## 1 Introduction  
Isolated attosecond pulses (IAPs) via high-order harmonic generation (HHG) have opened up a new branch of ultrafast science called attosecond physics [1]. In the past two decades, great advances have been made in attosecond physics, including the generation of IAPs with even shorter duration [2, 3] and the real-time observation of electronic dynamics in atoms, molecules, and solids (see, for example, [4, 5, 6, 7]). The first IAP source was demonstrated in the extreme ultraviolet (XUV) region (10-124 eV) using a few-cycle Ti:sapphire laser system (\(\lambda=0.8\) um) [8]. IAPs have been utilized with absorption spectroscopy to trace the ultrafast electron/ion dynamics in atoms and molecules. However, the efficient generation of IAPs driven by a Ti:sapphire laser has been limited to the XUV region. This is mainly due to the phase mismatch induced by free electrons from plasma because a high intensity greater than the ionization threshold of the medium is required for the generation of soft X-ray harmonics by a 0.8 um Ti:sapphire driving laser [9]. To extend the cutoff energy of HHG up to the water window region [2, 10, 11, 12, 13], [PERSON] and co-workers proposed and demonstrated the use of a mid-infrared (MIR) laser for HHG in 2008 [14, 15]. In contrast to the commonly used Ti:sapphire laser in the near-infrared region, the longer-wavelength MIR driving laser enables the HHG in the soft X-ray region with a neutral gas medium [14]. Then, the phase-matching technique [16, 17, 18, 19, 20] can be employed and the output HHG yield in the soft X-ray region can be significantly improved by the energy-scalingmethod [21; 22]. In addition, a longer-wavelength laser is advantageous for reducing the atto-chirp of IAPs, which is given as \(\propto\lambda^{-1}\)[23]. On the basis of these advantages, the wavelengths of driver lasers for generating IAPs have been shifting towards MIR wavelengths region [24]. In recent years, most HHG experiments with the aim of generating shorter IAPs in the soft X-ray region have employed few-cycle MIR laser pulses [2; 3; 12; 13]. Indeed, the IAP duration has reached \(43\pm 1\) as around the XUV region [3] and \(53\) as in the soft X-ray region [2]. However, IAPs generated using an MIR pulse face two principle problems even though the pulse duration was reached sub-50 as; one is the need for a method to compensate for attochirp, and the other is the reduced conversion efficiency due to the longer driver wavelength [25]. To compensate for the attochirp of HHG, a metal filter is generally employed [26]. We show the transmission and group delay dispersion (GDD) of different types of filters with a thickness of 1000 A in Figure 1. From Figure 1b, it is clear that these metal filters are not helpful for compensating the attochirp of IAPs owing to the low GDD values in the soft X-ray region. Moreover, it is difficult to use a thick filter to increase the GDD owing to the low transmissivity. Even though super broadband continuum soft X-ray harmonics that supports an IAP with a sub-10-as transform-limited (TL) duration can be generated by a single-cycle MIR laser; unfortunately, the actual IAP duration increases to over few-lens-as owing to the attochirp. Therefore, the reduction of the attochirp is extremely important to create an IAP with short duration. In addition, the lower conversion efficiency for a longer laser wavelength is a serious problem for generating an intense soft X-ray IAP. It has been demonstrated that the HHG efficiency is scaled as \(\lambda^{-5}\)-\(\lambda^{-6}\) with the driving laser wavelength \(\lambda\)[25; 27]. Thus, the HHG yield is low usually in the soft X-ray region, and the applications of soft X-ray IAPs have been generally demonstrated by an IR/soft-X-ray cross-correlation method or single photon absorption process. It is clear that the development of a high-energy MIR laser with a few-cycle duration is of paramount importance for creating an intense soft X-ray IAP.