We explore background-free options to detect mid-infrared (MIR) electric transients. The MIR field and a near-infrared probe interact via sum- (SFG) and difference-frequency generation (DFG) in an electro-optic crystal. An intuitive picture based on a phasor representation and rigorous numerical calculations are used for analysis. It turns out that separating photons generated either by SFG or DFG from the local oscillator via spectral filtering leads to a signal purely proportional the MIR intensity envelope. Background-free phase information may be extracted in a spectral window containing both SFG and DFG components and blocking the local oscillator background based on its orthogonal polarization. This variant leads to signal proportional to the square of the MIR field amplitude. It is limited by the finite efficiency of polarization filtering. The Hilbert transform as a conjugate variable to the electric field in the time domain turns out to play a fundamental role for the context discussed in this paper.

我们探索无背景选项来检测中红外（MIR）电瞬变。MIR场和近红外探头通过电光晶体中的和（SFG）和差频生成（DFG）相互作用。基于相量表示和直观数值计算的直观图片用于分析。事实证明，通过频谱滤波将SFG或DFG产生的光子与本地振荡器分离，会导致信号与MIR强度包络成正比。可以在包含SFG和DF​​G分量的光谱窗口中提取无背景的相位信息，并基于其正交极化来阻止本地振荡器背景。这种变化导致信号与MIR场振幅的平方成正比。它受到偏振滤波的有限效率的限制。