Titel : Absorption-,Emission- and Photoisomerisationdynamics of the dye DODCI, studied with picosecond- and femtosecondlaserspectroscopical methods

Author : Michael Wittmann

Supervisor : Prof. Dr. Alfons Penzkofer

Submitted : 14.06.1996

Abstract :

The organic dye DODCI (3,3'-diethyloxadicarbocyanine iodide) is widely used as saturable absorber for femtosecond pulse generation in passive mode-locked dye lasers. If illuminated by light, the dye forms isomers (N-isomer and P-isomer), which can be distinguished with spectroscopical methods.

The aim of this work is a better understanding of the isomerisation dynamics and the determination of spectroscopic parameters, which are important for the use of DODCI as saturable absorber or laser dye (excited-state absorption and relaxation times).

The dynamics of the isomerisation process was studied by recording the spectrally and temporally resolved fluorescence with a picosecond spectrometer-streak camera arrangement. The transfer occurs in the first excited singlet state via an intermediate state from the N-isomer to the P-isomer. The lifetime and energetic position of the intermediate state was determined.

The saturable absorption was measured with a tunable femtosecond dye laser amplifier system in the wavelength range from 580 nm to 630 nm for the N-isomer. Therefore the intensity-dependent transmission of a laser pulse was measured. For a better understanding of the bleaching behaviour, an energy level model was developed. The excited-state absorption cross sections and relaxation times were determined.

By accumulating the dye in the P-isomer state (by cw laser illumination), it was possible to study the bleaching of the P-isomer at 626 nm.

Both isomers show saturable absorption in the region between 580 nm and 630 nm, bleaching occurs also via the first excited-state to higher lying states.

The excited-state absorption was determined in the region of 500 nm to 750 nm. The dye was excited by a intense pump-pulse and probe with a delayed white light probe-pulse. The continuum was generated by focusing an intense laser pulse in a heavy water cell.

For wavelengths below 530 nm, DODCI shows revers saturable absorption, between 530 nm and 680 nm the dye is a saturable absorber. DODCI can be used as laser medium in the range of 580 nm to 680 nm.