHenningSarhanetal.2018, author = {Liebig, Ferenc and Henning, Ricky and Sarhan, Radwan Mohamed and Prietzel, Claudia Christina and Bargheer, Matias and Koetz, Joachim}, title = {A new route to gold nanoflowers}, series = {Nanotechnology}, volume = {29}, journal = {Nanotechnology}, number = {18}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0957-4484}, doi = {10.1088/1361-6528/aaaffd}, pages = {8}, year = {2018}, abstract = {Catanionic vesicles spontaneously formed by mixing the anionic surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt with the cationic surfactant cetyltrimethylammonium bromide were used as a reducing medium to produce gold clusters, which are embedded and well-ordered into the template phase. The gold clusters can be used as seeds in the growth process that follows by adding ascorbic acid as a mild reducing component. When the ascorbic acid was added very slowly in an ice bath round-edged gold nanoflowers were produced. When the same experiments were performed at room temperature in the presence of Ag+ ions, sharp-edged nanoflowers could be synthesized. The mechanism of nanoparticle formation can be understood to be a non-diffusion-limited Ostwald ripening process of preordered gold nanoparticles embedded in catanionic vesicle fragments. Surface-enhanced Raman scattering experiments show an excellent enhancement factor of 1.7 . 10(5) for the nanoflowers deposited on a silicon wafer.}, language = {en} } @article{LiebigHenningSarhanetal.2019, author = {Liebig, Ferenc and Henning, Ricky and Sarhan, Radwan Mohamed and Prietzel, Claudia Christina and Schmitt, Clemens Nikolaus Zeno and Bargheer, Matias and Koetz, Joachim}, title = {A simple one-step procedure to synthesise gold nanostars in concentrated aqueous surfactant solutions}, series = {RSC Advances}, volume = {9}, journal = {RSC Advances}, publisher = {RSC Publishing}, address = {London}, issn = {2046-2069}, doi = {10.1039/C9RA02384D}, pages = {23633 -- 23641}, year = {2019}, abstract = {Due to the enhanced electromagnetic field at the tips of metal nanoparticles, the spiked structure of gold nanostars (AuNSs) is promising for surface-enhanced Raman scattering (SERS). Therefore, the challenge is the synthesis of well designed particles with sharp tips. The influence of different surfactants, i.e., dioctyl sodium sulfosuccinate (AOT), sodium dodecyl sulfate (SDS), and benzylhexadecyldimethylammonium chloride (BDAC), as well as the combination of surfactant mixtures on the formation of nanostars in the presence of Ag⁺ ions and ascorbic acid was investigated. By varying the amount of BDAC in mixed micelles the core/spike-shell morphology of the resulting AuNSs can be tuned from small cores to large ones with sharp and large spikes. The concomitant red-shift in the absorption toward the NIR region without losing the SERS enhancement enables their use for biological applications and for time-resolved spectroscopic studies of chemical reactions, which require a permanent supply with a fresh and homogeneous solution. HRTEM micrographs and energy-dispersive X-ray (EDX) experiments allow us to verify the mechanism of nanostar formation according to the silver underpotential deposition on the spike surface in combination with micelle adsorption.}, language = {en} } @article{MatternPudellLaskinetal.2021, author = {Mattern, Maximilian and Pudell, Jan-Etienne and Laskin, Gennadii and Reppert, Alexander von and Bargheer, Matias}, title = {Analysis of the temperature- and fluence-dependent magnetic stress in laser-excited SrRuO3}, series = {Structural dynamics}, volume = {8}, journal = {Structural dynamics}, number = {2}, publisher = {AIP Publishing LLC}, address = {Melville, NY}, issn = {2329-7778}, doi = {10.1063/4.0000072}, pages = {9}, year = {2021}, abstract = {We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Gr{\"u}neisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime.}, language = {en} } @article{HerzogSchickGaaletal.2012, author = {Herzog, Marc and Schick, Daniel and Gaal, P. and Shayduk, Roman and von Korff Schmising, Clemens and Bargheer, Matias}, title = {Analysis of ultrafast X-ray diffraction data in a linear-chain model of the lattice dynamics}, series = {Applied physics : A, Materials science \& processing}, volume = {106}, journal = {Applied physics : A, Materials science \& processing}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0947-8396}, doi = {10.1007/s00339-011-6719-z}, pages = {489 -- 499}, year = {2012}, abstract = {We present ultrafast X-ray diffraction (UXRD) experiments which sensitively probe impulsively excited acoustic phonons propagating in a SrRuO3/SrTiO3 superlattice and further into the substrate. These findings are discussed together with previous UXRD results (Herzog et al. in Appl. Phys. Lett. 96, 161906, 2010; Woerner et al. in Appl. Phys. A 96, 83, 2009; v. Korff Schmising in Phys. Rev. B 78, 060404(R), 2008 and in Appl. Phys. B 88, 1, 2007) using a normal-mode analysis of a linear-chain model of masses and springs, thus identifying them as linear-response phenomena. We point out the direct correspondence of calculated observables with X-ray signals. In this framework the complex lattice motion turns out to result from an interference of vibrational eigenmodes of the coupled system of nanolayers and substrate. UXRD in principle selectively measures the lattice motion occurring with a specific wavevector, however, each Bragg reflection only measures the amplitude of a delocalized phonon mode in a spatially localized region, determined by the nanocomposition of the sample or the extinction depth of X-rays. This leads to a decay of experimental signals although the excited modes survive.}, language = {en} } @article{PavlenkoSanderMitzscherlingetal.2016, author = {Pavlenko, Elena S. and Sander, Mathias and Mitzscherling, S. and Pudell, Jan-Etienne and Zamponi, Flavio and Roessle, M. and Bojahr, Andre and Bargheer, Matias}, title = {Azobenzene - functionalized polyelectrolyte nanolayers as ultrafast optoacoustic transducers}, series = {Nanoscale}, volume = {8}, journal = {Nanoscale}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2040-3364}, doi = {10.1039/c6nr01448h}, pages = {13297 -- 13302}, year = {2016}, abstract = {We introduce azobenzene-functionalized polyelectrolyte multilayers as efficient, inexpensive optoacoustic transducers for hyper-sound strain waves in the GHz range. By picosecond transient reflectivity measurements we study the creation of nanoscale strain waves, their reflection from interfaces, damping by scattering from nanoparticles and propagation in soft and hard adjacent materials like polymer layers, quartz and mica. The amplitude of the generated strain epsilon similar to 5 x 10(-4) is calibrated by ultrafast X-ray diffraction.}, language = {en} } @article{BojahrHerzogMitzscherlingetal.2013, author = {Bojahr, Andre and Herzog, Marc and Mitzscherling, Steffen and Maerten, Lena and Schick, Daniel and Goldshteyn, J. and Leitenberger, Wolfram and Shayduk, R. and Gaal, P. and Bargheer, Matias}, title = {Brillouin scattering of visible and hard X-ray photons from optically synthesized phonon wavepackets}, series = {Optics express : the international electronic journal of optics}, volume = {21}, journal = {Optics express : the international electronic journal of optics}, number = {18}, publisher = {Optical Society of America}, address = {Washington}, issn = {1094-4087}, doi = {10.1364/OE.21.021188}, pages = {21188 -- 21197}, year = {2013}, abstract = {We monitor how destructive interference of undesired phonon frequency components shapes a quasi-monochromatic hypersound wavepacket spectrum during its local real-time preparation by a nanometric transducer and follow the subsequent decay by nonlinear coupling. We prove each frequency component of an optical supercontinuum probe to be sensitive to one particular phonon wavevector in bulk material and cross-check this by ultrafast x-ray diffraction experiments with direct access to the lattice dynamics. Establishing reliable experimental techniques with direct access to the transient spectrum of the excitation is crucial for the interpretation in strongly nonlinear regimes, such as soliton formation.}, language = {en} } @article{KielMoehwaldBargheer2011, author = {Kiel, Mareike and M{\"o}hwald, Helmuth and Bargheer, Matias}, title = {Broadband measurements of the transient optical complex dielectric function of a nanoparticle/polymer composite upon ultrafast excitation}, series = {Physical review : B, Condensed matter and materials physics}, volume = {84}, journal = {Physical review : B, Condensed matter and materials physics}, number = {16}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.84.165121}, pages = {6}, year = {2011}, abstract = {We determined experimentally the complex transient optical dielectric function of a well-characterized polyelectrolyte/gold-nanoparticle composite system over a broad spectral range upon short pulse laser excitation by simultaneously measuring the time-dependent reflectance and transmittance of white light pulses with femtosecond pump-probe spectroscopy. We extracted directly the ultrafast changes in the real and imaginary parts of the effective dielectric function, epsilon(eff)(r) (omega,t)and epsilon(eff)(i) (omega,t), from the experiment. This complete experimental set of information on the time-dependent complex dielectric function challenges theories modeling the transient dielectric function of gold particles and the effective medium.}, language = {en} } @article{BojahrHerzogSchicketal.2012, author = {Bojahr, Andre and Herzog, Marc and Schick, Daniel and Vrejoiu, Ionela and Bargheer, Matias}, title = {Calibrated real-time detection of nonlinearly propagating strain waves}, series = {Physical review : B, Condensed matter and materials physics}, volume = {86}, journal = {Physical review : B, Condensed matter and materials physics}, number = {14}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.86.144306}, pages = {5}, year = {2012}, abstract = {Epitaxially grown metallic oxide transducers support the generation of ultrashort strain pulses in SrTiO3 (STO) with high amplitudes up to 0.5\%. The strain amplitudes are calibrated by real-time measurements of the lattice deformation using ultrafast x-ray diffraction. We determine the speed at which the strain fronts propagate by broadband picosecond ultrasonics and conclude that, above a strain level of approx. 0.2\%, the compressive and tensile strain components travel at considerably different sound velocities, indicating nonlinear wave behavior. Simulations based on an anharmonic linear-chain model are in excellent accord with the experimental findings and show how the spectrum of coherent phonon modes changes with time.}, language = {en} } @article{SanderKocKwamenetal.2016, author = {Sander, Mathias and Koc, A. and Kwamen, C. T. and Michaels, H. and von Reppert, Alexander and Pudell, Jan-Etienne and Zamponi, Flavio and Bargheer, Matias and Sellmann, J. and Schwarzkopf, J. and Gaal, P.}, title = {Characterization of an ultrafast Bragg-Switch for shortening hard x-ray pulses}, series = {Journal of applied physics}, volume = {120}, journal = {Journal of applied physics}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-8979}, doi = {10.1063/1.4967835}, pages = {7}, year = {2016}, abstract = {We present a nanostructured device that functions as photoacoustic hard x-ray switch. The device is triggered by femtosecond laser pulses and allows for temporal gating of hard x-rays on picosecond (ps) timescales. It may be used for pulse picking or even pulse shortening in 3rd generation synchrotron sources. Previous approaches mainly suffered from insufficient switching contrasts due to excitation-induced thermal distortions. We present a new approach where thermal distortions are spatially separated from the functional switching layers in the structure. Our measurements yield a switching contrast of 14, which is sufficient for efficient hard x-ray pulse shortening. The optimized structure also allows for utilizing the switch at high repetition rates of up to 208 kHz. Published by AIP Publishing.}, language = {en} } @article{BojahrSchickMaertenetal.2012, author = {Bojahr, Andre and Schick, Daniel and M{\"a}rten, Lena and Herzog, Marc and Vrejoiu, Ionela and von Korff Schmising, Clemens and Milne, Chris and Johnson, Steven Lee and Bargheer, Matias}, title = {Comparing the oscillation phase in optical pump-probe spectra to ultrafast x-ray diffraction in the metal-dielectric SrRuO3/SrTiO3 superlattice}, series = {Physical review : B, Condensed matter and materials physics}, volume = {85}, journal = {Physical review : B, Condensed matter and materials physics}, number = {22}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.85.224302}, pages = {6}, year = {2012}, abstract = {We measured the ultrafast optical response of metal-dielectric superlattices by broadband all-optical pump-probe spectroscopy. The observed phase of the superlattice mode depends on the probe wavelength, making assignments of the excitation mechanism difficult. Ultrafast x-ray diffraction data reveal the true oscillation phase of the lattice which changes as a function of the excitation fluence. This result is confirmed by the fluence dependence of optical transients. We set up a linear chain model of the lattice dynamics and successfully simulated the broadband optical reflection by unit-cell resolved calculation of the strain-dependent dielectric functions of the constituting materials.}, language = {en} }