Kees Uiterwaal

In the Femtosecond Laser Group we investigate the interaction of matter with ultrashort (< 50 fs = 50 × 10^-15s) and intense (>10¹⁶ W/cm²) light pulses generated by lasers.

For such high intensities, easily attained in the focus of our Ti:sapphire laser beam, the electric field of the light pulse exceeds the electric field that holds atoms and molecules together. For instance, a typical atomic-scale electric field is ~10⁹ V/cm, and this corresponds to an intensity of ~10¹⁶ W/cm². Thus, target molecules usually ionize (release an electron) or even break up in the focus of an intense ultrashort laser beam. We collect and analyze the fragments quantitatively, and this gives us new insight into the underlying photodynamical pro-cesses.

We are currently looking into possibilities to modify both the target molecules and the laser beam. By modifying these two key ingredients of our experiments we hope to obtain more detailed information on the roles of the molecular structure and the temporal and spatial shape of the light pulse than would otherwise be possible.Using radio-frequency or microwave sources (to be built and tested), we want to modify the electronic structure of molecules such as O₂. Furthermore, we have plans to use holographic techniques. This would allow us to generate very exotic laser beams, such as the ones in which the electromagnetic energyflows in spirals! The effects of such odd properties of an intense laser beam on molecules are largely unexplored.

The four images on this page illustrate the holographic technique we use in our lab to generate a Laguerre-Gaussian mode LG_p=0^$ℓ=1$, often referred to as a "donut mode". In such a mode, each photon contains one h-bar unit of orbital angular momentum. Top right: theoretical amplitude and phase distribution in the donut mode. In the image, the brightness codes for the intensity, and the hue for the optical phase. This phase varies with the azimuthal angle φ as phase = $ℓ$ φ. The bottom right image shows a computer-generated phase hologram that can be used to generate light modes containing orbital angular momentum. Note the bifurcation. Top left: optical donut created with a HeNe laser and a computer-generated hologram. Bottom left: interferogram of donut mode with plane wave. Note the bifurcation.

We also plan to employ nonlinear optical techniques to modify the spectral content of our laser beam.This may allow us to control the competition between photoionization (loss of one or more electrons) and fragmentation (breaking up of the nuclear skeleton) in molecules.

The Femtosecond Laser Group group collaborates with the Max-Planck-Institute of Quantum Optics in Munich, Germany. This institute has lent us a unique time-of-flight ion mass spectrometer, which 'clips out' a micrometer-size three-dimensional volume from the heart of a laser focus. This allows quantitative measurements of intense-field atomic and molecular processes.

Currently, we have two lasers in the lab: an older 80-picosecond Nd:YAG laser (~80 mJ/pulse), and a state-of-the-art, all-solid-state, sub-50-femtosecond Ti:sapphire laser (~2 mJ/pulse).