Closing the terahertz gap: Tiny laser is an important step toward new sensors

A new imaging technology rapidly measures the chemical compositions of solids. A conventional image of a sample pill is shown at left; at right, looking at the same surface with terahertz frequencies reveals various ingredients as different colors. Such images would aid quality control and development in pharmaceutical manufacturing, as well as medical diagnosis and treatment.

https://www.eurekalert.org/multimedia/pub/207021.php

In a major step toward developing portable scanners that can rapidly measure molecules in pharmaceuticals or classify tissue in patients' skin, researchers have created an imaging system that uses lasers small and efficient enough to fit on a microchip.

The system emits and detects electromagnetic radiation at terahertz frequencies -- higher than radio waves but lower than the long-wave infrared light used for thermal imaging. Imaging using terahertz radiation has long been a goal for engineers, but the difficulty of creating practical systems that work in this frequency range has stymied most applications and resulted in what engineers call the "terahertz gap."

"Here, we have a revolutionary technology that doesn't have any moving parts and uses direct emission of terahertz radiation from semiconductor chips," said Gerard Wysocki, an associate professor of electrical engineering at Princeton University and one of the leaders of the research team.

Terahertz radiation can penetrate substances such as fabrics and plastics, is non-ionizing and therefore safe for medical use, and can be used to view materials difficult to image at other frequencies. The new system, described in a paper published in the June issue of the journal Optica, can quickly probe the identity and arrangement of molecules or expose structural damage to materials.

The device uses stable beams of radiation at precise frequencies. The setup is called a frequency comb because it contains multiple "teeth" that each emit a different, well-defined frequency of radiation. The radiation interacts with molecules in the sample material. A dual-comb structure allows the instrument to efficiently measure the reflected radiation. Unique patterns, or spectral signatures, in the reflected radiation allow researchers to identify the molecular makeup of the sample.

While current terahertz imaging technologies are expensive to produce and cumbersome to operate, the new system is based on a semiconductor design that costs less and can generate many images per second. This speed could make it useful for real-time quality control of pharmaceutical tablets on a production line and other fast-paced uses.

"Imagine that every 100 microseconds a tablet is passing by, and you can check if it has a consistent structure and there's enough of every ingredient you expect," said Wysocki.

As a proof of concept, the researchers created a tablet with three zones containing common inert ingredients in pharmaceuticals -- forms of glucose, lactose and histidine. The terahertz imaging system identified each ingredient and revealed the boundaries between them, as well as a few spots where one chemical had spilled over into a different zone. This type of "hot spot" represents a frequent problem in pharmaceutical production that occurs when the active ingredient is not properly mixed into a tablet.

The team also demonstrated the system's resolution by using it to image a U.S. quarter. Fine details like the eagle's wing feathers, as small as one-fifth of a millimeter wide, were clearly visible.

While the technology makes the industrial and medical use of terahertz imaging more feasible than before, it still requires cooling to a low temperature, a major hurdle for practical applications. Many researchers are now working on lasers that will potentially operate at room temperature. The Princeton team said its dual-comb hyperspectral imaging technique will work well with these new room-temperature laser sources, which could then open many more uses.

Because it is non-ionizing, terahertz radiation is safe for patients and could potentially be used as a diagnostic tool for skin cancer. In addition, the technology's ability to image metal could be applied to test airplane wings for damage after being struck by an object in flight.

In addition to Wysocki, the paper's Princeton authors are former visiting graduate student Lukasz Sterczewski (currently a postdoctoral scholar at NASA's Jet Propulsion Laboratory) and associate research scholar Jonas Westberg. Other co-authors are Yang Yang, David Burghoff and Qing Hu of the Massachusetts Institute of Technology; and John Reno of Sandia National Laboratories. Support for the research was provided in part by the Defense Advanced Research Projects Agency and the U.S. Department of Energy.

Speedy terahertz-based system could detect explosives

An artist’s embellishment of an image of the “gain medium” used to produce terahertz frequency combs. The different colors indicate that different wavelengths of oscillating terahertz radiation travel different distances through the medium, which has a different refractive index for each of them.

Image: Yan Liang/L2Molecule.com

Larry Hardesty | MIT News Office
May 20, 2016

http://news.mit.edu/2016/speedy-terahertz-based-system-could-detect-explosives-0520

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Spectroscopic system with chip-scale lasers cuts detection time from minutes to microseconds.

Terahertz spectroscopy, which uses the band of electromagnetic radiation between microwaves and infrared light, is a promising security technology because it can extract the spectroscopic “fingerprints” of a wide range of materials, including chemicals used in explosives.

But traditional terahertz spectroscopy requires a radiation source that’s heavy and about the size of a large suitcase, and it takes 15 to 30 minutes to analyze a single sample, rendering it impractical for most applications.

In the latest issue of the journal Optica, researchers from MIT’s Research Laboratory of Electronics and their colleagues present a new terahertz spectroscopy system that uses a quantum cascade laser, a source of terahertz radiation that’s the size of a computer chip. The system can extract a material’s spectroscopic signature in just 100 microseconds.

The device is so efficient because it emits terahertz radiation in what’s known as a “frequency comb,” meaning a range of frequencies that are perfectly evenly spaced.

“With this work, we answer the question, ‘What is the real application of quantum-cascade laser frequency combs?’” says Yang Yang, a graduate student in electrical engineering and computer science and first author on the new paper. “Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy.”

Different materials absorb different frequencies of terahertz radiation to different degrees, giving each of them a unique terahertz-absorption profile. Traditionally, however, terahertz spectroscopy has required measuring a material’s response to each frequency separately, a process that involves mechanically readjusting the spectroscopic apparatus. That’s why the method has been so time consuming.

Because the frequencies in a frequency comb are evenly spaced, however, it’s possible to mathematically reconstruct a material’s absorption fingerprint from just a few measurements, without any mechanical adjustments.

Getting even

The trick is evening out the spacing in the comb. Quantum cascade lasers, like all electrically powered lasers, bounce electromagnetic radiation back and forth through a “gain medium” until the radiation has enough energy to escape. They emit radiation at multiple frequencies that are determined by the length of the gain medium.

But those frequencies are also dependent on the medium’s refractive index, which describes the speed at which electromagnetic radiation passes through it. And the refractive index varies for different frequencies, so the gaps between frequencies in the comb vary, too.

To even out their lasers’ frequencies, the MIT researchers and their colleagues use an oddly shaped gain medium, with regular, symmetrical indentations in its sides that alter the medium’s refractive index and restore uniformity to the distribution of the emitted frequencies.

Yang; his advisor, Qing Hu, the Distinguished Professor in Electrical Engineering and Computer Science; and first author David Burghoff, who received his PhD in electrical engineering and computer science from MIT in 2014 and is now a research scientist in Hu’s group, reported this design in Nature Photonics in 2014. But while their first prototype demonstrated the design’s feasibility, it in fact emitted two frequency combs, clustered around two different central frequencies, with a gap between them, which made it less than ideal for spectroscopy.

In the new work, Yang and Burghoff, who are joint first authors; Hu; Darren Hayton and Jian-Rong Gao of the Netherlands Institute for Space Research; and John Reno of Sandia National Laboratories developed a new gain medium that produces a single, unbroken frequency comb. Like the previous gain medium, the new one consists of hundreds of alternating layers of gallium arsenide and aluminum gallium arsenide, with different but precisely calibrated thicknesses.

Getting practical

As a proof of concept, the researchers used their system to measure the spectral signature of not a chemical sample but an optical device called an etalon, made from a wafer of gallium arsenide, whose spectral properties could be calculated theoretically in advance, providing a clear standard of comparison. The new system’s measurements were a very good fit for the etalon’s terahertz-transmission profile, suggesting that it could be useful for detecting chemicals.

Although terahertz quantum cascade lasers are of chip scale, they need to be cooled to very low temperatures, so they require refrigerated housings that can be inconveniently bulky. Hu’s group continues to work on the design of increasingly high-temperature quantum cascade lasers, but in the new paper, Yang and his colleagues demonstrated that they could extract a reliable spectroscopic signature from a target using only very short bursts of terahertz radiation. That could make terahertz spectroscopy practical even at low temperatures.

“We used to consume 10 watts, but my laser turns on only 1 percent of the time, which significantly reduces the refrigeration constraints,” Yang explains. “So we can use compact-sized cooling.”

“This paper is a breakthrough, because these kinds of sources were not available in terahertz,” says Gerard Wysocki, an assistant professor of electrical engineering at Princeton University. “Qing Hu is the first to actually present terahertz frequency combs that are semiconductor devices, all integrated, which promise very compact broadband terahertz spectrometers.”

“Because they used these very inventive phase correction techniques, they have demonstrated that even with pulsed sources you can extract data that is reasonably high resolution already,” Wysocki continues. “That’s a technique that they are pioneering, and this is a great first step toward chemical sensing in the terahertz region.”