M. Wittmann

Spectral superbroadening of femtosecond laser pulses

The spectral superbroadening of femtosecond pulses in water, heavy water, ethanol, and fused silica is studied under strong focusing conditions. Efficient transient stimulated Raman scattering, impulsive stimulated Raman scattering, longitudinally phase-matched parametric four-photon interaction, and cascading light up-conversion and down-conversion are responsible for the spectral superbroadening. Self-phase modulation and cross-phase modulation broaden the laser and stimulated Raman lines.

Fluorescence spectroscopic analysis of indocyanine green J aggregates in water

Abstract Indocyanine green in water forms J aggregates, which are observed as a red-shifted narrow absorption band at 890 nm. The fluorescence quantum distribution of the indocyanine green J aggregates was studied by continuous wave laser excitation at 834 nm. A fluorescence quantum yield of Ф F ≈ 3 × 10 −4 was measured. The fluorescence peak coincides with the absorption peak (no Stokes shift).

Kerr lens effects in a folded-cavity four-mirror linear resonator

The Kerr-lens model locking of a folded-cavity four-mirror linear Ti: sapphire laser is simulated by applying the ABCD ray-tracing technique for a Gaussian beam. The intensity dependent Kerr-lens ABCD matrix of the Brewster-plate shaped active medium is derived. A split-step parabolic refractive index model is used. the astigmatism compensation condition is derived from the ABCD ray-tracing. Simulations are carried out for a symmetric resonator and an asymmetric resonator design. Hard-aperture loss mode locking and soft-aperture gain mode locking are discussed. The optimum design parameters (slit position for hard-aperture loss mode locking, pump beam focusing for soft-aperture gain mode locking, gain cavity spacing, gain medium position) for Kerr-lens mode locking are determined.

Generation of frequency tunable femtosecond pulses in a cw pumped linear dispersion-balanced passive mode-locked rhodamine 6G dye laser

Abstract Frequency tunable femtosecond light pulses are generated in a cw pumped linear group velocity dispersion balanced passive mode-locked rhodamine 6G dye laser by birefringent filter tuning. The saturable absorbers DODCI (duration Δ t L t L t L

Concentration-dependent absorption and emission behaviour of sulforhodamine B in ethylene glycol

Abstract Absorption cross-section spectra, refractive index dispersion, and fluorescence quantum distributions of sulforhodamine B in ethylene glycol are measured in the concentration region between 10 −5 and 0.5 mol/dm 3 . The fluorescence quantum yields are separated into monomer and closely spaced pair contributions. The absorption and stimulated emission cross-section spectra of the monomers and closely spaced pairs are resolved.

Photoisomerization dynamics of DODCI studied by spectrally resolved fluorescence decay measurements

Abstract The organic dye DODCI (3,3′-diethyloxadicarbocyanine iodide) in methanol and ethylene glycol is excited with picosecond light pulses at 527 nm (dominant N-isomer excitation) and the time dependence of the N-isomer and P-isomer fluorescence is monitored with a spectrometer-streak camera system. The slowed-down non-exponential N-isomer fluorescence and the time-delayed P-isomer fluorescence indicate S 1 -state photoisomerization via an intermediate state (M). The energetic position of this intermediate state is determined. The formation of a further photoisomer (P2-isomer) emitting at the short wavelength side of the N-isomer is observed. Additionally the molecular reorientation is analysed by time-dependent fluorescence anisotropy measurements.

Theoretical analysis of contributions of self-phase modulation and group-velocity dispersion to femtosecond pulse generation in passive mode-locked dye lasers

The influence of self-phase modulation (SPM) and group-velocity dispersion (GVD) on pulse development in a passive mode-locked dye laser is investigated numerically by using fast Fourier transformations. The situation of positive SPM is considered. Four regions of laser performance may be distinguished: (i) In the positive GVD region pulse broadening by GVD must be balanced by the pulse-shortening effect of the saturable absorber. (ii) In a region around zero GVD the laser is periodically self-quenching and the temporal and spectral pulse shapes change periodically similar to higher-order solitons. (iii) It follows a negative GVD region where stable pulses of smooth temporal and spectral shapes are generated similar to fundamental solitons. In this region the pulse duration is practically independent of the saturable absorber concentration, and the saturable absorber is needed mainly for background suppression. (iv) Further increasing the negative GVD, pulse broadening must be balanced by the saturable absor ber pulse shortening.

Spectroscopic Characterization of Semiconductor Doped Colour Filter Glasses

Fresh and photodarkened Schott RG645 and RG665 colour filter glasses are investigated in detail by absorption and emission spectroscopy. The CdSxSe1-x volume fraction is deduced from absorption measurements applying Maxwell Garnett theory. The cluster size is determined by Rayleigh scattering measurements. Groundstate and excited-state absorption cross-sections as well as the number density of interacting states are determined from nonlinear transmission measurements using a femtosecond dye laser. The temporal absorption recovery is studied. The fluorescence quantum distributions, quantum yields, and degrees of polarization are determined. A relaxation scheme in an energy band diagram is presented and dynamic parameters are deduced.

Femtosecond pulse generation around 650 nm in a passive mode-locked Kiton Red dye laser

Abstract Sub 100 fs light pulses have been generated in the spectral region between 640 and 660 nm (the shortest duration being 50 fs at 653 nm) by passive mode-locking of a Kiton Red dye laser with the saturable absorber DQTCI.

Saturable Absorption Dynamics of a Semiconductor Doped Glass

Semiconductor doped glasses are used as short-wavelength cut-off filters. They are applied in nonlinear optical studies1. Here the commercially available colour filter glass RG645 (CdSxSel-x microcrystallites embedded in crown glass) from Schott is investigated. Fresh (not exposed to laser light) and photodarkened samples (exposed to 2000 J cm-2 accumulated energy density of 5 ns pulses at 532 nm) are studied. Relevant parameters (composition, x; particle diameter, d; volume fraction of microcrystallites, p; number density of microcrystallites, N cr ; number density of CdSxSl-x formula units in particles, N 0) have been determined and are listed in Table 1. The microcrystallite radii are larger than the effective Bohr radius, a B (a B, CdS ≈ 3 nm, a B, CdS ≈ 4.5 nm). Therefore the microcrystallites behave like bulk semiconductor material and quantum size effects may be neglected1.