由强场中红外飞秒脉冲泵浦的氮离子能够以自由感应衰变（FID）的形式发出相干的前向辐射，该FID辐射可以被随后的800 nm飞秒脉冲有效抑制。这种擦除效应是对微弱FID辐射进行时间表征的独特工具，特别是在FID辐射与泵浦脉冲的谐波在频域重叠的情况下。基于密度矩阵与麦克斯韦方程的数值模拟结果，认为擦除效应主要是由于800 nm脉冲直接引发氮离子B态和A态相干性的变化，该变化进一步与800 nm脉冲耦合作用，引起了B态和X态之间相干性的减小，从而导致对应的391.4 辐射的减弱。

Strong field physics is an important frontier of current physics research. In recent years, the rapid development of ultrafast laser technology has enabled researchers to obtain laser sources with shorter pulse width, higher pulse energy, and wider tunable range. With the advancing of the research on the interaction between strong laser and matter from the traditional perturbation regime to the non-perturbative region, rich strong field physical phenomena such as tunneling ionization and high-order harmonics have been observed. When the high-intensity femtosecond laser propagates in in a transparent medium, it can form a light-plasma filament due to nonlinear optical effects including Kerr effect and plasma generation. In particular, the study of “air lasing” radiation produced by interaction of intense femtosecond laser pulses with air molecules has attracted many attentions in recent 10 years. Air lasing has the characteristics of bidirectional emission, high intensity, high coherence and remote generation, and holds potential application in the field of optical remote sensing. Traditional optical remote sensing collects scattered signals or fluorescent signals of lasers on the atmospheric targets. These optical signals do not have directionality, which poses challenges to the sensitivity of ground collection devices and limits detection accuracy and sensitivity. As a new concept light source, air lasing is expected to greatly improve the signal strength of optical remote detection due to its capacity of emitting coherent beams from the remote atmosphere to the ground. In recent years, several important progress have been achieved as to the understanding of the air lasing signal generated by the interaction of strong-field laser and the most important component of air, nitrogen. Regarding the underlying mechanism of the coherent emission of neutral nitrogen molecule, it is now accepted that the emission at 337.4 nm is Amplified Spontaneous Emission (ASE). In contrast, the lasing mechanism of N2+ is much more complex and is still hotly debated. Part of the debate stems from the mysterious fact that coherent emissions at 391.4 and 427.8 nm can always be observed regardless of the pump laser wavelength at 400 nm, 800 nm, 1 100 to 1 900 nm, or even 3 900 nm. Very recently, when pump lasers in the mid-infrared regime were used as drive pulses, the nature of the 391.4 and 427.8 nm emissions was attributed to a Free Induction Decay (FID) signal. In this study, we report for the first time the erasure effect of the FID signal pumped of nitrogen ions under an 800 nm control pulse. This effect provides a unique tool to measure the time-domain pulse width of the weak free induction decay signals, which is especially useful in the case where the FID radiation and the harmonics of the pump pulse overlapping in the frequency domain. In our study, by pumping nitrogen ions with strong-field mid-infrared femtosecond pulses (1 250/1 550 nm), the 391.4 nm radiation corresponding to state B2∑u+(ν'=0) and state X2∑g+(ν=0) transitions was observed in the forward direction. When pumped by 1 250 nm femtosecond pulses, the coherent 391.4 nm signal overlaps with the third harmonic in the spectral domain, while the spectrum of the 391.4 nm signal is clean when nitrogen gas was pumped by 1 550 nm femtosecond pulses since the the 391.4 nm wavelength deviates from the third and fifth harmonics. In our experiment, time-resolved measurement was performed by injection of an 800 nm control pulse. It was found that the subsequent 800 nm control pulse would have a significant suppression effect on the FID signal generated by the 1 250/1 550 nm pump laser. The suppression of the FID signal lasts approximately 3 ps. For 1 250 nm pump pulse, it was found that the FID signal was suppressed by about 75% for control pulse energies of 200 μJ and 300 μJ. While for the 100 μJ pulse, the FID signal is less suppressed. For 1 500 nm pump pulse, in addition to a similar erasure effect, at zero delay we observed a significant enhancement of the signal. This enhancement is attributed to the fact that the 1 550 nm pulse can excite the A-B coherence through a two-photon resonance process. While for the 1 250 nm pump pulse, its photon energy is far away from the A-B resonance with two photons, so no such enhancement is observed at the temporal overlap. Simultaneously, a similar erasing effect can be observed for 357.8 nm radiation corresponding to levels B (ν'=1) and X (ν=0) transitions when pumped by high-energy (530 μJ) 1 550 nm pulses. Based on numerical simulations, we attribute the mechanism of this erasure effect to the photoionization and reduction of the coherence between the B and X states due to the 800 nm control pulse. It was found that the intial B-X coherence couples with the 800 nm control pulse and leads to the change of B-A coherence, which in turn couples with the 800 nm pulse and results in reduction of the B-X coherence.