Terahertz Optical Transistors Beat Silicon

R. Colin Johnson
http://www.eetimes.com/document.asp?doc_id=1327330

PORTLAND, Ore.--Purdue University researchers have demonstrated a CMOS-compatible all-optical transistor capable of 4THz speeds, potentially over a 1000 times faster than silicon transistors.

Nano-photonic transistors processed at low-temperatures can be fabricated atop complementary metal oxide semiconductors (CMOS) to boost switching time by ~5,000-times less than 300 femtoseconds (fs) or almost 4 terahertz (THz), according to researchers at Purdue University. 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 ~300fs for a speed of ~4THz although it could be faster if you sacrifice some of the performance," doctoral candidate Nathaniel Kinsey told EE Times. Kinsey is working with Purdue University (West Lafayette, Indiana) professors Alexandra Boltasseva, a EE, and Vladimir Shalaev, the scientific director of nano-photonics at Purdue's Birck Nanotechnology Center.

"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 behavior makes them ideal for fabricating optical transistors atop CMOS chips, however in the past their slow electron-hole recombination time for emitting photons exceeded 100 picoseconds thereby limiting the speed with which signals could be modulated. Purdue University researchers have now cut that time to less than 1 picosecond--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 percent reflectance modulation levels with excitation and recombination times under one picosecond at low power--less than 4 miliJoules per square m2--when at the telecommunications wavelength of 1.3 microns.

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.

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)

"For now, our plans are to look towards integrated switching devices. We would like to develop an all-optical plasmonic circuit where we can take light from off-chip (a laser) and feed it efficiently on-chip where it can be modulated to carry data. This would require a plasmonic waveguide, and our all-optical transistor. Potentially we could even cascade several of these devices and work with multiple input wavelengths to achieve even larger bandwidths as is currently done with fiber optics. Developing such a device would allow us to explore how the device operates under high speeds and the stresses associated with this as well as to really see the limits of our materials."

Since the refractive index of AXO films is near zero, the researchers are also looking for ways to use metamaterials (whose index is less than zero) by using clouds of electrons called surface plasmons to help guide light around the all-optical chips. Pulsing lasers change the AZO's index of refraction which could in turn bend light in particular directions as it passes through. Also the amount of aluminum doping of the zinc oxide changes the conductive properties of the AZO material from a insulator at some wavelengths to a conductor at other wavelengths, making it possible to tune its characteristics.

Photonics test bench at Purdue University for testing its unique aluminum-doped zinc oxide (AZO) material.
(Source: Purdue University)

Also working on the project were doctoral candidates Clayton DeVault and Jongbum Kim as well as visiting scholar Marcello Ferrera from Heriot-Watt University (Edinburgh, Scotland).

Funding is being provided by the Air Force Office of Scientific Research, a Marie Curie Outgoing International Fellowship, the National Science Foundation, and the Office of Naval Research.

Top view of test set-up for nanophotonic transistors compatible with commercial telecommunications wavelength standards.
(Source: Purdue University)

For all the details see Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths in the journal Optica published by the Optical Society of America.

— R. Colin Johnson, Advanced Technology Editor, EE Times