Systematic investigation of ablation harmonics are performed for silver and indium targets, using the 40 mJ, 25 fs output
from the Advanced Laser Light Source. Optimum pre-pulse and main pulse conditions for ablation harmonics are
studied.
We demonstrate the first observation of significant resonance enhancement of a single high-order harmonic in the extreme ultraviolet region. Such intense harmonics are generated during the interaction of a femtosecond Ti:sapphire laser pulse with low-ionized indium ablation. A strong 13th harmonic (61.2 nm) with conversion efficiency of 8×10-5 and output intensity almost two orders of magnitude higher than neighbouring harmonics is demonstrated. Such an approach paves the way for efficient single-harmonic enhancement in the extreme ultraviolet range using different ablation sources.
We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited boron plasma produced by a prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. The harmonic generation in these conditions assumed to occur due to the interaction of femtosecond pulses with ions. The conversion efficiencies in the plateau region were varied between 10-7 to 8×10-5 depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the free-electrons-induced self-defocusing of main pulse.
Past works have used high-order harmonics in gas targets to demonstrate attosecond pulse generation. However, recent theoretical simulations have shown that solid-surface harmonics can also be used to produce attosecond pulses. Solid-surface harmonics are generated when a high-intensity femtosecond laser pulse irradiates a solid target surface at an oblique incidence angle. The conversion efficiency of this phenomenon increases rapidly with increasing pump laser intensity, and there is also no presently known upper limit in the pump intensity that can be used. Accordingly, this method possesses the potential for high-energy attosecond pulse generation. The main aim of this paper is to experimentally clarify the optimal conditions for highly efficient solid-surface harmonic generation. We demonstrate up to the 16th harmonic (49.1 nm wavelength) of a Ti:sapphire laser using modest pump intensities of 4×1016 W cm-2 irradiating a silicon wafer target. Investigations with low-order harmonics have revealed a large dependence of harmonic conversion efficiency on the target material. Furthermore, a drastic increase in the harmonic intensity has been observed by repetitively irradiating a metallic-coated target.
Backscattered 2 ω and 3 ω/2 harmonics were investigated during the interaction of femtosecond radiation ( λ=795 nm,
t=150 fs, 10-Hz pulse repetition rate) with various targets. The harmonics were generated inside the drilled hole without
changing the position of target from shot to shot. No 3 ω/2 harmonic was generated in the case of single shots
irradiated the fresh surface of the target. Various characteristics of harmonic radiation were analyzed and their
dependences on pump radiation parameters were discussed.
Simulations are performed to clarify the mechanisms that generate high spatial coherence x-ray lasers with the longitudinally pumped nickellike molybdenum scheme. Various factors that affect the x-ray laser output is also investigated, and we clarify the experimental conditions that maximize its performance.
We review the present status of our research on the generation of coherent soft x-ray radiation using high- intensity picosecond Nd:glass laser pulses. We adopt two completely different methods for this purpose, that is high- harmonic generation from solid-vacuum interfaces, and longitudinally pumped transient collisional excitation Ni- like Mo x-ray laser. Unexpected jetlike structures are observed in the visible emission of the longitudinally- pumped molybdenum plasma, extending over a length of several millimeters. Efficiency of harmonic generation from near- solid density plasma is found to increase by a factor of 2 to 3 when using prepulses.
The conditions necessary for large gain-length products in longitudinally-pumped Ni-like Mo x-ray lasers are clarified using a 1D hydrodynamics code. Ray tracing shows that for effective amplification within the high density ultra-high gain region, waveguides such as capillaries must be used. The effect of inhomogeneous pumping of the capillary on gain is also evaluated.
Investigations have been performed on the spatial distributions of high-order generation from solid surface plasmas. It is found that the divergence of the harmonics gradually increases at intensities between I(lambda) 2 equals 2 X 1015 and 2 X 1016 X cm-2 X micrometers 2 ps, 1054 nm laser pulse with typical maximum intensities of approximately 2 X 1017 W X cm-2. We experimentally observe that the divergence of the harmonics decreases as the order of the harmonic increases. We also measured that the divergence of the third harmonic emission is smaller for 2.2 ps width lasers as compared with those for 100 ps pulse width lasers.
Solid-state laser and x-ray laser research activities at the Institute for Solid State Physics of the University of Tokyo are reviewed. A description of the four-beam multi-TW neodymium- doped phosphate glass (Nd:Glass) laser system is given. Advanced laser research, such as the development of a table-top ultra-short pulse TW Nd:Glass system and phase conjugate system, are also reviewed. Studies on recombination pumped soft x-ray lasers with He-like ions are presently underway. Picosecond x-ray spectroscopy of laser produced aluminum plasmas revealed population inversions between the n equals 3 - 2 levels of He-like Al ions. Details on the first observation of amplification in a recombination pumped He-like soft x-ray laser, showing a time-resolved small-signal gain of 3 cm-1 for the 31D - 21P transition of He-like N, are also given.
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