Intense Femtosecond Laser

In our lab (located in B61 Behlen) we operate our Ti:sapphire laser system. This system, manufactured by Spectra Physics, delivers ultrashort light pulses with a repetition rate of 1 kHz (1000 pulses every second). Each single pulse has a duration of only 45 fs, and carries an energy of more than 2 mJ. The central wavelength of our pulses is close to 800 nm, i.e. their wavelength falls in the near-infrared. The picture on the left shows parts of the regenerative and multi-pass amplifiers of the laser system. The two dominating colors in this picture are green (the color of the pump beams) and red (the visible part of the broadband Ti:sapphire radiation).

Because of their extreme short duration, our laser pulses are really light bullets traveling through space. Their longitudinal dimension is (speed of light) × (pulse duration) = 15 micron! If we do not focus our pulses, their peak intensity is on the order of 10¹⁰ W/cm². When we do focus them, to a focal spot of, say, 50 micron, the peak intensity in the focus given by (energy) / (duration × focal spot area) exceeds 10¹⁵ W/cm². This sort of intensities has two important effects, both of which are studied in our lab. First of all, for intensities like this the optical properties of the medium the laser beam travels through can no longer be taken constant. To give an example, the index of refraction will start to depend on the intensity. On the other hand, a change in the index of refraction will have an influence on the beam path. So the medium and the radiation get closely interconnected in a nonlinear way. A dramatic demonstration such nonlinear effects (and other phenomena) can be given by focusing the laser in atmospheric air using a simple lens (focal distance ~15 cm). The picture in the background gives an impression of the visible colors generated in such a focus as they appear on a screen that is about 50 cm behind it. A second effect that we study is ionization of gaseous targets. The electric fields found in our focused pulses are similar in magnitude to the electric fields that are typically found in atoms and molecules and keep these species together. When we shine our focused pulses on a target gas, we typically observe ionization (release of one or more electrons) and/or fragementation (break-up of whole molecules). The resulting ions are detected using a time-of-flight ion mass spectrometer that we operate in collaboration with the Max-Planck-Institute of Quantum Optics in Garching (near Munich), Germany.

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