Linear field-resolved spectroscopy approaching ultimate detection sensitivity

Electric-field oscillations are now experimentally accessible in the THz-to-PHz frequency range. Their measurement delivers the most comprehensive information content attainable by optical spectroscopy – if performed with high sensitivity. Yet, the trade-off between bandwidth and efficiency associated with the nonlinear mixing necessary for field sampling has so far strongly restricted sensitivity in applications such as field-resolved spectroscopy of molecular vibrations. Here, we demonstrate electric-field sampling of octave-spanning mid-infrared waves in the 18-to-39 THz (600-to-1300 cm-1) spectral region, with amplitudes ranging from the MV/cm level down to a few mV/cm. We show that employing powerful short-wave mid-infrared gate pulses is key to approaching the ultimate detection limit of capturing all photons in the temporal gate, as well as to providing high linearity with respect to the detected mid-infrared field. This combination of detection sensitivity, dynamic range and linearity enables exploitation of the full potential of emerging high-power waveform-controlled infrared sources for (non-)linear spectroscopy of solids, liquids and gases.