Michael Woerner

Generation of single-cycle THz transients with high electric-field amplitudes.

Single-cycle terahertz (THz) transients in the frequency range 0.3-7 THz with electric-field amplitudes of more than 400 kV/cm are generated by four-wave mixing of the fundamental and the second harmonic of 25 fs pulses from a Ti:sapphire amplifier in ionized air. These transients are fully characterized by electro-optic sampling with ZnTe and GaP crystals. One can tune the center frequency of the THz transients by varying the length of the incident pulse. The electric-field amplitude increases linearly with the incident pulse energy.

Generation, shaping, and characterization of intense femtosecond pulses tunable from 3 to 20 µm

We report on an intense mid-infrared light source that provides femtosecond pulses on a microjoule energy level, broadly tunable in the 3–20-µm wavelength range with pulse durations as short as 50 fs at 5 µm. The pulses are generated by phase-matched difference-frequency mixing in GaSe of near-infrared signal and idler pulses of a parametric device based on a 1-kHz Ti:sapphire amplifier system. Pulse durations are characterized with different techniques including autocorrelation measurements in AgGaS2, two-photon absorption in InSb, and cross-correlation measurements with near-infrared pulses in a thin GaSe crystal. A subsequent zero-dispersion stretcher of high transmission allows for optimum pulse compression, a more detailed amplitude and phase characterization and, ultimately, amplitude shaping of the mid-infrared pulses.

Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction

Reversible structural changes of a nanostructure were measured nondestructively with subpicometer spatial and subpicosecond temporal resolution via x-ray diffraction (XRD). The spatially periodic femtosecond excitation of a gallium arsenide/aluminum gallium arsenide superlattice results in coherent lattice motions with a 3.5-picosecond period, which was directly monitored by femtosecond x-ray pulses at a 1-kilohertz repetition rate. Small changes (ΔR/R = 0.01) of weak Bragg reflexes (R = 0.005) were detected. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.

Direct field-resolved detection of terahertz transients with amplitudes of megavolts per centimeter

Phase-matched difference-frequency mixing in a thin GaSe crystal within the broad spectrum of 25-fs pulses from a Ti:sapphire oscillator multipass amplifier system permits the generation of few-cycle electric field transients, frequencies up to 30 THz, and amplitudes of more than 1 MV/cm. The field transients generated at a 1-kHz repetition rate are directly measured by electro-optic sampling by 12-f probe pulses from the 75-MHz repetition-rate Ti:sapphire oscillator in combination with a novel electronic gating technique.

Broadband phase-matched difference frequency mixing of femtosecond pulses in GaSe: Experiment and theory

Phase-matched difference frequency mixing within the broad spectrum of single 13 fs pulses from a cavity-dumped, mode-locked Ti:sapphire laser in a GaSe crystal allows the generation of femtosecond mid-infrared pulses, continuously tunable in the wavelength range from 7 to 20 μm. Pulse durations down to 95 fs at 9.3 μm are directly measured. Model calculations including the full dispersion of GaSe show that the chirp of the pump pulses allows to change both the pulse duration and conversion efficiency obtained in the nonlinear process, in agreement with the experiment.

Femtosecond infrared pulses tunable from 9 to 18??µm at an 88-MHz repetition rate

Femtosecond mid-infrared laser pulses that are continuously tunable in the wavelength range from 9 to 18mum are demonstrated. These nearly bandwidth-limited pulses are generated by phase-matched difference-frequency mixing within the broad spectrum of 20-fs pulses from a mode-locked Ti:sapphire laser in GaSe. A direct determination of the pulse duration at 11.5mum gives a value of 140 fs. The average mid-infrared power of 1muW is ~100 times greater than that for infrared generation by non-phase-matched optical rectification.

Internal motions of a quasiparticle governing its ultrafast nonlinear response.

A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fröhlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties—relevant for transport on nanometre length and ultrashort timescales—are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Phase-resolved two-dimensional spectroscopy based on collinear n-wave mixing in the ultrafast time domain

We present a novel approach for femtosecond two-dimensional (2D) spectroscopy in the midinfrared combining a collinear beam geometry and phase-resolved detection. Two phase-locked pulses of variable time delay tau interact with the sample. The transmitted electric fields are measured in real time t by electro-optic sampling. 2D spectra are generated by Fourier transforming the signal along the two time axes tau and t. In the 2D spectra, nonlinear signals originating from different orders n in the electric field are separated. Such decomposition of the overall response is demonstrated by mapping the nonlinear response of intersubband transitions in GaAs/AlGaAs multiple quantum wells.

Microfocus Cu K ? source for femtosecond x-ray science

We demonstrate a subpicosecond 1 kHz femtosecond x-ray source with a well-accessible quasi-point size (10??m diameter) providing Cu K? emission with a maximum flux of 6.8×1010? photons?s for continuous operation of 10 h. A new geometry that essentially facilitates the adjustment and diminishes the temporal jitter between the x-ray probe and the laser pump pulse is implemented for time-resolved diffraction experiments.

Controlled shaping of ultrafast electric field transients in the mid-infrared spectral range

We experimentally demonstrate amplitude and phase shaping of femtosecond mid-infrared pulses in a range centered about 14 mum . Single pulses with a tailored optical phase and phase-locked double pulses are generated by phase-matched difference-frequency mixing in a GaSe crystal of near-infrared pulses shaped with a liquid-crystal modulator. The electric field transients are directly measured by free-space electro-optic sampling, yielding pulse durations of 200-300 fs. Our data are in good agreement with a model that describes phase-matched optical rectification.