Recently, researchers from the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) have revealed the competitive behavior of bound electrons and free electrons in Epsilon-Near-Zero (ENZ) materials irradiated by ultrafast laser pulses.  

They proposed the principle and method of synchronously tailoring the saturated absorption (SA) and reverse saturated absorption (RSA) of ENZ materials, which greatly broadened the ability to control the nonlinear optical response at the ENZ wavelength. Relevant study has been published in Photonics Research on April 15. 

ENZ material means that in a specific wavelength range, the real part of the dielectric permittivity of the material approaches zero (ε~0). Theoretically, the limited dielectric permittivity change can obtain a great refractive index change. Therefore, compared with other nonlinear materials, ENZ materials have a larger nonlinear optical response, which is a research hotspot in the field of nonlinear optics. 

The researchers studied the nonlinear absorption response of a typical ENZ material (indium tin oxide) under the irradiation of a femtosecond laser. The results show that only SA behavior was observed at ENZ wavelength (1,440 nm) whereas a conversion behavior from SA to RSA at the non-ENZ wavelength (1030 nm) with increasing incident laser intensity.  

They analyzed the principle of the transition between SA and RSA in the non-ENZ wavelength. They believed that the SA effect originates from the bleaching process of free electrons in the conduction band, and the RSA effect originates from the multiphoton absorption of the bound electrons in the valence band. 

In addition, the researchers proposed thin film annealing technology to suppress free electron concentration, which achieves regulation of electron bleaching and multiphoton absorption competition. This method realizes the regulation from 120% saturated absorption to 95% reverse saturated absorption at ENZ wavelength. 

Starting from the basic properties of ENZ material, the researchers analyzed its nonlinear absorption mechanism and obtained a method for synchronously regulating ultrafast laser SA and RSA, which was expected to develop new applications in ultrafast optical field adaptive regulation, optical switching, and optical limiting. 

This work was support by the National Natural Science Foundation of China and the Strategic Priority Research Program of CAS. 

Fig. 1. Open aperture Z-scan results of as-deposited indium tin oxide films under excitation of 1,030 nm at different input fluences. (a), (b) 1,030 nm, (c) 1,440 nm. (Image by SIOM)

Fig. 2. Dependence of normalized transmittance of indium tin oxide films (asdeposited and annealed) on optical intensity using different pump sources. (a) 1,030 nm, (b) 1,440 nm. (Image by SIOM)