Jan Thøgersen

Coherent anti-Stokes Raman scattering microscopy with a photonic crystal fiber based light source

Coherent anti-Stokes Raman scattering (CARS) microscopy is demonstrated using a light source consisting of the output from a photonic crystal fiber pumped by a standard Ti:sapphire oscillator and the fundamental oscillator beam.

X-Ray Diffraction from Isolated and Strongly Aligned Gas-Phase Molecules with a Free-Electron Laser

We report experimental results on x-ray diffraction of quantum-state-selected and strongly aligned ensembles of the prototypical asymmetric rotor molecule 2,5-diiodobenzonitrile using the Linac Coherent Light Source. The experiments demonstrate first steps toward a new approach to diffractive imaging of distinct structures of individual, isolated gas-phase molecules. We confirm several key ingredients of single molecule diffraction experiments: the abilities to detect and count individual scattered x-ray photons in single shot diffraction data, to deliver state-selected, e.g., structural-isomer-selected, ensembles of molecules to the x-ray interaction volume, and to strongly align the scattering molecules. Our approach, using ultrashort x-ray pulses, is suitable to study ultrafast dynamics of isolated molecules.

Supercontinuum generation in ZBLAN fibers—detailed comparison between measurement and simulation

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.

Continuous-wave wavelength conversion in a photonic crystal fiber with two zero-dispersion wavelengths.

We demonstrate continuous-wave wavelength conversion through four-wave mixing in an endlessly single mode photonic crystal fiber. Phasematching is possible at vanishing pump power in the anomalous dispersion regime between the two zero-dispersion wavelengths. By mixing appropriate pump and idler sources, signals in the range 500-650 nm are obtained in good accordance with calculated phasematching curves. The conversion efficiency from idler to signal power is currently limited to 0.3% by the low spectral density of the pump and idler sources at hand, but can be greatly enhanced by applying narrow line width lasers.

Tunable light source for coherent anti-Stokes Raman scattering microspectroscopy based on the soliton self-frequency shift

We present a photonic crystal fiber (PCF)-based light source for generating tunable excitation pulses (pump and Stokes) that are applicable to coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The laser employed is an unamplified Ti:sapphire femtosecond laser oscillator. The CARS pump pulse is generated by spectral compression of a laser pulse in a PCF. The Stokes pulse is generated by redshifting a laser pulse in a PCF through the soliton self-frequency shift. This setup allows for probing up to 4000 cm(-1) with a spectral resolution of approximately 25 cm(-1). We characterize the stability and robustness of CARS microspectroscopy employing this light source.

IR microscopy utilizing intense supercontinuum light source

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.

Fiber laser-based light source for coherent anti-Stokes Raman scattering microspectroscopy

We demonstrate an alternative light source for CARS microspectroscopy based on a fiber laser and a photonic-crystal fiber. The light source simultaneously delivers a near-transform-limited picosecond pump pulse at 1033.5 nm and a frequency-shifted, near-transform-limited femtosecond Stokes pulse, tunable from 1033.5 nm to 1400 nm. This corresponds to a range 0 - 2500 cm(-1), so that Raman-active vibrations in this frequency range can be probed. The spectral resolution is 5 cm(-1), given by the spectral width of the pump pulse. The frequency range that can be probed simultaneously is 200 cm(-1)-wide, given by the spectral width of the Stokes pulse. The achievable pulse powers are 50 mW for the pump and 2 mW for the Stokes pulse. The repetition rate is 35 MHz. We demonstrate the capability of this light source by performing CARS microspectroscopy and comparing CARS spectra with Raman spectra.

Spectral compression of femtosecond pulses in photonic crystal fibers

We demonstrate efficient spectral compression of femtosecond pulses near the zero-dispersion wavelength in nonlinear photonic crystal fibers (PCFs). The highest measured compression factor is 21, in which case the spectral brightness increases by a factor of 5. We numerically model the pulse propagation and find good agreement with the experiment. We argue that the fibers studied allow for spectral narrowing of more than 2 orders of magnitude. With dispersion-shifted PCFs, efficient spectral compression can take place across the visible and near-infrared part of the spectrum.

Hydration Dynamics of Aqueous Nitrate

Aqueous nitrate, NO3(-)(aq), was studied by 2D-IR, UV-IR, and UV-UV time-resolved spectroscopies 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 nondegenerate asymmetric-stretch vibrations on a 300-fs time scale 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, was 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.

Temperature dependent relaxation and recombination dynamics of the hydrated electron

The ultrafast solvation and recombination dynamics of the hydrated electron generated by two-photon ionization of water at 4.65 eV is studied by transient absorption spectroscopy as a function of temperature in the range from 277 K to 355 K. The part of the spectral blue shift which is observed in the absorption spectrum of the hydrated electron after 1 ps is purely continuous and is accurately described by the well known analytical expression for the temperature dependent absorption spectrum of the ground state hydrated electron. This indicates that thermal relaxation or more likely solvation of the hydrated electron predominantly causes the blue shift. The survival probability of the hydrated electron shows a strong temperature dependence, which is satisfactory explained by the temperature dependent mobility and reaction rates of the species involved in the recombination. This implies that the average initial separation between the hydrated electron and the ionization site of 〈r0〉=1.0±0.1 nm does not de...