A. Laubereau

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.

Evidence for biexcitons and dynamic Stark effect in J-aggregates from femtosecond spectroscopy

Abstract J-aggregates in aqueous solution are investigated at room temperature with tunable fs pulses using near-resonant excitation at 565 nm. The coherent response of the J-band is observed at short times leading to a precursor of the dynamic Stark effect similar to recent findings for excitonic transitions in semiconductors. Approximately 150 fs after the excitation in blue-shifted absorption peak is measured that presents direct evidence for the formation of biexcitons. Intensity-dependent decay constants in the range 13 s -1 give experimental support to the excitonic annihilation process discussed in previous work. Our analysis shows that hopping models for the incoherent exciton transfer via nearest-neighbour interaction are not consistent with the experimental findings.

Parametric generation of femtosecond and picosecond pulses for spectroscopic applications

Parametric three-photon interaction provides continuously tunable ultrashort pulses in extended spectral regions from the UV to the mid-IR. We focus on parametric systems operating in the pulsed mode. Optical parametric oscillators pumped by intense microsecond trains of subpicosecond pulses exhibit favorable properties. We present data on a pulsed optical parametric oscillator that has a wide tuning range from 0.7 to 1.8 μm, a conversion efficiency of a few percent, and a minimum pulse duration of as low as 65 fs. Larger tuning ranges are covered by subsequent downconversion or parametric amplification. We discuss the generation of ≲1-ps pulses at wavelengths between 2 and 18 μm. Finally, the potential of parametrically generated IR pulses is demonstrated by several applications.

Resonant vibrational dephasing investigated by high-precision femtosecond CARS

Abstract Using three-colour coherent anti-Stokes Raman scattering on the femtosecond time scale, the concentration dependence of the dephasing time T2 of the ν2 mode of acetone is carefully studied at room temperature for isotopic dilution and the solvent CCl4. Theoretical arguments suggest that the measured isotopic dilution effect of 10.8±1% represents the contribution of resonant energy transfer via the repulsive part of the intermolecular potential to the total dephasing rate of the neat liquid. The steric factor S accounting for the smaller efficiency of non-collinear interactions as compared to head-on collisions is determined from the experimental data to be S=0.12±0.01, in agreement with theoretical expectations.

Stable generation of subpicosecond pulses by feedback-controlled mode locking of a Nd:glass laser

A high-performance subpicosecond Nd:glass laser with feedback-controlled mode locking is described. Depending on operation conditions, the laser oscillator generates single pulses of 0.80-psec duration and 10-μJ energy or microsecond trains of approximately 400 pulses with 1.3-psec duration and 0.8-mJ total energy. Stable performance, a repetition rate of 6–10 Hz, and a peak-to-background intensity ratio of >5 × 105 are reported.

Intense bandwidth- and diffraction-limited picosecond pulses with large tuning range

A singly resonant optical parametric oscillator is devised with type II phasematching of a KTP crystal and pumping by the second harmonic of a feedback-controlled modelocked Nd: YLF laser. Pulses of 1.5 ps duration and 50 nJ energy are generated over the tuning range 1.2–1.9 μm with time-bandwidth product <0.5 and beam divergence <1 mrad. Subsequent parametric amplification and down-conversion in two LiNbO3 crystals yield single pulses of 2.1 ps and 20 μJ in the wavelength region 2.3–4.2 μm. A repetition rate of 70 Hz and rapid computer-controlled wavelength setting make the pulse generation device well suited for spectroscopic applications.

Coherent pulse propagation in the infrared on the picosecond time scale

Abstract Coherent propagation of resonant ultrashort pulses in gases is investigated theoretically and experimentally at medium pressure values. Drastic changes of the pulse wings extending over several ten picoseconds are predicted and experimentally observed using tunable pulses of 4 ps produced by a parametric generator device and picosecond IR light gates. The dephasing time T 2 of the R(3) vibration-rotation transition of HCl is measured to be T 2 × p = 21.5 ps × bar in the pressure range p = 1 to 3 bar. Quantitative agreement between theory and experiment demonstrates that a novel transient spectroscopy in the infrared can be accomplished by coherent pulse reshaping.

Feedback-controlled mode-locking operation of Nd-doped crystal lasers

Feedback-controlled mode locking (FCM) of Nd:YAG-, YAP-, and YLF-laser oscillators with superior pulse properties is demonstrated. Microsecond trains of ultrashort laser pulses are generated with durations of 11 psec (YAG), 9.8 psec (YAP), and 6.7 psec (YLF), respectively, governed by the fluorescence linewidths. Inserting an antiresonantly tuned etalon in the cavity yields stable operation with even shorter pulses of 6.4, 5.9, and 4.7 psec, respectively, and a reproducibility of ±5%.

Feedback control of an actively-passively mode-locked Nd:Glass laser

Improved performance of a pulsed solid-state laser by electrooptic amplitude control is reported. A sequence of ≃200 pulses is reproducibly generated with 10 Hz repetition rate, 3 ps duration and 300 μJ total energy. The microsecond pulse trains are particularly interesting as a pump source of pulsed fs dye lasers.

Femtosecond relaxation dynamics in the electronic ground state of dye molecules studied by polarization-dependent amplification spectroscopy

Abstract A novel technique of time-resolved polarization spectroscopy permits the investigation of the ground-state dynamics of dye molecules in liquid solution. Relatively long vibrational relaxation times varying from 1.5 to 3.1 ps are reported for rhodamine 6G in different solvents. Evidence is presented for rapid intramolecular energy redistribution within the vibrational manifold on a time scale shorter than 100 fs. Analysis of the experimental data indicates that the dephasing of the vibronic transitions is mainly determined by these redistribution processes.