High harmonic generation (HHG) is a highly nonlinear phenomenon in strong field light-matter interaction. HHG process can be interpreted by the classical three-step model. First, coulomb potential is suppressed by the laser field and the electron is ionized. Then, the electron is accelerated. At last, the laser field is reversed and parts of electrons are recombined with the parent nucleus. The high energy photons are radiated. HHG has been an attractive topic for its preeminent applications,such as attosecond XUV pulse generation. In the past decades, the linearly polarized attosecond pulse has been studied in many works. A linearly polarized attosecond pulse is one-dimensional, and the range of its applications is greatly restricted. The non-linearly polarized laser pulse has been a hot spot in recent years. Very recently, the bichromatic counterrotating circularly polarized (BCCP) field has attracted a great deal of attention. It consists of two coplanar counter-rotating circularly polarized fields having the angular frequencies  and 2. This field has two advantages for HHG: (1) the generated individual harmonics are circularly polarized, and (2) the conversion efficiency of HHG needs to not be compromised. In parallel with the realization of generation of circularly polarized harmonics, it was shown that the polarization state of individual harmonics can be controlled in a recent experiment. However, in practical experiment, the controllability of the ellipticity of the attosecond pulse is widely demanded. And such a practical method is very challenging.

The ultrafast optics group led by Prof. Peixiang Lu presents a novel approach to generate attosecond XUV pulses with tunable ellipticity from aligned molecules irradiated by a BCCP laser field. By rotating the BCCP field, the attoseond XUV pulse varies from being left elliptically polarized to right elliptically polarized. The rotation of the BCCP field can be easily achieved by adjusting the relative phases between the two components of the BCCP laser field. This method relaxes the experiment conditions and it benefits a broad range of applications.

This work is published on Opt. Lett. Vol. 42, NO. 6, 1027 (2017). This work was supported by National Natural Science Foundation of China (NSFC) (11234004, 11404123, 11422435, 11627809, 11574101).

Figure: The schematic of the driving field and the molecular orbital; (a ) and (c) the attosecond pulse; (b) and (d) the electric field of the pulse.