Investigation of Ultrafast Optical Limiting in a Doped-CdO Based Device

Traditionally metals such as gold and silver have been used for plasmonic applications due to their large carrier densities which result in plasma frequencies ranging from the ultraviolet to the visible wavelength regime. However, metals are ultimately limited with regards to their applications in infrared optics due to their static carrier densities as well as high optical losses. Doped cadmium oxide (CdO) has been shown to be an attractive candidate to fill this void due to having low optical losses in a variable wavelength range extending from the near to mid infrared regimes dependent on chemical doping. Further, epsilon-near-zero (ENZ) materials such as CdO have been shown to have exceptionally high non-linearities at ENZ which, when combined with its easily tunable ENZ, make CdO an attractive candidate for nonlinear optical applications in the near to mid infrared.

In this study, we demonstrate an ultrafast optical limiting device based on the combination of high nonlinearity at the ENZ of doped CdO and a frequency-selective metallic surface consisting of an array of gold cross antennae. Upon high flux intraband excitations with a mid infrared laser, the carriers in doped CdO exhibit a change in effective mass due to the non-parabolic conduction band. This ultimately results in a spectral redshift of the plasma frequency that causes a nonlinear change in the refractive index of the CdO film effectively detuning the plasmonic resonance of the antenna from the resonance in CdO leading to a decrease in transmission. We investigate the physical mechanism behind this decrease in transmission by performing mid-infrared pump probe experiments in which we optically excite the device at the ENZ wavelength of CdO with intense fluences and probe the time dependent transmission response at and around the ENZ wavelength. These measurements provide insight into the dominant mechanisms that result in the shift in plasma frequency by deciphering the relative roles of electronic scattering and effective mass changes.