Avalanche effect and gain saturation in high harmonic generation

TL;DR

This paper demonstrates that high harmonic generation in helium gas acts as an amplifier exhibiting avalanche and saturation effects, confirmed through experiments and modeling with attosecond pulse trains in the extreme ultraviolet range.

Contribution

It provides a theoretical and experimental demonstration of avalanche and saturation effects in high harmonic generation as an optical amplifier for EUV pulses.

Findings

Amplification occurs only with perfectly synchronized seed pulses.

Helium gas acts as an effective amplifier for EUV pulse trains.

Experimental results agree with model calculations.

Abstract

Optical amplifiers in all ranges of the electromagnetic spectrum exhibit two essential characteristics: i) the input signal during the propagation in the medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth and ii) the amplification saturates at increasing input signal. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of both the avalanche and saturation effects in the amplification of extreme ultraviolet attosecond pulse trains. We confirm that the amplification takes place only if the seed pulses are perfectly synchronized with the driving strong field in the amplifier. We performed an experimental study and subsequent model calculation on He gas driven by intense 30-fs-long laser pulses, which was seeded with an attosecond pulse train at 110 eV generated in a separated Ne gas jet. The comparison of the performed calculations with the measurements clearly demonstrates that the pumped He gas medium acted as an amplifier of the extreme ultraviolet pulse train.