We present a quasi-cw laser in vacuum ultraviolet region at megahertz repetition rate. The narrowband pulses generated from an ytterbium-fiber laser system at 33 MHz repetition rate at the central wavelength of 1074 nm is frequency-converted by successive stages of LBO crystals and KBBF crystals. The generated radiation at 153 nm has the shortest wavelength achieved through phase-matched frequency conversion processes in nonlinear optical crystals to our knowledge.
The temporal characteristics of the harmonic emission from solid targets irradiated with intense laser pulses is
examined in detail. In the case where the CoherentWake Emission mechanism is dominant it is found that indeed
the harmonics thus produced possess a frequency chirp resulting in non Fourier-Transform-Limited pulses. A
simple model explains the underlying physics while Particle-In-Cell simulations support the conclusions drawn.
The interaction of relativistically intense (Iλ<sup>2</sup>>>1.3 10<sup>18</sup>Wcm<sup>-2</sup>μm<sup>2</sup>) laser pulses with a near step-like plasma density
profile results in relativistic oscillations of the reflection point. This process results in efficient conversion of the incident
laser to a phase-locked high harmonic spectrum, which allows the generation of attosecond pulses and pulse trains.
Recent experimental results on efficiency scaling, highest harmonic generated and beam quality suggest that very high
focused intensities can be achieved opening up the possibility of ultra-intense attosecond X-ray interactions for the first