  Mid-infrared (mid-IR) spectroscopy provides a way to study structures and
dynamics of complicated molecules in condensed phases. Therefore, developing
compact and broadband mid-IR spectrometer has been a long-standing challenge.
Here, we show that a highly coherent and broadband mid-IR frequency comb can be
generated by using an intra-pulse difference-frequency-generation with a train
of pulses from a few-cycle Ti:Sapphire oscillator. By tightly focusing the
oscillator output beam into a single-pass fan-out-type periodically-poled
lithium niobate crystal and tilting the orientation of the crystal with respect
to incident beam, it is shown that mid-IR frequency comb with more than an
octave spectral bandwidth from 1550 cm-1 (46 THz) to 3650 cm-1 (110 THz) and
vanishing carrier-envelop offset phase can be generated. Then, using two
coherent mid-IR frequency combs, we demonstrate that ultrabroad mid-IR dual
frequency comb spectroscopy of both aromatic compounds and amino acids in
solutions is experimentally feasible. We thus anticipate that our mid-IR
frequency combs could be used to further develop ultrafast and broadband IR
spectroscopy of chemically reactive and biological molecules in condensed
phases.
