Two-octave mid-infrared pulses via chirp-optimized cascaded intra-pulse difference frequency generation.

Intra-pulse difference frequency generation (IDFG) has been widely applied for generating few-cycle carrier-envelope-phase (CEP) stable mid-infrared (MIR) pulses, whose maximal spectral bandwidth reaches around one octave, limited by the phase-matching bandwidth of crystals. Here, we propose and demonstrate a chirp-optimized cascaded IDFG (CC-IDFG) scheme that overcomes the phase-matching bandwidth limitation of nonlinear crystals. By carefully manipulating the chirp of a supercontinuum pulse in 0.5-1 µm, an ultra-broadband pulse in 1-4 µm is generated by CC-IDFG employing cascaded BBO, MgO:LiNbO, and BIBO crystals, which supports a sub-cycle MIR pulse. Our study presents an attractive method for generating ultra-broadband pulses, which is significantly helpful for applications such as arbitrary waveform tailoring, isolated attosecond pulse generation, and ultrafast electron dynamics control in materials.