摘要： Linear near-infrared (NIR) spectroscopy provides information on the molecular composition, structure, and conformation, affording remarkable potential for high-resolution, in-depth, label-free biological spectro-microscopy. Broadband measurements in NIR have been carried out in the frequency domain. However, in this approach, the detection of small absorption differences is limited to the intensity noise of the source, and the detection dynamic range. We propose the concept of molecular fieldoscopy to alleviate above-mentioned limitations. Here, phase-coherent, broadband ultrashort pulse in NIR range excites molecules, afterward the transmitted electric field that carries the molecular response is directly detected by electro-optics sampling (EOS). The excitation pulse has tens of femtoseconds temporal duration, which is considerably shorter than free induction decay (FID) of the molecules under inspection. Therefore, the excitation pulse is temporally separated from the molecular response, allowing background-free measurement, and suppressing the influence of the intensity noise of the source. Moreover, measuring the complex electric field permits for extracting the full spectral phase information of the molecular response, adding a new dimension to the obtained spectroscopic data. In this work, we report about field-resolved overtone spectroscopy of water molecules in the liquid phase by using EOS to detect its first overtone vibrational mode of O-H bond appearing at 1900-1950 nm. In this respect, a carrier-envelope phase (CEP) stable, ultra-broadband spectrum covering from 700 nm to 2500 nm was directly generated from a home-built, diode-pumped Yb:YAG thin-disk regenerative amplifier. The broadband spectrum was splitted into two spectral regions: i) spectral region covering 700-1400 nm, and ii) NIR spectral region covering 1600-2500 nm. Thereafter, both regions were amplified to 25 μJ of energy, in a single-stage optical parametric amplifier and compressed to 4.8 fs and 18 fs, respectively. The 18 fs, NIR pulses were guided through a 500 μm-thick mixture of acetic acid and water. Eventually, the transmitted light was detected by EOS, containing a 50 μm-thick BBO (Type II) crystal, where the 4.8 fs pulses were used as a probe. In the absence of any sample, the measured electric field of the excitation pulses goes to zero at temporal delays above 200 fs (Fig.1a)). By adding the sample, a background-free, molecular response is observed for temporal delays beyond 200 fs, as shown in Fig.1b). The NIR response of water molecules can be clearly detected in water concentrations as low as 3%. In summary, we presented the new concept of NIR molecular fieldoscopy covering the entire molecular finger-print region. Femtosecond molecular excitation allows for background-free detection of the molecular response and higher detection sensitivity, while the heterodyne detection in EOS enhances the detection dynamic range.