Transparent conducting oxides (TCOs) are compounds of metals and oxides. Due to their wide bandgap (> 3 eV), they are transparent in the visible and near-infrared (NIR) regime, being at the same time highly conductive as a result of their high free-carrier concentration (1019-1021 cm-3) of appreciable mobility (around 50 cm2V-1s-1). Their electrical and optical properties are tailorable during fabrication, being highly dependent on the deposition conditions. TCOs have been widely employed in optoelectronic applications, with the most well-known representative being indium tin oxide (ITO).

Recently, TCOs have attracted considerable attention for serving as dynamically configurable materials in integrated photonic applications due to uniquely exhibiting near zero permittivity values in the NIR. This property allows them to manifest the epsilon-near-zero (ENZ) effect when integrated into waveguide structures, drastically modifying the profile of the propagating mode. Both the intensity and the phase velocity of the guided-mode can be controlled by switching TCO between its ENZ and dielectric (metallic) regime, paving the way for dynamically controlled TCO-based photonic components. 

The switching mechanism is based on externally inducing changes in the TCO free-carrier concentration, usually achieved through the field effect. Specifically, a capacitor-like structure is formed by integrating a bilayer of a TCO material and an insulator into a waveguide, inducing under bias the accumulation or depletion of free carriers at the TCO/insulator interface, locally modifying the TCO permittivity. Both amplitude and phase modulators have been demonstrated by integrating a bilayer of ITO and hafnium dioxide (HfO2on well-established silicon-photonics platforms, providing efficient, low-loss, and high-speed modulation designs. 

TCO-based metamaterial absorbers are also attracting high interest for NIR applications, being able to tune the reflectance of an impinging wave depending on the biasing conditions of a thin TCO layer, integrated into the metamaterial unit cell. Switching between perfect absorbance and a highly reflective state results in the intensity modulation of the impinging wave, with phase modulation schemes also being reported.


Electro-optic ITO/HfO2 modulator

Fig. 1: (a) ITO permittivity as a function of its electron concentration. (b) Si-rib electro-optic modulator. (c) Electron-concentration curves along the biased n-Si/HfO2/ITO contact . (d)-(e) ITO permittivity and optical electric field along the n-Si/HfO2/ITO contact with respect to the concentration distributions in (c).

(Click on image to enlarge)


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