XUV Interferometry Using High Order Harmonics: Application to Plasma Diagnostics

Abstract

High order harmonic generation in gases is now a widely studied non linear process. The research in this domain shows two main directions. On the one hand, researchers are working on the different fundamental aspects involved in high order generation. On the other hand, they have developed several applications of this unique XUV source. High order harmonics (HOH) present a good beam quality, which allows to focus the harmonic radiation to very tight spots¹–³, a high brightness and a high repetition rate (up to 1 kHz)⁴ –⁶. Since high order harmonic generation is a field driven process, the properties of the laser that is used to generate the harmonics are partially transferred to the XUV radiation. As a result, the harmonic pulse presents a short duration, equal to or shorter than the generating laser pulse duration ⁷ ,⁸ depending on the harmonic order. The harmonic radiation is also naturally synchronized with the fundamental driving IR laser. These two last properties are very important for all experiments of the pump probe type. Up to now high order harmonics have already been used in atomic and molecular spectroscopy⁹,¹⁰. This type of experiments can also be applied in solid state physics for instance to probe the relaxation time of electrons that have been excited in the conduction band of a dielectric material¹¹. Another important property of the harmonic source is the good temporal¹² and spatial¹³–¹⁵ coherence that can be obtained in some generation conditions. Moreover, a very unique property in this XUV range is the possibility to produce two mutually coherent sources either separated in space or in time. In the following, we discuss the generation of these phase-locked harmonic sources and their applications to plasma diagnostic.