Research Index / High-Field Technology / Ultrafast Photon Source

Focal spot after optimization of deformable mirror: 1/e2 radius is 0.65 micron. Sub-10-fs pulses are also generated from the system using a hollow fiber and chirped mirror compression. These pulses can be focused to 1.3 micron to deliver an anticipated 1018 W/cm2, bringing relativistic studies with single-cycle, single-wavelength pulses within reach.

Using supplemental money from an MRI grant (NSF Major Research Instrumentation Program), a kilohertz laser has been built that produces millijoule (mJ) pulses at a kilohertz (kHz) repetition rate, while providing ultrashort duration.  Following amplification from the nanojoule (nJ) to the mJ level in two stages of a multi-pass amplifier, 3 mJ pulses were obtained with 21- to 24-fs duration.  These pulses are directed to a telescope, a deformable mirror and an off-axis paraboloid to produce intensities in the relativistic domain.  Rather than using the wavefront measurement methods discussed in previous reports, a genetic algorithm was used to optimize the wavefront of the focusing beam by maximizing the output of an SHG crystal.  This method has allowed us to obtain a 1.3-micron focus (see figure at right) and to produce ionization of Ar up to the 11th or 12th stage.  This gives direct evidence for focused intensity of a few times 1018 W/cm2.  The threshold for relativistic effects involving electrons is 2x1018 W/cm2.

High harmonic generation (HHG) has continued to be a subject of interest.  By applying the (I lambda2) scaling principle, it is apparent that shorter wavelengths might be generated by longer wavelengths.  To demonstrate this principle, an OPA was constructed to produce ultrashort pulses at 1.55 micron.  Experiments indicate that the scaling law does hold for HHG and that further experiments with greater pulse intensity should provide tunable coherent X-Rays beyond the range accessible with an 800-nm laser.

Applying the pulses from the Ultrafast Photon Source to the study of laser-solid interaction, preliminary results indicate that a few-micron hard X-ray source can be made.  Such a source is expected to have applications in diagnostic X-Rays for mammography, provided that scaling to higher average powers can be demonstrated.

Using transient absorption, experiments are under way to investigate ground and excited state reaction dynamics in simple polyenes.  In addition, an investigation of the specific influence of solvent-solute interactions on vibrational relaxation and thermalization has been set up.  These experiments will utilize the wide bandwidth available from the kHz Ultrafast Photon Source.

Research Index / High-Field Technology / Ultrafast Photon Source

Copyright Center for Ultrafast Optical Science, University of Michigan

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