Torsten Siebert

Standoff spectroscopy via remote generation of a backward-propagating laser beam

In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N2 and O2. This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.

Identification of Conical Structures in Small Aluminum Oxide Clusters: Infrared Spectroscopy of (Al2O3)1−4(AlO)+

The vibrational spectroscopy of the electronically closed-shell (Al 2O 3) n (AlO) (+) cations with n = 1-4 is studied in the 530-1200 cm (-1) range by infrared predissociation spectroscopy of the corresponding ion-He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C 3 v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al-O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions.

A Complete Reactant-Product Analysis of the Oxygen Transfer Reaction in [V4O11·C3H6] : A Cluster Complex for Modeling Surface Activation and Reactivity

The anionic V4O11 cluster is presented as a gas-phase system of low dimensionality for modeling surface activation of molecular oxygen and the reactivity toward unsaturated hydrocarbons. Together with the charged cluster aggregates and fragments taking part in the reaction, neutral reactant and product species are monitored via multiphoton ionization for the first time within the instrumentation of tandem mass spectrometry and ion trap reactors. This novel approach allows for a comprehensive analysis of the photoinduced oxygen transfer reaction to propene within the defined aggregate complex [V4O11 x C3H6]- that simulates coadsorption and activation under fully controlled conditions.

Anharmonic Backbone Vibrations in Ultrafast Processes at the DNA-Water Interface.

The vibrational modes of the deoxyribose-phosphodiester backbone moiety of DNA and their interactions with the interfacial aqueous environment are addressed with two-dimensional (2D) infrared spectroscopy on a femto- to picosecond time scale. Beyond the current understanding in the harmonic approximation, the anharmonic character and delocalization of the backbone modes in the frequency range from 900 to 1300 cm(-1) are determined with both diagonal anharmonicities and intermode couplings on the order of 10 cm(-1). Mediated by the intermode couplings, energy transfer between the backbone modes takes place on a picosecond time scale, parallel to vibrational relaxation and energy dissipation into the environment. Probing structural dynamics noninvasively via the time evolution of the 2D lineshapes, limited structural fluctuations are observed on a 300 fs time scale of low-frequency motions of the helix, counterions, and water shell. Structural disorder of the DNA-water interface and DNA-water hydrogen bonds are, however, preserved for times beyond 10 ps. The different interactions of limited strength ensure ultrafast vibrational relaxation and dissipation of excess energy in the backbone structure, processes that are important for the structural integrity of hydrated DNA.

Optically reconfigurable azobenzene polymer-based fiber Bragg filter

Optically writable, thermally erasable surface relief gratings in thin Disperse Red 1 polymethyl methacrylate azopolymer films were used to demonstrate an arbitrarily reconfigurable fiber Bragg filter. Gratings were optically written on azopolymer-coated side-polished fiber blocks, and a write-erase-write cycle was demonstrated. Finite difference time domain simulations reveal that this optically reconfigurable device concept can be optimized in a silicon-on-insulator waveguide platform.

Femtosecond coherent Raman spectroscopy

Femtosecond time-resolved CARS spectroscopy is applied in order to prepare and monitor coherent states of different samples mainly in its electronic ground but also excited states. The time evolution prepared by such methods give information on the dynamics of molecular vibrations. In a first example, the fs CARS transients of iodine are investigated. Depending on the timing of the laser pulses different dynamics are reflected in the transient CARS signal. Second, we report on selective excitation of the vibrational modes in the electronic ground state of polymers of diacetylene by means of a femtosecond time-resolved CARS scheme. Control is achieved by varying the timing and the phase shape of the exciting laser pulses. Finally the electronic ground state dynamics of biologically relevant porphyrine molecules are studied with transient CARS spectroscopy.

Ultrafast vibrational dynamics of the DNA backbone at different hydration levels mapped by two-dimensional infrared spectroscopy

DNA oligomers are studied at 0% and 92% relative humidity, corresponding to N < 2 and N > 20 water molecules per base pair. Two-dimensional (2D) infrared spectroscopy of DNA backbone modes between 920 and 1120 cm−1 maps fluctuating interactions at the DNA surface. At both hydration levels, a frequency fluctuation correlation function with a 300 fs decay and a slow decay beyond 10 ps is derived from the 2D lineshapes. The fast component reflects motions of DNA helix, counterions, and water shell. Its higher amplitude at high hydration level reveals a significant contribution of water to the fluctuating forces. The slow component reflects disorder-induced inhomogeneous broadening.

Femtosecond pump-probe and four-wave mixing spectroscopies applied to simple molecules

Abstract A femtosecond three color pump/dump-probe scheme in combination with a time-of-flight (TOF) mass selective detection unit has been applied to study vibrational wave packet dynamics in the electronic ground state of cold K 2 molecules. A spectral analysis of the time domain signal reveals two different wave packet contributions: one from a stimulated Raman process and one from stimulated emission pumping. Also femtosecond time-resolved degenerate four-wave mixing (DFWM) spectroscopy is performed in order to investigate molecular dynamics in iodine molecules in the gas phase. Depending on the timing of the laser pulses different dynamics are reflected in the DFWM transient signal. By the use of time-evolution diagrams, the varying contribution of ground and excited state dynamics can be explained conclusively.

Supercontinuum pulse shaping in the few-cycle regime

The synthesis of nearly arbitrary supercontinuum pulse forms is demonstrated with sub-pulse structures that maintain a temporal resolution in the few-cycle regime. Spectral broadening of the 35 fs input pulses to supercontinuum bandwidths is attained in a controlled two-stage sequential filamentation in air at atmospheric pressure, facilitating a homogeneous power density over the full spectral envelope in the visible to near infrared spectral range. Only standard optics and a liquid crystal spatial light modulator (LC-SLM) are employed for achieving pulse compression to the sub 5 fs regime with pulse energies of up to 60 μJ and a peak power of 12 GW. This constitutes the starting point for further pulse form synthesis via phase modulation within the sampling limit of the pulse shaper. Transient grating frequency-resolved optical gating (TG-FROG) allows for the characterization of pulse forms that extend over several hundred femtoseconds with few-cycle substructures.

Poor man’s source for sub 7 fs: a simple route to ultrashort laser pulses and their full characterization

A practicable and economic method for the generation and full characterization of laser pulses ranging down to sub 7 fs duration with energies spanning the full microJ domain is presented. The method utilizes a self-induced and self-guiding filamentation of titanium-sapphire based, amplified pulses in air for spectral broadening, a standard chirp mirror compression scheme and transient grating frequency resolved optical gating for determining the spectral phase over the full visible to near infrared range. In this manner, few-cycle laser pulses with a high quality in the spatial beam profile have been generated in an robust arrangement with a minimal amount of standard optical components for their full characterization. The optical scheme demonstrates an uncomplicated, versatile access to this regime of pulsed laser radiation accompanied by a comprehensive analysis.