Ultrafast superheating and melting of bulk ice.

The superheating of a solid to a temperature beyond its melting point, without the solid actually melting, is a well-known phenomenon. It occurs with many substances, particularly those that can readily be produced as high-quality crystals. In principle, ice should also be amenable to superheating. But the complex three-dimensional network of hydrogen bonds that holds water molecules together and gives rise to unusual solid and liquid properties strongly affects the melting behaviour of ice; in particular, ice usually contains many defects owing to the directionality of its hydrogen bonds. However, simulations are readily able to 'create' defect-free ice that can be superheated. Here we show that by exciting the OH stretching mode of water, it is possible to superheat ice. When using an ice sample at an initial temperature of 270 K, we observe an average temperature rise of 20 +/- 2 K that persists over the monitored time interval of 250 ps without melting.

The precursors of the solvated electron in methanol studied by femtosecond pump-repump-probe spectroscopy.

Using UV photoionization and delayed near-infrared reexcitation pulses, a novel time-, frequency-, and polarization-resolved pump-repump-probe spectroscopy is conducted in the probing range of 450-2400 nm with improved experimental accuracy. Both the generation process and relaxation dynamics following selective repumping of intermediate species of the solvated electron are investigated and analyzed self-consistently with the help of a kinetic model. New insight in the intermediates of the trapped electron is gained leading to a unique microscopic picture.

All-optical phase locking of two femtosecond Ti:sapphire lasers: a passive coupling mechanism beyond the slowly varying amplitude approximation.

Two independently tunable femtosecond Ti:sapphire lasers are passively synchronized with a stable relative carrier-envelope offset phase. By heterodyning the spectral overlap of the two frequency combs, we observe multiple regimes for the cavity length difference in which the relative round-trip phase slip is effectively locked to zero. The strong correlation of the femtosecond pulse trains is maintained over minutes without any external stabilization, and relative cavity length variations of 50 nm are compensated. The phase synchronization relies on phase-dependent cross-phase modulation, taking full advantage of the nonresonant optical nonlinearity of the shared gain medium, which is much faster than the optical cycle.

Subthreshold carrier-LO phonon dynamics in semiconductors with intermediate polaron coupling: a purely quantum kinetic relaxation channel.

Femtosecond transmission spectra of highly polar CdTe are compared to more covalent GaAs contrasting semiclassical kinetics with two-time quantum kinetics based on the Dyson equation. Nonequilibrium heavy holes in CdTe show ultrafast energy redistribution via the Fröhlich mechanism even if photoexcited below the LO phonon energy. This subthreshold relaxation is a genuine quantum kinetic effect. It gains importance if the polaron self-energy is comparable to the phonon energy. Conservation of the free-particle energies is not required under these conditions.

Novel precursors of solvated electrons in water: evidence for a charge transfer process.

Femtosecond midinfrared spectroscopy of water (heavy water) after two photon excitation at 9 eV provides clear evidence for two short-lived precursors of the hydrated electron preceding the well-known "wet electron." The measured first intermediate with peak absorption at 2.9 (4.1) microm is proposed to represent an O(H, D):e(-) complex. The subsequent solvation proceeds via e(-):[(H, D)2O](n) complexes at approximately 1.6 (2.0) microm in the electronic ground state, involving an increasing number of water molecules during the first 200 fs and followed by the wet electron.

Widely tunable two-color mode-locked Ti:sapphire laser with pulse jitter of less than 2 fs.

An improved laser setup for reliable dual-wavelength cross mode locking is reported. We obtain two perfectly synchronized pulse trains that are independently tunable over a wavelength interval as wide as 100 nm and with pulse durations below 30 fs. The pulses are shown to be cross correlated with interferometric accuracy, thus demonstrating an astonishing potential of the nonlinear coupling mechanism.

Tunable femtosecond-pulse generation by an optical parametric oscillator in the saturation regime.

We demonstrate the generation of powerful, bandwidth-limited pulses in a KTP optical parametric oscillator synchronously excited by the pulse train of a f lash-lamp-pumped, mode-locked Nd:YLF laser. Operating the optical parametric oscillator a factor of 4 above threshold and optimizing the cavity length yields pulses of 260 +/- 30 fs. The shortening by a factor of 15 relative to the pump is accompanied by extended pulse wings. Tunability of the device is shown in the wavelength range 1.2 -1.6 microm.

Generation of synchronized, independently tunable 120-fs light pulses with optical parametric oscillators.

We report the generation of powerful, widely tunable femtosecond radiation by parallel synchronous pumping of two singly resonant optical parametric oscillators with an additive-pulse mode-locked Nd:glass laser. For a single-pulse energy of 10 nJ, a tuning range of 0.9-1.5 microm is achieved with pulse durations of 120 +/- 20 and 200 +/- 20 fs. The time jitter is estimated to be +/-50 fs.

Broadly tunable femtosecond pulses generated by optical parametric oscillation.

A singly resonant optical parametric oscillator is demonstrated that uses a beta-barium borate crystal and is synchronously pumped by microsecond pulse trains of a Nd:glass laser (0.8 psec, 527 nm). Broadly tunable pulses are generated in the wavelength range 0.7-1.8 microm with a duration of 160-260 fsec and an energy conversion of 3%. Steep pulse wings are observed, with a 1/e decay time of approximately 90 fsec. Under special conditions, parametric pulses as short as 65 +/- 7 fsec are obtained.

Generation of intense 25-fsec pulses by a pulsed laser system.

A pulsed femtosecond dye laser is demonstrated with relaxed stability requirements, improved output reproducibility, and significant pulse shortening. Starting with a sequence of approximately 350 pump pulses of a Nd:glass laser (repetition rate 6 Hz, duration 1.3 psec), pulses of 25 fsec and 10 nJ are generated at 566 nm. A non-colliding-pulse, mode locked ring laser is used with dispersion compensation and the dyes Rhodamine 6G, DQOCI, and DTCI. The evolution of the pulse parameters as a function of cavity round trips is investigated.

Picosecond coherent anti-Stokes Raman spectroscopy of molecules in free jet expansions.

By using time-resolved coherent Raman scattering, closely spaced vibrational resonances are simultaneously excited and subsequently monitored over long, nanosecond time intervals. Coherent superposition of molecular transitions leads to dramatic oscillations of the picosecond anti-Stokes scattering signals. Experimental data are presented for the nu(1) mode of CH(4) in supersonic expansions at low rotational temperatures (T(ROT) >/= 25). Our timedomain data are in good agreement with spectroscopic results. The corresponding frequency resolution of less, similar3 x10(-3) cm(-1) suggests that this method of Fourier-transform Raman spectroscopy may prove useful in high-resolution molecular spectroscopy.

Sensitive detection of IR photons with picosecond time resolution.

Parametric upconversion is shown to be a powerful detection scheme for picosecond pulses in the IR. Tuning characteristics, spectral bandwidth, and solid angle of acceptance of the process are discussed. The linear response of the setup is demonstrated over a large range of 8 orders of ten in agreement with theoretical arguments. Using pump pulses of 1 x 10(-5) J, IR signals with 1 x 10(-15) are detected with a time resolution of approximately ~10 psec.