We present a multi-octave, mid-infrared supercontinuum source spanning from 3.6μm and extending into long-wave infrared region out to 11μm with an exceptionally high conversion efficiency of 8.2% and output power of 39 mW. These results were enabled by intra-pulse difference frequency generation involving a femtosecond Thulium doped fiber laser, an indium fluoride fiber and a zinc germanium phosphide crystal.
High harmonic generation (HHG) is a useful source of coherent light in the extreme ultraviolet (EUV) region of the spectrum. However, both the conversion efficiency and the highest achievable photon energy have in the past been limited in the past by the inability to phase-match the frequency conversion process. In this paper, we summarize recent results on the development of new techniques for phase-matching the high-harmonic conversion process. We also summarize finding from three series of experiments that make use of the coherent EUV light generated using HHG: 1) probing of acoustic dynamics in materials; 2) monitoring of chemical dynamics at surfaces using photoelectron spectroscopy; and 3) time-resolved plasma imaging.
We present a simple setup for obtaining high resolution, sub-micron images using high harmonic generation (HHG) in a hollow-core waveguide as a light source. We demonstrate imaging with illumination at a wavelength of 30 nm using an all-reflective, double-multilayer mirror setup and a CCD camera as a recording device. For the magnifications of up to 50x used here, the all-reflective setup has advantages over zone plate microscopes because of the much larger working distances that allow for imaging of plasmas. This setup has also a throughput that is higher by at least a factor of three compared to zone-plate microscopes, and presents the additional advantage of preserving the temporal pulse width of the harmonics because diffractive optics are not used. This work demonstrates the feasibility of high-spatial-resolution, time-resolved, EUV imaging of plasmas and other objects using a tabletop compact light source.
As ultrafast laser technology advances, it is of importance to evaluate the potential of sub-100-fs laser pulses for laser surgery. We have extended the investigation of laser- induced optical breakdown on hard and soft tissues down to laser pulse widths of 20 fs. Powerful 20-fs to 100-ps pulses from a single Ti:sapphire oscillator/amplifier laser source at 800 nm were focused in vitro onto the surface of fresh human corneas and human enamel to a spot of 60 - 70 micron in diameter. The threshold for ablation was determined by increasing the pulse energy while monitoring scattered probe light at ejected ablation particles. Our experiments show a slower decrease of the threshold fluence in dependence of the pulse width in the femtosecond regime than in the picosecond regime. Unlike previously suggested, no saturation behavior could be observed at the shortest available pulse widths. For the shortest pulses with 20 fs width, we measured a threshold of 0.38 J/cm2 and 0.42 J/cm2 for cornea and enamel, respectively. For the longest pulses at 100 ps, the threshold fluence was 4.3 J/cm2 and 2.06 J/cm2, respectively. Comparison to theoretical models and to previous data determines the contribution of multi-photon and avalanche processes. Our results suggest an optimum laser pulse width of several hundred femtoseconds for most applications in ultrashort pulse laser surgery.
We report on a Ti:sapphire amplifier system which produces 21 fs, 1 mJ pulses at 1 kHz. A nanojoule-level seed pulse makes 8 passes through a highly doped Ti:sapphire crystal for a total gain of approximately 106. The crystal is pumped with 12 mJ of 527 nm light which is converted into amplified pulse energy with an efficiency exceeding 10%. We have used this laser system to generate ultra-short UV light pulses through third harmonic generation. By focusing the 21 fs pulses in argon and other gases (including air), we obtain UV pulses with energy 1 (mu) J and wavelength 267 nm. The duration of the UV pulses was measured to be approximately 16 fs, representing the shortest pulses in this wavelength range measured to date.
Full characterization of the electric field amplitude and shape of an ultrashort optical pulse is important for a number of fields of research, such as high-field atomic and plasma physics, and the generation of ultrashort-pulse XUV and soft x-ray radiation. We have used the technique of frequency-resolved optical gating (FROG) to characterize low-energy pulses as short as 13 fs duration, and high peak-power pulses as short as 25 fs. We discuss experimental considerations for implementing FROG for very short pulses, and the limitations of the technique.
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