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CMOS optical transistor boasts speeds up to 4THz

Posted: 06 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Purdue University? CMOS? optical transistor? AZO? nanophotonic?

A team of researchers from Purdue University researchers has showcased a CMOS-compatible all-optical transistor capable of 4THz speeds, potentially over a 1000 times faster than silicon transistors. According to the scientists, the nano-photonic transistors processed at low-temperatures can be fabricated atop complementary metal oxide semiconductors (CMOS) to boost switching time by nearly 5,000-times less than 300fs or almost 4THz.

The aluminum-doped zinc oxide (AZO) material from which these optical transistors are fabricated has a tunable dielectric permittivity compatible with all telecommunications infrared (IR) standards.

"The limiting time is about 300fs for a speed of roughly 4THz although it could be faster if you sacrifice some of the performance," said doctoral candidate Nathaniel Kinsey. Kinsey is working with Purdue University professors Alexandra Boltasseva, an EE, and Vladimir Shalaev, the scientific director of nano-photonics at Purdue's Birck Nanotechnology Centre.

"What is important," Kinsey continued, "is that electrical transistors are limited by the RC delay time while the limiting mechanism for our 'all optical transistor' is recombination time. These are entirely different mechanisms and the latter could enable much more freedom in engineering performance and responses to reach faster switching speeds than the electrical counterpart."

The transparent conducting oxides making up these photonic transistors are CMOS-compatible materials with low optical loss that can be processed at temperatures low enough for back-end-of-line (BEOL) fabrication. Their metal-like, versatile and tunable behaviour makes them ideal for fabricating optical transistors atop CMOS chips, however in the past their slow electron-hole recombination time for emitting photons exceeded 100ps thereby limiting the speed with which signals could be modulated. Purdue University researchers have now cut that time to less than 1ps, speedy enough for optical transistors that outperform silicon. The AZO films were fabricated with deep-level defects with an ultra-high carrier concentration enabling demonstrations by the researchers of 40 per cent reflectance modulation levels with excitation and recombination times under one picosecond at low power, less than four miliJoules per square m2, when at the telecommunications wavelength of 1.3?m.

The AZO plasmonic oxide material is predicted by the researchers to be capable of 10-times faster communications speeds at all popular telecommunications wavelengths. The all-optical technology uses light for both the data stream and the control signals that modulate the data, rather than use electrical signals to control the modulation like today. The AZO films can be engineered to either increase or decrease the reflection index to encode the 1s and 0s during data transmissions. Their next step is to fabricate a working device in a simple application.

Plasmonic oxide material

Artists rendering of a new "plasmonic oxide material" that could make optical communications over 10 times faster than conventional technologies. (Credit: Nathaniel Kinsey) (Source: Purdue University)

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