In an ultrafast nonlinear optical interaction, the electric field of the emitted nonlinear signal provides direct access to the induced nonlinear transient polarization or transient currents and thus carries signatures of ultrafast dynamics in a medium. Measurement of the electric field of such signals offers sensitive observables to track ultrafast electron dynamics in various systems. In this work, we resolve the real-time phase of the electric field of a femtosecond third-order nonlinear optical signal in the molecular frame. The electric field emitted from impulsively pre-aligned gas-phase molecules at room temperature, in a degenerate four-wave mixing scheme, is measured using a spectral interferometry technique. The nonlinear signal is measured around a rotational revival to extract its molecular-frame angle dependence from pump-probe time-delay scans. By comparing these measurements for two linear molecules, carbon dioxide and nitrogen, we show that the measured second-order phase parameter (temporal chirp) of the signal is sensitive to the valence electronic symmetry of the molecules, whereas the amplitude of the signal does not show such sensitivity. We compare measurements to theoretical calculations of the chirp observable in the molecular frame. This work is an important step towards using electric field measurements in nonlinear optical spectroscopy to study ultrafast dynamics of electronically excited molecules in the molecular frame.

中文翻译：

---

分子框架中的超快时间相位分辨非线性光谱


在超快非线性光学相互作用中，发射的非线性信号的电场提供了对引起的非线性瞬态偏振或瞬态电流的直接访问，因此在介质中携带超快动力学的特征。对此类信号电场的测量提供了灵敏的可观测值，以跟踪各种系统中的超快电子动力学。在这项工作中，我们解析了分子框架中飞秒三阶非线性光信号的电场的实时相位。使用光谱干涉技术测量简并四波混频方案中室温下脉冲预排列气相分子发射的电场。围绕旋转复兴测量非线性信号，以从泵浦探针延时扫描中提取其分子框架角度依赖性。通过比较二氧化碳和氮气这两种线性分子的测量结果，我们发现测量的信号二阶相位参数（时间啁啾）对分子的价电子对称性敏感，而信号的幅度则不敏感。表现出这样的敏感性。我们将分子框架中可观察到的线性调频脉冲的测量结果与理论计算结果进行比较。这项工作是利用非线性光谱中的电场测量来研究分子框架中电子激发分子的超快动力学的重要一步。