High-order harmonic generation (HHG) by the interaction of intense laser pulses with atomic [1], [2] and molecular [3], [4] gases has been researched extensively for producing coherent attosecond pulses [5], [6]. These ultrashort pulses offer unprecedented temporal resolution in observing and controlling electron and nuclear dynamics [7], [8], [9], [10]. In the past ten years, most HHG works focused on the linearly polarized isolated attosecond pulses. The researchers put forward lots of constructive schemes, such as the polarization gating [11], three-color field control [12], and spatially inhomogeneous field [13] and so on. Compared with linearly polarized attosecond pulses, circularly or elliptically polarized attosecond pulses have additional degrees of freedom and abundant applications [14], [15], [16], such as the study of optically induced spin and orbital momentum transfer.

To date, various laser schemes have been suggested for generating circularly polarized harmonic. One useful scheme is the elliptically polarized laser pulse [17], [18]. However, as the ellipticity of laser pulse increases, the harmonic efficiency drops dramatically, and the harmonic ellipticity is small. Yuan et al. [19] theoretically investigated the single circularly polarized attosecond pulse generation. The harmonic efficiency in this method is still unimproved. Kfir et al. [20] experimentally proposed that the interaction between the single atom and the counter-rotating bichromatic driving field can be used for generating elliptically polarized high-order harmonics with considerable intensity. In recent work, the polarization control of the isolated attosecond pulse has been achieved by the non-collinear scheme in experiments [21], [22]. The orthogonally polarized two-color (OTC) laser fields, involving only a simple optical set-up of frequency doubling crystal, can be used to control the ellipticities of the odd and even order harmonics [23], [24], [25], [26]. In addition, the two-color fields contain much more control parameters, which helps to enhance the harmonic efficiency and reduce the duration of attosecond pulse [5], [27], [28]. Except for changing the driving laser fields, researchers also try using the specified targets [29], [30], [31], [32].

The ellipticity-tunable high-order harmonic has attracted experimental and theoretical attention. The experiment performed by Fleischer et al. [33] presented the polarization state of high harmonics driven by bichromatic elliptically polarized beams. Zhang et al. [34] theoretically demonstrated that an attosecond XUV pulse with controllable ellipticity can be generated by changing the relative phase of the bichromatic counterrotating circularly polarized field. Zhu et al. [35] theoretically illustrated that the ellipticity of harmonic from the He atom can be controlled through adjusting the intensity ratio of the laser fields. In practical applications, the continuously controlled polarization state of isolated attosecond pulses is still desired [36], [37], [38], [39].

In this paper, we suggest a method to produce a single attosecond pulse with a continuously controlled polarization state. The ellipticity of HHG from the H2+ molecule is investigated by using the OTC laser fields. Based on solving the time-dependent Schrödinger equation (TDSE), we show that the ellipticity of harmonic depends on the internuclear distance R of H2+. In our laser conditions, the ellipticity of harmonic is near zero at R=2a.u., while, when R=6a.u., a resonant transition from the 1sσg to 2pσu states is triggered by the fundamental field, which can lead to the generation of elliptical harmonics. We present the electron current induced by the electron wavepackets of H2+ at R=6a.u., which shows a clear picture of the dynamic process of HHG. With specific intensity of the OTC field (4×1014 W/cm2), the ellipticity tunable isolated attosecond pulse with pulse duration of 293 as can be obtained by synthesizing the harmonic spectrum with different orders.