Realization of Polarization Control in High-Order Harmonic Generation

Ultrafast optical pulse generation plays important role in pump/probe measurement for the fastest dynamics study in atoms, molecules and materials. Among all of nonlinear effects to generate short pulses, high-order harmonic generation (HHG) opens a path towards the production of the shortest light pulses, with durations in the order of the attosecond time scale. The generation mechanism of HHG is easily understood by the semi-classical three-step model: an electron is ionized and accelerated by an intense driving electrical field; after half-cycle of the driving pulse, the electron is pulled back, re-colliding with the parent ion [1,2]. The kinetic energy accumulated during propagation in the continuum is released as high-energy -or high-frequency-radiation, ranging from the extreme ultraviolet (EUV) to the soft X-rays. Due to the nature of the HHG process, attosecond bursts are emitted every half cycle of the infrared (IR) laser field, resulting in the generation of an attosecond pulse train when multi-cycle driving pulses are used. This fact deprives the attosecond time scale of the HHG radiation, since the envelope width of the APT is usually in the range of several femtoseconds. Many approaches have demonstrated the production of isolated attosecond pulses, giving access to study attosecond dynamics [3,4]. However, those methods are limited to linearly polarized fields. When an elliptically polarized driving laser field is applied, the electronic wavepacket is driven away from the parent ion, decreasing the probability of recombination and thus significantly dropping the harmonic yield. The availability of circularly polarized or elliptically polarized attosecond pulses is particularly interesting for the study of dynamics in circular dichromic metrology of magnetic materials, chiral molecules and electronic spin of matter waves [5,6]. The great challenge of HHG still remains in this community: how to control the polarization state of attosecond pulses. Though large facilities, such as synchrotrons or x-ray free electron lasers (XFEL), provide alternative access to circularly polarized EUV or X-ray radiation with high brightness (by means of specially designed undulator systems [7]), their access is limited. In addition, the shortest pulse duration reported to date in XFELs is in the order of tens of femtoseconds, which imposes a limitation for investigating fast dynamics of electron spin motion. Therefore, polarization control of attosecond pulse on tabletop EUV light source becomes essential. In this talk, I will review the recent progress to produce elliptically or circularly polarized attosecond pulses. In particular, we show that today complete polarization control of HHG is available, through adjusting the ellipticity of two input driving pulses with counter-rotating helicity in a non-collinear geometry [8]. This solves the great dilemma of polarization control in HHG, paving the way towards attosecond metrology in chiral systems and magnetic materials.