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Breaking the size and speed limit of modulators: The workhorses of the internet

Electro-optical modulators are the workhorses of the internet. They convert electrical data from computers and smartphones to optical data streams for fiber optic networks, enabling modern data communications like video streaming. The new invention is timely since demand for data services is growing rapidly and moving towards next generation communication networks. Taking advantage of their compact footprint, electro-optic converters can be utilized as transducers in optical computing hardware such as optical artificial neural networks that mimic the human brain and a plethora of other applications for modern-day life.


Electro-optical modulators in use today are typically between 1 millimeter and 1 centimeter in size. Reducing their size allows increased packaging density, which is vital on a chip. While silicon often serves as the passive structure on which photonic integrated circuits are built, the light matter interaction of silicon materials induces a rather weak optical index change, requiring a larger device footprint. While resonators could be used to boost this weak electro-optical effect, they narrow devices' optical operating range and incur high energy consumption from required heating elements.


Researchers developed and demonstrated for the first time a silicon-based electro-optical modulator that is smaller, as fast as and more efficient than state-of-the-art technologies. By adding indium tin oxide (ITO) -- a transparent conductive oxide found in touchscreen displays and solar cells -- to a silicon photonic chip platform, the researchers were able to create a compact device 1 micrometer in size and able to yield gigahertz-fast, or 1 billion times per second, signal modulation.



As an oxide semiconductor with a wide band gap, indium tin oxide (ITO) is transparent and electrically conductive due to its high electrical conductivity and good transmittance in the visible wavelength region. It has wide applications in the transparent electrodes such as various optoelectronic devices, antistatic conductive coatings, solar cells, electrochromic devices. Recently, ITO film has been used as high-temperature strain sensors, thermocouples, and thermoelectric devices due to their excellent chemical stability, phase stability, and resistivity to oxidation at elevated temperatures.


Ping C. conducted the world’s first ion beam sputtering to prepare ITO film. The film was annealed at different temperatures in a flow of nitrogen or air, respectively. The effect of annealing process on the microstructure, optical and electrical performances of the ITO film was studied. The results showed that the crystallinity and optical properties of the ITO film were improved with increasing annealing temperature. The transmittance of the ITO film in visible regions increased up to 96% after it was annealed at 500  in nitrogen or air.


It was found that the resistivity of ITO film depended on both the annealing temperature and the annealing atmosphere, which showed a close relationship to the atomic percentages sum of oxygen vacancy andSn4+, which was quantified by XPS result.


Read the full paper at the journal of Composite Materials Research: