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Since its first experimental demonstration in 1960 the laser became an indispensable tool for investigating the nature of matter across a large wavelength range. To further extend this range nonlinear optical effects, e. g. second or third harmonic generation to name only a few, can be employed to convert the laser light into spectral regions, which cannot be addressed by lasers due to the lack of suited gain materials. Especially, the generation of extreme ultraviolet (XUV) radiation or even X-rays directly with a laser is hindered by the wavelength dependence of the Einstein coefficients. During the last years, the strong-field process of high harmonic generation (HHG) has been of strong interest, because it allows for the conversion of laser light into the XUV spectral region, potentially resulting in pulses with attosecond pulse durations. A plethora of applications emerged, investigating ultrafast processes in atoms or molecules on an attosecond timescale. The requirements on the driving laser to generate such isolated attosecond pulses (IAPs) via HHG are very stringent. It has to provide gigawatt level peak powers, few-cycle pulse durations and a constant carrier-envelope phase (CEP) from pulse to pulse. In addition, the driving laser should deliver high average powers to compensate for the very low conversion efficiency (~ 10−6) of HHG, which impairs acquisition times, statistics or signal-to-noise ratios of any subsequent application. The objective of this thesis is to develop a laser system based on optical parametric chirped pulse amplification (OPCPA) delivering CEP-stable sub two-cycle pulses with peak powers sufficient to drive the strong-field process of HHG. Strategies to reduce detrimental thermal effects are developed to increase the output average power to unprecedented values. Strong emphasis is put on the investigation and the mitigation of effects limiting the achievable compressed pulse duration and CEP stability at ultra broadband operation of OPCPA systems. The result is a high average-power source of precisely wave-form controllable two-cycle pulses. This unique laser system is applied to the process of HHG to generate XUV radiation with different spectral and temporal properties. The excellent stability, even at high average power operation, allowed to increase the repetition rate of IAPs by two orders of magnitude compared to conventional sources.
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High average-power few-cycle OPCPA system for strong-field applications, Ernst Stefan Demmler
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- 2016
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