Four-wave-mixing microscopy is widely researched in both biology and medicine. In this paper, we present a simplified near-degenerate four-wave-mixing microscopy (SNDFWM). An ultra-steep long-pass filter is utilized to produce an ultra-steep edge on the spectrum of a femtosecond pulse, and a super-sensitive four-wave-mixing (FWM) signal can be generated via an ultra-steep short-pass filter. Compared with the current state-of-the-art FWM microscopy, this SNDFWM microscopy has the advantages of simpler experimental apparatus, lower cost, and easier operation. We demonstrate that this SNDFWM microscopy has high sensitivity and high spatial resolution in both nanowires and biological tissues. We also show that the SNDFWM microscopy can achieve an ultra-sensitive detection based on the electron-resonance effect. This method might find an important application in tracking of nano drugs in vivo.

作为一种非线性光学效应，二次谐波产生(Second Harmonic Generation,SHG)因其良好的偏振敏感性在获得物质成分、结构、特性等信息方面具有广泛应用。尽管前人利用SHG光谱或SHG显微成像方法探索研究了纳米线的精密定位或追踪问题，但是，结合使用SHG光谱与SHG显微成像方法实现纳米材料结构与晶轴空间取向方面的研究鲜见报道。本研究分别使用SHG光谱与SHG显微成像方法研究了CdS纳米线空间取向问题。首先，基于全光学分析方法从实验上和理论上研究了硫化镉（CdS）纳米线SHG光谱强度随入射光偏振方向变化的规律，并详细分析了晶轴方位角γ，ω，φ对CdS纳米线SHG花型图的影响。其次，通过理论计算与实验测量结果相互验证，成功确定了单根CdS纳米线的三个晶轴取向。最后，利用偏振相关的SHG显微成像方法研究了单根CdS纳米线的空间取向，发现单根CdS纳米线不同部位具有不同的SHG响应。研究结果为SHG光谱与显微成像在纳米材料空间高精度定位研究提供了新的思路与重要参考，并为纳米材料在生物医学方面的潜在应用提供了重要启示。

In general cases of strong field excitation, the Stark effect has a significant influence on transient two-photon transitions, and the analytic description of this process is quite challenging. By combining analytical solutions and numerical simulations, the transient two-photon transition processes excited by weak and strong chirped pulses are systematically investigated, showing the important influences of parameters such as light field intensity, chirp factor, and detuning on the time-domain evolution of two-photon transition probabilities. Firstly, an approximate analytical expression is derived for the amplitude of the time-domain two-photon transition probability by using the second-order perturbation theory. This analytical solution indicates that the transient two-photon transition process under weak field excitation is similar to the Fresnel rectangular edge diffraction effect. As the light field intensity increases, the influence of the Stark effect on two-photon transitions also intensifies. Secondly, through a series of approximations, the approximate analytical solutions of the Schrödinger equation under strong field interactions are obtained. The analytical solutions show that the strong field Stark effect induces energy level to split, which disrupts the symmetry of the time-domain two-photon transition probability distribution, and its frequency domain process is similar to the “double-slit interference” effect. The research results indicate that the efficiency of population transfer during strong field excitation is closely related to the light field intensity, while the chirp factor can not only regulate the efficiency and time position of population transfer but also change the oscillation frequency of the population probability in the time domain. This work offers new insights into describing the time-domain evolution of the population probability under strong field excitation and lays a scientific basis for research on two-photon microscopy imaging.