  We report a newly designed mid-infrared-pump visible-probe measurement
system, which can measure phase-sensitive responses to a mid-infrared pulse
along the oscillating electromagnetic field. In this system, the pump light is
a phase-locked mid-infrared pulse with temporal width of 100 fs, which is
produced via difference frequency generation (DFG) from two idler pulses of two
optical parametric amplifiers (OPAs) that are excited by the same Ti:sapphire
regenerative amplifier. The probe pulse is a visible pulse with temporal width
of 9 fs, and it is generated from a custom-built non-collinear OPA. By
measuring the electric-field waveforms of mid-infrared pump pulses with
electro-optic sampling and evaluating their carrier envelope phase (CEP) and
the temporal positions of their envelopes relative to ultrashort visible probe
pulses, we are able to perform double feedback corrections that eliminate both
the following sources of drift. The CEP drift in mid-infrared pulses
originating from fluctuations in the difference of optical-path lengths of the
two idler pulses before the DFG is corrected by inserting a wedge plate in one
idler path, and the drift in pump-probe delay times due to fluctuations in the
difference of the overall optical-path lengths of the pump and probe pulses is
corrected with mechanical delay lines. In this double-feedback system, the
absolute carrier phase of mid-infrared pulses can be fixed within 200 mrad and
errors in the measurement of phase-sensitive responses can be reduced to within
1 fs over a few tens of hours.
