Nonlinear optical limiting (NOL) is an effective means to protect high energy laser, which can achieve high transmittance of weak signal light and high attenuation of strong damage light. The NOL plays an important role in protecting personnel and optical detection equipment in numerous high-power laser applications such as laser processing and manufacturing [1], laser medical treatment [2], and high-power laser experimental research [3,4]. Some materials based on nonlinear mechanisms such as nonlinear absorption [[5], [6], [7]], nonlinear scattering [8,9] and nonlinear refraction [10,11] have been explored by many researchers for designing NOLs. However, these NOL methods protect the equipment from strong laser damage, but also reduce the transmission of weak signal light, which cause a low maximum transmittance of signal light.

To solve this problem, various materials, such as fullerene [12], graphene [13] and metal nanoparticles [14,15], have been studied to find NOL methods with higher signal light transmittance. In previous work, we have studied various NOL materials based on nonlinear absorption and nonlinear scattering, such as graphene [16,17], 1-D photonic crystals [18] and C70 [19], which has good protection for the 1064 nm laser. However, these methods still have low transmittance (less than 90%) when the signal light is weak. Therefore, it is greatly significant to study NOL materials with higher transmittance in weak signal light. W. Man et al. designed a series of suspensions with different polarizability and found that different polarizability of suspension has a gain or attenuation effect on laser transmission [20], which indicates that the suspension with negative polarizability has higher transmittance under weak light conditions and the suspension with positive polarizability has lower transmittance under intense light conditions. Besides, according to the optical Kerr effect, the refractive index of the material will change with the increase of the light intensity [21], which shows that the polarizability of the suspension can change with the change of light intensity, to achieve high transmittance under weak light and low transmittance under strong light. J. Hermann pointed out that the optical Kerr effect can cause the self-focusing or self-defocusing effect of the beam propagation inside the material, thus affecting the optical properties of the material [22,23]. We will further explore the effects of self-focusing on materials during laser transmission and the microscopic dynamics.

In this paper, we propose a brand-new NOL method based on the polarizability inversion suspension to achieve tunable optical nonlinearity that vary with light intensity. Under weak light conditions, the transmittance of the suspension will increase with the increase of the light intensity, so as to achieve the gain of the light transmission of the weaker signal. When the light intensity exceeds a certain value, the transmittance of the suspension decreases with the increase of the light intensity, and then the attenuation of the strong damaged light is realized. We analyze the NOL mechanism of the proposed new limiting method and provide a mathematical model for the relationship between the transmittance of the suspension and the incident light intensity. In addition, we also prepared PS (polystyrene)-CS2 sample suspension and carried out verification experiments. We measure the optical limiting effect of PS-CS2 suspension at four different concentrations (3.45, 4.72, 5.82 and 6.80 g/L) and three different thicknesses (5, 10 and 20 mm) at the wavelength of 1064 nm. Each set of results is consistent with our theoretical inference. Among them, the 10 mm thick PS-CS2 suspension with a concentration of 4.72 g/L has the best optical limiting effect, in which the linear transmittance is 81.9%, the maximum transmittance is 91.7% and the lowest transmittance is 29.9%, which is better than most NOL materials at 1064 nm.