February 09, 2017

Princeton shrinks tabletop terahertz equipment down to a few square millimeters

Researchers at Princeton University have drastically shrunk much of the equipment for terahertz wave generation: moving from a tabletop setup with lasers and mirrors to a pair of microchips small enough to fit on a fingertip.

Terahertz electromagnetic pulses lasting one millionth of a millionth of a second may hold the key to advances in medical imaging, communications and drug development. But the pulses, called terahertz waves, have long required elaborate and expensive equipment to use.

In two articles recently published in the IEEE Journal of Solid State Circuits, the researchers describe one microchip that can generate terahertz waves, and a second chip that can capture and read intricate details of these waves.

"The system is realized in the same silicon chip technology that powers all modern electronic devices from smartphones to tablets, and therefore costs only a few dollars to make on a large scale" said lead researcher Kaushik Sengupta, a Princeton assistant professor of electrical engineering.

Terahertz waves are part of the electromagnetic spectrum — the broad class of waves that includes radio, X-rays and visible light — and sit between the microwave and infrared light wavebands. The waves have some unique characteristics that make them interesting to science. For one, they pass through most non-conducting material, so they could be used to peer through clothing or boxes for security purposes, and because they have less energy than X-rays, they don't damage human tissue or DNA.

Princeton University researchers have drastically shrunk the equipment for producing terahertz — important electromagnetic pulses lasting one millionth of a millionth of a second — from a tabletop setup with lasers and mirrors to a pair of microchips small enough to fit on a fingertip (above). The simpler, cheaper generation of terahertz has potential for advances in medical imaging, communications and drug development. (Photos by Frank Wojciechowski for the Office of Engineering Communications)

IEEE Journal of Solid State Circuits - On-Chip THz Spectroscope Exploiting Electromagnetic Scattering With Multi-Port Antenna

IEEE Journal of Solid State Circuits - Dynamic Waveform Shaping With Picosecond Time Widths

Terahertz waves also interact in distinct ways with different chemicals, so they can be used to characterize specific substances. Known as spectroscopy, the ability to use light waves to analyze material is one of the most promising — and the most challenging — applications of terahertz technology, Sengupta said.
To do it, scientists shine a broad range of terahertz waves on a target then observe how the waves change after interacting with it. The human eye performs a similar type of spectroscopy with visible light — we see a leaf as green because light in the green light frequency bounces off the chlorophyll-laden leaf.

The challenge has been that generating a broad range of terahertz waves and interpreting their interaction with a target requires a complex array of equipment such as bulky terahertz generators or ultrafast lasers. The equipment's size and expense make the technology impractical for most applications.

Researchers have been working for years to simplify these systems. In September, Sengupta's team reported a way to reduce the size of the terahertz generator and the apparatus that interprets the returning waves to a millimeter-sized chip. The solution lies in re-imaging how an antenna functions. When terahertz waves interact with a metal structure inside the chip, they create a complex distribution of electromagnetic fields that are unique to the incident signal. Typically, these subtle fields are ignored, but the researchers realized that they could read the patterns as a sort of signature to identify the waves. The entire process can be accomplished with tiny devices inside the microchip that read terahertz waves.

"Instead of directly reading the waves, we are interpreting the patterns created by the waves," Sengupta said. "It is somewhat like looking for a pattern of raindrops by the ripples they make in a pond."

Daniel Mittleman, a professor of engineering at Brown University, said the development was "a very innovative piece of work, and it potentially has a lot of impact." Mittleman, who is the vice chair of the International Society for Infrared Millimeter and Terahertz Waves, said scientists still have work to do before the terahertz band can begin to be used in everyday devices, but the developments are promising.

"It is a very big puzzle with many pieces, and this is just one, but it is a very important one," said Mittleman, who is familiar with the work but had no role in it.

On the terahertz-generation end, much of the challenge is creating a wide range of wavelengths within the terahertz band, particularly in a microchip. The researchers realized they could overcome the problem by generating multiple wavelengths on the chip. They then used precise timing to combine these wavelengths and create very sharp terahertz pulses.

In an article published Dec. 14 in the IEEE Journal of Solid State Circuits, the researchers explained how they created a chip to generate the terahertz waves. The next step, the researchers said, is to extend the work farther along the terahertz band. "Right now we are working with the lower part of the terahertz band," said Xue Wu, a Princeton doctoral student in electrical engineering and an author on both papers.

"What can you do with a billion transistors operating at terahertz frequencies?" Sengupta asked. "Only by re-imagining these complex electromagnetic interactions from fundamental principles can we invent game-changing new technology."

The paper "On-chip THz spectroscope exploiting electromagnetic scattering with multi-port antenna" was published Sept. 2, and the paper "Dynamic waveform shaping with picosecond time widths" was published Dec. 14, both by IEEE Journal of Solid State Circuits. The research was supported in part by the National Science Foundation's Division of Electrical, Communications and Cyber Systems (grant nos. ECCS-1408490 and ECCS-1509560).

Abstract- On-chip THz spectroscope exploiting electromagnetic scattering with multi-port antenna

This paper presents a new methodology to extract spectral information of radiated THz signals to enable a fully integrated broadband THz spectroscope in silicon. A classical down-conversion spectrum analysis architecture requires onchip generation of LO signals covering GHz-THz frequencies to analyze an incident spectrum covering that range. This paper presents a method to exploit the interaction between the front-end antenna and the incident signal to extract spectral information eliminating the need for extremely wideband LO generation and the entire receiver architecture following the antenna. The central premise is that the incident THz signal excites a spectrum-dependent current distribution on the antenna surface and this work presents a method to measure and then estimate the incident spectrum from the impressed current distribution on an onchip antenna. The chip is implemented in 0.13 μm SiGe BiCMOS technology and measures 2.6 mm×1.9 mm. Measurement results are presented for various incident spectra between 40-330 GHz. In addition, the paper presents a method for extracting time-domain information by exploiting the variable nonlinearities of the integrated detectors. By modifying a classical single-port antenna into a 2D multi-port scatterer, the paper presents a synthesizer-free THz spectroscope which consists of an integrated scatterer and multitude of low-power sensors capable of subwavelength measurement of near-field interactions which are exploited for spectral estimation.

Abstract - Dynamic waveform shaping with picosecond time width
In this paper, we present a scalable architecture in silicon that allows synthesis and dynamic waveform shaping of periodic mm-wave signals, either generated on-chip or quasi-optically through radiation in free space capable of producing pulse trains with picosecond time signatures. This is achieved through an architecture that allows extraction of multiple harmonics above fmax with simultaneous amplitude and phase control. Signal synthesis is achieved through controlled interference of multiple traveling waves with rich harmonic components and delays. The first example, presented in this paper, is where the signal synthesis is achieved on-chip demonstrating a pulse train at the output with a measured pulsewidth of 2.6 ps and a 0.46-mW output power. The second example is a four-element array with integrated antennas, which allows signal synthesis in space and is demonstrated to show reconfigurable radiation of pulse trains with a 2.6-ps pulsewidth, pure tones at a fundamental frequency of 107.5 GHz with an effective isotropic radiated power (EIRP) of 4.6 dBm and a second harmonic of 215 GHz with EIRP of 5.0 dBm, as well as any combination of these two harmonics with arbitrary amplitudes and delays. To the best of the authors’ knowledge, this paper demonstrates the sharpest on-chip and radiated pulses with dynamic waveform shaping in any integrated circuit technology. This can open the door to innovations in broadband terahertz imaging, sensing, and spectroscopy.

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