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Advancements in Terahertz Wave Technology: MIT’s Innovative Approach

Terahertz waves, which occupy a unique position in the electromagnetic spectrum between radio waves and infrared light, offer the potential for enhanced data transmission, superior medical imaging, and advanced radar capabilities. However, generating these waves effectively using semiconductor chips presents significant challenges, particularly when it comes to integrating them into electronic devices.

Conventional methods for generating terahertz waves often fall short in terms of radiating power, typically requiring cumbersome and costly silicon lenses to enhance signal strength. These lenses, often larger than the generating chip itself, complicate the integration of terahertz sources into compact electronic devices. To address these challenges, researchers at MIT have developed a novel terahertz amplifier-multiplier system that significantly increases radiating power without the need for bulky silicon lenses.

By incorporating a thin, patterned sheet of material on the back of the chip and leveraging high-power transistors from Intel, the team has created a more efficient and scalable chip-based terahertz wave generator. This innovation paves the way for creating terahertz arrays that could vastly improve applications such as security scanners aimed at detecting concealed objects and environmental monitors designed to identify airborne pollutants.

“To maximize the utility of terahertz wave sources, scalability is key. A terahertz array could involve hundreds of chips where silicon lenses cannot be feasibly integrated due to space constraints. We’ve showcased a promising alternative approach that supports the development of scalable and cost-effective terahertz arrays,” notes Jinchen Wang, a graduate student in the Department of Electrical Engineering and Computer Science (EECS) at MIT and the lead author of the research paper discussing this terahertz radiator.

Wang collaborated with fellow EECS graduate students Daniel Sheen and Xibi Chen, along with Steven F. Nagel, managing director of the T.J. Rodgers RLE Laboratory, and senior author Ruonan Han, an associate professor in EECS leading the Terahertz Integrated Electronics Group. Their work will be presented at the upcoming IEEE International Solid-States Circuits Conference.

Understanding Terahertz Waves

Terahertz waves facilitate the transfer of larger amounts of information compared to conventional radio waves, due to their higher frequencies, while still maintaining the ability to safely penetrate numerous materials unlike infrared light. One prevalent technique for generating these waves involves a CMOS chip-based amplifier-multiplier chain that increases the frequency of radio waves to reach the terahertz spectrum.

Despite this method’s potential, a significant hurdle arises from the dielectric constant, which affects how electromagnetic waves propagate through different materials. Given that the dielectric constant of silicon is considerably higher than that of air, there is a tendency for terahertz waves to be reflected rather than transmitted efficiently at the silicon-air interface, leading to loss of signal strength. Traditional methods have often resorted to using bulky silicon lenses to compensate for this loss.

A New Solution

The groundbreaking approach taken by MIT researchers involved utilizing a concept from electromechanical theory called matching, which aims to harmonize the dielectric constants of silicon and air to reduce signal reflection. They achieved this by applying a thin sheet of material with a dielectric constant intermediate between that of silicon and air to the back of the chip. This alteration enhances the transmission of terahertz waves out of the chip.

To create the matching sheet, the researchers selected an inexpensive, commercially available substrate material and employed a laser cutter to create minute holes, effectively reducing the dielectric constant to the desired level. Wang explains, “The process of perforating the sheet injects air into it, lowering its overall dielectric constant and helping to optimize signal transmission.”

Additionally, the design of their chip features advanced Intel transistors, which offer higher maximum frequency capabilities and improved breakdown voltage compared to standard CMOS transistors. Combining these innovative elements allowed the MIT chip to generate terahertz signals with an impressive peak radiation power of 11.1 decibel-milliwatts, outperforming existing state-of-the-art devices.

Scalability Challenges Ahead

One of the significant challenges faced during development was managing power and temperature levels while generating terahertz waves, as established CMOS chip design standards do not apply effectively in this context. Additionally, the team had to devise a scalable method for incorporating the matching sheet within a manufacturing environment.

Looking ahead, the researchers aspire to showcase the scalability of their innovations by fabricating a phased array of CMOS terahertz sources, which would enable dynamic steering and focusing of powerful terahertz beams within a compact and cost-effective device.

This research initiative has received support from entities including NASA’s Jet Propulsion Laboratory and the Strategic University Research Partnerships Program, along with contributions from the MIT Center for Integrated Circuits and Systems. The fabrication of the chip was facilitated through the Intel University Shuttle Program.

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