Carrier-envelope phase-dependent high-harmonic generation in the water window using a few-cycle infrared light source

Most of strong-field experiments such as high harmonic generation (HHG), attosecond pulse generation, and their applications rely on femtosecond Ti:sapphire lasers around 800 nm. Currently the photon energy range in HHG is limited to 200 eV or less for spectroscopic applications. To overcome this barrier, an IR source is promising because the ponderomotive potential is proportional to the square of a laser´s wavelength, therefore, we expect the extension of an electron kinetic energy toward ~keV [1]. However an infrared (IR) light source must meets all requisites for attosecond pulse generation in soft X-rays such as few-cycle pulse duration, carrier-envelope phase (CEP) stabilization, mJ output energy, and high repetition rate. Although many IR systems have been constructed so far, CEP-sensitive strong field phenomena have not been exploited so far because of difficulty in meeting all the requisites. We have constructed a novel optical parametric chirped pulse amplifier (OPCPA) by use of BiB₃O₆ crystals and Ti:sapphire lasers. The OPCPA produces 9.0-fs, 550-μJ, CEP-stabilized optical pulses around 1.6 μm at 1 kHz [2]. In this presentation, using this light source, we report on the generation of an attosecond continuum in the water window and their CEP dependences, which can be understood by half-cycle cutoffs [3].